Saturday, May 3, 2014

Power System Protection Course MOTOR PROTECTION


PROTECTION - GENERAL..................................................................................
MECHANICAL OVERLOADING.......................................................................
SHORT CIRCUIT...................................................................................................
EARTH FAULTS....................................................................................................
SINGLE PHASING................................................................................................
MOTOR WINDING TEMPERATURE PROTECTION.......................................
STARTING TIME...................................................................................................
OVERLOAD PROTECTION..............................................................................
INSULATION FAILURE..................................................................................
DIFFERENTIAL PROTECTION....................................................................
LOSS OF SUPPLY..............................................................................................
SYNCHRONOUS MOTORS.............................................................................


For many years a comprehensive system of motor protection has been considered essential for vital services and industrial processes to safeguard machines and their cables from damage caused by overcurrents.  Mechanical overload, stalling, single-phasing and short-circuits are some of the potential hazards.  All of these result in overcurrents and are usually detected by either time delayed or instantaneous overcurrent relays.  The vast majority of electric motors, whether operating on high or low voltage, is of the squirrel-cage induction type, and many except the very largest are direct-on-line started.


Most motors are switched on and off by contactors, and in most cases it is necessary to provide back-up fuses to protect the contactor itself, as well as the circuit, against high-level faults such as short-circuits which are beyond the breaking capacity of the contactor.  The fuses have special ratings both to ensure discrimination in relation to the tripping characteristic of the contactor and to prevent their being blown by the starting current of the motor.  They do not protect the motor against overloads; this is a function of the contactor.


Like any other piece of electrical plant a motor, whether HV or LV, must be protected against overcurrent and usually against earth fault.  In larger motors over-temperature may also need to be monitored in both the cooling air and the windings themselves.

Overcurrent in a motor can be caused by any of five conditions:
·         mechanical overloading
·         short-circuit
·         stalling
·         single-phasing
·         earth fault or leakage.


A motor may be overloaded mechanically by either overloading the driven end (e.g. pump or compressor) beyond its rating or by some internal mechanical malfunction such as a stiff bearing.  Either may cause a rise of active current above the full-load rated current of the motor.  Mechanical overloading is probably the commonest cause of overcurrent in a motor.

The motor is protected by an inverse-time overcurrent device which will cause the contactor to trip if the overcurrent is sufficiently high and persists.  The device usually takes the form of a thermal element in each phase, either directly or CT-operated.  It has an inverse-time characteristic which is more nearly matched to the thermal behaviour of the motor itself than that of the inverse-time electromagnetic overcurrent relay described earlier.  It must allow the large starting current (up to five times full-load current) to flow during the run-up period without operating, but it must trip the motor if even a small overcurrent persists for a longer period.  A typical setting of such a device would be 110% full-load continuous current with the appropriate time setting.  For short starting times the inverse-time characteristic must be such that the starting current and run-up time are taken into account.  In this respect it must be remembered that high-inertia loads such as a motor-generator set or a compressor take much longer to run up than, say, a centrifugal fan.

For the majority of LV motors and a few HV motors the inverse-time device is thermal.  For the smaller LV motors it is in series with the motor itself, but for the others it is a separate relay operated through CTS.  For most HV motors on the later platforms however, the device is wholly electronic but with a similar characteristic; it too is a separate relay, CT-operated (see Figure 12.1(b)).  Where these relays are separate the overcurrent device is combined with certain other features into a single ‘Motor Protection Relay’ which is further discussed later.

A characteristic of inverse-time relays which is particularly noticeable in thermal relays, and which has to be taken into account in allowing for starting current, is ‘overshoot’ (or ‘overrun’).  This means that if the relay is energised with something more than its minimum operating current it may close its contacts even after the current has subsequently fallen below the operating level.  For example, a motor could be tripped after it had safely started and reached full speed, even though the relay had not operated during the starting period.  This can have a considerable effect on the discrimination that can be achieved between starting and overload currents, unless complications are added to the protection scheme.

