AC Motors and Drives

Level 4

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LO 2 Describe the construction features and operation of AC

Induction motors

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For industrial and mining applications, 3-phase AC induction motors are the prime movers for the vast majority of machines. These motors can be operated either directly from the mains or from adjustable variable frequency drives. In modern industrialized countries, more than half the total electrical energy used in those countries is converted to mechanical energy through AC induction motors. The applications for these motors cover almost every stage of manufacturing and processing. Applications also extend to commercial buildings and the domestic environment. They are used to drive pumps, fans, compressors, mixers, agitators, mills, conveyors, crushers, machine tools, cranes, etc.

3-phase AC induction motors

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2.1a Main constructional features of 3 phase A.C. squirrel cage and wound rotor induction motors

Construction of 3-Phase AC induction motors

Although the basic design of induction motors has not changed very much in the last 50 years, modern insulation materials, computer based design optimization techniques and automated manufacturing methods have resulted in motors of smaller physical size and lower cost per kW. International standardization of physical dimensions and frame sizes means that motors from most manufacturers are physically interchangeable and they have similar performance characteristics.

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The reliability of squirrel cage AC induction motors, compared to DC motors, is high. The only parts of the squirrel cage motor that can wear are the bearings. Slip-rings and brushes are not required for this type of construction. Improvements in modern pre-lubricated bearing design have extended the life of these motors

Although single-phase AC induction motors are quite popular and common for low power applications up to approx. 2.2 kW, these are seldom used in industrial and mining applications. Single-phase motors are more often used for domestic applications.

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The AC induction motor comprises 2 electromagnetic parts: Stationary part called the stator Rotating part called the rotor, supported at each end on

bearings The stator and the rotor are each made up of: An electric circuit, usually made of insulated copper or

aluminium, to carry current A magnetic circuit, usually made from laminated steel, to

carry magnetic flux

Basic construction

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The stator The stator is the outer stationary part of the motor, which consists of: The outer cylindrical frame of the motor, which is made either of welded sheet steel, cast iron or cast aluminium alloy. This may include feet or a flange for mounting. The magnetic path, which comprises a set of slotted steel laminations pressed into the cylindrical space inside the outer frame. The magnetic path is laminated to reduce eddy currents, lower losses and lower heating. A set of insulated electrical windings, which are placed inside the slots of the laminated magnetic path. The cross-sectional area of these windings must be large enough for the power rating of the motor. For a 3-phase motor, 3 sets of windings are required, one for each phase. 8

Stator and rotor laminations

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The rotor This is the rotating part of the motor. As with the stator above, the rotor consists of a set of slotted steel laminations pressed together in the form of a cylindrical magnetic path and the electrical circuit. The electrical circuit of the rotor can be either: Wound rotor type, which comprises 3 sets of insulated windings with connections brought out to 3 slip-rings mounted on the shaft. The external connections to the rotating part are made via brushes onto the slip-rings. Consequently, this type of motor is often referred to as a slip-ring motor.

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Wound rotor

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Squirrel cage rotor type, which comprises a set of copper or aluminium bars installed into the slots, which are connected to an end-ring at each end of the rotor. The construction of these rotor windings resembles a ‘squirrel cage’. Aluminium rotor bars are usually die-cast into the rotor slots, which results in a very rugged construction.

Even though the aluminium rotor bars are in direct contact with the steel laminations, practically all the rotor current flows through the aluminium bars and not in the laminations. 12

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Rotor bars are made from aluminium in a lamination stack

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The other parts

The other parts, which are required to complete the induction motor are: Two end-bells to support the two bearings, one at the drive-

end (DE) and the other at the non drive-end (NDE) Two bearings to support the rotating shaft, at DE and NDE Steel shaft for transmitting the torque to the load Cooling fan located at the NDE to provide forced cooling for

the stator and rotor Terminal box on top or either side to receive the external

electrical connections Name plate

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Assembly details of a typical AC induction motor

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Assembly details of a typical AC induction motor

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https://www.youtube.com/watch?v=CBFE-Bt7RjY

2.2a Principle of operation and operational characteristics of 3 phase A.C. induction motors

What is the operating principle of a 3ph induction motor?

An electric motor converts electrical energy into a mechanical energy which is then supplied to different types of loads. A.C. motors operate on an a.c. supply, and they are classified into synchronous, single phase and 3 phase induction, and special purpose motors. Out of all types, 3 phase induction motors are most widely used for industrial applications mainly because they do not require a starting device. A 3 phase induction motor derives its name from the fact that the rotor current is induced by the rotating magnetic field, instead of electrical connections. The operating principle of a 3 phase induction motor is based on the production of Rotating Magnetic Field. 18

Production of a rotating magnetic field

The stator of an induction motor consists of a number of overlapping windings offset by an electrical angle of 120°. When the primary winding or stator is connected to a three phase alternating current supply, it establishes a rotating magnetic field which rotates at a synchronous speed. The direction of rotation of the motor depends on the phase sequence of supply lines, and the order in which these lines are connected to the stator. Thus interchanging the connection of any two primary terminals to the supply will reverse the direction of rotation.

