Looking for some thoughts on a 3 phase motor tripping the thermal. I have a coolant pump motor on a industrial machine that randomely trips the thermal motor protection. It's on an Australian built machine that operates on 415 volts nominal, the thermal protection is a Moeller PKZ MO -10. I checked the load on all three legs while I was on site and had readings that were consistent in the 6.2A range(+/- .2) The nameplate lists 6.6A . I can see a slight difference since the actual operating voltage is a bit different than the motor nameplate. I realize that this thermal is oversize and does need to be changed in order to properly protect (owner changed pumps awhile back and got a smaller pump). I did find some excessive arcing on one of the contacts inside the contactor. Could that possible interruption be enough to cause a single phasing occurance? Does the thermal function on the principle that when a phase is lost or interrupted the other legs amperage exceeds the setting and trips?Don't do alot with industrial applications, how forgiving is a thermal unit in a situation where intermittent arcing may occur as opposed to a total loss of the leg? Thanks for any help.
3 Phase motor tripping thermal
- Thread starter nizak
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- Placerville, CA, USA
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- Retired PV System Designer
Anything which causes extra heat to the thermal overload, whether it is high ambient temperature, arcing in the same enclosure, or other heat sources can cause the thermal overload to trip at a lower current.
Not sure whether your overload will trip on overcurrent for a single winding or if it effectively adds up the current in all windings.
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
Yes, that PKZM0 unit has what is called "differential trip compensation" which changes the trip point if there is no current flowing though one leg of the 3 phases. People call it "single phase protection" but that's kind of a misnomer. It's really just biasing the OL trip point to compensate for the added heating effects in the motor due to single phasing.
But...
If you really had no current flowing through one pole, the motor would not start. That protection is for when a RUNNING motor loses one phase. It's more likely that the arcing evidence you saw points to a more serious problem of "chattering" of the contactor, or if not that, at least short-cycling. That would also cause the OL to trip and could be due to a defective contactor, but often it is indicative of a system that is demanding that the pump cycle on and off too fast. What controls that pump? You mention that it was replaced with a smaller one, perhaps it is too small and has to cycle on and off too often?
electric_cal
Member
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- California
I would consider replacing both the contactor and the motor protective circuit breaker. You mentioned that the owner had replaced the original motor with this smaller motor. That would point to a previous failure of the original motor, which could be the cause of the arcing you found in the contactor. It would also appear that by installing a smaller motor, it is causing the machine to cycle on and off at a greater rate which could lead to a build up of heat in the protective circuit breaker during the start cycle which is not being allowed to dissiapate properly.
Thanks for the input. I will check and see what the difference of flow is between the 2 pumps and maybe something has been changed in the programming of the machine as far as how frequently the coolant has to be dispersed.Highly possible that this is an ongoing problem that could have been present with the first pump failure.I will be changing the contactor and thermal device and hopefully put the issue to bed.To my knowledge, the coolant pump turns on and off per the control program that is being used for whatever style of tool they are grinding. Some machining is done dry and some use coolant.
kwired
Electron manager
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What was changed? A smaller pump, a smaller pump motor, combination of both? The actual output of the two pumps will be the biggest factor of current flow here outside of problems from damaged, worn, etc. contactor possibilities mentioned. Pump physical size doesn't necessarily mean it will have less flow either unless all other physicalities remain the same. The amount of flow is what loads the motor, and an oversized pump could still be installed and then compensated for by restriction of flow in the lines. But at same time it sounds like the current is below the setting of the device so if it is the pump output that is overloading things you are not catching it at a time when the load increases for some reason.
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