PROTECTION OF

MOTORS

AC Motors – Classification:• Synchronous Motors• Induction motors

– Squirrel cage • Single cage • Double cage

– Slip ring induction motors. • Induction motors are also available which work in single-phase.

– Split phase induction motor – Capacitor induction motors, – Universal induction motors, – shaded pole induction motors, – Repulsion motors.

Induction Motor:

Operating principle: • Works on the induction principle, so is called an

induction motor. • It has a rotor and a stator. • AC supply is given to the stator. • Stationary coils in the stator produces magnetic flux

rotating in nature when supplied with polyphase supply.

• Rotating flux induces currents when linked with shorted rotor conductors and produce a current.

• This current interacts with the rotating magnetic field of the stator, which produces a mechanical force.

Protection of induction motor:• Over load Protection:

– Thermal or magnetic overload relays are incorporated in the motor starter to provide protection against overloads.

– Taking notice of transient over loads lasting ten to fifteen seconds that occur during starting period.

– Whenever the overload relay trips due to overload, it has to be reset.

– Both manual and automatic resetting is possible by suitably adjusting the tripping mechanism.

– But due care should be taken while choosing the one so as to avoid any damage to the motor.

• The common causes for overloads – Excessive belt tension, – Rubbing of rotor and stator, – Single phasing, machine jam, – Low voltage, – Hard bearings.

• Short circuit protection:– Provided by Fuses – Grounding of coils

• Single phasing in induction motor:– This causes the motor windings to be heated up and burnt. – Causes:

• Blown out fuse in transformer primary, • Blown out fuse in the motor branch circuit, • Loose or poor connection in the switch/starter/contactor. • Badly soldered winding connections in the motor. • Open circuit within the winding.

– Standstill motor will not start in single phasing condition and gives humming sound.

– If the fault occurs when the motor is running, it will continue to run with squeezing sound till it gets isolated by the protective device.

– Protection against single phasing is provided in the cases of large motors and critical duty motors.

• Protection against reversal of direction:– Required when reversal of direction causes danger, like in lifts.

• Any electrical equipment during its operation bound to develop some faults in it. A fault is a defect in an electrical circuit due to which the flow of current is diverted from the intended path. Certain distinctive feature of a fault is

• Fault impedance is very low.

• Fault currents are generally high.

• During the faults, the voltages of three phases become unbalanced and the supply to the neighboring circuits is affected.

• Fault currents being excessive, they can damage not only the faulty equipment, but also the installation through which the fault current is fed.

• Therefore it is necessary to disconnect the faulty part as early as possible.

• Faults cannot be totally avoided but can be minimized by

• Improved system design,

• Improved quality of components,

• Better and adequate protective system

• Better operation and maintenance

• Adequateness of protection is judged by considering the following aspects:

• Rating of the protected machine.

• Location of the protected machine

• Probability of abnormal condition due to internal and external causes

• Cost of machine, its importance in the process

• The type of protection used for a particular motor depends on the switchgear used for its control (starting, stopping, speed variation etc)

• In general the basic protections provided for every motor are

• Thermal overload protect

• Short circuit protection

• The switchgear used for motor control falls into two distinct classes

• Contactor starters with HRC fuse and thermal over current relays

• Circuit breakers and associated protective relays.

•  

• Contractors and fuses are used for motors upto approximately 150kW For larger motors circuit breakers are used.

• Direct acting overload trips devices such as thermal overload relay can be incorporated with the contactor starter. The protection against short circuits is proved by HRC fuse. The fuse selection depends on starting current. The fuse should blow at currents more than those which can be interrupted by the contactor.

• In case of voltage loss the coil is de-energized and the contactor opens. The motor has to be started again. Hence the contactor starter provides no volt release. Generally start, stop, reverse buttons are provided along with the starter..Large motors are provided with various relaying schemes and a circuit breaker. The circuit breaker is air break type or vacuum or SF6.

• Air break type circuit breakers are more popular. The closing mechanisms are manually operated or solenoid operated or spring closing type Solenoid closing is suitable for remote controlled motors and larger motors. Generally overload trip devices operating direct on the tripping mechanisms form an integral part of the circuit breaker.