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Rotating magnetic field animation

The number of poles and the frequency of the applied voltage determine the synchronous speed of rotation in the motor’s stator. Motors are commonly configured to have 2, 4, 6 or 8 poles. The synchronous speed, a term given to the speed at which the field produced by primary currents will rotate, is determined by the following expression.

𝑛𝑆 = 120 ∗𝑓

𝑃 (𝑟𝑝𝑚)

𝑛𝑆 = synchronous speed of rotation (rpm) 𝑓 = supply frequency (Hz) 𝑃 = number of poles on the stator

P 50 Hz 60Hz

2 3000 rpm 3600 rpm

4 1500 rpm 1800 rpm

6 1000 rpm 1200 rpm

8 750 rpm 900 rpm

10 600 rpm 720 rpm

12 500 rpm 600 rpm

14 428.57 rpm 514.29 rpm 21

Production of magnetic flux A rotating magnetic field in the stator is the first part of operation. To produce a torque and thus rotate, the rotor’ bars must be carrying some current. In induction motors, this current comes from the rotor conductors. The revolving magnetic field produced in the stator cuts across the conductive bars of the rotor and induces an EMF.

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The rotor windings in an induction motor are either closed through an external resistance or directly shorted. Therefore, the EMF induced in the rotor causes current to flow in a direction opposite to that of the revolving magnetic field in the stator, and leads to a twisting motion or torque in the rotor.(Interaction). As a consequence, the rotor speed will not reach the synchronous speed of the RMF in the stator. If the speeds match, there would be no EMF. induced in the rotor, no current would be flowing, and therefore no torque would be generated. The difference between the stator (synchronous speed) and rotor speeds is called the slip.

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The slip varies with the load. An increase in load will cause the rotor to slow down or increase slip. A decrease in load will cause the rotor to speed up or decrease slip. The slip is expressed as a percentage and can be determined with the following formula:

s (%) = ns - nr

ns

x100

where:

ns = the synchronous speed in rpm

nr = the rotor speed in rpm

torque on rotor

synchronous speed

rotor speed

rotor slip

The difference between the speed of the rotating stator field and the rotor speed is called slip. The smaller the slip, the closer the rotor speed approaches the stator field speed.

Slip

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Slip

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The rotation of the magnetic field in an induction motor has the advantage that no electrical connections need to be made to the rotor. What results is a motor that is: Self-starting Explosion proofed (because of the absence of slip rings or

commutators and brushes that may cause sparks) Robust in construction Inexpensive Easier to maintain

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Motor torque and power

Work is carried out whenever a force - any force - causes motion. Work equals force times distance. For linear movement, power is expressed as work at a given point in time.

𝑃 = 𝑊 ∗ 𝑡 𝑊 ∗ 𝑠 = 𝐽 When it comes to rotation, power is expressed as torque (T) times rotating speed (ω).

𝑃 = 𝑇 ∗ 𝜔 (𝑵𝒎 ∗𝑟𝑎𝑑

𝑠=

𝐽

𝑠= 𝑊)

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The speed of a rotating object is determined by measuring the time it takes for a given point on the rotating object to make a complete revolution from its starting point. This value is generally expressed as revolutions per minute min-1 or RPM. If, for example, an object makes 10 complete revolutions in one minute, it has a speed of 10 min-1 which also is 10 RPM. So, rotational speed is measured as revolutions per minute, that is min-1.

Rotational speed

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1 𝑅𝑃𝑀

60= 1 𝑅𝑃𝑆 = 2𝜋

𝑟𝑎𝑑

𝑠

𝑇 = 𝑃

𝜔=

𝑃

2𝜋 ∗ 𝑛= (𝑊𝑠 = 𝑁𝑚)

If the speed is in rpm, then

𝑇 = 𝑃

𝜔=

𝑃 ∗ 60

2𝜋 ∗ 𝑛 (𝑁𝑚)

𝑇 = 9549.3 𝑃(𝑘𝑊)

𝑛(𝑟𝑝𝑚)= (𝑁𝑚)

Torque

If the speed is in rpm and power in kW, then

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Example of torque calculation

𝑇 =𝑃 ∗ 60

2𝜋𝑛=3000 ∗ 60

2𝜋 ∗ 1437= 20 𝑁𝑚 or

𝑇 = 9549.3 3

1437= 20 Nm

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Torque speed characteristic

The turning force applied to a pump is torque, not power. Power [kW or HP] blends torque with speed to determine the total amount of work to be carried out within a given time span.

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Starting torque (ST) / Locked-rotor torque (LRT) The torque produced when power is applied to a motor at rest, i.e. when the motor is energised at full voltage and the shaft is locked in place. This is the torque used to start accelerating the load. Pull-up torque (PUT) This term is used for the lowest point on the torque speed curve for a motor which is accelerating a load up to full speed. Most motors do not have a separate pull-up torque value, as the lowest point is found at the locked-rotor point. As a result, pull-up torque is the same as starting torque/locked-rotor torque for the majority of all motors.

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Breakdown torque (BT) The maximum torque that an AC motor develops with rated voltage applied at rated frequency without causing sudden drops in speed. This is also known as pull-out torque or maximum torque. Full-load torque (FLT) The torque required to produce rated power at full-load speed.

