dc boost/ ir comp

[richerdick]

Hi, is there a rule of thumb for the maximum amount of IR comp that can be used without risk of burning out the motor windings. i am having trouble with some 240v three phase motors that run for prolonged periods of time at freuencies as low as 3hz. They have been in service for some time and occasionally tend to stall, i was thinking of tweaking the IR comp setting up a bit it is currently set to 12v what would be a resonable maximum and what is the rule of thumb for working this out. I need to be quite cautious because we are talking about 48 motors so i would not want to change settings that cause problems on all of them.
The motors have no forced ventalation they are cooled by the shaft mounted fan.
could i just increase the ir comp and then measure the temperature of the motor over a period of time. Would this prove that i am not likely to have problems?

 

[waross]

At 12 volts and 3 RPM you are right on the V/Hz. curve. It won't take much voltage boost to saturate and overheat the motors. Running that slow without external cooling and stalling is not good. I am surprised that your motors are not burning out. I would either look for a mechanical solution or live with the occasional stall. The expensive solution is larger motors and external cooling fans.
respectfully

 

[DickDV]

richerdick, there may be a cheaper easier solution. If the power train includes a belt or chain drive, change one or both pulleys to increase the speed reduction by 50%. Then reprogram the drive to run the motors 50% faster. That would be 90hz in North America and 75hz elsewhere.

Doing this increases the starting torque 50%, increases the motor speed compared to the machine speed 50%, drops the torque the motor sees 50% and significantly improves cooling.

I am assuming that max speed on the load is 50 or 60hz on the motor. You will not hurt the motor doing this as long as it is a four or six pole motor. If it is two pole, I would only go to 75hz.

Good luck.

 

[sreid]

Measure the motor temperature rise and see if there is any "Head Room" for an increased temperature rise.

Set the drive to display motor current and see what it is.

Increase the low RPM boost so you see a 10% current increase.

Check the motor temperature.

 

[richerdick]

unfortunately these are all motor gearboxes so there is no scope for mechanical alterations unfortunately this is just a bad design. what sort of temperature can an induction motor be expected to run at if ambient is say 21 celcius?

 

[itsmoked]

For that many motors how about a simple custom bolt-on cooling system? Design one that is very easy to install. 50 pieces is enough to get a reasonable price too. Then you might be able to up the voltage(enough) without any problems.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com