DRIVE SYSTEM AND POWER ELECTRONICS ENT 289 Lecture 4:

DC Motors

Contents

• Introduction

• Principle of Operations

• Counter-Electromotive Force (C-EMF)

• Mechanical Power (P) and Torque (T)

• Speed of Rotation

• Speed Control

• Types of DC Motors

• Stopping a Motor

• Summary

Introduction

Several limitations:

• Regular Maintenance - commutator susceptible to mech wear

• Expensive – apart from motor have to consider dc converter

• Heavy

• Speed limitations

• Sparking – ionizing sparks from commutator

DC Motors – also known as brushed DC motors

• Electrical energy--- mechanical energy

• They drive devices such as hoists, fans, pumps.

• The torque-speed characteristic of the motor must be

adapted to the type of the load it has to drive.

Classification of Electrical Motors

Principle of Operations

Inducing a force on a conductor

There are 2 conditions which are necessary to produce a force on a conductor

I) The conductor must be carrying a current

II) The conductor must be within a magnetic field

• When this conductor exist, a force will be applied to the conductor, which will attempt to move the conductor in a direction perpendicular to magnetic field. This is the basic theory by which all DC motor operate

• Factors that determine the direction of rotation in a dc motor is a direction of armature current and direction of magnetic flux in field. This relationship is best explained by using FLEMING’S LEFT-HAND RULE FOR MOTORS

Left hand rule for motors

CEMF

• Motor armature resistance is very low

• When switch closed, a large current flows in the armature (20-30 times greater than nominal full load current of motor)

• Individual armature conductors, subjected to a force that add up to produce powerful torque, causing armature to rotate

Counter-Electromotive Force (CEMF)

• As the loop sides cut the magnetic field, (due to armature rotation) a voltage is induced in them, the same as it was in the loop sides of the dc generator. This induced voltage causes current to flow in the loop

• In the case of a motor, the induced voltage E0 is called Counter –Electromotive Force (CEMF) because its polarity always acts against the source voltage Es

• CEMF can never becomes as large as the applied voltage

• If there were no such thing as CEMF, much more current would flow through armature, and motor would run much faster

Direction of battery current Es

E0

The net voltage acting in armature circuit in Fig. A is (ES-E0) Volts

I = (ES-E0)/R

When the motor at rest, the induced voltage E0 = 0, and so the starting current is

I = ES /R

C-EMF (V) Source voltage

Armature resistance

Problem 1:

The armature of a permanent-magnet dc motor has a resistance of

1 Ω and generates a voltage of 50 V when the speed is 500 r/min.

If the armature is connected to a source of 150 V, calculate the

following:

a. The starting current

b. The counter-emf when the motor runs at 1000 r/min and also at

1460 r/min

c. The armature current at 1000 r/min and also at 1460 r/min

Problem 2

• The armature of a permanent-magnet dc motor has a resistance of 3.5 Ω and generates a voltage of 50 V when the speed is 500 r/min. If the armature is connected to a source of 200 V, calculate the following:

a. The starting current

b. The counter-emf when the motor runs at 1000 r/min and also at

1460 r/min

c. The armature current at 1000 r/min and also at 1460 r/min

Problem 3

• The armature of a permanent-magnet dc motor has a resistance of 3.5 Ω and generates a voltage of 60 V when the speed is 600 r/min. If the armature is connected to a source of 200 V, calculate the following:

a. The starting current

b. The counter-emf when the motor runs at 1200 r/min and also at

1500 r/min

c. The armature current at 1200 r/min and also at 1500 r/min

Acceleration of motor

• As speed increases, CEMF increases, hence (Es-E0) value diminishes and current I drops progressively

• Motor continue to accelerate until reach maximum speed

• At no load, this speed produce E0 slightly less than Es, and hence (Es-E0) and I would become zero

• The driving force would cease and mechanical drag would slow motor down

• As speed decreases net (Es-E0) increases and so does current I

• Speed will cease to fall when torque developed by armature current equal to the load torque

Mechanical power p [W] and torque T [N.m]

• The Power and Torque of a dc motor are two of its most important properties

Mechanical power p [W] and torque T [n.m]

