Electric Motor Runs backwards, Cap OK 1

[rboisson]

Hi All,

I have a 110v motor for my home bandsaw that is running backwards. I've researched the issue and looked for what the problem may be. In the process, I purchased and installed a new start capacitor with no change. The motor starts very well but consistently in the wrong direction. If I remove the start capacitor and give the motor a good spin in the correct direction, it runs fine.

I've opened the motor up, and it doesn't appear that it is intended to be reversible. I am able to see the various windings attached to the power leads, but I don't know which wires should be reversed to change the motor direction. Can anyone take a look at the attached picture and let me know which leads should be changed?? Or am i just out of luck?

Motor Pic (2.2mb)

What would have caused the motor to change start direction?

Thanks!

 

[jraef]

Most likely...

There is a centrifugal switch in your motor that switches the start cap in and out. At rest the switch is supposed to be closed so that when you apply power, the start cap is in the circuit and creates a virtual phase shift in the magnetic field that causes it to spin in the desired direction. Once it gets to speed, the centrifugal switch opens and removes that cap, but the motor is already spinning so it keeps spinning in that direction.

If that switch fails to re-close after the motor is shut down, the start cap, even if good, will not be in the circuit. So then what happens is that the run winding field field is randomly going to spin the motor in either direction. Murphy's Law then takes over to dictate that it will always be the WRONG direction.

If there is no centrifugal switch, then the function is being done by an external device called a "potential relay" that is looking at the back emf of the motor to decide when to switch out the start cap. But he same holds true, it is likely failing open.


"Will work for salami"

 

[rboisson]

Thanks for the suggestion. I am familiar with the centrifugal switch motors, having fixed one recently on a compressor. I haven't found anything looking like a centrifugal switch on the motor. So that would either lead to the potential relay that you mentioned, or that this motor is a capacitor run motor instead of a capacitor start motor.

Where would a potential relay typically be located on a motor? I haven't seen any other components, unless the relay is buried somewhere within the windings.

As to your comment on the failure state of the switch, I am certain that it did not fail open, as there is a VERY large difference when you remove the capacitor from the leads. With the capacitor in the circuit, the motor starts easily every time, only running backwards. I have started and stopped the motor at least 100 times and it is consistently always the same direction (Wrong). When I remove the capacitor, the motor does not start at all. I can start the motor in either direction simply by spinning it either way. It will continue running in the direction I spin it.

So to me, this is not an indicator of a switch failing open. In my past experience with the capacitor start compressor, when the switch failed closed, the motor did not achieve full running speed and made a heck of a racket when it was turning. This leads me to think that this is a capacitor run motor with the capacitor wired incorrectly.

As a side thought, is there any way to use a different value capacitor to change the phase and start the motor in a different direction?

Thanks!

 

[OperaHouse]

Did you wake up one morning and this was running backwards? Or did you just get this. The only way I know that these motors can reverse is if you reverse the frame with respedt to the rotor. The rotor has a spiral twist to the assembly. Alrwenately,the capacitor field windings can be reversed if they are accessable.

 

[rboisson]

It was a castaway from my work. They used it for a few years and then the blade guide broke on it. It sat for a few more years and then they decided they didn't want the hassle of fixing the blade guide so they replaced the whole saw. Based on what they say, it never had any problems before the blade guide broke. I'm not 100% convinced that it ever ran the right direction. It would have seen very light use prior to breaking.

A little more research, and I think that I can reverse the direction of the motor by flipping the capacitor from wire E to wire C.

I'll give it a try and cage up the capacitor while testing to make sure nothing goes awry.

Thanks!

 

[MiketheEngineer]

Turn the motor around??

 

[rboisson]

Not possible with this setup, and certainly not worth the hassle on this bandsaw. This is a 9" Craftsman benchtop model. I'm really fixing it in the hopes of selling it or handing it off to someone in my family who needs it. I've got a 14" Jet on a floor stand for myself. I'm looking for cheap and easy solutions or the thing will be recycled completely.

 

[Motorwinder]

A single phase motor uses the start winding to set rotation. Not the capacitor. You would have to take it to a motor shop so they can lift the connection and reconnect so the start winding kicks it in the other direction.

 

[Motorwinder]

Or lift it your self and break the connection where the start winding taps into the run winding. Bringing the lead out, then you could run it in either direction.

 

[Compositepro]

From the photo it is apparent there is no centrifugal switch and you almost certainly have a motor with only one run/start capacitor. These motors have relatively low starting torque. To reverse the motor you have to disconnect and reverse both ends of the start winding. One end is now connected to one of the power leads and the other end to the capacitor. Switch these two. Seeing what you have already done, this should be no problem for you. I've done this to several drill presses and added reversing switches so they could be used as mixers.

 

[rboisson]

Sounds good. I'll get to it tonight or at the latest, this weekend. I'll post my results.

Thanks!

 

[rboisson]

Hey Compositepro,

Your comment worked perfectly. The motor is back together and running perfectly, in the correct direction Thanks for your help!

Thank you to the rest of the posters as well! I appreciate the assistance.

 

[racookpe1978]

Well, ya know you could have looked fer a reverse-direction saw blade ... 8<)

 

[Compositepro]

Racookpe, I know your joking, but did you know that the teeth on band saw blades are reversible by turning the band inside-out? When you uncoil a new blade you have to check that it uncoiled correctly.

 

[John2025]

Every yougster who's ever worked in a toolroom learns that mistake the hard way (me included). Nothing like laughter and derision from the old guys to make the lesson stick. Nowe that I'm one of the old guys, I feel like it's my duty to ridicule the new kids