Whereas the contactor with its inverse-time overcurrent device (thermal or electronic) provides overload protection for the motor, such contactors cannot in general clear a fault of short-circuit proportions.  For this they must be backed up by series HRC fuses.

When used with motors such fuses must have special characteristics.  They must have a continuous rating which will allow them to pass the full-load current of the motor continuously, and they must also allow the considerably greater starting current to pass for the period of the run-up time without melting the fuses.


A short-circuit in a motor circuit will cause a severe overcurrent.  One of the more vulnerable places to short-circuit in a motor is the incoming cable box where a too-small radius or imperfect jointing could lead to weakness.  As many process motors are located in hazardous areas, it is essential to clear the circuit in the quickest possible time under these conditions.

The overcurrent produced by a short-circuit will operate the inverse-time device in a relatively short time, but in general not short enough to cause a trip before the fuse blows.  Indeed it is important that the contactor should NOT operate as it is not rated to clear fault currents.  Where HV or LV motor feeders are provided with back-up fuses, it is these that will blow first and clear the fault very quickly. Where very large motors are fed through circuit-breakers and there are no back-up fuses, the inverse-time protection will be backed up by a high-set instantaneous overcurrent relay element to trip the circuit-breaker instantaneously without waiting for the inverse-time element to operate.  Its setting will be well above the motor overload and starting current levels.


Earth faults, especially those occurring in solidly earthed systems, will also give rise to severe overcurrents in the affected phases.  They may be dealt with by an earth-fault relay which trips the contactor.  With small motor starters where the thermal overcurrent devices are direct-connected, the relay is usually energised by a core-balance CT through which the three cables pass. With larger motors where there are CTS, these may be used to provide an earth-fault signal by their spill current.  The earth-fault relay may be separate or may be part of the composite ‘Motor Protection Relay’.

The same consideration will apply to earth faults in HV motors.  Nearly all HV systems are resistance-earthed, which limits earth-fault currents to a low level that will not actuate the fuses or high-set instantaneous relays.  Here also an earth-leakage relay operated through CTS is necessary, but the fault current is then well within the breaking capacity of the contactor.


Stalling can occur if the motor becomes heavily overloaded - for example by a mechanical seizure of a bearing or of the driven element, or it may be unable to start against an excessive load.  In all these cases the stalled motor draws its ‘locked rotor’ current (that is, its maximum starting current) as long as the supply remains connected, and severe overheating results.  The condition is aggravated by the lack of ventilation while the rotor is stationary.  There is also a temptation to make repeated attempts to start if unsuccessful the first time.

More rarely, stalling can occur if the whole power network goes unstable and begins to run down, causing all motors throughout the system to lose speed.  If the system recovers while the run-down is proceeding, all the motors in the system will find themselves running at a large slip and all taking nearly their full starting currents.  The combined effect on the generators of all these simultaneous starting currents will be to depress the system voltage to such a level that some of the larger motors may not develop sufficient torque to recover against their loads.  They will then continue to run down and stall.  This is sometimes called the ‘Patrickson Effect’.

The long period of drawing ‘starting current’, though not actually starting, will appear as a normal overcurrent and should, in principle, be dealt with by the motor thermal overcurrent protection, but difficulty arises when this protection has to discriminate between normal starting current (which is present during the run-up time but then disappears) and the stalled motor current (which persists).

This problem is particularly difficult if the run-up time of the motor is of the same order as the motor stall (or locked rotor) withstand time.  It is still worse in the case of high-inertia loads, where the run-up time may well be longer than the stall withstand time. For these situations ‘stall relays’ are sometimes used, especially with HV motors.

Stall relays are of two types: one using electric sensing of the motor starting current, and the other using detection of actual rotation.  The former uses a current-sensitive element and a timer.  On a normal start the current-sensitive element energises, but the timer prevents its causing a trip unless the normal run-up time has elapsed.  On a genuine stall the timer will trip the motor after run-up time has expired if the overcurrent is still present.  This type of relay is often fitted with a ‘thermal memory’ which prevents a restart until sufficient cooling time has elapsed.  The stall relay is sometimes included with other elements in a combined ‘Motor Protection Relay’.