Pa = IEs

Pa Electrical Power Supplied to the Armature

Total current supplied to the armature [A]

(armature current)

Eo = Zn / 60

Es = E0 + IR

Supply voltage

C-emf induced in a lap wound armature [V]

Pa = I Es

= I (E0+IR) = E0I + I2R

Heat dissipated in the armature

Electrical power that is converted into mechanical power

#Mechanical power of the motor is exactly equal to the product of the c-emf multiplied by the armature current

Value of the Induced Voltage

The voltage induced in a dc generator having a lap winding is given by:

Eo = Zn / 60

Eo = voltage between the brushes [V]

Z = total number of conductors on the armature [1 turn = 2 conductors]

n = speed of rotation [rev/min]

= flux per pole [Wb]

P = E0I

Where:

P = mechanical power developed by the motor [W]

Eo = induced voltage in the armature (cemf) [V]

I = total current supplied to the armature [A]

Turning our attention to torque T, mechanical power P is given by the expression:

P = nT/9.55

n is the speed of rotation [r/min]

nT/9.55 = E0I = ZnI/60 T = ZI / 6.28

Torque [N.m]

Number of conductors on the armature (normally fixed value)

Effective flux per pole [Wb]

Armature current

Constant [exact value = 2]

The torque developed by a lap-wound motor:

[N.m]

A constant to take care a units [exact value = 30/]

Hence, T proportional to .Ia

Speed of Rotation n [r/min]

IR Drop is always small compared to the supply voltage Es (when motor drives load between no-load and full load)

This means that c-emf E0 is very nearly equal to Es

Es = Zn / 60

Replacing E0 by Es

This important equation shows that the speed of the motor is directly proportional to the armature supply voltage and inversely proportional to the flux per pole

Es = E0 + IR

Supply voltage

C-emf induced in a lap wound armature [V]

IR drop

n = 60 Es

Z [rev/min]

Due to armature resistance

n also can be said to be proportional to E0 /

The circular symbol represents the armature circuit, and the squares at the side of the circle represent the brush commutator system. The direction of the arrows indicates the direction of the magnetic fields.

DC motor connections

Externally- excited DC motor

Shunt-DC motor Series DC motor

Compounded DC motor Cumulatively -compounded Differentially

-compounded

Types of DC Motors

Figure a shows an externally- excited DC motor. This type of DC motor is constructed such that the field is not connected to the armature. This type of DC motor is not normally used. Figure b shows a shunt DC motor. The motor is called a "shunt" motor because the field is in parallel to the armature. Figure c shows a series DC motor. The motor field windings for a series motor are in series with the armature. Figures d and e show a compounded DC motor. A compounded DC motor is constructed so that it contains both a shunt and a series field. Figure d is called a "cumulatively-compounded" DC motor because the shunt and series fields are aiding one another. This connection is known as short shunt. Figure e is called a "differentially-compounded" DC motor because the shunt and series field oppose one another. This connection is known as short shunt.

Problem 1.

A shunt motor rotating at 1500r/min is fed by a 120 V source. The line current is 51 A and the shunt-field resistance is 120 ohm. If the armature resistance is 0.1ohm, calculate the following: a) The current in the armature. b) The counter-emf c) The mechanical power developed by the motor.

Shunt Motor

Problem 2

A shunt motor rotating at 1800r/min is fed by a 150 V source. The line current is 51 A and the shunt-field resistance is 120 ohm. If the armature resistance is 0.1ohm, calculate the following: a) The current in the armature. b) The counter-emf c) The mechanical power developed by the motor.

Shunt Motor

For all DC motors, T proportional to Φ.Ia For shunt connection, Φ is assumed constant since Ish is assumed constant. Hence, T proportional to Ia (shown in graph T vs Ia) n proportional to (Es-IaRa)/Φ For shunt connection, n proportional to Es-IaRa (shown in graph n vs Ia) Since T proportional to Ia, Ia =kT n proportional to Es – kTRa (shown in graph T vs n)

Shunt Motor

Shunt Motor

• Even though there is slight drop of speed as load increased, armature reaction will weaken the field as armature current increased: This field weakening will increase the motor speed and thus compensate for speed decrease due to voltage drop.

