DC Motor Control

Alan A.

Member
Join Date
Jan 2004
Location
Alabama
Posts
1
This does not have any thing to do with PLCs, but maybe you can help me out alittle. We have 3 DC motors, 6.7 KW 2240 rpm, 440 volt arm @ 16.8 amps, 220 volt field @ 0.699 amps and 50 vdc tach feed back. The customer wants to run these motors at about 22 rpm. Has anyone had any luck running a DC motor at these speeds. These motors are on a Russia made, machine shop Bar machine. We are trying to use Fincor DC Drives, Model 3460. We do not think we can control at this low of speed??? Or I should say we are not having any luck at it.
 
You really want to do this?

I suspect DickDV will have a better cut on this than I ever will, but I really don't think you have much of a chance with this app. Trying to run that motor at about 1% of its intended nameplate speed has got to be frustrating. I suspect the drive is not capable of doing much to help you either. You would be better off to put in a gear reducer and get the motor back up to where it can perform for you. Or, change the motor and get one that runs nearer the rpm you need. You are probably getting a pretty sever cogging or unstable speed the way your are going now. Just my thoughts on the matter. You might also try going over to Ron Doran's site at Industrial Electrical & Maintenance forum. There are guys who frequent his site that may know more about this and they sometimes don't come here. Good luck with your machine.
 
My 10c (my 2c is free)

Try either:
1, Gearbox
2, Change motor ot a permenent magnet synchronous motor such as SEW DY series. I have used an SEW servo motor in the range of 50 to 3000 rpm (same motor).

Another question, you want to run this motor at 22 rpm. Will you ever want to run it back up to 2200rpm? If so, this will change your solution. Standard DC motors cannot do this.

I don't think you can do what you want with what you've got.

Hope this helps

Doug
 
Well now I aint sure I totally agree with all this. DC motors like this dominated the variable speed area for many years. I have worked with printing presses, paper converters, tubewinders etc that all used motors similar to this with no gearbox AND were capable of fractional rpm...depending.

Two things come to mind first and foremost.
1. The motor better have a blower(cooling) mounted on it, at low speeds the motor has no way to dissipate the heat.
2. Fincor is good for general DC motor control but I have not seen one that was that good at precision control. They are fair drives tho and if other options, like what I mention next, are added they can be better than fair.

As was stated a gearbox may offer the option you need..but maybe not just ANY GEARBOX. On the market are variable gearboxes...ie you can adjust the ratio. Take a 10 to 1 variable ratio gearbox (some can even be controlled electrically) and you have your range from 2240 to 224 rpm...at 224 rpm its fairly easy to run at 10% which is approximately 22 rpm. The variable gearbox may allow you the low and high speed options if needed. These type gearboxes replace older sheave drives and some other forms of variable speed control gearboxes/drive systems.

WHat you may want to do is get Fincor or there local rep to send an application engineer out to assist in the project. This may be free or at most a nominal fee required but can offer lots of info. May also talk to other drive reps and get there input, these guys have an engineering staff to go to for application issues that would cost a fortune to get otherwise.

I mention them because they are alot cheaper than more elaborate DC digital drives that can offer fractional rpm, full torque at zero speed etc etc.

Y'all do come and visit if in the neighborhood.

I said that jokingly. pertaining to internet, but then looked again at the thread and noticed that ALAN A is in my neighborhood in reality.
 
Last edited:
Hi Alan A !

What are the power & torque requirement at the machine end at 22 rpm?
By running the DC motor below its base speed will definately
bring down the output power in fraction ratio of the speed.

So as suggested by RSDORAN Variable Speed gear box would be good option if the mechanical setup allows for.
 
Maybe its just my contrarian nature but, whenever someone says it can't be done, my creative juices really start flowing. And I've paid dearly for it too sometimes, I might add!

Anyway, first some basic stuff. Doing the math, I find that this motor develops 7hp at rated speed and torque. That torque calculates to 21ft-lbs.

It is the nature of wound field DC motors to develop full rated torque rather easily right down to zero speed. There is little challenge in controlling speed at very low speeds either as long as the torque isn't dancing around high/low and plus/minus. For the application at hand, I will need to know more about the torque behavior of the machine to get very detailed with solutions.

I hope that the application requires the full speed output of the motor as well as the low speed. Otherwise, a 7hp motor is being asked to do only .09hp of work (21ft-lbs @ 22rpm). Seems a bit of a waste! If high speed isn't needed, a gearbox and much smaller drive/motor is advised.

It wasn't mentioned but the motor field winding style is important. A shunt field is good for motors that need to develop the same torque in both directions. A Stab shunt or comp shunt motor has both series and shunt fields so the motor develops more torque in one direction than the other. It is very important that the two fields are wired properly or the motor will not be stable and will not develop full torque in either direction.

The next issue is the tach which I assume is a 50vdc/1000rpm unit. This is an analog device with 1% accuracy typical. Since we are trying to run at 22rpm, that's only 1.1vdc which can get lost in the noise very easily. Is the tach wiring carefully shielded? At these speeds, an optical encoder would be far better but I don't know if the Fincor drive can deal with an encoder.

Ron Doran mentioned an auxiliary blower for cooling if full torque is needed at these low speeds. It's very important from a heat standpoint but probably is not a factor in getting the motor to settle down.

It is essential that the Current Minor Loop be well-tuned as well as the Speed Major Loop. These have a huge effect on speed stability. Use the drive AutoTune features if it has them.

If the torque isn't changing much and you can stand about 2% speed regulation (that's from no load to full load), you might try operating with Armature Feedback only. That leaves speed loop tuning and tach issues out of the picture. If torque is changing a lot, this will not give good results.

How about coming back with field winding style, load torque characteristics, speed range needed, and confirmation of tach output specs. We can probably help you further with that information.
 

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