motor failures 1

[balerguy]

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?

 

[dpc]

No, that's probably not an issue. The service you are describing sounds like a three-phase 240/120 V "wild-leg" connection, except that your voltage to ground should be on the wild leg should be close to 208V, I think. A 230 V three-phase motor should run OK on this system.

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.

 

[balerguy]

Sorry it took so long to get back.. It's along way from anywhere so it took me time to get back to the machine.
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

 

[ScottyUK]

No change between laden and offload amps?

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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If we learn from our mistakes I'm getting a great education!

 

[balerguy]

my mistake. There is a differance between no load and load.
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?

 

[davidbeach]

You really need a third transformer in the bank. Due to loading you are not getting 120[°] between phases. For your motor and transformer connections, voltages to ground are irrelevant, the motor only sees the line to line voltages.

 

[jraef]

Although your at-load voltage seems fairly well balanced, you have a 30.5% current imbalance! Something is wrong here. You either have a bad connection or bad measurements, and you seem fairly confident in your measurements. Since you have already replaced the motor and experienced the same failure, I would be looking hard at the wring and the contacts on the motor starter.

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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[dpc]

Is this current only the motor current?

 

[rbulsara]

BG:
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.

 

[ScottyUK]

An idle thought (shoot it down if you want; I ain't sufficiently awake yet) - the phase with high voltage is driving the motor into saturation, causing the high current observed. There's enough IR drop in the motor cable and starter due to the higher current so that the voltage at the motor appears fairly balanced even though it is quite badly imbalanced at the source.

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!

 

[RRaghunath]

Could it be that your system has lot of harmonics and that is responsible for the unbalance. Then, the failure could be due to heating caused by harmonics and if you have negative phase sequence currents that will show up as cooked-up rotor of the motor.
But, then why only this motor every time? as jraef suggested, wiring of this motor and specifically the contactor needs to be looked at.

 

[waross]

Hi Scotty, the OP mentioned California. Standard motor voltage is 230 Volts typically run from a 240 Volt source.
But, is this possibly a 200 Volt motor?

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?

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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Jimmy Carter