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The effect of varying rotor resistance on the torque–speed characteristic of a wound-rotor induction motor.

Torque speed characteristics of wound (slip rings) motor

It is possible to insert resistance into the rotor circuit of a wound rotor because the rotor circuit is brought out to the stator through slip rings. As the rotor resistance is increased, the pullout speed of the motor decreases, but the maximum torque remains constant.

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It is possible to take advantage of this characteristic of wound-rotor induction motors to start very heavy loads. If a resistance is inserted into the rotor circuit, the maximum torque can be adjusted to occur at starting conditions. Therefore, the maximum possible torque would be available to start heavy loads. On the other hand, once the load is turning, the extra resistance can be removed from the circuit, and the maximum torque will move up to near-synchronous speed for regular operation.

Briefly outline of the three main load types

Constant power The term "constant power" is used for certain types of loads where you need less torque as the speed is increased and vice versa. Constant power loads are usually found within metal-processing applications, e.g. drilling, milling, and similar processes.

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Constant torque As the name suggests, "constant torque" means that the amount of torque necessary to drive a machine is constant regardless of the speed involved. One example would be conveyors.

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Variable torque and power "Variable torque" is one of the most relevant category and is found in loads where low torque is required at low speeds and greater torque is needed as the speed increases. Centrifugal pumps and blowers are a typical examples. Having established that centrifugal pumps feature variable torque, we should sum up some of the characteristics of the centrifugal pump. The use of variable speed drives is determined by specific physical laws. In this case, these laws are known as affinity laws and describe the relationship between pressure differences and flows.

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Firstly, the rate of flow in a system is directly proportional to the speed. This is to say that if the pump runs 25% faster, the rate of flow will be 25% greater.

Secondly, the head of the pump will vary as the square of the change in speed. If the speed increases by 25%, the head increases by 56%.

Thirdly, and interestingly, power is proportional to the change in speed cubed. This means that if the required speed is reduced by 50%, this equals an 87.5% decrease in power consumption. 40

When a motor accelerates from zero to full speed, the torque produced can vary considerably. The amount of torque required by a given load also varies with speed. To match the motor to the relevant load, you need to ensure that the amount of torque available from the motor exceeds the torque required by the load at all times.

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In the example of a centrifugal pump has a full-load torque of 70 Nm, which corresponds to 22 kW at a nominal speed of 3000 min-1.

Example of a centrifugal pump

In this particular case, the pump requires 20% of the full-load torque when starting, i.e. approximately 14 Nm. After start-up, the torque drops slightly, and then increases to full-load value as the pump picks up speed.

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Matching motors and loads

When you need to establish whether the torque capability of a particular motor meets the requirements of a given load, you can compare the motor's speed-torque curve with the speed-torque curve of the load. The torque produced by the motor must exceed the torque requirements of the load at all times, including during acceleration and full speed.

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Starting current

As the motor accelerates, it starts by drawing a line current which corresponds to (550-800)% of the rated current. As the motor draws nearer to its rated speed, the line current diminishes. As one might expect, the motor loss is high during this initial start-up phase, so it should never be too long to avoid overheating.

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Run-up time

When we want to identify the correct motor size for pump loads, as is the case for centrifugal pumps, we should only concern ourselves with providing adequate torque and power at the nominal operation point, because the starting torque for centrifugal pumps is rather low. The run-up time, however, is short because the torque available for the acceleration is rather high. 45

In many cases, sophisticated motor protective systems and monitoring systems require a run-up time to be able to take the locked-rotor current into consideration. The run-up time for a motor and pump is estimated by means of the following formula:

tstart = the time it will take a pump motor to reach full-load speed n = motor full-load speed Itotal = Inertia that need to be accelerated that is motor shaft, rotor, pump shaft, and impellers Tacc = Acceleration torque. The real acceleration torque is the motor torque minus the torque of the pump at different speeds 46

Tacc can be estimated by the following formula:

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Number of starts per hour

Sophisticated motor monitoring systems can monitor the number of starts per hour for a given pump and motor. The reason why it is necessary to monitor the number of starts is that every time the motor starts and accelerates, the motor consumes a high starting current. The starting current heats up the motor. If the motor does not cool down, the continuous load from the starting current will heat up the motor’s stator winding considerably. Consequently, either the motor breaks down or the lifespan of the insulation system is reduced. Normally, the motor supplier is responsible for how many starts per hour the motor can handle. As regards the motor, the number of starts per hour can be calculated.

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Starting time

Motor data: Full-load speed (n) = 3000 min-1

Full-load torque = 11 Nm Locked-rotor torque (240% of full-load torque) = 26 Nm Breakdown (340% of full-load torque) = 37 Nm Inertia of motor shaft, rotor and fan (Imotor) = 0.0075 kgm2

Pump data: Inertia of pump shaft and impellers (Ipump) = 0.0014 kgm2

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𝐼𝑡𝑜𝑡𝑎𝑙 = 𝐼𝑚𝑜𝑡𝑜𝑟 + 𝐼𝑝𝑢𝑚𝑝 = 0.0075 + 0.0014 = 0.0089 𝑘𝑔𝑚2

𝑇𝑎𝑐𝑐 =𝐿𝑅𝑇 + 𝐵𝑇 − 𝐹𝐿𝑇

2=26 + 37 − 11

2= 26 𝑁𝑚

𝑡𝑠𝑡𝑎𝑟𝑡 =3000 ∗ 2𝜋 ∗ 0.0089

60 ∗ 26= 0.11 𝑠𝑒𝑐

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Power and efficiency h (eta)

There is a direct link between the power drawn from the electricity supply net, the power which the motor can supply to the pump via the shaft end, and the power delivered to the pump – which creates flow and pressure. This is often described in terms of electric power input, shaft power, and hydraulic power output.

