Correct Setting For Short Circuit Instantaneous Protection in Induction Motors 1

[powering2002]

Dear all,
Last week Our Boiler Feed Pump Motor was tripped few seconds after started. From The Event Recorder in The Relay Panel, it was found that the cause was Short Circuit Trip.The current at that time were 1506 A (Phase A),1125 (Phase B), and 1115 (Phase C) . It exceeded the Short Circuit Instantaneous Setting value, which is 1500 A. So, we checked the motor. We performed PI test and megger Test. The Insulation Resistance was Good. It exceeded 4 MegaOhm. The PI value was 5,54. Then we started the motor again. and it ran well. My question are:
1. Is our instantanous short circuit trip setting was correct?
2. Is this high starting current caused by DC offset?
3. Does we need to increase the instantanous short circuit trip setting to avoid this false tripping?

Thanks for your attention.
Note: here is the specification of our BFP motor:

KW : 950
AMPERE : 161
VOLT : 4000
RATING : CONT.
STARTING : FULL VOLT
FORM : VCCN
HZ : 50
RPM : 2960
AMBIENT TEMP : 40° C
POLE : 2
STATOR INSULATION CLASS : B
STANDARD SPECIFICATION : JEC 37

 

[waross]

Under Canadian codes, a thermal magnetic breaker may be 250% of rated full load current. That would be 161 Amps x 250% or a 400 amp breaker.
Typically the magnetic trips on a moulded case breaker may be set to 10 X the rating or 4000 Amps for a 400 Amp breaker.
A magnetic only circuit breaker may be set to 1300% of motor full load current. That would be 161 Amps x 1300% or 2093 Amps. The Canadian code allows this setting to be increased beyond 1300% in the event the motor trips on starting.
I would increase the instantaneous setting to 1600 Amps for an installation that has been in service for some time with only one nuisance trip.
I would increase the setting to the maximum allowed for a moulded case breaker or 2000 Amps for a new installation to avoid nuisance trips in the future.
In the event of nuisance trips in the future at a 2000 Amp setting, I would first check the condition of the motor. If there are no problems with the motor I would then increase the setting to at least 100 Amps above the motor current at the time of the trip.
Note: Motors typically experience starting transients on one or more phases. This transient is much less than 1/4 cycle but will be greater than the instantaneous trip setting and cause a nuisance trip. For this reason the instantaneous setting of the breaker may be greater than the normal 800% starting current:

Instantaneous-trip circuit breakers are a magnetic-only trip device without time delay that may be provided with
a dampening means to accommodate the transient motor inrush current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[automatic2]

CEC provides for increasing the setting of an inverse time breaker for hard starts, but is silent on such an allowance for instantaneous breakers.

 

[LionelHutz]

Was the motor re-started or possibly rotating when started? Your instantaneous setting is 930% which is a little on the low side to accomodate the initial inrush before magnetizing current is established. I would tend to agree with Waross and say 2000A would be more suitable.

 

[waross]

28-210 Instantaneous-trip circuit breakers (see Appendix B)
Appendix B: Rule 28-210(b)
The intent of this Rule is to allow an increase in the trip setting above 1300% for motors with high locked rotor
currents that will trip on the asymmetrical inrush at the 1300% rating. For example, a motor with 800% locked
rotor current would result in a trip setting of up to 1720% of full load current. Higher locked rotor currents are
common in energy-efficient motors.

When used for branch circuit protection, instantaneous-trip circuit breakers shall be
(a) part of a combination motor starter or controller that also provides overload protection; and either
(b) rated or adjusted, for an ac motor, to trip at not more than 1300% of the motor full load current or at not
more than 215% of the motor-locked rotor current, where given.......
Rated full load current is 161 Amps.
The present setting is 1500 Amps. That is 932%
This may be increased to a limit of 1300% or 2093 Amps.
If there are still asymmetric trips, this setting may be increased further to 215% of locked rotor current.
Assuming a locked rotor current of 800% this clause will allow an increase in the setting to 2769 Amps.
With the event recorder showing an instantaneous current of 1506 Amps, I am sure that the settings could be adjusted to a value that will eliminate nuisance trips without violating Canadian Codes.
I use the Canadian code as an example and because I have a little familiarity with this code.
This is a world wide forum. If you inform us of the codes that you comply with, I am sure that someone with local knowledge will step in and help.
In the meantime I am sure that an increase in your setting of +100 Amps or +200 Amps will be safe and will probably eliminate nuisance trips.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

You guys are going on about Energy Efficient motors and MCCB setting etc., I think you failed to notice that this is Medium Voltage (4000V). There is no such thing as a fixed trip MCCB in MV, his breakers will be Power Circuit Breakers or Vacuum Circuit Breakers where each of the trip settings will be completely separate. Also, the issues regarding higher than normal magnetic inrush on EE motor designs applies only to NEMA frames, i.e. LV and below 300HP. MV motors are not built the same way. The motor mfr should be consulted on what the correct magnetic inrush setting for that motor should be. But, "a few seconds after started" would NOT be the Instantaneous (I) trip setting causing this, but more likely he has an LSI or LSIG trip unit and it tripped on the S function, Short Time trip setting.

"Will work for (the memory of) salami"

 

[powering2002]

@LionelHutz,
After trip, we didn't start the motor immediately. We Checked the motor. we conducted PI test and Megger test. The Reslut is OK. We started again the motor three hours after the trip incident.

Our motor is TOSHIBA Motor. using JEC-37 Standard. Unlike the American motors which has NEMA Code to determine the Locked Rotor Current, we don't find any information on the Motor name plate (specification) that can help us in determining the Locked Rotor Current. Could you help us to find another method to determine the Locked Rotor Current based on the information (motor Specification) I've mentioned above?
Thanks..

 

[waross]

Hi Jeff. I'll split the difference on this one.
Yes I may have been was remiss in using MCCBs and moulded case breakers as an example for a medium voltage motor.
However the Canadian Code does not consider voltage class when specifying trip settings. I stand by that information.
From the currents shown by your event recorder, I would estimate your locked rotor current at about 1100 to 1150 Amps.
I believe that the 1506 Amps shown on A phase to be a first cycle asymmetric transient the magnitude of which depends on the point on wave at which the power was applied.
If that motor was in Canada the trip settings would be increased to 2000 Amps.
Perhaps one of the IEC experts may suggest settings in compliance with IEC codes.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

No, I was asking if the motor was rotating during the start which caused the trip, not the start which after investigating.

I also completely agree with Waross about the 1506A current being the initial inrush transient. I've never seen a medium voltage motor data sheet showing a LRC even close to 935%.

 

[ScottyUK]

2000A seems like a reasonable guess; I might be tempted to try 1800A first and increase if needed. This isn't an exact science after all.