Controlling DC motor with DC signal

I am looking to use a DAQ to output an analog signal which would be used as a reference voltage for a linear voltage regulator to control the velocity of my motor. Because of the motor's size (12V 44A), I will need an external power source.

I would have liked to use PWM but because of the sensitivity of the environment to Radio Frequency Interference PWM will not be an option.

I have only used PWM in the past so any help on this subject would be of great use. Has anyone done this before? Any pointers or tips?

Thanks,

Mosher

Mosher,
You're right, PWM would be the usual way, perhaps check into what they use for a handicapped scooters. If your motor has a shunt field you could vary that to control over a limited range, that's how it was done before PWM (stronger field, lower speed).

If you have PM field you will have to loose some serious Watts with a Series transistor or perhaps a combination of fixed resistor/transistor in series/parallel.
No doubt someone else will have a better solution.

Good luck
Roy

A simple way to do this is to have the DA converter drive the reference pin of a LM 317 voltage regulator or similar. this will give you control from about 1.25 volts to a few volts below your unregulated supply. Very low parts count, the regulator is robust and protected, and the scaling is easy since the differential is always 1.25 volts. I've used this for tester stimulus channels.

Regards
cww

I think you may want a Xantrex or an Agilent (formerly HP) DC power supply. I am guessing at your needs though, since you haven't really said what the application is. (An actuator a car? A test system on a motor production line? A home robotics project?). This is a conventional permanent magnet DC brushed motor, is it?

If you want to talk about your application in more detail, I can probably give some more definite advice. At present, I am only guessing at what you are trying to do. I have worked with a lot of motors in this power range, and I can say with confidence that the best power supply for one application is entirely the wrong one for another application.

You also need to be more specific about the RFI interference criteria. Is this radiated RFI, or conducted RFI? If the latter, then how would the RFI be conducted to whatever it is you are worried about? Is it the motor that you don't want to expose to RFI, or is is something else? A more detailed description of the application would help.

I currently have a HP DC power supply. I don't have it right on hand right now but I believe it is something like 0-60V 0-120A.

The application is a motion control of a trolley on a straight and level track. The fear of RFI is because the trolley is used as a lowering station to calibrate acoustic transducers and receivers.

Unfortunately I know very limited things about the motor I have been given, it was just one lying around and was a suitable size to move the trolleys. I am assuming it is a brushed motor... there is something odd about this motor though (or I have never seen it before). The motor has 3 pins common, CW rotation and CCW rotation, does that tell me anything about what type of the motor it is?

I have tried looking up information on the net about the motor but it looks like none can be found.

Thanks for your ideas, and I will be looking into all of them in the next little while.

Mosher

Are you sure it's DC? With 3 wire connection it could be the type of AC motor where a capacitor is connected between CW & CCW.

It might be a DC motor with 2 sets of series fields, perhaps.

A picture is worth 1000 words!

Roy

I've used the 0-20V 0-120A and 0-60V 0-60A versions of that power supply. That is a top-notch power supply. Xantrex is cheaper and just as good, but I don't know of any that are actually better than the one you have. It is also very expensive (I think around $4000).

RFI from the output side that power supply should not be a problem. If you put a scope on the output, you will see that it is very quiet. The motor itself however will generate some RFI while it is moving due to commutation noise. That should be mostly contained to the DC circuit though rather than radiated. If you aren't driving the instrumentation under test from the same power supply, then RFI problems are not likely.

All the same, you will probably want to route cables, etc. away from the instrumentation under test. Radiated RFI intensity will be inversely proportional to the square of the distance. If you are mounting the motor in a fixed position and driving the trolley via a cable or belt, this makes RFI problems even less likely.

The motor sounds like a conventional DC brushed motor. Given the voltage and current rating, it may be one from an automobile. However, I can't think of any in that size range that have a three pin configuration for reversing. Have you actually tested the motor yet? Usually, the extra pin is for two speed operation. Sometimes the third pin does nothing and is just there because the auto maker wants to use the same connector for different applications. Of course the motor might not be from an automobile so the above may not apply. Are there any markings on the motor? If you can put a few photos of it up on an image host (e.g. http://www.tinypic.com or some other image host) then I could try to see if I recognise it.