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Three-phase motors windings The windings are connected in star(Y)-connection or in delta(Δ)-connection according to IEC 60034-8. This is done by wiring the terminal board as shown in the wiring diagram below. The marking of the terminal board is also defined in IEC 60034-8.

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Star(Y)-connection By short-circuiting the terminals W2, U2 and V2 and connecting the mains to the W1, U1 and V1 you get a star(Y)-connection.

Current: Iphase = Iline

Voltage: Vphase = Vline / √3

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Delta(Δ)-connection When connecting the end of a phase to the start of another phase you get a delta(Δ)-connection

Current: Iphase = Iline / √3 Voltage: Vphase = Vphase

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The direction of rotation for the motor shaft is defined in IEC 60034-8 as either CW (clockwise) or CCW (counter-clockwise), when looking into the shaft. The direction of rotation can also be changed at the terminal board. When dealing with a three-phase motor, this is done by interchanging two of the line cables, e.g.: switch L1 and L2. When dealing with a single-phase motor always check the wiring diagram, before attempting to change direction of rotation.

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IEC 62114 Electrical insulation systems - Thermal classification Insulation classes (temperature classes) and temperature rise (ΔT) are defined in IEC 62114. As a standard, EFF 2 motors are made to operate in ambient temperatures up to 40˚C, and EFF 1 motors are normally made to operate in ambient temperatures up to 60˚C. The maximum acceptable temperature rise at rated load and voltage is according to class B. This implies that the motors are considered to be cold because their maximum temperature rise is 80K.

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The limit temperatures only apply for operation at the installation and for nameplate data for continuous operation. During operation in different operating conditions and with different supply voltages, limits for temperature increase or temperature limits at rated duty point can be exceeded.

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CE marking The CE mark is the manufacturer’s/importer’s proof that the product meets the requirements stated in the relevant EU Directives and EN standards All products included in EU Directives and which are sold within EEA countries (EU member states, Norway, Iceland and Liechtenstein) have to carry the CE mark. The objective of the CE marking is to ensure and harmonise the safety level as to mechanical and electrical risks in connection with machines or electric devices in the EU. Once the product carries the CE mark, no country within the EEA can prohibit or prevent the product from being sold or installed.

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Speed control of induction motor

A properly designed variable-frequency induction motor drive can be very flexible. It can control the speed of an induction motor over a range from as little as 5 percent of base speed up to about twice base speed. However, it is important to maintain certain voltage and torque limits on the motor as the frequency is varied, to ensure safe operation.

𝑛𝑠 = 120𝑓

𝑃 (𝑟𝑝𝑚) so the only ways in which the synchronous

speed of the machine can be varied are by changing the electrical frequency by changing the number of poles on the machine. Slip control may be accomplished by varying either the rotor

resistance or the terminal voltage of the motor.

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Speed control by changing the line frequency

When a motor is running at speeds below its base speed, it is necessary to reduce the terminal voltage applied to the stator for proper operation. The terminal voltage applied to the stator should be decreased linearly with decreasing stator frequency. This process is called de-rating. If it is not done, the steel in the core of the induction motor will saturate and excessive magnetisation currents will flow in the machine.

According Faraday’s law: 𝑣 𝑡 = −𝑁𝑑∅

𝑑𝑡

∅ 𝑡 =−1

𝑁 𝑣 𝑡 𝑑𝑡 =

−1

𝑁 𝑉𝑚𝑠𝑖𝑛𝜔𝑑𝑡 =

𝑉𝑚

𝜔𝑁𝑐𝑜𝑠𝜔𝑡 =

𝑉𝑚

2𝜋𝑓𝑁𝑐𝑜𝑠𝜔𝑡

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To avoid excessive magnetisation currents, it is customary to decrease the applied stator voltage in direct proportion to the decrease in frequency whenever the frequency falls below the rated frequency of the motor. When the voltage applied to an induction motor is varied linearly with frequency below the base speed, the flux in the motor will remain approximately constant. Therefore, the maximum torque that the motor can supply remains fairly high. However, the maximum power rating of the motor must be decreased linearly with decreases in frequency to protect the stator circuit from overheating. The power supplied to a three-phase induction motor is given by If the voltage VL is decreased, then the maximum power P must also be decreased, or else the current flowing in the motor will become excessive, and the motor will overheat.

𝑃 = 3𝑉𝐿𝐼𝐿𝑐𝑜𝑠𝜑

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Variable frequency speed control in an induction motor

The family of torque–speed characteristic curves for speeds below base speed, assuming that the line voltage is de-rated linearly with frequency.