The other type of relay uses a shaft rotation detector.  This form is most suitable when the motor safe stall withstand time is very close to the motor run-up time, or even less as in the case of the very large gas re-injection motors on certain offshore installations.  The rotation method is a more accurate indication of a stall condition.  However it does no incorporate any ‘memory’ to protect against quick restarting, and it must be used in conjunction with some form of lock-out protection.


A problem special to 3-phase motors is single-phasing.  Any such induction motor needs three phases to produce its rotating field and to provide the necessary starting torque, but once running, the removal of any one of the phases will not necessarily stop it.  It will however reduce the driving torque and will also increase the current in the two remaining phases.  If the motor were already well loaded it would eventually trip on sustained overcurrent.  If, however, the mechanical loading on the motor were not too high, there might still be sufficient torque to drive the load.  Also the current, although increased, might still not be enough to actuate the inverse-time overcurrent relay set, typically, to 110% full-load current.  The situation could therefore pass unnoticed except for a high-pitched whine from the motor, and no actual harm would result.

There is a much greater risk, however, when attempting to start.  If the single-phasing had occurred while the motor was running partially loaded and had not been noticed, the motor would have been stopped normally when the operation was complete, but still in its single-phased state.  If later an attempt were made to restart it, there would be excessive starting current but no starting torque, and it would remain stationary.  As the overcurrent relay is set to allow adequate run-up time, this situation could persist for a dangerously long time with no ventilation in the stationary motor.  Still worse, the operator might make several attempts to start, and each time the relay would reset and allow full run-up time afresh.  Eventually the motor would probably burn out before the overcurrent relay disconnected it.  Therefore if a motor fails to start after two attempts, the operator must make no further attempts to start it until the cause has been found and corrected.

A general 'Rule of Thumb' that can be applied is that motors over 50kW are only suitable for two successive starts (even if successful) when cold.  Another one-start attempt is allowable after a cooling period of 30 minutes at standstill.  No more than three starts may be attempted in any one hour.

Damage to a motor by single-phasing is caused by overheating of the windings due to the prolonged excessive currents in two of the phases.  It is normally prevented by the protection system.  In the case of small motors provided with thermal overcurrent protection, the three thermal elements are mounted in such a way that unequal heating produces a differential movement which causes the contactor to trip.

With some HV and larger LV motors where protection is through current transformers and where thermal overcurrent protection is used, the single-phasing protection is provided by these same overcurrent elements where unequal heating produces the differential movement.

With motors which are protected by electronic relays the device includes a special element which detects the single-phase condition, whether the motor is running or being started. It is referred to as a ‘single-phasing’ or ‘Negative-Phase-Sequence’ (NPS) relay.

Single-phasing can be caused by the blowing of one of the three back-up fuses, by the possible welding of contactor contacts or, less probably, by the open-circuiting of one line due to damage or vibration.  It will not occur however with HV fuses where they are fitted with external striker-pin contacts which trip the contactor if any one of them blows.


Over-temperature protection is sometimes used, in addition to the thermal overcurrent protection afforded by the Motor Protection Relay, to safeguard the windings of a motor.

Three main types of temperature sensor are used:
·         thermocouple
·         Resistance Temperature Device (RTD)
·         thermistor

The sensing elements are normally embedded in the winding insulation, usually in the overhang.


A major problem in motor design and protection is to ensure that the starting current can flow for long enough to accelerate the motor without bringing out the motor’s overcurrent protection, while at the same time not impairing the close protection given to the motor while it is running.

For this purpose a time-quantity ‘tE’ is considered.  This is defined as the time taken for the motor’s windings, while carrying the starting current IA continuously, to be further heated from the maximum temperature reached in rated service and in a maximum ambient temperature to the limiting allowable temperature.  The tE time of the motor should be given on its nameplate.

The thermal relay for protecting a squirrel-cage motor should be selected so that the tripping time read from the thermal relay’s time/current characteristic using the IA/IN ratio (ratio of starting to normal rated current) is not larger than the motor’s stated tE time.

