Silly question on a AC electric motor.

[hutch--]

I just had a gadget arrive to use for sharpening tungsten tipped saw blades and while all of the bits are there, the electric motor runs the wrong direction. If it was a DC brush motor it would be a simple swap the wires around but its a mains 240 volt 50 cycle motor that has permanent magnets in it so I guess its an AC brush motor. It has been so long since I touched one of them that I forget how to reverse the running direction. After the switch is a .47 mf capacitor on each side, a couple of diodes on a circuit board and two wires going to the motor.

What I forget is whether you can simply reverse the direction by swapping the 2 wires going to the motor or whether you have to swap wires in the guts of the motor. Anyone remember how these small AC brush motors actually work ?

[dedndave]

diodes ?
it may be a DC motor
can you draw a schematic ?
it doesn't have to be to scale - lol

[dedndave]

on second thought....
you may not need to draw it

this is one of the many rectifier topologies


if there are only 2 wires going to the motor, it is likely a DC motor
an AC motor with a starting capacitor will typically have field and armature windings, so there will be more than 2 wires

[donkey]

Geez Dave you really need a capacitor on that bridge to get rid of the ripple ! I've been studying power supplies as part of my new hobby in electronics (specifically microcontrollers), I'm going to be trying to build a 5v switching supply this weekend :)

[dedndave]

switchers are a lot of fun
be sure to wear safety glasses   :bg

truthfully, even a simple shunt regulator circuit isn't as simple as it sounds
to make it work correctly over full line and load conditions requires a lot of calculations

[hutch--]

This is the small board inside the motor end cover, it may just be a crappy little half wave rectifier running at mains voltage and the motor may just be a DC motor. Never seen stuff like this before.

[NPNW]

Friend says its single phase. Isn't sure if you could change the wires after the diode. What model is it?

[dedndave]

well - it probably wouldn't hurt to reverse a couple wires - lol
just don't run it long if it starts growling at you

the rectifier circuit may be for the relay
i can't tell where some of the wires are going, and i can't see the circuit board "runs", either

the capacitor hardly looks big enough to be a starter

don't suppose you have a meter ?
i think it's a DC motor

John brings up a good point....
have you looked everywhere for the reversal switch ?   :bg

the red and blue wires off the right are obviously DC (marked + and -)

notice that the diodes are different
maybe - one for the motor and one for the relay ?
hmmm - just noticed the third one

[NPNW]

It's a single phase motor, the capacitors are for starting it, they shut off once the motor is running. Without seeing the nameplate, I have no idea how to change rotation, other than it is done in the motor housing.

This is from a electrician friend.

[dedndave]

make and model ?
we might find info online...

[NPNW]

http://www.eng-tips.com/viewthread.cfm?qid=254253

They are talking about several motors. Might help.

[hutch--]

Done, just reversed the wires after all of the switch gear and capacitors and it runs the right way now. It appears to have been an error in assembly as there was a color code swap with the wiring.

I have not seen high voltage DC motors in this type of context before, you see plenty of AC brush motors in ordinary power tools.

[NPNW]

Wonder how these worked Hutch?

[dedndave]

yah - hand drills and saws don't have starter windings because they are wound on magnetic cores
in those types of AC motors, the magnetic poles determine the rotation direction
also - starter capacitors are non-polar - so you kinda get the impression that one is for filtering DC

[donkey]

Can't really tell if the circuit is a rectifier unless you can see the traces on the board but the electrolytic (10uF) capacitor would blow pretty quick in an AC circuit so I guess it must be. However, the other problem is that at 240v there is no transformer and the diodes wouldn't last long (there's no resistor to drop the voltage across), there would have to be some sort of step down transformer and usually at least a zener and resistor to limit the voltage.