ksk paper 6 - mccbs for motors

MOULDED CASE CIRCUIT BREAKERS FOR THREE PHASE SQUIRREL CAGE MOTOR STARTER

APPLICATIONS SOME SELECTION CONSIDERATIONS

by: K. Sivakumar, Manager-Training, Larsen & Toubro Limited, Switchgear Training

Centre, Coonoor.

(Paper sent for publication in the July 2012 issue of Electrical India magazine)




Centre, Coonoor.


Introduction: Motors AC three-phase squirrel cage induction motors, in particular consume

about 75% of the total electric power consumed in any industry. And, they are the vital elements

in any production process, being the prime movers of the various mechanical and process drive

applications. As such, smooth starting and protection of such motors are of prime concern to any

electrical system design engineer and/or switchgear application engineer.

Motor Starting Current: Any electrical design engineer and/or switchgear application engineer is

only too well aware of the starting current or the locked rotor current encountered while starting

any induction motor. The magnitude of such starting current can vary from about 5 times to 8

times of the rated full load current of the motor, depending upon the motor design &

construction. The magnitude of the starting current for any given motor is given by the motor

manufacturer in the motor catalogue or data sheet.

While the magnitude of the starting current is the phenomenon of the motor, the duration of the

starting current is the phenomenon of the connected load or the driven load. For example, if for a

given motor, the starting current is 6 times its rated full load current, then, if the motor is driving a

centrifugal pump, this starting current of 6 times the rated full load current, might last only for

about 10 seconds or less. Else, if the motor is driving a centrifugal blower, this starting current of 6




Centre, Coonoor.


times the rated full load current, might last, for about 20 seconds or even more, depending upon

the inertia of the blowers rotor.

Careful consideration must be given to both the magnitude and the duration of the starting

current, while selecting starter components like the short circuit protective device (SCPD),

switching device like a contactor, overload protective device (OLPD), etc., so that there are no

nuisance trippings of the OLPD and/or the SCPD and no damage to the starter components.

And, many a times electrical designers and switchgear application engineers do pay due attention

to this issue of starting current (magnitude & duration) and do select the starter components

accordingly. Thus, nuisance tripping and/or damage to the starter components are avoided.

But....

Motor Inrush Current: There is a much more severe and unrealised component of the motor

starting current, which still causes nuisance tripping and/or damage to starter components. It is

the magnetising inrush component of the motor starting current. Any electrical person is aware

that a transformer draws a very high magnetising inrush current at the time of switching-on. This

is due to the core excitation requirements. One must also have studied that an induction motor is

nothing but a transformer, with a short-circuited, rotating secondary. Havent we? Thus, there

would be inrush current while switching-on an induction motor too, as in the case of a

transformer. Note that this inrush current can be experienced by the motor even during transition

from star to delta in an open-transition type star-delta starter.

TYPICAL CURVE OF MOTOR CURRENT DURING A D.O.L. START

12

6 IrM

IrM

TTIIMMEE

CCUURRRREENT




Centre, Coonoor.


TYPICAL CURVE OF MOTOR CURRENT DURING AN OPEN TRANSITION STAR-DELTA START

One can call this, the transient component of the motor starting current. (This could be a

misnomer, as motor starting current itself is transient in nature). May be, one can call the inrush

current as a sub-transient component, the starting current as the transient component and the

running current as the steady-state component of motor current. The magnitude of such

magnetising inrush current in motors would vary, depending upon the point of the voltage

waveform at which the motor is energised, the amount of residual flux in the motor, etc. It could

reach a value as high as 12 times the motor rated current. And, the duration could be a few cycles

too.

Under such conditions, the SCPD provided in the motor starter might sense this high magnitude

current as a fault & could cause nuisance tripping and the motor could never be started. Due

consideration must be given to the inrush current too, while selecting switchgear components for

motor starter feeders.

Various Starter Component Combinations: Today, an electrical system engineer has a wide choice

of switchgear components, while designing a motor starter. One can choose between:

i) Switch + Fuse + Contactor + Overload Relay




Centre, Coonoor.


ii) MCCB + Contactor + Overload Relay

iii) MPCB (with built-in overload & short circuit protection) + Contactor

Among the above three combinations, we will consider the MCCB + Contactor + Overload Relay

combination in this paper, because many a times, it is observed that there is ample scope for

wrong selection of MCCB for this application.

MCCBs for Motors: It is observed that when a customer wants to provide an MCCB for his motor

feeder, either as a fresh application or as a replacement for an existing Switch-Fuse combination,

he simply calculates (or refers to a switchgear selection chart) the required rating of such an

MCCB. Then he simply procures the most economical type of MCCB available in the market place,

without realising that it might not be suitable for the given motor application. Thus, he ends up

having nuisance tripping of the motor and/or damage to the starter components, thus causing

avoidable production downtimes.