The present day tendency is to employ motors to the limit of their thermal margins and to cater for this a relay with an inverse-time characteristic, similar to the thermal time characteristic of the motor, is used.  The characteristic must allow the motor starting current to flow for a time in excess of the motor starting time.
It may be that a short resume' of the operation of the three-phase induction motor would be useful at this point.  The three-phase voltage produces current in the stator winding which sets up a rotating magnetic field.  This field flux cuts the short-circuited rotor conductors and induces a current in them.  The interaction of the current and flux produces a torque which causes rotation.  Figure 10.1 shows a torque-speed curve for a typical motor and superimposed on this curve is the torque-speed curve of a fan.  Underneath is the current-speed curve for the motor.  As can be seen the current is at starting current level until about 80% speed is reached.

Figure 10.1  -  Current/Speed, Torque/Speed and Current/Speed Characteristics

The torque increases until it reaches a maximum, in this case at 90% speed, and the value at this point is known as pull-out torque.  A further increase in speed causes the torque to decrease until it would become zero if 100% speed could be reached.  At zero speed the torque is in excess of that demanded by the fan and therefore the motor accelerates.  The speed increases steadily as the excess torque is roughly the same value up to 30% speed.

After 40% speed there is a large excess of torque and so the machine accelerates quickly until it delivers the amount of torque required by the fan at A.  If the fan dampers were closed then the required torque is far less; the excess torque and therefore the acceleration is greater and the machine runs up to speeds quicker and delivers the amount of torque required by the fan at B.  If vertical lines are dropped from points A and B the load current will be indicated on the current-speed curve.  The actual current is greater than that indicated which does not include the magnetising current.

From the torque-speed curve it can be seen that the risk of stalling is greatest up to 30% speed where the difference between the motor and load torque is least.  If there was a reduction in motor torque which could happen if the voltage was depressed to a level where it equalled the fan torque at that speed then the motor would not accelerate and would draw starting current.

The speed at which pull-out torque occurs depends on the ratio of rotor resistance to rotor reactance.  Rotor reactance is proportional to the rotor frequency which in turn depends on the difference between the speed of the rotor and the speed of the rotating field which has been produced in the stator.  This difference is the slip.  Therefore rotor reactance is proportional to slip frequency.

Pull-out torque occurs when Rotor Resistance = Rotor Reactance when R2 = SX2 where S is the slip.

In the case shown in the curve the X/R ratio is 10/1 and therefore the pull-out torque will occur at

S =             = 0.1

With slip-ring motors it is possible to introduce resistance into the rotor by connecting in a resistance bank.  This will change the position of the pull-out torque.  For example, if resistance is added so that the total resistance is equal to the reactance at 50 Hz then the pull-out torque will occur at S = R/X = 1, i.e.  at motor standstill.  This will produce a relatively high torque to accelerate the rotor quickly but with the load shown would run at only 82% speed.

If, when the motor achieved 30% speed, the value of external resistance was reduced to just below half then the pull-out torque would occur at N = 0.5, i.e.  half speed.  Finally the external resistor would be reduced to zero and the condition would be as shown in Figure 10.1.  The torque-speed curves for the three resistance steps are shown in Figure 10.2.

The above is an accepted method of starting slip-ring motors but the change in external resistance values would be carried out smoothly to give the best acceleration.



One of the most widely used relay for a.c. motor protection is the thermal relay which consists of three heaters supplied by three current transformers measuring stator current.  The heaters are in the proximity of bimetallic strips which when heated produce a torque to move the relay contacts towards a fixed contact.  The deflection is propor­tional to current squared and therefore a motor operating at full load would move the contact three-quarters of the way towards a final contact set at 115%.  Hence the operating time would only be a quarter of the time required to operate the relay if the motor was running light.  Whilst this is obviously a desirable feature it should be remembered that the thermal time constants of the bimetal and motor differ widely and if the motor load is varying the bimetal will respond in seconds whereas the motor temperature change will take minutes or even hours.

