240V PWM to control 120V DC shunt motor 1

[zurc]

Hi,

We are studying the possibility of supplying with a 240V (mains) PWM signal, 50% duty cycle, a 120V/450W DC shunt motor. We are not experts on this area, so we are concerned about the possible damage that overvoltaging may cause in the windings.

Could anybody confirm if this approach will work, or if it will damage the motor?

Thanks!

 

[JERRYROBINSON]

Doubling the applied Voltage on a D.C. machine will double the RPM. Most small motors are not capable of running at double speed.

 

[zurc]

Sure, but we will reduce the duty cycle to 50% or less (of the armature and the field) in order to "compensate" the voltage increase

 

[Skogsgurra]

You will be absolutely fine with 240 V 50 %. Insulation levels do not start to be critical before you reach 400 or 500 V.
Make sure you have a good protection if the controller gets mad and delivers 100 % - that will probably not be good for the motor.
Also, make sure your carrier frequency is high enough so you do not get excessive armature current ripple.

Gunnar Englund

http://www.gke.org

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

 

[zurc]

Thanks Gunnar,

We are still testing the best frequency, but seems that a value between 1KHz and 10KHz works fine.

 

[sreid]

Remember that 240 VAC full wave rectified and filtered gives 240 VDC. How will you handle the field voltage (or is it a PM motor?).

 

[zurc]

Sreid, field voltage is applied in a 50% duty cycle as well, same as the armature voltage, so they are "syncronized". Apparently, it worked, but we have to do more tests (measure speed and torque, and current) yet.

 

[Skogsgurra]

I think that you actually meant something like "240 VAC full wave rectified and filtered gives 335 VDC"

Gunnar Englund

http://www.gke.org

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

 

[sreid]

Sogsgurra,

Yes, 340 VDC.

 

[zurc]

Yep, 340VDC, but...we are not filtering the supply, so we get an average value of 240V. I know about rattling risks and so on, but the motor will only work 2 secs every ten minutes. Our final customer decided not to filter the supply so....

 

[itsmoked]

curious zuru; what kind of process is a motor doing with that duty cycle?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[zurc]

Well, basically, the motor is actuating over a big contactor, and the maximun cycle is limited to that one, in order to avoid overstress on the contactor.

 

[Skogsgurra]

A thought that you probably also have "thinked": Could not a transformer take the voltage down to a more suitable level? Sounds easier and safer. Probably also cheaper. The 450 W is not much and with such a low duty cycle, you could easily get away with a much smaller transformer.

Gunnar Englund

http://www.gke.org

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

 

[zurc]

A transformer is not a solution, due to size constraints, but we do have 240V motors as well, so we could use them and forget about PWM stuff. But if we could make that work, we could reduce the number of motors referenced in our warehouse...

Thanks for the support