The family of torque–speed characteristic curves for speeds above base speed, assuming that the line voltage is held constant.

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Speed control by changing the line voltage

The torque developed by an induction motor is proportional to the square of the applied voltage. If a load has a torque–speed characteristic, then the Speed of the motor may be controlled over a limited range by varying the line voltage. This method of speed control is Sometimes used on small motors driving fans.

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Electric Motor Classifications by Electrical Type

2.1 b & 2.2b Main constructional features, principle of operation and operational characteristics of single phase A.C. induction

motors

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Single-phase AC motors are as ever-present as they are useful -- serving as the prime power sources for a seemingly limitless array of small-horsepower applications in industry and in the home. Where three-phase power is unavailable or impractical, it's single-phase motors to come in. A squirrel-cage motor connected to a single-phase line develops no starting torque, but having been started by some external means, it runs approximately like a poly-phase motor. The many types of single-phase motors are distinguished by the means by which they are started.

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The fundamental single-phase AC induction motor consists of two basic parts: 1. Stator. The stator is constructed of a set of stacked laminated discs which are surrounded by a stator winding. This winding is connected to the power supply (voltage, phase and frequency) and produces a magnetic field that revolves around the motor at a speed designated “synchronous.” 2. Rotor. The rotor is connected to the output shaft and consists of a shorted aluminium winding which is cast into slots and stacked and joined at both ends of the stack with end rings. The rotor acts as a conductor which when placed in the magnetic field of the stator winding creates a magnetic field of its own and interacts with the magnetic field of the stator, producing torque.

Construction of the single phase motor

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The first principle applies to the magnetic field created by the stator and the second applies to the rotor as it rotates within the stator field.

Flux Patterns Produced in Stator

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The interaction of these two magnetic fields produces a mechanical force on the wire which is the basis for the production of torque.

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Principle of operation of single phase motor

https://www.youtube.com/watch?v=awrUxv7B-a8

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A standard single-phase stator has two windings placed at an angle of 90° to each other. One of these windings is known as the main winding, while the other is called the auxiliary winding or starting winding. Depending on the number of poles, each winding may be distributed across several sub-coils.

An example of a 2-pole, single-phase winding with four sub-coils in the main winding and two sub-coils in the auxiliary winding.

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Basic types of single-phase induction motors

Theoretically, a single-phase motor could be started by spinning the motor mechanically and applying power immediately. In actual practice, however, all motors use automatic starting of some sort. Single-phase induction motors are often known by the names of the starting method used.

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Split-Phase Motors (SP) or Resistance start/Induction run motors

A split-phase motor’s components are a main winding, start winding and a centrifugal switch.

The main (run) winding is designed for operation from 75% synchronous speed and above. The main winding design is such that the current lags behind the line voltage because the coils embedded in the steel stator naturally build up a strong magnetic field which slows the build up of current in the winding.

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The start winding is not wound identically to the main, but contains fewer turns of much smaller diameter wire than that of the main winding coils. This is required to reduce the amount the start current lags the voltage.

When both windings are connected in parallel across the line, the main and start winding currents will be out of time phase by about 30 degrees. This forms a sort of imitation of a weak rotating flux field which is sufficient to provide a moderate amount of torque at standstill and start the motor.

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The total current that this motor draws while starting is the vector sum of the main and start winding currents. Because of the small angle between these two, the line current during starting (inrush current) of split-phase motors is quite high. Also the small diameter wire in the start winding carries a high current density, so that it heats up very rapidly. A centrifugal switch mechanism (or relay) must be provided to disconnect the start winding from the circuit once the motor has reached an adequate speed to allow running on the main winding only.

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General Performance Characteristic (SP Motor)

SP motors are well suited for small grinders, fans, and other applications with low starting torque and power needs from 60 W to 250 W. They are not suitable for applications which require high torques or high cycle rates.

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Steel Housing Single-Phase Split-Phase Start Fractional Horsepower Induction Motor

Model Output HP

Voltage V

Frequency Hz

Speed rpm

Current A

YUG-7112 1/3 220 50 2800 3

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Capacitor-Start Induction Run Motors (CSIR)

This is the largest group of single-phase motors. It should be noted that the capacitor-start motor utilizes the same winding arrangement as the split-phase motor, but adds a capacitor in series with the start winding. The main (run) winding current remains the same as in the split-phase case, but the start winding current is very much different. With the capacitor in the circuit, the starting current now leads the line voltage, rather than lagging as does the main winding. The start winding is also different, containing slightly more turns in its coils than the main winding and utilizing wire diameters only slightly smaller than those of the main.

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The new result is a time phase shift closer to 90 degrees than with the split-phase motor. A stronger rotating field is therefore created and starting torque is higher than with the split-phase design. Also the vector Sum of the main and Start Winding currents is lower, resulting in a reduction in the inrush current as compared to the split-phase design.

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Again, a centrifugal switch and mechanism (or relay) must be used to protect the start winding and capacitor from overheating. When the capacitor-start motor is running near full load RPM, its performance is identical to that of the split-phase motor.

General Performance Characteristic (CSIR Motor)

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CSIR motors have a relatively high starting torque, between 50 to 250 per cent of the full-load torque. This makes them a good single-phase motor choice for loads that are difficult to start, e.g. for conveyors, air compressors, and refrigeration compressors.