Because the bimetallic strips are heated indirectly, i.e. by heat radiated from the heaters as opposed to heating by passing current through the bimetal, there is a time delay in its response.  This means that the moving contact will continue to move towards tripping even after the motor-starting current has disappeared.  To avoid operation the relay operating time must be at least twice the motor starting time.

These relays often have a special contact arrangement which operates if the current in any phase differs from the current in the other phases by more than 12%.

The reason that unbalanced phase currents require disconnection of the machine is that any unbalance in the current results in a negative phase sequence component which produces a rotating field in the opposite direction to the rotating field produced by the applied system voltage.  This counter-rotating field will cause induced currents in the rotor of almost twice normal system frequency resulting in overheating and possible damage.

Apart from the condition where one phase of the supply is missing completely, for example, owing to a blown fuse it may be thought that any unbalance in the system is small.  This may be true in terms of voltage but as the negative phase sequence voltage will be applied to the standstill impedance of the motor the current will be substantial.  If the negative phase sequence voltage was 5% then, if the starting current is 6 times full-load current, the negative phase sequence current will be 3%.  In the case where one phase is missing completely the positive and negative sequence currents will be the same.

Other conditions which can cause unbalanced voltages are heavy single-phase loading or a blown fuse in a power factor correction capacitor circuit.
Overload and unbalanced load are conditions associated with the situation external to the motor.  Overload is caused by an increase in the mechanical load whereas unbalanced currents are caused by the supply.

If tripping has been initiated by either of these conditions, indicated by operation of the flag relay marked "Thermal", it is usually in order to restart the motor.  Only one restart should be attempted and during the starting period the relay should be used to diagnose the type of fault, overload or unbalanced current if it is still present.

As shown in Figure 10.1 the starting current of a direct-on-line motor is practically constant at short-circuit level during most of the run-up period and there is, therefore, no means of detecting a stalled condition by current level alone.  The thermal relay will trip the motor eventually, but because the time is long it may be too slow to prevent damage.  In this case a single element stalling relay is used.  This relay has a directly heated bimetal which gives a low overrun and therefore the operating time can be set close to the maximum run-up time.  In some cases the run-up time is greater than the allowable stall time this means that the condition can only be resolved by the addition of a speed-measuring relay.


Protection against short-circuits is accomplished by instantaneous overcurrent relays but as these must have a setting greater than the starting current of the motor only a limited amount of the stator winding is protected.  For example, if a short-circuit occurs at the motor terminals the current is the full system fault current whereas if the short-circuit was at the star point no fault current would flow as the star point itself is a short-circuit.  Therefore along the length of the winding there is a reduction of fault current from maximum to zero.  The decrease is not linear but roughly proportional to the square of the distance from the star point.  Figure 10.3 shows this and also that an instantaneous relay which is set to avoid operation under starting conditions will only operate for faults just beyond the motor terminals.  In fact these overcurrent relays can only be regarded as protection for the cable and the motor terminal box.

In a delta-connected motor there is, of course, no point in the winding where a short-circuit would produce zero current.  Neverthe­less there is a large reduction in fault current for short-circuits away from the motor terminals.


The detection of short-circuits between windings is accomplished by the earth-fault relay.  This is residually connected and has a setting of 10% which means that practically all of a star-connected winding will be protected.  It is assumed that any insulation failure will result in an earth fault which is a reasonable assumption in that all the stator windings are in the close proximity to the stator iron.

The relay itself should have a voltage setting with a stabilising resistor but in some cases this is not required as stability is required only to the level of the motor starting current.

Operation of either the instantaneous overcurrent or the earth- fault relays operates a flag indicator marked 'Instantaneous".  When the motor has been tripped by these relays no attempt must be made to restart until an insulation test has been carried out.


A typical motor protection relay is available with different time ranges, namely 14 20 and 30 minutes.  These do not describe the relay in a very precise manner as they refer to points on the curves which are asymptotic.  The times would have more meaning if they were quoted at a higher multiple of setting current.  The 15- and 20-minute relays are the standard.  A 30-minute relay would be used where the starting period is long or the machine is started frequently.

