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Motor protection depending on size and voltage level (on photo 3-phase asynchronous motorLeroySomer P280 S-8, 55 kW)

Motor protection depending on size andvoltage level

Motor protect ions vary widely depending on the size of the motor and voltage level involved,thus only the more common ones are discussed in this technical art icle.

Protection Index1. Motor Instantaneous Over-current Protect ion2. Motor Timed Over-Current Protect ion3. Thermal OverLoad4. Motor Ground Fault Protect ion5. Motor Stall Protect ion6. Motor Over-Fluxing Protect ion

1. Motor Instantaneous Over-current ProtectionInstantaneous over-current is usually the result of fault condit ions (phase to phase, phase toground), in which current f low will great ly exceed normal values. Damage due to windingoverheat ing and burning damage associated with large fault currents can occur without thistype of protect ion.

These types of faults can be rapidly detected by a dif ferent ial protect ion scheme using CoreBalance CTs as will be discussed later and cleared before major damage results. In thesesituat ions, fast act ing electromagnet ic relays will be used to t rip the af fected motor.

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2. Motor Timed Over-Current ProtectionCont inuous operat ion of an electric motor at currents marginally above its rated value canresult in thermal damage to the motor.

The insulat ion can be degraded, result ing in reduced motor life through eventual internal motorfaults. Typically, an electric motor has a service factor rat ing listed on its nameplate. Thisnumber represents the cont inuous allowable load limit that can be maintained withoutsustaining damage to the motor. For example, a typical electric motor is designed to withstanda cont inuous overload of about 15% without sustaining damage and has a service factor =115%.

Continuous operation at or above this value will result in thermal damage. To protectagainst motor damage, we mustensure that this condit ion is not reached, hence we must t ripthe motor before the overload limit (service factor) is reached.

The relay most commonly used for this purpose is the induct ion disc relay. In this relay (Figure1), the current in two coils produces opposing magnet ic f luxes, which create a torque on a disc.As the motor current increases, so does the torque on the disc.

When the torque overcomes the spring torque, the disc begins to rotate. When the movingcontact meets the stat ionary contact on the disc, the t rip will operate.

Figure 1 - Induction Disc Relay

Tap sett ings and t imecharacterist ic adjustmentscan be made to alter thet ime delay of the relay. Themajor benef it of theinduct ion disk t imed overcurrent relay is that thespeed of rotation isproportional to the motorcurrent.

Hence major over-currentcondit ions will t rip thesupply breaker almostinstantaneously, whilecurrents just above ratedload will cause operat ionafter several seconds (orminutes).

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3. ThermalOverLoadAnother common type ofrelay used for t imedoverload protect ion is athermal overload relay. Inthis type of relay, the motorcurrent or a f ract ion of thecurrent through a currenttransformer is connected to an in-line heater. Figure 2 shows asimplif ied thermal overload relay. The heater (heated by I2R action) is used to heat a bimetallicstrip, which causes the displacement of a relay contact . A bimetallic strip consists of twodif ferent materials bonded together, each having dif ferent thermal expansion propert ies.

As the materials are heated, one side will lengthen more than the other, causing bending.

Normal operat ing currents or short durat ion overload condit ions, will not cause the bimetallicelement tobend enough to change the relay contact posit ions.

Excessive currents will cause increased heat ing of the bimetallic strip, which will cause relaycontacts to open and/or close, t ripping the motor.

Figure 2 - Thermal Overload Relay

The thermal overload relayhas an inherent react ion t ime,since the heater andbimetallic element take t imeto heat. Care must be takento match the current heat ingcharacterist ics of the motoror else the motor could bedamaged during the lockedrotor start ing condit ions.

This type of relay can beused for direct protectionagainst excessive motorcurrent caused by electricalfaults and motor overloads.Also, it is of ten used incombinat ion with the t imedover-current protect ion.

Thermal overload relays using in-line heaters and bimetallic strips, provide an alarm in the caseof cont inuous overload. This provides an opportunity for the operator to correct the problembefore it reaches trip level magnitude.

As we have stated, thermal over-load trips can occur during repet it ive starts on a motor orduring motor over-loading. Thermal overload trips will seal-in to prevent the motor contactorfrom closing. This lock-out will require manual reset before the motor can be re-started. Theoperator or at tendant will have to physically conf irm that the motor has had suff icient t ime tocool down and that the cause for the overload has been removed. If the operator is conf identthat there is not a permanent fault on the motor the relay can be reset.

Note however, that if an instantaneous over-current t rip has occurred, no at tempt at closingthe motor contactor should be made. An instantaneous trip will only occur if there is a fault inthe motor or supply cable and this must be corrected before any at tempt to reset the relay.

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4. Motor Ground Fault ProtectionIn the detect ion of ground faults, as with the detect ion of instantaneous over-currents, it isextremely important that the fault be detected and cleared quickly to prevent equipmentdamage. Insulat ion damaged by heat (f rom extended overload operat ion), brit t leness ofinsulat ion (due to aging), wet insulat ion or mechanically damaged insulat ion can cause groundfaults.