AC Electric Motors 1 Phase Capacitor Start - Induction Run Type 110Volt 50HZ for High Starting Torque Applications

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Permanently Split Capacitor Motors (PSC)

The windings of the PSC motor are arranged like those of the split-phase and capacitor-start designs, but a capacitor capable of running continuously replaces the intermittent duty capacitor of the capacitor-start motor and the centrifugal switch of both the split-phase and capacitor-start motors. The main winding remains similar to the previous designs, current lags the line voltage.

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The design of PSC motors means that they cannot provide the same initial boost as motors with separate capacitors. Their starting torques are quite low, between 30 to 80% of rated load, so they cannot be used for applications which are hard to start. This is offset by their low starting currents - usually less than 200% of rated load current – which makes them the perfect choice for applications with high cycle rates.

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However, the real strength of the permanently split capacitor design is derived from the fact that the start winding and capacitor remain in the circuit at all times and produce an approximation of two-phase operation at the rated load point. This results in better efficiency, better power factor than in equivalent capacitor-start and split-phase designs.

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General Performance Characteristic (PSC Motor)

Different starting and running characteristics can be achieved by varying the rotor resistance.

Permanent-split capacitor motors can be used for many different applications, depending on their design. Low-inertia loads such as fans and pumps would be a common example.

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US Motors PSC, Direct Drive Fan, 1/5 HP, 1-Phase, 1050 RPM Motor Permanent Split Capacitor Direct Drive Fan & Blower 5.6 Diameter Open Air Over (OAO) Without capacitor

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Capacitor Start-Capacitor Run Motors (CSCR)

These motors have a run capacitor and an auxiliary winding permanently connected in parallel with the main winding. In addition, a starting capacitor and a centrifugal switch are also in parallel with the run capacitor. The switch disconnects as the motor accelerates. It should be noted that the capacitor start-capacitor run motor utilises the same winding arrangement as the permanently split capacitor motor when running a full load speed and the same winding Arrangement as a capacitor-start motor during start up.

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The advantage of the capacitor start-capacitor run design is derived from the fact that the start winding and capacitor remain in the circuit at all times (similar to PSC type motor) and produce an approximation of two-phase operation at the rated load point, plus with an additional capacitor in series with the start winding circuit (similar to the capacitor-start type motor), the starting current now leads the line voltage, rather than lagging as does the main winding, dramatically increasing starting torque. Capacitor start-capacitor run motors feature a low running current due to an improved power factor caused by the run capacitor.

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This results in better efficiency, better power factor, increased starting torque than in equivalent capacitor-start and split-phase designs. The capacitor start-capacitor run motor is basically a combination of the capacitor-start and PSC motor types and is the best of the single-phase motors.

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General Performance Characteristic (CSCR Motor)

CSCR motors are the most powerful single-phase motors and can be used for quite demanding applications, e.g. high-pressure water pumps and vacuum pumps and other high-torque applications which require 1.1 to 11 kW.

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Open view of Capacitor start / Capacitor run Motor

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Shaded Pole Motors

The shaded pole motor differs widely from the other single-phase motors. All of the other designs contain a main and start winding, differing only in details of the starting method and corresponding starting circuitry. The shaded pole motor is the most simply constructed and therefore the least expensive of the single-phase designs. It consists of a run winding only plus shading coils which take the place of the conventional start winding.

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Construction of a typical shaded pole motor

The stator is of salient pole construction, having one large coil per pole wound directly in a single large slot. The shading coils are short circuited copper straps which are wrapped around one pole tip of each pole.

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The placement and resistance of the shading coil is chosen so that, as the stator magnetic field increases from zero at the beginning of the AC cycle to some positive value, current is induced in the shading coil. As previously noted, this current will create its own magnetic field which opposes the original field. The net effect is that the shaded portion of the pole is weakened and the magnetic centre of the entire pole is located at point “a”. As the flux magnitude becomes nearly constant across the entire pole tip at the top of the positive half cycle, the effect of the shading pole is negligible and the magnetic centre of the pole shifts to point “b”. As slight as this shift is, it is sufficient to generate torque and start the motor.

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Therefore, the shaded pole motor efficiency suffers greatly due to the presence of winding harmonic content, particularly the third harmonic which produces a dip in the speed torque curve at approximately 1/3 synchronous speed . In addition there are losses in the shading coils. These factors combine to make the shaded pole the least efficient and noisiest of the single-phase designs. It is used mostly in air moving applications where its low starting torque and the third harmonic dip can be tolerated.

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General Performance Characteristic (Shaded Pole Motor)

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Summary of Five Single-Phase Motor Types

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Single-phase AC motors are not all equal. There are five basic types, all with different operating characteristics and capabilities. The differences between each motor type are great enough that it is important for the user to understand each motor type, where it makes sense to apply them and how to apply them.

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Special conditions

A number of special conditions apply to single phase motors compared to three-phase motors. Single-phase motors should never run idle because they become very warm at small loads, therefore it is not recommended to run the motor less than 25% of full-load. When the motor runs with an asymmetrical rotating field, the current in one or both windings may be bigger than the mains current. These currents cause a loss, and so one or both windings (which is often seen in case of no load) will become too warm, even though the mains current is relatively small.