In setting the thermal section of the relay there are two adjustments which allow the correct setting to be made.  A rough adjustment by changing the turns ratio on the auxiliary current transformer and a fine adjustment by alteration of the fixed contact position.

The possible settings on the auxiliary CT are 80% 90% or 1000/0 and a setting corresponding to the ratio of full load current to line CT primary current should be chosen.

45kW motor FL current = 84 A, CT ratio 100/1,

                        = 0.84A set to 80%.

55 kW motor FL current = 98 A, CT ratio 100/1,

                        = 0.98A set to 100%.

75 kW motor FL current  t36 A, CT ratio 150/1,

                        = 0.91 set to 90%.

75 kW motor FL current = 136 A, CT ratio 200/1,

                        = 0.68 set to 80%.

The auxiliary CT, which incidentally utilises the magnetic circuit of the instantaneous overcurrent relay as a core, changes the overall ratio of the current transformer circuit.  The l50/1 CT with the auxiliary CT on a 90% tap gives an overall ratio of
0.9       x 150/1 = 135/1.

The adjustment of the fixed contact depends on the duty of the motor.  If the motor load is fairly constant, say a fan or a pump, then the contact can be set fairly close to the full-load current value, say at 110%.  If the load fluctuates, for example a conveyor then a wider setting may be needed, a setting of 115% or even more.

This does not mean the fixed contact will be set at 110% or 115% although in most cases it will be fairly close to these values.

The contact should be set, for a 110% setting to:

FL current
CT primary current x aux CT tap
110% x

in the case of the 37 kW motors

100 x 80
110% x                             = 1.16 set, to 116%

or the other motors

100 x 100
110% x                             = 1.08 set, to 108%

150 x 90
110% x                             = 1.11 set, to 111%

200 x 80
110% x                             = 0.94 set, to 94%

or for a 115% setting

200 x 80
115% x                             = 0.98 set, to 98%

The setting of the instantaneous overcurrent relay must be about l½ times the motor starting current in order to avoid operation during initial peak of the starting current which can be more than twice the steady short-circuit current but has a duration of less than one cycle.
It would be more correct to say that the relay setting should be about 1l3 times the motor short-circuit current.  This is the same as starting current in the case of direct-on-line motors but not when the motor is started by a method which limits the starting current.  The fact is that an induction motor will contribute current to a system fault at a level equal to the short-circuit current.  This is the initial current at the moment of fault but quickly dies away.

Therefore even though the current has been limited during starting to, say twice full-load current an external fault will cause a current of 6 to 8 times full-load current to flow from the motor.  The instantane­ous overcurrent relay must be set so that it does not operate under these circumstances.  The actual setting is the times full-load figure on the scale multiplied by the tap setting on the auxiliary CT.

The success of this type of relay is undoubtedly due to its simplicity in setting and the ability to check its performance whilst in service.  The moving contact arrangement carries a pointer which indicates the percentage load.  From observation of the panel ammeter and a knowledge of the overall CT ratio the correct operation of the protection can be verified.  The overall CT ratio is the line CT ratio, which can usually be deduced from the ammeter range multiplied b~ the auxiliary CT tap which is indicated on the relay nameplate.

For example, the 45 kW motor, CT ratio 100/1 auxiliary CT tap 80%.  If the panel ammeter was indicating 68 A, on the percentage load scale

100 x 0.8
                                       x 100% = 85%

should be indicated.

There are a number of electronic relays which protect the motor in the same way as the thermal relay which are capable of matching the motor characteristic more accurately.  In addition to adjustments for current level the operating time can be adjusted as well as the settings for unbalanced current and earth fault.


