Motor windings failure

[koshyeng]

We recently burnt motor windings. It is a 600HP motor with drive used for extruders.

My vendor says it failed due to high temperatures while running at low speeds.

I am a Chemical Eng. I always thought at high speeds, temperature in the windings will rise and if reaches above insulation class temp, it will eventually burn the windings.

Can someone help me understand how can we get high temperatures at low speeds?
Appreciate your help.

 

[dpc]

At lower speeds, there is less air moving across the motor windings for cooling.

Motor nameplate ratings are based on operation at line frequency (and a sine wave), not operation from an inverter.

 

[oldfieldguy]

If you're running your motor off variable speed control, you need to have supplemental cooling for the windings.

old field guy

 

[fangas]

I agree with DPC & Old Field Guy.


Usually, you can get a separate fan option for large motors just for that reason. Also, RTD implants for winding overtemp are available. Whether it's being rewound or replaced, if it's not too late, those options will pay for themselves.

FYI. A shaft driven fan, while sufficient at rated speeds, is ridiculously inefficient. Slow it down, it tends to become uselessly inefficient.

Ed

 

[burnt2x]

If I may add a question: Was your motor rated for inverter duty? Maybe?

 

[ozmosis]

What is your AC supply voltage?
What is your minimum speed/frequency?
Extruder motors would typically require force-ventilation when used on VFD.

 

[7anoter4]

First of all 600 HP [450 kW] is a huge motor for a usual extruder. The voltage has to be above 600 V. I have to admit I know only small extruders for cable manufacturing industry.
The extruder is provided with heaters in order to soften the material and also a screw to ground [crush the material] and create the pressure in the outlet die. Since the force required to this process is merely constant [for a well softened material], the torque required from motor will be constant and then P=k*Tq*rpm= the required power, will decrease with the rpm, indeed.
The screw is heated by grounding process and also by the heat produced by heaters.
As the screw is direct connected to the motor shaft and[ I don't think is a water cooling system provided, or it is controlled only by extrusion process only], so the cooling of the whole body is the ventilator job. As dpc and oldfieldguy already said the ventilator cooling flow decreases with velocity square. If the velocity decrease in such dramatic way even if the required power will decrease with rpm the ventilator flow will decrease with rpm^2. If the rpm decrease, somewhere, the motor will be warmer than permissible due to lack of ventilation.
I think [from my experience with cable extruder] you have to increase the take-up speed instead to reduce the screw velocity [if it is possible].

 

[ozmosis]

7anoter4
the voltage doesn't have to be above 600V but if it is above 500Vac then special precautions on the output of the VFD is required: dv/dt filter as an example.
If not, this could lead to quicker breakdown of the insulation due to the very high p-p voltages.

 

[7anoter4]

Thank you, ozmosis, for your remarks. You are also right the voltage does not have to be above 600 V if the motor starts through VFD. I am still in DOL stage-as our policy is to avoid harmonics how much it is possible and VFD is one of the sources. And since the insulation break-down electric field intensity depends upon the frequency such a filter is vital.

 

[7anoter4]

A more accurate statement will be as follows :the flow will decrease only proportional with rpm but the heat transfer factor will drop also approximately with the square root of air velocity that means the air flow will take less calories per flow unit and at the end the result will be as the flow drop with rpm square[approx.].

 

[edison123]

koshyeng

The motor driven cooling fan output reduces as square of the speed and if your load current does not fall in the same ratio, then the winding will overheat and possibly fail.

You need to provide more motor details here

1. Is this a separately ventilated motor ? (i.e. externally driven cooling fan)

2. Is the motor open type or closed type

3. If closed type, is the secondary cooling agent air or water ?

4. Was the winding originally designed for VFD duty ?

5. Motor nameplate details

6. Your load currents at various speeds