Motor Overheating with VFD

[Gluzchuk]

HI, we have a extruder, it has a motor 400HP with the Altivar 71 VFD.
THe original motor was burn and we rewind it, but it has overheating 100C, torque at 80% and the current is 400 amp, it is low beacause support 590 amp.
So that, we buy a new motor and it has the same issues...........
Sometime PTC is tripping.....
Could you help me?
Thany you for your advice.

 

[edison123]

That's more load related than to the motor, whose output is linear to the speed, as Jeff said.

In such a scenario of load power proportional to the speed squared, wouldn't the motor be underloaded and not overloaded at lower speeds?



Muthu

http://www.edison.co.in

 

[Gr8blu]

Motor may be electrically underloaded at reduced speed (drawing less than rated nameplate power, for example) but can still overheat because the cooling is not sufficient to get rid of the developed heat. To operate on a "constant torque" application (where power required varies directly with speed), the machine must have separately powered ventilation because it has to move the same amount of coolant at the reduced speed - and the rotort5 moving slower is not going to do that on its own. For a "variable torque" application (where the power and speed are not linearly connected, such as a fan or pump), it is possible to operate over a fairly wide speed range without a separate fan - but the critical intersection occurs between 25 and 80 percent of rated nameplate speed. In this range, a really good understanding of what the load requirement is becomes necessary because it is very easy to require more power than the machine can cool on its own. This is why the motor manufacturers always ask about the application and the intended operating speed (range).

Converting energy to motion for more than half a century

 

[LionelHutz]

It has a separately powered cooling fan. That's been stated multiple times by the OP already. Given the motor is 400hp @ 1500rpm the reduced speed results in the motor being capable of approximately this much,
800rpm = 213hp
100rpm = 27hp
1000rpm = 267hp

You mentioned PTCs so I expect the 100*C you stated was winding temperature. 100*C on the winding isn't overheating it, at all. As an example, NEMA allows a class F insulated motor with a 1.0 service factor to have a winding temperature rise of 105*C. This is to be determined by measuring the winding resistance change so it's measuring the average winding temperature rise. Add the allowed maximum 40*C ambient temperature and the motor winding could be running 145*C. This is 10*C below the 155*C rating of class F insulation which allows for hot spots in the windings. All motors are using class F or the even better class H insulation today. If the motor is class H insulated then it's ever further away from being overheated.

 

[edison123]

Gr8blu: I was questioning your power is proportional to speed squared and your motor HP calculations based on it. Motor HP is proportional to speed, not speed squared.

This is a separately cooled motor as confirmed by OP, so shaft powered self cooling is a moot point.

Muthu

http://www.edison.co.in

 

[Gluzchuk]

Thank you everyone.