• The characteristics of a shunt-wound motor give it very good speed regulation, and it is classified as a constant speed motor.

• Shunt motors are used in industrial and automotive applications such as to drive centrifugal and reciprocating pumps, light machine tools and drilling machines.

• Shunt motors are used where constant speed is required at low starting torque.

Series Motor

PROBLEM 1

A 15hp, 240V, 1780 r/min DC series motor

Has a full-load rated current of 54A. Its

Operating characteristics are given by the

Per-unit curves.

Calculate:

1. The current and speed when the load torque is 24Nm.

2. The efficiency under these conditions.

Series Motor

PROBLEM 2

A 15hp, 240V, 1878 r/min DC series motor

Has a full-load rated current of 55A. Its

Operating characteristics are given by the

Per-unit curves.

Calculate:

1. The current and speed when the load torque is :

• 30Nm.

• 40Nm

• 50Nm

2. Explain your observation based on question 1

Series Motor • For all DC motors,T proportional to Φ.Ia

• DC series motor are used where high starting torque is required.

• Since for series connection an increase in Ia will also increase series field current Ise by the same amount and hence, increasing Φ.

• For series motor T proportional to Ia2 up to saturation point where afterwards T will be proportional to Ia (graph T vs Ia)

• On light loads, flux will be very small so motor will be operating at high speeds (graph T vs n)

• As load increases, flux will also increase and the speed of motor will decrease.

Series Motor

• Series motor should never be started without mechanical load because the tendency to run away at no load.

• The advantage of a series-wound motor is that it develops a large torque when operated at low speed which make it suitable for starting heavy loads

• It is often used for industrial cranes and winches where very heavy loads must be moved slowly and lighter loads moved more rapidly. Other suitable use is for traction in electric trains.

Compounded motor

• A compound motor has both series field and shunt field compound motors are of two types.

• If the series field flux and shunt field add each other, it is called cumulative compounding.

• If the series field flux opposes the shunt field flux, it is called differential compounding.

• The motor is used in applications where intermittent high starting torque is required. Loads such as presses, punch shears, and rolling mills are often driven by compounded motors.

Ward-Leonard speed control system (type of armature speed control)

If flux per pole is kept constant, speed depends only on Es

Motor field current is constant. If Generator field current is

varied by us, ES varies; by connecting the motor

armature M to a separately excited variable voltage DC

generator G ; hence speed of motor varies. This is the

speed control.

Motor speed can be varied from zero to maximum in either

direction.

Used in steel mills, paper mills, mines and high rise

elevators.

Speed Control of Motor

MOTOR

Field Current is constant; so, Φ is constant

If resistance in Arm Rheostat increased,

Fixed value of Es is reduced by voltage

drop in rheostat and hence n is reduced

(1) Enables to reduce speed below nominal speed

(2) Recommended for small motors

(3) Lot of power wasted in the rheostat such that efficiency is low

(4) Poor speed regulation : IR drop (loss) is high as I increase which causes

substantial drop in speed with increasing mechanical load (not robust).

Speed control using an armature rheostat

(Armature Speed Control)

Place a rheostat in series with the

armature

EO = Z Φ n / 60

if IX changes, Φ changes

Φ reduces if resistance of Rheostat

increases.

Then n increases.

(1) Better for speed control of higher level speed

(2) Field loss is much less; IX is usually small ( 1-2 amps)

(3) Efficiency is better than that of armature speed control.

(4) Too much of reduction of field flux current increases the speed to

dangerously high level.

Speed control using a field rheostat

(Field Speed Control)

Starting a shunt motor

• Since at starting, CEMF = 0, hence if rated normal voltage applied to armature terminals, the starting current would be dangerously high.

• High starting current produces high mechanical stress and heating that would damage the winding.

• Large voltage drop at the supply line due to this would affect other machines working in the same supply line.

• Reduced voltage should be applied to the motor at the time of starting to limit the starting current to safe value.