Figure 3 - Three Phase Ground FaultProtection

Figure 4 - Single Phase Ground FaultProtection

Ground fault protect ion schemes use dif ferent ial protect ion to detect and clear the faultedequipment. For motors, the common method is to use a Core-Balance CT as illustrated inFigure 3. The output of the core-balance CT will be the dif ference or imbalance of currentbetween the three phases.

If no ground fault is present, no current imbalance is present; hence no current will f low in theprotect ion circuit .

If a ground fault develops, a current imbalancewill be present and a current will f low in theprotect ion circuit , causing it to operate to t ripthe supply breaker.

Figure 4 shows a similar protect ion scheme,with each of the windings of the motorprotected individually (this scheme is notnormally installed in small motors, but mayappear in the protection of very large motors).

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5. Motor Stall ProtectionStalling or locking the rotor, is a situat ion in whichthe circuits of a motor are energized but the rotoris not turning. Motors are part icularly suscept ibleto overheat ing during starts, due to high currentscombined with low cooling air f lows (due to thelow speed of the motor, cooling fans are deliveringonly small amounts of air).

This is also why some larger motors have a limit onthe number of at tempted motor starts before acooling of f period is required. However, stallcondit ions can occur during normal operat ion. Forexample, mechanical faults such as a seizedbearing, heavy loading or some type of foreign object caught in a pump could be possiblecauses of motor stalling.

The loss of a single phase while the motor is not rotat ing or under high load, is anothersituat ion in which a motor may stall.

The typical start ing t ime of a motor is less than ten seconds. As long as this start t ime is not

Figure 5 - Stalling Relay

exceeded, no damage to a motor will occur due to overheat ing from the high currents.Duringoperat ion, a motor could typically stall for twenty secondsor more without result ing inexcessive insulat ion deteriorat ion.

We use a stalling relay to protect motors during starts, since a standard thermal relay has toomuch t ime delay. A stalling relay will allow the motor to draw normal start ing currents (which areseveral t imes normal load current) for a short t ime, but will t rip the motor for excessive t ime athigh currents.

A stalling relay uses the operat ing principle of a thermal overload relay, but operates fasterthan a standard thermal relay.

A schematic representat ion of astalling relay has be been provided inFigure 5 for reference.

By passing a port ion of the motorcurrent direct ly through the bimetallicelements in this relay, the heat ing isimmediate, just as would beexperienced within the windings ofthe motor.

This type of relay is usuallyoperat ional only when the motorcurrent is above 3 times the normaloperating current and is switchedout when the current is below 2 t imesthe normal operat ing current. Thisswitching in/out is achieved by theuse of an addit ional relay contact .

When the motor is operat ing normally, the current in this protect ion scheme passes throughthe resistor and bypasses the bimetallic elements.

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6. Motor Over-Fluxing ProtectionAs you can recall f rom the module on motor theory, the current drawn by a motor is roughlyproport ional to the core f lux required to produce rotat ion. Moreover, the f lux in the core isroughly proport ional to the square of the slip speed.

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Obviously over-f luxing is most severe during the locked rotor or stall condit ion when the slip isat the maximum. The stall relay previously discussed protects against this.

However, there is another condit ion where we can enter into a state of over-f luxing the motor.If one of the three phases of the supply has high resistance or is open circuit (due to a blownfuse, loose connect ion, etc.), then the magnet ic f lux becomes unbalanced and the rotor willbegin to slip further away from the stator f ield speed.

The rotor (shaft) speed will decrease while the supply current will increase causing windingover-heat ing as well as core iron heat ing. Also intense vibrat ion due to unbalanced magnet icforces can cause damage to the motor windings and bearings.

This open-phase condit ion is oddly enough called single phasing of the motor, even thoughtwo phases are st ill connected. If the motor cont inues to operate with an open supply line, thecurrent in the remaining two healthy leads will exceed twice the current normally seen for agiven load. This will result in rapid, uneven heat ing within the motor and damage to insulat ion,windings, reduced machine life and thermal distort ion.

If torque required by the load exceeds the amount of torque produced, the motor will stall. Themotor will draw locked rotor current rat ings, which are, on average, 3-6 times full load current.This will lead to excessive heat ing of the windings and will cause the insulat ion to be damaged.If the open circuit is present before the motor start is at tempted, it is unlikely that the motor willbe able to start rotat ing.

The phase-unbalance relay used to protect against this scenario is similar in design to the stallrelay, but is set for about 20% of the full load current. A rough representat ion of the operat ionof the relay is included in Figures 6 and 7 for reference only.

If any one of the phases in the motor loses power, the heater will cool down. The bimetallicstrip will turn, causing the unbalance contacts to close and the motor to be tripped. This relaywill also protect against thermal overload, as the heaters cause the bimetallic strips to close theoverload trip contact .

You will also see a compensat ing bimetal element, which will compensate for ambienttemperature changes, thus prevent ing unnecessary t rips.

Figure 6 - Phase Unbalance and Overload Protection

Figure 7 - Phase Unbalance and Overload Protection

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Resource: Science and ReactorFundamentals Electrical i CNSCTechnical Training Group

Motor protection depending on size and voltage levelProtection Index1. Motor Instantaneous Over-current Protection2. Motor Timed Over-Current Protection3. Thermal OverLoad4. Motor Ground Fault Protection5. Motor Stall Protection6. Motor Over-Fluxing Protection