Electric motor direction change

[newbie31]

Hello. I have to admit this is probably way too simple for most in this forum but it's not my specialty so I am trying to learn. Bare with me please. My question is, I am trying to change the drection of a motor I pulled from a small orbital sander. I would like the motor to operate in both directions. I realize that it wasn't designed to, but I also know it can be re wired to do so. Can anyone give me an idea of how to do this and any schematics or simple diagrams of how a motor like this is normally wired and what wires in it need to be changed. It's obviously an AC motor operating at 120 volts. Any help would be greatly appreciated. I am trying to learn all I can about electric motors and this forum is just superb. Hats off to all the pro's who take the time to help.

Thank you in advance.

Michael

 

[motorsdirect]

Hi Michael.
check out this page for some useful diagrams:

http://www.lmphotonics.com/single_phase_m.htm

otherwise,leave a few more details about the motor and wiring you have got and I will try to help.

Jeff

http://www.motors-direct.co.uk

 

[buzzp]

Must be single phase, hey?

 

[motorsdirect]

Good point....could be three phase I suppose.In which case,If you reverse one pair of the incoming supply lines, you will reverse the direction of the motor!!
Whoops
Jeff

http://www.motors-direct.co.uk

 

[jraef]

I seriously doubt it is 120V 3 phase. If it is a hand-held orbital sander it is most likely a universal motor, i.e. a DC motor with a built-in rectifier. Reversing it would entail a more complex DC power unit than what you already have because it would need to reverse the polarity. You could do it with a system of switches, but it would probably be cheaper to buy an inexpensive reversing drill motor because it already has that built-in.

"Venditori de oleum-vipera non vigere excordis populi"

 

[newbie32]

Thanks for all the input so far. I will get the exact information on the motor and post it. I don't know a whole lot about electric motors but I too would suspect it's not 3 phase. I didn't think they used many 3 phase motors until things get really big ie 8hp or more. This thing probably only draws 3-4 amps. Anyway **jraef** do you know if they often use rectified motors in these types of applications? I thought it would just be a cheap a/c motor but if they use rectifiers and such, I suppose like you said it would be more difficult to reverse. I called a place that does that sort of thing. They say they can do it for $50 switch and all but I want to learn how it's done. In this application, I need it to basically remain in the original housing, so using a reversing drill motor isn't in the cards, although I agree, that would make sense.

 

[jraef]

If it is a hand-held power tool, yes they are almost always a universal motor. That IS the definition of a "cheap AC motor" by the way, because they are mass produced specifically for power tools. Turn off the lights and run it in the dark. If you see any blue sparking going on in the motor housing, that is from the brushes of the universal motor. In order to reverse it you will need to know exactly how it is wired now, and since these are usually OEM motors designed specifically for one tool manufacturer, you will need to get ahold of an internal wiring diagram to know what to hook up to. If you already knew enough about how universal motors worked you may be able to figure it out without a diagram eventually, but if you make a mistake you may smoke the motor, so what do you save? The $50 sounds cheap enough to me if you put any $ value on your own time. If it were me, it would probably take me 1-2 hours to figure it out, wire it and test it. I don't sell my time that cheap.

"Venditori de oleum-vipera non vigere excordis populi"

 

[newbie32]

Hello Jraef. Thanks for the lesson. I am now quite sure it is a universal motor. The $50 isn't the issue, my time is definitely worth more than that but I am trying to learn more about these as I tend to work with electrical motors a lot. So basically I guess, I had better find the wiring diagram for this thing, then I can probably figure it out. They told me it only takes them 20-30 min to rewire plus the switch. Sounds simple enough to me once you know what to change.


By the way. What does that line in your posts mean?

 

[motorsdirect]

Hi...
Its all getting clearer now....sounds like a series wound type of motor....in which case,,,reversing the brush leads should reverse the rotation.....although....most armature motors will run slightly better in one direction than the other!!

Jeff

http://www.motors-direct.co.uk

 

[UKpete]

newbie32, Universal motors will work without a rectifier. They are capable of running with ac or dc.

A permanent magnet commutator motor only works on dc and will reverse if you reverse the motor leads. A Universal motor (so named because it can run from ac or dc) differs only in that the field flux is supplied by stationary coils instead of a permanent magnet, these field coils being connected in series with the armature.

The direction of a Universal motor can be reversed by reversing EITHER the direction of current in the rotor (the armature) via the brushes, or in the stator (field windings). If you reverse the current in BOTH you get no change in direction, hence when you apply ac to the motor, the rotating force will be in the same direction for both half cycles of ac because you are simultaneously reversing current direction in both armature and field.

So as motorsdirect says, you can simply switch over the brush leads (or the field winding leads) to reverse it. This applies even if the motor has an integral rectifier (they are sometimes fitted, possibly this is to reduce sparking - maybe someone else could comment on this).

Note the motor will generally run on lower voltage when driven of dc (the inductance limits the current on ac). You can experiment with a low voltage dc power supply.

 

[jraef]

There ya go. These guys obviously know more about universal motors than I. I was mainly just getting you going in the right direction.

My sig line roughly means "Snake-oil salesmen cannot thrive without an ignorant populace".

"Venditori de oleum-vipera non vigere excordis populi"