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Thread: Motor tripping circuit breaker

  1. #1
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    Motor tripping circuit breaker

    We have a 480V 150hp motor that keeps tripping a mag only circuit breaker in a combination starter. The circuit breaker is an AB HMCP breaker rated at 250A with the dials set to 2500A. The motor has a FLA of 175A with a Locked rotor code of G.

    Figuring the locked rotor code I come up with a maximum of about 1136A starting current or locked rotor current. This value is certainly under the 2500A setting on the breaker. Also I know that there may be an inrush factor assosciated with the motor, and I have seen this previously to be about 1.6 x LRC so using this I get about 1817A. This is still under the 2500A setting on the breaker.

    All the voltages on the starter look o.k. and are about 480V. The breaker tripping is internitent. The application is a water pump that is running as a backup to another similar pump. We had problems starting when other pump was causing this pump to freespin backwards, but we have since solved that problem and pump is not spinning at all when we go to start and get the intermitent trips.

    Any ideas on what could be causing the breaker trip?

  2. #2
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    Quote Originally Posted by philly View Post
    We have a 480V 150hp motor that keeps tripping a mag only circuit breaker in a combination starter. The circuit breaker is an AB HMCP breaker rated at 250A with the dials set to 2500A. The motor has a FLA of 175A with a Locked rotor code of G.

    Figuring the locked rotor code I come up with a maximum of about 1136A starting current or locked rotor current. This value is certainly under the 2500A setting on the breaker. Also I know that there may be an inrush factor assosciated with the motor, and I have seen this previously to be about 1.6 x LRC so using this I get about 1817A. This is still under the 2500A setting on the breaker.

    All the voltages on the starter look o.k. and are about 480V. The breaker tripping is internitent. The application is a water pump that is running as a backup to another similar pump. We had problems starting when other pump was causing this pump to freespin backwards, but we have since solved that problem and pump is not spinning at all when we go to start and get the intermitent trips.

    Any ideas on what could be causing the breaker trip?
    Either a fault or a failed breaker, likely the trip unit whcih usually fails to the defailts (Minimum setting)

    You will need to do some testing, winding resistance and megger tests on the motor, megger the cables, trip testing of the breaker. Can all be done by a testing company in a couple hours.

    If you have a spare breaker swap it out, I will test it for free if you can ship it, otherwise find a testing company, I can recommend one in your area if you like.

  3. #3
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    Quote Originally Posted by zog View Post
    Either a fault or a failed breaker, likely the trip unit whcih usually fails to the defailts (Minimum setting)

    You will need to do some testing, winding resistance and megger tests on the motor, megger the cables, trip testing of the breaker. Can all be done by a testing company in a couple hours.

    If you have a spare breaker swap it out, I will test it for free if you can ship it, otherwise find a testing company, I can recommend one in your area if you like.
    We meggered both motor and feeders and they checked out o.k.

    For testing of the breaker, would this be a primary injection test of some sort?

    Is it true that the locked rotor current is the maximum current seen under a locked rotor condition (slip =1) however the "inrush current" occuring in the first .01s to magnetize the motor can be even larger than the locked rotor current? I've seen somewhere that this value can be as high as 1.6x LRC. This value is dependent on where on the waveform motor is energized, as well as how much residual magnetizim is remaining in motor.

  4. #4
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    The strange part is, that we have an identical motor and starter (same breaker) in another bucket that is having the exact same problem.

    Basically as I mentioned these pump are on parallel lines in the field which feed the same header. Only one is running at a time and when one shuts down the other one kicks on and vise versa. Each pump has a check valve on its output.

    Pump A's check valve is broken, so when pump B is running it is pushing water through pump A causing its rotor to run backwards. When the rotor is spinning backwards and we shut down pump B and start pump A, pump A trips due to the rotor spinning backwards. We've pretty much chalked this tripping up due to the fact of the backwards spinning rotor.

    However Pump B's check valve does work, and therefore the motors rotor does not spin backwards when Pump A is running. However when Pump A is shutdown and pump B is started (5s delay between start of one pump and start of the other) pump B is tripping the breaker almost instantly. We are confused here b/c as I mentioned the rotor is stationary. An to make things even more confusing, when both pumps are stopped you can start either pump sucessfully without a problem.

    On sucessull starts we have measured peak current at about 1055A. However I dont know if the meter is actually fast enough to catch the peak current which may be higher, and this 1055A is just the first reading the meter can produce after peak current has already elapsed. When the breaker trips, we do not record any current, and I'm thinking the meter is not fast enough to see this inrush current.

  5. #5
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    Quote Originally Posted by philly View Post
    For testing of the breaker, would this be a primary injection test of some sort?
    Yes

    Quote Originally Posted by philly View Post
    Is it true that the locked rotor current is the maximum current seen under a locked rotor condition (slip =1) however the "inrush current" occuring in the first .01s to magnetize the motor can be even larger than the locked rotor current? I've seen somewhere that this value can be as high as 1.6x LRC. This value is dependent on where on the waveform motor is energized, as well as how much residual magnetizim is remaining in motor.
    That sound scorrct to me but I am not a motor guru, Jraef would be a good guy to answer that one.

  6. #6
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    If the motors are new energy efficient types, it is a known issue that in a few motor designs the magnetization current, in that "connect at the worst possible moment" scenario, can be as high as 2000% FLA. There is a paper available that was part of the "Motor Challenge" program from the DOE a few years ago promoting the replacement of older motors with newer EE types, but it was cautioning users that they may run into this issue. One of the strategies used to increase the motor efficiency is to use more copper to reduce the I2R losses. To do this, they need less core material (bigger slots) and they also use grain oriented high grade silicon steel to reduce eddy current losses. But these measures sometimes have the combined side effect of resulting in inrush currents being much higher than previously encountered.