• Abnormal Operating Conditions and causes of failures in Induction Motors:

• Mechanical overloads– Sustained overloads,-prolonged starting or

locked rotor– Stalling

• Abnormal supply conditions– Loss of supply voltage –unbalanced supply

voltage– Phase sequence reversal of supply voltage– Overvoltage – under-voltage – under frequency

• Faults in starting supply/circuit– Interruptions in phases –blowing of fuse/ single

phasing– Short circuit in supply cable

• Internal faults in motor itself– Caused by any of the above – Phase to phase faults-phase to earth faults

• Failure o phase (open circuit) mechanical failure

• The abnormal conditions are summarized below:

• Prolonged overloading: – It is ceased by mechanical loading, short time

cyclic overloading,. Overloading results in temperature rise of winding and deteriorating of insulation resulting in winding fault. Hence motor should be provided with overload protection.

• Single phasing: – One of the supply Ines gets disconnected due to

blowing of fuse or open circuit in one of the three supply connections. In such cases the motor continues to run on a single phase supply. If the motor is loaded to its rated full load, it will draw excessive currents on single phasing, the winding get overheated and damage is caused. The single phasing causes unbalanced load resulting in excessive heating of rotor due to negative sequence component of unbalanced current. Static single phasing relays are becoming very popular.

• Stalling:– If the motor does not start due to excessive

load, it draws heavy current. It should be immediately disconnected from supply

• Stator earth faults: – Faults in motor winding are mainly caused y

failure of insulation due to temperature rise.

• Phase to Phase faults:– There are relatively rare due to enough

insulation between phases. Earth faults are relatively more likely.

• Inter turn faults: – These grow into earth faults. No separate

protection is generally provided against inter turn faults

• Rotor faults:– These are likely to occur in wound rotor motors

due to insulation failure

• Failure of bearing: – This causes locking up of rotor. The motor

should be disconnected. Bearing should be replaced.

• Unbalanced supply voltage:– This causes heating up of rotor due to negative

sequence currents in staor winding

• Supply under voltage: – The under voltage supply cause increase in

motor current for the same load.

• Fault in starter or associated circuit:– The choice of protection for motor is depends

upon the size of the motor, its importance in the plant and nature of load.

•  

• Protection Requirements:

• Motor protection should be simple and economical. Cost of protective system should be within about 5% of motor cost. The motor protection should not operate during starting and permissible overloads. The choice of motor protection scheme depends upon the following:

• Size of motor, rated voltage, kw

• Type: squirrel cage or wound rotor

• Type of starter, switchgear and control gear

• Cost of motor and driven equipment

• Importance of process, whether essential service motor or not

• Type of load, starting current, possible abnormal condition etc.

Abnormal condition

Alternate forms of protection from which choice is made.

Remarks

Overloads Over load release

Thermal overload relays Inverse over current relays

Miniature circuit breaker with built in trip coils

Overload protection given for almost all motors

Should not trip during starting currents.

Phase faults and earth faults

HRC fuses High set instantaneous over

current relays Differential protection

Differential protection becomes economical for motors above abut 1000 kw.

Below this high set instantaneous protection is preferred.

Under voltage Under voltage release

Under voltage relays

Under voltage release incorporated with every starter

Under voltage relay used in certain applications

Unbalanced voltage

Negative phase sequence relays

Only in special applications

Reverse phase sequence

Phase reversal protection Generally at supply point

Prevents reversal of running. Single phasing Usual thermal overload relays

Special single phase preventer

Recently developed static single phasing devices becoming popular.

Unbalance protection Stalling Thermal relays

Instantaneous OC relays

Instantaneous Trip

Rotor faults Instantaneous Over current relays

Only for wound rotor motors

Switching surges RC surge suppressor 100 ohm, 0.1 micro Farad connected between phase and ground.

• Some of indications and inferences that can be drawn by observing the faulty motor:

• If al the coils are burnt equally, the motor has failed due to overloading only.

• Bursting of a coil may be due to rubbing of rotor or locked rotor., This occasionally occurs due to malfunctioning of brakes.

• Burning of only one or two sets of coils indicate the single phasing and loose contacts

• Rubbing of rotor occurs due to damaged bearing or unbalanced loads

• Bunt mark at terminal block or at one or two places, indicates over voltages or starting the motor without heating basically insulation failure.

