motor failures
motor failures
(OP)
I've got a can crusher in the recycling industry with a 7 1/2 hP motor that has had 3 failures since we've put it in operation. This motor operates a blower that "typically" runs at close to full load motor amps.
First one failed, the machine was replaced, then the newer (still a early 90' vintage) machine had the same motor fail. That motor was replaced and the replacement ran approx. 6 hrs before it to failed.
Moisture may be a problem as this machine is opeating outside on the California coast but I also found the electrical supply is a 4 wire delta system with one leg reading 230v to ground while the other 2 are 120V. Could that be that the cause, that we are installing the incorrect type of motor?
First one failed, the machine was replaced, then the newer (still a early 90' vintage) machine had the same motor fail. That motor was replaced and the replacement ran approx. 6 hrs before it to failed.
Moisture may be a problem as this machine is opeating outside on the California coast but I also found the electrical supply is a 4 wire delta system with one leg reading 230v to ground while the other 2 are 120V. Could that be that the cause, that we are installing the incorrect type of motor?





RE: motor failures
But.. if there is a large amount of 120 V load in comparison with the three-phase load, there could be a voltage imbalance between the three phases and this could why the motor is failing.
Can you (safely) measure the phase-phase (240V) voltage for all three phases while the motor is running? Post the voltages and we can determine the level of imbalance.
These small motors also generally have a lot of ways to connect them, so you may want to check the motor nameplate for the voltage rating and connections.
RE: motor failures
The motor that failed was wired correctly (low voltage)
The Ph-Ph voltage readings are as follows:
Without load
Line A to B 250 volt
Line A to C 247 volt
Line B to C 242 volt
Under load
line A to B 241 volt
Line A to C 238 volt
Line B to C 238 volt
The Phase to Ground reading are as follows
A to ground 217 volt
B to ground 120 volt
C to ground 120 volt
Amp readings are as follows
no load
Line A 17.5
Line B 12.2
Line C 13.5
Under load
line A 17.5
Line B 12.2
Line C 13.5
I replaced the motor and made one adjustment to the machine. I will monitor tomorrow to see if any of the electrical values change during operation. Thank you for your help.. BG
RE: motor failures
The voltage imbalance might be cause for concern - were the laden voltages measured at the source or at the motor terminals?
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RE: motor failures
the no load readings are as follows
no load amps
A 17.45
B 13.49
C 13.50
under load Amps
A 17.62
B 13.08
C 13.84
After monitoring today the voltage does not vary more then 1 volt during 4 hours of run time
Amp loads vary less then 1/2 amp (a phase 17.05 to 17.62)
Question: Why does the A phase have higher amp readings thent he other two even though the phase to ground readings show that phase has the high voltage (217 volt)?
I thought the higher the voltage the lower the amp load?
RE: motor failures
RE: motor failures
Rule of thumb (off the top of my head, so someone correct me if I'm wrong) is that motor winding insulation life decreases by 50% for every 10 degrees of temperature rise over the motor's rated rise. So a high current imbalance like that on a fully loaded motor like yours is going to increase the motor motor winding temperature quite a bit and your expected motor life is going to be short.
Oh wait, you already know that!
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RE: motor failures
RE: motor failures
I would repeat ScottyUK's question again..why is there virtually no difference between load and no load current?
What is the difference between the two conditions? Since this is a blower, is there a damper which opens/closes for loading or no loading?
A true no-load condition would be the motor de-coupled from the blower. Damper may be too leaky to make a difference.
Some more questions too:
What do you mean by "typically" it runs close to full load? Does it get over loaded often? For how long?
Are there caps connected to this motor?
You mentioned moisture, what type of motor is it? TEFC?
I have a feeling the motor is undersized and it gets overloaded often..as for the voltages, what dpc said.
RE: motor failures
That was the reason for asking if the on-load voltage measurements were at the motor or at the source.
The other possibility is that the motor is badly saturating due to high voltage and that's why the current doesn't change much from no load to full load - it's virtually all magnetising current with little active current doing useful work.
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If we learn from our mistakes I'm getting a great education!
RE: motor failures
But, then why only this motor every time? as jraef suggested, wiring of this motor and specifically the contactor needs to be looked at.
RE: motor failures
But, is this possibly a 200 Volt motor?
Within limits and for balanced voltages only.
That is, if all three phases are slightly over voltage, the current will be slightly less, but one or two phases over voltage causes so much extra current that the slight current reduction from higher voltage is swamped by the high currents caused by the unbalance.
You may be able to buy some safety factor by restricting the air inlet of the blower. Watch the current and continue to increase the restriction until the current drops 3 or 4 amps.
Bill
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