Tiny DC motor control

[panelman]

Gents

We normally deal with nice 3 phase 400V motors but have a little project on the books where we need to provide simple speed (pot on the panel door) to a couple of tiny DC motors. They are PM 50V 1.7A 60W. Control can be fairly crude and motors aren’t heavily loaded.

Looking around the choices seem to be

A card which takes 110V, rectifies it and then uses electronics to PWM the DC out to the motor. (

http://www.kbelectronics.com/data_sheets/kbic.pdf

or similar)

Or

Something similar the first option but the DC is supplied by a standard 48V PSU. (

http://www.quasarelectronics.com/3067-dc-motor-speed-controller-4-100v-7.5a.htm

or similar)

There isn’t a lot of difference in cost, the second type is cheaper but balances out once you add the cost of the 48V psu.

For simplicity’s sake I’m leaning towards the first option but have slight concerns because the motors are pulsed with rectified 110V AC (about 155V DC?) the brushes and insulation are going to be seeing voltages about 3 times nominal. Is this going to shorten life appreciably? Cause extra noise? Any other downside?

The motors come with a little 2.2uF + 2 x 22nF + 470R + 2 x 1.8mH filter, is this going to be enough or do I need to add a choke too.

All comments & suggestions welcome

 

[wayne440]

Given that the motor is rated for 50 volts, my advice is to limit the applied voltage that figure. After a brief look, I don't see anything in the data given for the first device that indicates the output is anything other than 0-90VDC. It may very well use PWM in there someplace, but the document clearly says "0 – 90 VDC output" in several places. If you provide a means to prevent adjustment of the output to values above 50 volts, I don't see why it wouldn't work. Just in case I'm missing something glaringly obvious, will you tell me how you deduce that "... the motors are pulsed with rectified 110V AC (about 155V DC?).."?

 

[Skogsgurra]

A look at the device and the connection diagram shows that there is no transformer between grid and motor. There are no huge capacitors and the efficiency doesn't seem to be too bad (a linear regulator would need much bigger heat-sinks than shown in the pictures). All this leads to one conclusion; the devices are SCR controllers.

An SCR, or thyristor, controller lets parts of the mains cycles through and the shorter the voltage 'cut outs' the lower the average voltage across the motor. So, if you read voltage vith an instrument, you will probably see voltages that correspond to what the data sheet says.

But, if you look at the voltage with an oscilloscope, you will see parts of the sine wave with peaks around 150+ That may be more than your 50 V motor can take - or the motor doesn't matter. It depends. If the motor has a high armature reactance, it will probably be just fine. If not, there will be a high ripple current that can make the brushes arc or the torque less smooth or heat the motor too much. Insulation is usually not a problem - but if the motor data sheet says that insulation is less than some 1000 V, you may need to be careful.

I would go for the low voltage pure DC option. Or at least run a thorough and prolonged test if I chose the SCR option.

Gunnar Englund

http://www.gke.org

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

 

[panelman]

Thanks for the input

The filter has little chokes & caps in it which I guess would get rid of some of the peaks & ripple, I was thinking of adding a 10 or 20 mH choke in there too

 

[cjhut]

Check out

http://www.dartcontrols.com.

We use these for our fractional HP PM dc motors. Everything is built in to tune in overload protection etc. We use the model CM50 with a 10 or 5kohm (can't remember) $15.00 knob pot. These have been a very durable unit for us. I have about 15 in our plant and they have been in use for about 10 years- very rugged and cost effective too.

 

[panelman]

thanks, I think that's much the same as the KB unit I mentioned earlier, do you run motors rated at less voltage than the controller will provide?