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Example of asymmetrical operation, where the current in the two phases is bigger than the mains current

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Example of run of currents as a function of the load. Please note that in the operating and starting phases the currents are bigger than the mains current at 0% load.

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Voltage issues

It is important to be aware that voltages can be higher than the mains voltage inside the motor. This is also true for symmetrical operation.

Example of voltage internally in the 1-phase motor. At Vsupply = 230 V voltages may be VC =370 V and VA = 290 V

2.3 Various starting methods for A.C. induction motors

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Starting methods

Today, various methods for starting motors are available. Changes, such as higher starting currents for new energy-efficient motor designs, mean greater focus on starting methods. This is closely linked to the fact that power quality has become a much more important issue in recent years, which has in turn led to greater emphasis on voltage transients associated with the start-up of large motors.

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The principle objective of all methods of motor starting is to match the torque characteristics to those of the mechanical load, while ensuring that the peak current requirements do not exceed the capacity of the supply. Many starting methods are available, each of which has slightly different characteristics. As regards those starting methods which are to reduce the locked-rotor current, the run-up time must not be too long. Excessive run-up times will cause unnecessary heating of the winding.

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Inrush current or locked-rotor current? When a motor is energized, the resulting initial current transient is known by various names: inrush current, starting current, or locked-rotor current. Mostly, these terms all refer to the same thing: a very large current – five to ten times the full-load current – flows initially. This surge current drops as the motor accelerates up to its running speed.

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Different starting methods are used in order to reduce the starting current to comply with local laws and regulations. Naturally, avoiding huge voltage drops on the mains is a significant objective in its own right. The most common starting methods: Direct-on-line starting, Star-delta starting, Auto-transformer starting, Stator series resistors Soft starting Frequency converter starting.

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Direct-on-line starting As the name suggests, direct-on-line starting means that the motor is started by connecting it directly to the supply at rated voltage. Direct-online starting, (DOL), is suitable for stable supplies and mechanically stiff and well-dimensioned shaft systems – and pumps qualify as examples of such systems.

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Advantages DOL starting is the simplest, cheapest and most common starting method. Furthermore it actually. It is the obvious choice wherever the supply authority’s current limiting restrictions allow for its use. Power plants may have varying rules and regulations in different countries; for example, three-phase motors with locked-rotor currents above 60 A must not use direct-on-line starting in Denmark. In such cases, it will obviously be necessary to select another starting method. Motors that start and stop frequently often have some kind of control system, which consist of a contactor and overload protection such as a thermal relay.

Small motors which do not start and stop frequently need only very simple starting equipment, often in the form of a hand-operated motor protection circuit breaker.

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Drawbacks However, the limitation of this method is that it results in a high starting current, often several times the rated current of the motor. Also the starting torque is very high, and may result in high stresses on the couplings and the driven application. Full voltage is switched directly onto the motor terminals. For small motors, the starting torque will be 150% to 300% of the full-load value, while the starting current will be 300% to 800% of the full-load current or even higher. Even so, it is the preferred method except when there are special reasons for avoiding it.

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Current – speed and Torque – speed characteristics

Control wiring diagram 110

Typical circuit diagram of DOL starter

Power wiring diagram

Control wiring diagram 111

Typical circuit diagram of F/R DOL starter

Power wiring diagram

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Star-delta starting The objective of this starting method, which is used with three-phase induction motors, is to reduce the starting current. In starting position, the stator windings is connected in star (Y). In the running position, the stator windings is reconnected to delta (Δ) once the motor has gained speed.

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Advantages Normally, low-voltage motors over 3 kW will be dimensioned to run at either 400 V in delta (Δ) connection or at 690 V in star (Y) connection. The flexibility provided by this design can also be used to start the motor with a lower voltage. Star-delta connections give a low starting current of only about one third of that found with direct-on-line starting. Star-delta starters are particularly suited for high inertias, where the load are initiated after full load speed.

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Drawbacks They also reduce the starting torque to about 33%. This method can only be used with induction motors that are delta connected to the supply voltage. If the changeover from star to delta takes place at too low a speed, this can cause a current surge which rises almost as high as the corresponding DOL value. During the even small period of switch over from start to delta connection the motor looses speed very rapidly, which also calls for higher current pulse after connection to delta.

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Current – speed and Torque – speed characteristics

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Typical power circuit diagram of Star-Delta starter

F1 Fuses F2 Thermal cut-out K1 Main contactor K2 Delta contactor K3 Star contactor M1 Three-phase motor

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Typical control circuit diagram of Star-Delta starter

S1 Stop push button NO S2 Start push button NC K4 timer

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Comparison of DOL and star-delta starting

The DOL starting method features a very high locked-rotor current which eventually flattens and becomes constant. The star-delta starting method features a lower locked-rotor current, but peaks during the starting process as the changeover from star to delta is made.

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Auto-transformer starting

As the name clearly states, auto-transformer starting makes use of an auto-transformer coupled in series with the motor during starting.