While selecting an MCCB for motor application, it MUST be ensured that the Instantaneous

Release setting in the MCCB is set to a value higher than the highest anticipated magnetising

inrush current, while switching-on the motor.

How to calculate this value?

Typical Magnitude of Staring Current (Ist): 6 to 8 times the motor rated current (IrM)

Tolerance for Locked Rotor Current as per IS 325 (Table-1): + 20%.

So, the maximum anticipated starting current: 1.2 x 8 = 9.6 IrM

Transient inrush current (peak): 2 x 9.6 IrM = 13.57IrM

Also, vide IS/IEC 60947-2, Clause 7.2.1.2.4, there is a tolerance of +/- 20% in the values of current

for tripping of the circuit breaker by short-circuit releases. This means that, even with the -20%

tolerance, the MCCB shall not trip for the above magnitude of the magnetising inrush current. This

also means that the effective setting of the instantaneous release setting in the MCCB should be

above 13.57 / 0.8 = 16.96 IrM; But, this is peak value. Whereas the instantaneous setting in the

MCCB is in RMS value. Hence, the above value has to be converted to RMS value; (i.e.) 16.96 /

1.414 = 11.99 IrM; Thus, the instantaneous setting in the MCCB has to be above 11.99 IrM or say, it

should be 12 IrM. Assuming that the rated operational current (In) of the MCCB is the same as that

of the rated full load motor current (IrM), then, the instantaneous setting in the MCCB shall be set

to 12 In.

Selection of MCCB: Now, assuming that the user selects a conventional feeder protection MCCB

the instantaneous release setting in these MCCBs range from 3 In to 9 In. Even with MCCBs with

modern Electronic / Numerical Protection Releases, the instantaneous setting range is 6 to 10 In.




Centre, Coonoor.


Thus, there is a chance that the MCCB might trip during switching ON of the motor, due to the

inrush current, even if kept at the maximum possible instantaneous setting.

It is to be noted that such spurious tripping might not happen all the times. Depending upon the

instant of switching and the residual flux in the motor, if the inrush current is more than the

instantaneous setting made in the MCCB, then only it would trip. Otherwise, it wouldnt. Hence,

one might hear complaints from the user about this strange arbitrary tripping behaviour of the

MCCBs during motors starting (sometime it trips, sometime it doesnt).

To ensure that the MCCB wouldnt trip at all for the above inrush conditions, one must select an

MCCB, whose instantaneous release setting is more than the worst case magnetising inrush

current, as calculated above. One might opt for a special Motor Protection Type MCCB, whose

instantaneous setting is fixed at 12 In.

Contactor & Relay selection with Motor Protection Range MCCBs: It is also observed many times,

that whenever a customer wants to replace his Switch-Fuse Combination in a motor starter feeder

with an MCCB, he simply removes the Switch-Fuse Combinations and replaces the same with an

equally rated MCCB. Unfortunately, the user does not pay any attention to the adequacy of the

short time withstand capabilities of the contactor & the overload relay, with the revised SCPD

(i.e.) MCCB.

It is to be noted that when a particular frame of contactor and relay were suggested for use with a

switch-fuse combination unit, they were sized based on the I2t let through energy of the Fuse,

which would clear a short circuit. As HRC fuses are fast acting (typically operating within 4 milli-

seconds for a heavy short circuit fault), the energy let through would also be less during a short

circuit and thus the contactor and relay would be subjected to a lesser degree of short circuit

stresses. Hence, they would be sized accordingly.

But, when the switch-fuse combination is replaced with an MCCB of equal rating, now during a

short circuit, the MCCB would clear the fault. Even with the most advanced current limiting type of

MCCBs, the fault clearing time would be around 10 milli-seconds and thus the energy let through

would be higher during a short circuit, than that with a HRC Fuse. Now, the smaller contactor and

the relay which were actually selected for the shorter let-thro with HRC fuses could not handle

the increased let-thro, now with MCCBs and thus would damage. Thus, what was Type-2

Coordination with Switch-Fuse combination gets converted into Type-1 Coordination with MCCB

now. Thus, it leads to damage of starter components and increased production downtimes.

To avoid this, the adequacy of the contactor and relay to tolerate the increased let-through energy

- during a short circuit with an MCCB as a back-up switching and protection device must be

verified & established. This can be easily established by truly following the Type-2 Coordination

based Switchgear Selection charts with MCCB + Contactor + Overload Relay combination, which is

published by the switchgear manufacturer.




Centre, Coonoor.


Conclusion: It is hoped that the reasons for the arbitrary spurious trippings of MCCBs during motor

starting are well understood now. It is also hoped and wished that proper selection guidelines be

followed by customers and end users, while opting for an MCCB as a back-up switching &

protection device in motor starter feeder applications.

ksk paper 6 - mccbs for motors

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