    The "curiouser and curiouser" aspect of this is your observation of it happening only after the other pump has been recently running. Maybe the fact that the water flow through the pipes when the other pump was running is acting as a heat pump, changing the temperature of the pump, motor shaft and eventually the motor? You may have the perfect storm of issues here! Just a SWAG really.

    PS: Found the paper, here's the link.
    Excerpt:
    ...That is because peak inrush can be as high as 2.8 times the RMS locked rotor current and may exceed the circuit protector current setting.
    Last edited by Jraef; 05-20-10 at 04:38 PM. Reason: Added PS:

  7. #7
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    Quote Originally Posted by zog View Post
    Jraef would be a good guy to answer that one.
    See? Knew he would come through

  8. #8
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    What does art This is how I interpret the NEC:

    430-52 says about the maximum setting of a mag only device if the motor trips an MCP breaker of set following table 430-52? You are allowed to go to 17X. 175 x 17= 2975 from what I interpret the NEC.

    If that doesn't get the job done take a look at 430-52, (C) Rating or Setting, Exception No. 2, where the rating specified in Table430.52, as modified by exception 1, is not sufficient for starting current of the motor:
    (c) The rating of an inverse time circuit breakers shall be permitted to be increased but shall in not case exceed 400 percent for full load currents of 100 amperes or less or 300% for full load currents greater than 100 amperes.

    Yes, inverse time breakers (THERMAL MAGNETIC) are allowed. Since the motor should already have an OLR tne inverse time element of a breaker really has no purpose. Being that you are allowed to use a TM breaker you are now given the opportunity to use the magnetic (instantaneous) of the TM breaker. Knowing that a TM breaker of this size would normally have an instantaneous adjustment between 5 and 10X if you select a breaker based upon the 300% allowed take that rating and multiply by 10 would give you the instantiations trip setting.

    Would this get you out of trouble and still be within the NEC requirements?

  9. #9
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    I agree with Jraef, and would like to add just a couple things. First, the protection device is most likely a Eaton-CH HMCP, not AB. As AB does not physicallt make them, even if their name is on it.

    Second, the current in question is the asymmetrical current. I plotted the motor curve with the 250A HMCP, and if the asymmetrical factor is greater than 2, it is likely the MCP is going to trip, especially if it has been doing this often, there can be some weakening of the device, i.e. tripping below setting.

    Also, in this case, a higher voltage is actually working against you. Motor starting is the oddball when it comes to voltage and current relationship. We typically think of the steady state relationship of V=I*Z, so as V goes up, I goes down. But during starting Z is transient, and therefore as V goes up, so does I, and vice versa (softstarter).

    If the motor nameplate is 460V, and the motor bus voltage is stiff at 480V or even slightly above that, then the assymetrical factor may not even need to be 2 for it to trip.
    "Just because you're paranoid, doesn't mean they're not out to get you"

  10. #10
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    Quote Originally Posted by templdl View Post
    What does art This is how I interpret the NEC:

    430-52 says about the maximum setting of a mag only device if the motor trips an MCP breaker of set following table 430-52? You are allowed to go to 17X. 175 x 17= 2975 from what I interpret the NEC.

    If that doesn't get the job done take a look at 430-52, (C) Rating or Setting, Exception No. 2, where the rating specified in Table430.52, as modified by exception 1, is not sufficient for starting current of the motor:
    (c) The rating of an inverse time circuit breakers shall be permitted to be increased but shall in not case exceed 400 percent for full load currents of 100 amperes or less or 300% for full load currents greater than 100 amperes.

    Yes, inverse time breakers (THERMAL MAGNETIC) are allowed. Since the motor should already have an OLR tne inverse time element of a breaker really has no purpose. Being that you are allowed to use a TM breaker you are now given the opportunity to use the magnetic (instantaneous) of the TM breaker. Knowing that a TM breaker of this size would normally have an instantaneous adjustment between 5 and 10X if you select a breaker based upon the 300% allowed take that rating and multiply by 10 would give you the instantiations trip setting.

    Would this get you out of trouble and still be within the NEC requirements?
    I believe the NEC has a maximum of 17x full load of the motor. I dont think you can install a thermal magnetic breaker and then used adjustments off of it.

    Quote Originally Posted by kingpb View Post
    I agree with Jraef, and would like to add just a couple things. First, the protection device is most likely a Eaton-CH HMCP, not AB. As AB does not physicallt make them, even if their name is on it.

    Second, the current in question is the asymmetrical current. I plotted the motor curve with the 250A HMCP, and if the asymmetrical factor is greater than 2, it is likely the MCP is going to trip, especially if it has been doing this often, there can be some weakening of the device, i.e. tripping below setting.

    Also, in this case, a higher voltage is actually working against you. Motor starting is the oddball when it comes to voltage and current relationship. We typically think of the steady state relationship of V=I*Z, so as V goes up, I goes down. But during starting Z is transient, and therefore as V goes up, so does I, and vice versa (softstarter).

    If the motor nameplate is 460V, and the motor bus voltage is stiff at 480V or even slightly above that, then the assymetrical factor may not even need to be 2 for it to trip.
    Can you explain why there is an asymmetrical current as part of the inrush current? What causes the DC offset and the asymmetrical current?

    I read in another post that the settings on a instantaneous breaker will not necessarily protect a motor if it is adjusted to a lower setting. Even at a lower setting, once a fault occurs the motor is toast! Any truth to this?

    What happens if the motor can still not be started even with a device set to 17x FLC.

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