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The auto-transformer starter contains transformers, often featuring two or more voltage reductions, which reduce voltage to provide low-voltage starting by tapping off the secondary voltage of the auto-transformer, usually at approximately 50% – 80% of full voltage. More than one tapping can be used, depending on the starting torque/current required. Of course, reduced voltage to the motor will result in reduced locked-rotor current and torque, but this method gives the highest possible motor torque per line ampere. At no point in time is the motor not energised, so it will not loose speed as is the case with star-delta starting. The time of the switch between reduced and full voltage can be adjusted to suit specific requirements.

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Current – speed and Torque – speed characteristics

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Drawbacks Besides a reduced starting rotor torque, the autotransformer starting method has yet another disadvantage. Once the motor has started running, it is switched over to the mains voltage – this will cause a current pulse.

Torque versus voltage

𝑉𝑜𝑙𝑡𝑎𝑔𝑒2 2

𝑉𝑜𝑙𝑡𝑎𝑔𝑒12 =

𝑇𝑜𝑟𝑞𝑢𝑒2𝑇𝑜𝑟𝑞𝑢𝑒1

𝑉2

2

𝑉12 =

𝑇2𝑇1

The values for starting torque are reduced at a rate corresponding to the square of the reduction in volts.

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Torque – speed characteristics at various voltages

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Typical power circuit diagram of Auto-transformer starter

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Typical control circuit diagram of Auto-transformer starter

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Stator’s Series Resistance starter

In this method, a voltage-dropping resistance is placed in series with the motor during starting. The impedance seen by the power system then is that of the resistance plus that of the motor.

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Starting Characteristics: Motor terminal voltage is reduced from line voltage. Motor current equals line current. Starting torque is reduced by the square of the terminal voltage. Applications: Usually on low voltage (less than 600 v). Where current reduction requirements are low, or where load torque during acceleration is minimal. Not often used with large motors because of the high heat loss in the resistors.

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Typical wiring circuit diagram of Stator’s Series Resistors starter

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Soft starting A soft starter is, a device which ensures a soft start of a motor.

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Advantages Soft starters are based on semiconductors. Via a power circuit and a control circuit, these semiconductors reduce the initial motor voltage. This results in lower motor torque. During the starting process, the soft starter gradually increases the motor voltage, thereby allowing the motor to accelerate the load to rated speed without causing high torque or current peaks. Soft starters can also be used to control how processes are stopped. Soft starters are less expensive than frequency converters.

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Current – speed and Torque – speed characteristics

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Possible wiring diagram

R, S, T terminals of soft starter are input terminals while U, V, W are output terminals. QF-auto air breaker, KM-contactor, RJ-over heating protection relay, RD1-fuse,

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Drawbacks They do, however, share the same problem as frequency converters: they may inject harmonic currents into the system, and this can disrupt other processes. The starting method also supplies a reduced voltage to the motor during start-up. The soft starter starts up the motor at reduced voltage, and the voltage is then ramped up to its full value. Run-up time and locked-rotor current (starting current) can be set.

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Frequency converter starting (VSD or VFD) Frequency converters are designed for continuous feeding of motors, but they can also be used for start-up only.

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Current – speed and Torque – speed characteristics

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Drawbacks Even so, frequency converters are still more expensive than soft starters in most cases; and like soft starters, they also inject harmonic currents into the network.

Advantages The frequency converter makes it possible to use low starting current because the motor can produce rated torque at rated current from zero to full speed. Frequency converters are becoming cheaper all the time. As a result, they are increasingly being used in applications where soft starters would previously have been used.

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P2.4 Basic calculation of A.C. induction motor parameters from its name plate

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Output power = 18.5 kW 3 phase supply voltage = 400/690 Δ/Y ( +/- 5%) Frequency = 50 Hz Line current = 32/18.55 A Rotational speed = 2935 rpm Power factor = 0.91

To calculate: 1. Input power (kW) 2. Total power loss (W) 3. Efficiency (%) 4. Slip (%) 5. Torque(Nm) 6. Frequency in the rotor circuit 7. Number of poles

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1. 𝑃𝑖𝑛 = 3 ∗ 𝑉𝐿 ∗ 𝐼𝐿 ∗ 𝑐𝑜𝑠𝜑

𝑃𝑖𝑛 = 3 ∗ 400 ∗ 32 ∗ 0.91 = 20174.93 W = 20.18 kW 2. 𝑃𝐿𝑜𝑠 = 𝑃𝑖𝑛 − 𝑃𝑜𝑢𝑡 = 20175 − 18500 = 1675 𝑊

3. 𝜂 =𝑃𝑜𝑢𝑡

𝑃𝑖𝑛=

18500

20175= 0.917 = 91.7%

4. 𝑇 =𝑃𝑜𝑢𝑡∗60

2𝜋𝑛𝑟=

18500∗60

2𝜋∗2935= 60.2 (𝑁𝑚)

5. 𝑠 =𝑛𝑠−𝑛𝑟

𝑛𝑠∗ 100% =

3000−2935

3000∗ 100% = 2.17%

6. 𝑓𝑟 = 𝑠 ∗ 𝑓 = 0.0217 ∗ 50 = 1.1 𝐻𝑧

7. 𝑃 = 120𝑓

𝑛𝑠= 120

50

3000= 2

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