If you are switching relays to reverse the motor, then you will want to make sure the current limit is set to zero on the power supply before you do so. If you are controlling the power supply through an analogue signal, then you can control both the voltage and the current limit (there are independent voltage and current command inputs). Before switching the relay, set the current limit (current command) to zero, then switch the relay, then set the current limit back to whatever you are operating at. You will need to allow a time delay for the relay to switch (probably about 100 msec for a motor contactor). This ensures that the relay is switched without creating an arc (and RFI). If you are controlling the power supply via GPIB, you can control the voltage and current through GPIB commands as well.

You are correct, it is the 0-20V 0-120A HP 6011A. I did not realize that it had the capability to accept an analog DC signal to control either the CC or CV (I'm just borrowing the power supply for the application so I'm not terribly familiar with it). This may be the ticket to solve my problem.

So far I have tried controlling the voltage of a 0-200V 0-17A HP that I had lying around it works quiet nicely! A bit slow for control applications but that’s not what it was meant for I don’t imagine.

I will add some power mosfets to control direction and emergency stops of the motor lines.

For the future:
I am fairly new to this field so some of my questions might be quite amateur. With RFI I didn't realize that it would only be in the circuit and it wouldn't be emitted. Is there any literature you can point me to, which will tell me more about this. I was told because I am operating the PWM at such a high freq and using such a high current that it would emit a large amount of interference.

I don't think the power supply has GPIB or... HP-IB. It has the analog ports for remote voltage and current control. I could use this to set the voltage to zero while switching directions but I was planning on having a set CC and varying the CV with the DAQ analog output 0-5V and you can only control either the CV or CC but not both at the same time.

As for the motor, I am not allowed to use a camera within my building so I will just have to write what is on the label:

-------------------------
Ofrans
CODE:30371 No:02548
W:1500 V: 24
A: 88 IP:56
ROT:R/L RPM:2200
-------------------------

I have used both input terminals and they do turn it opposite directions.

Thanks for your input,

Mosher

Appears to be an Italian boat winch, that makes sense. Perhaps you can track down the manual

Roy

With regards to the motor, it appears to be a 24V motor, so it's definitely not an automotive motor. The name plate you give shows it is 24 volt, 88 amp, 1500 watts, and 2200 rpm. I suspect that it is an AC motor. Since you said that it also runs on DC, then it is probably a series motor (since this type can use both AC and DC).

As for your test with the 0-200V 0-17A HP, I think that you will find that the power supply was operating in current limiting mode whenever the motor was accelerating. This would result in slower acceleration. The motor should accelerate much faster with the 120A power supply.

I believe that GPIB is an option with these power supplies. It won't be present unless the power supply was ordered with it.

As for conducted versus radiated RFI, the radiated RFI affects an electronic circuit when the circuit acts as an antenna and picks up the emissions and turns them into electric current. Conducted RFI just skips the radiation and antenna step and is conducted directly into the affected circuit. It can be a more serious problem because its signal strength is much stronger (because it isn't attenuated by having to travel through the air). Just don't power the instruments from the motor power supply and you should be OK so far as conducted RFI is concerned. Conducted RFI is more of a problem with automobiles, where all sorts of noise is injected into the 12V power that runs everything in the car.

As for the power supply itself, the HP power supply is a switch mode supply, but it is a very good switch mode supply. They are designed for lab and test use and have a lot of filtering on their outputs (as well as very careful design). This is why they cost thousands of dollars. The PWM supplies that people are warning you about are the cheap ones (an order of magnitude cheaper) normally used to drive industrial motors. When you are driving the motor, there will be noise in the DC circuit. It will be noise coming out of the motor though, not from the power supply.

To deal with RFI, what you need to do is to design the system to try to avoid the problems (which is what you appear to be doing). When you have it built, check the instrumentation circuits with an oscilloscope for noise. If you find a source of noise, then you need to find the source and figure out how to deal with it. Just be warned that at some point you will be looking at RFI from the local radio or TV station transmissions. It's not something that you can eliminate completely, just something to reduce to an acceptable level.