Motor windings failure
Motor windings failure
(OP)
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.
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.





RE: Motor windings failure
Motor nameplate ratings are based on operation at line frequency (and a sine wave), not operation from an inverter.
RE: Motor windings failure
old field guy
RE: Motor windings failure
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
RE: Motor windings failure
RE: Motor windings failure
What is your minimum speed/frequency?
Extruder motors would typically require force-ventilation when used on VFD.
RE: Motor windings failure
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].
RE: Motor windings failure
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.
RE: Motor windings failure
RE: Motor windings failure
RE: Motor windings failure
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