Motor Overload

[sow]

We have a squirrel cage motor that is failing on its rotor bars around 1 year after commissioning. I understand that during commissioning there were some difficulties getting the pumps to start without a “failed to start” error on the soft start, and that the pump supplier employed some interesting methods to overcome this.

The motor drives a pump with a constantly rising power curve as flow increases. I am pretty sure that the flow is very high during start up and therefore the pump shaft power input exceeds the motor shaft power output.

I am in two minds about the mode of this failure, I see two possibilities on start up of the pumps.

1) The motor reaches full speed. To my mind the torque at full speed would increase with power as P = constant * Torque * speed. This would increase the torque on the shaft and therefore the rotor bars. In addition to this if slip were to increase then so would torque.

2)The motor would not reach running speed as the torque required would exceed the "rated" torque of the motor

Either way I would imagine that the there would be thermal effects due to the larger current drawn.

Any comments or observations or alternative theories would be greatly appreciated

Thanks

Sow

 

[itsmoked]

How big is the motor?

What were the "[green]interesting methods[/green]"?

How exactly are the rotor bars failing?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[sow]

The motors are 350kW

The methods are as yet unknown, and it is only really chatter to be honest.

The rotor bars are failing mechanically. Cracks have appearing on the rotor bars, close to the end rings.

 

[jraef]

Rotor bar failures are usually the result of torque transients.

...therefore the pump shaft power input exceeds the motor shaft power output

I am assuming that you meant the pump power requirement[/] is exceeding the motor shaft power input. But be that as it may, I doubt this is your problem. I have yet to see a pump that requires more power at low speed than it does at full speed, with or without flow. The soft starter is reducing your torque, so it is difficult to imagine that even with immediate flow, you are creating more torque than the motor can handle.

The first thing I would check would be the fluid status at startup. Is there a check valve or some sort of back-flow preventer? If not, or if there is one and it isn't working correctly, is there a chance that the fluid is back-flowing when you energize the pump? If that happens and it is spinning the pump backwards, even for a moment, it may be the source of a torque transient significant enough to damage the rotor bars.

More details about the plumbing would be helpful.

 

[edison123]

Does your load exceed the motor rating ? If it doesn't, then I would say it is poor rotor bar design and/or poor workmanship.

 

[sow]

The question boils down to this -

The motors / pumps are up to full speed, they are out on thier curve, hence the power input (requirement)to the pump exceeds the rated power output of the pump.

What would be the affect of this?

 

[edison123]

Continuous Overload. Motor fail.

 

[sow]

Thanks for the replies -

The motor would be in this state for around 10 seconds every time the motor starts.

Could the overload result in

1) an overtorque or increased mechanical stress on the rotor bars.
2) thermal stresses on the rotor bars.
3) any other modes of failure in the rotor bars or other part of the motor?

Thanks

 

[waross]

Normal starting under load should not cause problems or damage to the motor. Overload usually refers to a continuous condition, not starting effects. The starting process of getting a motor from rest up to full speed results in much higher current (6-10 times rated current) than the load of most centrifugal pumps.
Generally it would take abnormal torque transients to damage a motor.
Such transients may result from such issues as:
1> A catastrophic failure of the driven equipment that results in an almost instantaneous stop.
2> Energizing a back spinning motor.
3> Momentary power outages.
Another issue may be extended starting times, resulting in heat stress cracks in the bars.
Have the motors actually failed? Without seeing the actual bars it is not possible to say but it may be that the cracks are surface heat cracks that are normal for that motor and can be ignored.
I would investigate the possibility of momentary power outages. A very short power loss (from a few cycles to a few seconds, depending on the load characteristics) can impose severe current and mechanical (torque) transients on a motor.
When the power fails, the motor becomes an induction generator. Because of both the slip and the deceleration of the motor the generated frequency will be lower than the supply frequency. If the power is reapplied the situation is analogous to a generator being connected to the line out of synchronism. This will often result in current and torque transients much greater than maximum normal currents and torques. Smaller motors will often survive. Large motors have a lower survival rate.
From the information given I suspect power dips. I wouldn't be concerned with normal starting conditions unless you have frequent starts and/or extended start times. Even then I would not be concerned with torque under starting conditions, only heating of the bars.

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

 

[sow]

Waross - the pumps start up to full speed ok. Im not concerned about the increased current at this point, as we have soft starts to start the pumps. The issue is that the motor is overloaded for a "normal" operation after the pumps are up to full speed (and voltage) for around 10 seconds until the pumps are back on the curve. We definitley have a number of full depth cracks in the rotor bars. Therefore I was wondering if this overloading (on power) would increase the torque and the thermal stres to the point it could cause premature failure of the rotor bars...

 

[waross]

Understood. I think that your momentary overload will be a small fraction of the stress and torque from a normal start. Look for power dips. Very short.

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

 

[burnt2x]

sow,
as jraef said:

More details about the plumbing would be helpful.

Please check the recirculation circuit(line). Was there any history that the control valve for recirculation malfunctioned? My experience with this kind of problem was that the recirculation circuit went bad and caused overloading to the drive motor.

 

[sow]

the flow discharges into an empty main for 10 seconds at start up- the pumps are out on the curve and at full speed for this time and the power curve is rising.

The question was, would a temporary overload in power on the motor equate to a failure of the motor (by torque / thermal or other effects)

Thanks for all of your replies.

 

[edison123]

[blockquote]the pumps are out on the curve and at full speed for this time and the power curve is rising.[/blockquote].

What exactly do you mean by this ? Please post the motor name-plate details, the starting current and duration and the running current before we decide it is a sustained overload or not.

A cage rotor is quite failure resistant unless it was badly designed or manufactured. The stator would fail earlier than a cage rotor due to overload.

 

[edison123]

rats... stooopid html tags...

 

[roydm]

So you are pumping well over the normal into an empty header, what happens once it's full, large shock wave?
Have you considered installing one of the self acting pump control valves, these can be adjusted to slowly load the pump. They also close slowly on controlled shutdown and act as a check valve. They really take a lot of stress off the pump.
A couple of sites, I have used both.
http:/

http://www.singervalve.com

http://www.cla-val.com

Good Luck
Roy

 

[Skogsgurra]

Cracks in bars and near the ends can also be the result of the soft-starter not having a contactor bypassing it after start. Some soft-starters do not make the full mains voltage available to the motor. If that is the case (and if the mains voltage is low from the beginning) then you will have excessive slip without seeing that in the motor current.

Excessive slip always means more rotor heat and will result in cracked rotor bars. I have a few cases where this happened.

A check to see that motor voltage at full load is within tolerances could help rule this problem out. But, do measure at the peckerhead. Not at the motor control center.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...