The type of protection described would only be applied where the motor is supplied via a circuit-breaker.  In many cases the switching of the motor is by a contactor which, although capable of making fault current, cannot interrupt fault current.  Therefore fuses are used to clear the fault instead of the instantaneous overcurrent relays.  The earth-fault relay is used as it will detect low-level faults and trip the contactor hut for high-fault levels the fuse 'would operate faster than the earth-fault relay.  It may be that a time delay is to be introduced into the earth-fault relay circuit to ensure that the fuse operates faster.  Figure 10.4 shows a typical composite characteristic where 160 A fuse is used in conjunction with a relay thermal element and earth-fault relay to protect a 550kW motor with a full-load current of 120 A.  From 132A, 110% full load current to 700A the motor is protected by the thermal relay which would trip the contactor.  Above 700A the 200A fuse would clear the fault.  Similarly earth fault from 12A to 3000A would be cleared via the contactor and above 3000A by the fuse.  The contactor will never be called upon to break a fault current beyond its capability.


Differential protection on a phase by phase basis is shown in Figure 10.5.  This type of protection is eminently suitable and will detect faults on practically the whole of the winding but is generally only used on large motors.  The leads between the current transformers in the motor neutral terminal box and the relay which is associated with the switchgear, may be long and therefore could have a high resistance.


However.  as the most onerous condition under which stability is required is the motor starting current a fairly low relay setting.  and reasonably small current transformers can he used


When the supply is removed from an induction motor its back e.m.f.  will decay exponentially and virtually disappear in a seconds.  During that time there will also be a slight decrease in speed so that the phase of the back e.m.f.  moves away from the position which it occupied before the removal of the supply.  The result is that the locus of the back e.m.f.  traces a spiral as shown in Figure 10.6.  The figures are arbitrary and are not meant to represent any particular motor.

If the voltage was restored before 0.4 s then the voltage applied to the motor would be less than system voltage because of the back e.m.f.  and the current would be less than short-circuit current.  After 0.4 s the voltage between the applied voltage and the hack e.m.f.  is greater than the applied voltage and the short-circuit current would be correspondingly greater.  If the voltage was restored after 0.8s the short-circuit current would be 1½-, times normal.  This means that the mechanical forces exerted on the rotor would be over twice the normal starting forces and could cause damage to the rotor structure.


For this reason undervoltage relays are used on large machines to ensure that the machines are disconnected if the loss of voltage exceeds say 0.3 s.  The relay used is either an attracted armature relay with a time-delay relay or an induction relay

During a system fault there is a loss of supply to all motors connected to the system until the fault is cleared by unit protection.  The loss of supply will be of the of the order of 120 to 250ms, the protection-operating time plus the circuit-breaker opening time.  Even if the fault persisted for a longer time there is not much danger of the high short-circuit current.  This is because the motor will be contributing current to the fault and consequently- the decay of the back e.m.f.  is far more rapid.  It will in all probability have disappeared in less than 0.5 s.


The protection for synchronous motors is the same as that for induction motors but with the addition of a relay to detect loss of synchronism and loss of supply.

For loss of synchronism an out-of-step relay is applied to motors which could he subjected to sudden overloads.  The motor could pull out of step because of an increase in mechanical load or if there is a reduction in supply voltage.  When pole slipping occurs the stator current increases and the power factor changes to a very low value and it is this condition that the out-of-step relay detects and trips out the motor during the first slip cycle.

The relay coil is connected to a bridge circuit which compares the current from one phase with the voltage from the other two phases.  The relay is energised by the voltage and is in the operated position at all times when the current is zero or under normal load condition.  When the motor pulls out of step the current is such that it subtracts from the current produced by the voltage to such an extent that the net current falls below relay drop-off level.  A non-linear resistor increases the overall tripping area of the characteristic to ensure correct operation.  This type of relay will also detect the loss of field.


If the supply to a synchronous motor is interrupted for more than say, 0.3s, then there is a danger that if the supply is restored the motor may be out of step and therefore an undervoltage relay is required to trip the machine.  This relay will also prevent starting and running under abnormally low voltage conditions.  Other protection devices are underpower or reverse power relays which are usually induction relays and are identical except that the former closes its contacts when the forward power is less than say, 3% and the latter closes its contacts when the reverse power exceeds 3%.  The reverse power relay should always be preferred as it is more stable to momentary swings of power but it depends for operation on the protected motor generating to other loads connected to the same busbar.  If there are no other loads then an underpower relay must be used.
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