# Using PM DC motor as a generator



## GE11 (Oct 24, 2011)

I have a PM DC motor that I want to couple to a gas engine to make a DC generator out of it. I know a PM DC motor will make a generator by turning it. But is using it as a generator "bad" for the life of the machine? Like in other words if it were intended to be a generator by spec from the factory would it be made any different than it being a motor?

Here is one thing in noticed, when I had the engine running it and no load on the terminals I peeked at the brushes and saw small sparks on the brushes, even with no load... Why??


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## GE11 (Oct 24, 2011)

By the way this is a 4 pole machine... 4 brushes ..... 4 magnets


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## Salty9 (Jul 13, 2009)

An electric current is created by a conductor moving in a magnetic field and vice versa. It's similar to the magnetos on small ICE engines.


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## major (Apr 4, 2008)

GE11 said:


> I have a PM DC motor that I want to couple to a gas engine to make a DC generator out of it. I know a PM DC motor will make a generator by turning it. But is using it as a generator "bad" for the life of the machine? Like in other words if it were intended to be a generator by spec from the factory would it be made any different than it being a motor?


No and No. Probably. But it depends on a lot of things, like the application particulars (duty cycle, RPM, durability, life, etc.). So if it was designed to run intermittently as a motor and you run it continuously as a generator, then yes, life in calendar days will be shorter. 



> Here is one thing in noticed, when I had the engine running it and no load on the terminals I peeked at the brushes and saw small sparks on the brushes, even with no load... Why??


Well, hard it tell from here  At no load, terminals open circuit, there is no net armature current and should be no current going in and out of the brushes, so one would think. However when the armature is spinning in the magnetic field there is still voltage generated in the armature coils. The brushes will span more than a single comm bar so will short out adjacent armature coils. Ideally, those coils should be moving in an area of zero flux and have zero voltage generated in them which would lead you to believe that sparking should not occur. However, that coil is connected to the adjacent coil on the other side which does have voltage and from multiple coils. So as the comm bar enters and leaves the brush face, it has voltage which is abruptly forced to change and will cause discharges (sparks). The current path can be across the brush face and around the shorted coil.

How's that for a wild ass theory 

major


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## GE11 (Oct 24, 2011)

major said:


> No and No. Probably. But it depends on a lot of things, like the application particulars (duty cycle, RPM, durability, life, etc.). So if it was designed to run intermittently as a motor and you run it continuously as a generator, then yes, life in calendar days will be shorter.
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Thanks alot for the time and your help.

I am looking to use it in a situation where some of the time it is "motoring" and some of the time it is "generating". So I sould be OK in doing this as long as I stay within the RPM ratings and the Current ratings continous ratings that is..


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