• After the motor picks up speed and sufficient CEMF developed, then normal rated voltage could be applied.

• This method is achievable using starters: • Three point starter

• Four point starter

Stopping a motor

Electromechanical brake:

(1) Dynamic braking

(2) Plugging

Not simple to stop a large DC motor when a large DC motor coupled to a heavy

inertia load.

Must apply a braking torque to ensure a rapid stop.

One way to brake the motor is by simple mechanical friction, in the same way

we stop a car (car brake). Another way is through natural deceleration method

by removing power supply from the motor. This method is called coassting.

Or by more elegant method:

Consists of circulating reverse current in the armature, so as to brake the motor

electrically

Dynamic Braking

• Neglecting the armature IR drop, Eo = Es

• The direction of the armature current I1 and the polarity of the cemf Eo are as shown

1 Normal condition

2 Open the switch

• Motor continues to run, but its speed will gradually drop due to friction and windage

losses

• Shunt field still excited, induced voltage Eo continues to exist, falling at the same

rate as the speed.

• The motor is now a generator whose armature is on open circuit

1

2

3

• Voltage Eo will immediately produce an armature current I2

• However, this current flows in the opposite direction to the original current I1,

reverse torque is developed

• It follows that a reverse torque is developed whose magnitude depends upon I2

• The reverse torque brings the machine to a rapid, but very smooth stop

• Resistor R is chosen so that the initial braking current is about twice the rated

motor current. The initial braking torque is then twice the normal torque of the

motor

3 Connect to external resistor

Plugging

• We can stop the motor even more rapidly by using a method called plugging

• It consists of suddenly reversing the armature current by reversing the terminals of

the source

• The net voltage acting on armature circuit becomes (Es + Eo) not (Es – Eo). The

cemf become adds to the supply voltage Es

• This net voltage would produce reverse current, perhaps greater than full

load armature current. Can destroy segments and brushes

• To prevent - limit the reverse current by introducing a resistor in series with

reversing circuit

• As soon as the motor stops, we must immediately open the armature circuit,

otherwise it will begin to run in reverse

Reversing circuit

Resistor

Speed versus time for various braking method

Comparison to permanent magnet motors • Field motors requires coil and field current to produce flux

• Energy consumed, heat produced, and relatively large space taken up by field poles are disadvantages to dc motor

• If permanent magnets are used, these disadvantages can be overcome

• Another advantage is effective air gap is increased many times. Thus, armature reaction is reduced, improving commutation and overload capacity of motor

• Long air gap reduces inductance of armature, hence responds quickly to changes in armature current

• PM motors are particularly advantageous in capacities below 5hp

• Drawbacks of PM motors is relatively high cost of magnets and inability to obtain higher speeds by field weakening

DC Motor Theory Summary

• There are two conditions necessary to produce a force on a conductor

- The conductor must be carrying current.

- The conductor must be within a magnetic field.

• The function of torque in a DC motor is to provide the mechanical output to drive the piece of equipment that the DC motor is attached to.

• Torque is developed in a DC motor by the armature (current-carrying conductor) being present in the motor field (magnetic field).

• CEMF is developed in a DC motor by the armature (conductor) rotating (relative motion) in the field of the motor (magnetic field).

• The function of the voltage that is developed in a DC motor (CEMF) opposes the applied voltage and results in the lowering of armature current.

• The speed of a DC motor may be changed by using resistors to vary the field current and, therefore, the field strength.

• In a shunt-wound motor, the field is in parallel, or "shunts" the armature.

• In a series-wound motor, the field is in series with the armature.

• A compounded DC motor is constructed so that it contains both a shunt and a series field.

• A shunt-wound DC motor has a decreasing torque as speed increases.

• The characteristics of a shunt-wound motor give it very good speed regulation, and it is classified as a constant speed motor, even though the speed does slightly decrease as load is increased.

• A series-wound motor has a rapidly increasing torque when speed decreases. As load is removed from a series-wound motor, the speed will increase sharply.

• The advantages of a series-wound motor are that it develops a large torque and can be operated at low speed. It is a motor that is well-suited for starting heavy loads.

THE END!!!!!!!!!!!!