# What makes a DC motor 'reversible'



## major (Apr 4, 2008)

booksix said:


> I see motors on ebay and other places listed as "reversible". I was under the impression that most electric motors could work in reverse. If this is not the case, what is different or what do I need to look for to be sure I'll be able to reverse my motor?


Hi Brett,

To reverse rotation on the DC motor, one has to switch the relative polarity between the armature and field. So, if it is a two terminal motor with the field connected to the brush internally, it will rotate the same direction, regardless of the polarity (+ or -) connected to the two terminals. Reversible motors will have four terminals, two for the armature (A1 & A2) and two for the field (S1 & S2). The field to armature connection is made external to the motor. So, connecting A1 to S1, and power to the other two, will rotate one direction. Connecting A1 to S2 and power to the remaining two will cause opposite rotation.

A similar thing with shunt or SepEx, except field and armature are not in series. PM motors are easy because the field (permanent magnet) always is the same polarity (N, S), so one just has to reverse the electrical polarity on the two terminals, which are connected to the brushes inside and then to the armature thru the comm.

Hope that explains it,

major


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## Georgia Tech (Dec 5, 2008)

Look for motors with 4 connections this will in general mean they will run in reverse....


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## booksix (Aug 26, 2008)

ok, cool. I actually knew that much but I wanted to make sure there wasn't internal issues to look for.

I asked because I'm on my way to WI for christmas and I might head to a forklift 'yard' in IL for a pair of 9" motors. They have 10 acres of lifts!!! I have to sort through, find what I want and they'll pull them while I wait. I'll probably rebuild whatever I find. Hope I come out with something good!


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