How slow can I run my DC motor?

[OEMsparky]

I've got a 90Vdc 1/8hp gearmotor (Dayton 4z383) that has an output speed of 30rpm. I have an application that would require me to run it as slow as 2 rpm - so I'd be applying only about 6 volts to it. I believe that there is a problem with running dc motors too slow and overheating them - how slow is too slow. If I need to change ratio or speed it up, how far do I have to go to be in the "green zone" again?

Thanks.

 

[ian69san]

Need to know some more info
Whats the gear ratio of the unit?
What form factor is the control?
What's the load on the gearmotor?

I've had a small 130v Bodine on the bench and it runs OK at 6 volts but thats off load

 

[Skogsgurra]

You have three things to consider when running a DC motor at low speeds:

First. Cooling. Will the air circulation be adequate at that speed? You have to test to find out. If the motor gets too hot - than external cooling is needed.

Second. At that speed, the collector can overheat locally, since the brushes will stay on each segment for a prolonged time. The time constant of these little bronze pieces is very short.

Third. The winding is usually not designed to be carrying current for a long time. If the motor runs very slowly, the commutation between different parts of the winding does not happen very often. And that can lead to overheating the coil that is carrying current. It definitely does so if you stall the motor for en extended time.

But since this is something like 6 or 7 percent of nominal speed, I think that the motor will be OK. Make sure it doesn't stall.

Gunnar Englund

http://www.gke.org

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

 

[IRstuff]

Why not just gear it down and avoid these problems?

TTFN

 

[itsmoked]

You say, "as low as 2RPM".

"How fast do you need it to go?", and we can tell you how much to gear it down.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[OEMsparky]

Thanks for the input guys. I did get in touch with the mfg and am told that they normally are OK with a 1/20 speed reduction from nominal but that the 1/17 that this application represents could be considered "on the edge" and may cause problems. Changing the gearmotor to a larger reduction is the easy answer but one we were trying to avoid in order to keep this drive the same as some past machines. However, in light of the mfg's comments I think we'll do just that and go to the next size reduction which puts us back in the acceptable range.

Thanks again.

 

[itsmoked]

If you can afford it conservative is always good! Good luck.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

I do not quite get it.

Quote: normally are OK with a 1/20 speed reduction from nominal but that the 1/17 that this application represents could be considered "on the edge" Unquote.

The 1/20 is a larger speed range than the 1/17 that you need. So, according to that statement, you should be fine.

But do not expect speed to follow voltage linearly. The internal resistance in these small motors is quite high. So do not be surprised if you need about 10 V instead of 6 V to get your 2 PRM out of it. The main question is how it is loaded. If your load changes a lot, then a new gear ratio is needed. If it is more or less constant, a voltage reduction should be OK.

Gunnar Englund

http://www.gke.org

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

 

[waross]

We are assuming that your motor has a permanent magnet field. If it has a shunt field the field should be held at full voltage and only the armature voltage varied.
respectfully