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Elninio
07-06-2008, 12:06 PM
I want this motor to spin in the reverse direction, permanently. I will be using it in a tool grinder. If i remove the capacitors, it will give me the freedom to choose the direction it rotates by giving it a push start in the direction I choose.

The reason it starts in the direction it does it because the capacitors (in the motor) act as an impedance which shifts the AC phase in the direction it will rotate. It is offset by 90 degrees (I assume), if I add more capactors in series I can get the phase offset by 270 degrees, which would turn the shaft in the backward direction. But this would have altered the total impedance of the circuit. How can I offset the phase without adding more resistance to the circuits. Does it even matter?

J Tiers
07-06-2008, 12:24 PM
Generally, reversing the phase of the START winding (the one with the capacitor) will reverse the motor. You have to stop it and re-start with the start winding wires reversed.

A "DPDT" switch will do the reversing. Used to run my lathe that way.

davidh
07-06-2008, 01:23 PM
yesterday i installed a new field winding in a dewalt chopsaw and instaled the field backwards. the damn blade turned backwards. took it apart and turned the fields around ( not in the rotation way ) and it solved the problem. i suppose i could have reversed the 4our wires in the connections but they were packed pretty tight in the assembly. learned something new again. goes to show you are never to old to learn.

RobbieKnobbie
07-06-2008, 01:34 PM
Am I understanding this clearly? If you reverse the two wires going to the start windings (the ones connected to the centrifugal switch) the motor will run backwards?

No way, it can't be that simple. Really?

J Tiers
07-06-2008, 01:43 PM
Yes. The rotating field set up by the phase shift in the start winding will be reversed, because you have turned the "temporary north" poles to "temporary south" poles, and vice-versa.

That reverses most motors where the windings are all brought out. In fact, that type usually says right on the wiring plate to reverse one set of wires for reverse rotation.

The basic motor will run either way depending on which way it is started.

I just wired up a SouthBend for the Dad-in law that way 2 days ago. Reverses fine, but you have to let it stop before you flip the switch "on" for reverse.

Mind, this is for INDUCTION motors, single phase, with start windings.

Chopsaws and small grinders etc usually have universal motors, and then you reverse the field (or armature) as davidh found out.

Elninio
07-06-2008, 04:58 PM
Am I understanding this clearly? If you reverse the two wires going to the start windings (the ones connected to the centrifugal switch) the motor will run backwards?

No way, it can't be that simple. Really?

I've always thought this was the case for DC motors, but for AC motors, the capacitors do the phase change. Is this what you were referring to? I am also confused.

J Tiers
07-06-2008, 05:12 PM
The single-phase induction motor STARTS as a "two phase" motor, using some means to shift the start winding phase to form a "second phase" which is "displaced" from the phase of the run winding. *

When you reverse the wires, you move the "pulling force" from , say, being CW of the "run" winding poles, to being CCW of those poles. That gives a "kick" to the opposite direction, and reverses the start rotation.

Once the motor starts, the start winding is disconnected by a switch. After that, the motor depends on its inertia to rotate it into a position where the next pole can pull on it.

If a very heavy load slows the motor enough, it won't rotate inertially far enough, and torque "breaks down" and the motor will slow until the start winding cuts in again. This is usually bad, leading to a burned-out motor as the start winding or the capacitor, exceeds its allowable percent duty, and is damaged.

Anyway, the start winding merely does exactly what you were doing when you gave the motor a 'start" one way or the other.


* The start winding may have a capacitor in series, OR a resistor. The "resistor" may be simply from using smaller wire.

Either way, the run winding and the start winding are out of phase, and until the switch cuts out the start winding, there is an actual force turning the motor. After that it is dependent on inertia to turn it into a position where the next pole can pull the rotor.

Elninio
07-06-2008, 09:35 PM
The single-phase induction motor STARTS as a "two phase" motor, using some means to shift the start winding phase to form a "second phase" which is "displaced" from the phase of the run winding. *

When you reverse the wires, you move the "pulling force" from , say, being CW of the "run" winding poles, to being CCW of those poles. That gives a "kick" to the opposite direction, and reverses the start rotation.

Once the motor starts, the start winding is disconnected by a switch. After that, the motor depends on its inertia to rotate it into a position where the next pole can pull on it.

If a very heavy load slows the motor enough, it won't rotate inertially far enough, and torque "breaks down" and the motor will slow until the start winding cuts in again. This is usually bad, leading to a burned-out motor as the start winding or the capacitor, exceeds its allowable percent duty, and is damaged.

Anyway, the start winding merely does exactly what you were doing when you gave the motor a 'start" one way or the other.


* The start winding may have a capacitor in series, OR a resistor. The "resistor" may be simply from using smaller wire.

Either way, the run winding and the start winding are out of phase, and until the switch cuts out the start winding, there is an actual force turning the motor. After that it is dependent on inertia to turn it into a position where the next pole can pull the rotor.

Ah, gotcha. Do all motors have start windings? How do I know what the start windings look like?

ftl
07-06-2008, 10:07 PM
Most 1/10 HP and larger single phase motors (aka: split phase) have start windings. Very small motors use different techniques called shaded poles, but these have very low starting torque and are unlikely to be what you are dealing with.

You do not actually have to identify the starting vs the run windings as reversing the wiring of either winding will do what you want. The absolute direction each winding is connected in is not important, just the relative direction. Reversing either winding should reverse the motor.

Use an ohmeter or test lamp to identify the four wires that should come out of the field windings. There should be two windings that are not connected to each other. Reverse the connections to one winding.

The start winding is usually made of thinner wire and has a higher resistance if you want to try to identify it.

Hope this helps.