• Partially burnt insulation all over the wining or crumbled insulation indicates distorted wave forms. E g. PWM drives, Induction furnaces etc.

Protections generally used in field:Current Driven Earth leakage Below 5% Core balance CT and Relay Earth fault 2-10% 4 CT or earth Return CT Unbalance 10-80% 3Phase CT and Relay Overload 70-110% Thermal overload relay Over current 120-200% IDMT Relay Stalling 200-1000% Motor protection relay Short circuit 200-4000% Instantaneous relays Fuses Differential 2-20% Basically for earth fault within a

particular zone. CT on both sides Voltage Driven Under Voltage Below 85% UV Relay Over Voltage Above 110% Over Voltage Relay Unbalance Below10% Single Phase Preventers Single Phasing Cumulative 10% Distorted wave form

• Bimetal Overload Devices:

• Triple pole bimetal relays are generally employed.

• Bending of one or more bimetal strips causes movement of common lever which in turn operates the trip contacts in case of overloads.

• The bimetal strips are either heated directly by current flown through them or by special heater coil through which motor current flows.

• In case of bigger motors, they are connected in the secondary circuit of CTs.

• Bimetal relays can be usually be set in a certain range.

• Most of them are provided with additional bimetal strips to enable ambient temperature compensation.

• Bimetal strips can be self setting type or hand resetting type.

• Selection of Bimetallic Relay:

• Characteristics of motor and characteristics of relay

• Nature of loading

• Type of starting

• Staring current

• Protection against overloads,

• Protection against single phasing.

• Short circuit Protection by HRC Fuses:

• Short circuit protection of motor, connecting feeder and starter requires careful study. The overload protective device and short circuit protective devices employed for motor protection shall be well coordinated.

• The range of currents between 1.5 to 10 times rated current is generally termed as overload range.

• Fault currents exceeding 10 times the rated current can be considered as short circuit currents and those should be covered by short circuit protecting devices

• The short circuit protection is provided by one of the following:

• HRC fuse – short circuit release opening the circuit breaker

• Instantaneous high set cover current relay which trips the circuit breaker.

• Protection of large motors:– Large motors need protection against various

abnormal conditions as follows:– Faults in windings and associated circuits– Reduction loss of supply voltage– Excessive overloads– Phase unbalance, and signel phased– Phase reversal– Switching over voltages, surges

• Types of relays available for motor protection: – Thermal protection only– Thermal protection, Instantaneous over current

protection– Thermal Instantaneous three phase over

current, instantaneous unbalance, single phasing.

– Thermal, Instantaneous three phase over current, Instantaneous unbalance, single phasing and Instantaneous earth fault

• Overload protection of Induction motors:

• It can be grouped as – Those which respond to motor current, e.g. bimetal relays, eutectic

alloy relays, electromagnetic relays, static relays.

– These relays open the control circuit of the main contactor or close the strip of circuit breaker.

– Those which respond to winding temperature, e.g. resistor devices embedded in slots, thermostats, Thermistors, etc

– Such devices are embedded in slots and serve to supervise the winding temperature and trip the switching device.

• The current sensing overload protecting devices can sense the following abnormal conditions;– Overloads, under voltage– Single phasing – Locked rotor, stalling – Heavy starting– Continuous overload.– Heavy braking.

• Following conditions can be sensed only by embedded thermal devices;– Temperature rise due to higher ambient

temperature– Temperature rise duet o failure of cooling– Temperature rise due to other causes

• Protection against unbalance:

• Causes of unbalance:– Single phase loads on distribution service line– Blown out fuse in power factor correcting plant– Short circuit within or outside the motor – Phase failure by blown fuse single phasing.

• Protection:– Bimetallic relays arranged to trip faster for unbalance currents

– Single phase relays sensing over current in heavily loaded phases

– Phase unbalance relays.

– Protection against single phasing

– Single phasing preventer.

• Phase reversal protection– Electromagnetic relays with discs making contacts at certain

direction of rotation and opening in the opposite direction of rotation.

• Faults in rotor winding:– The faults in rotor results in variation in rotor

current and in turn it reflects in stator currents. The stator over current protection can be set accordingly.