Reversing a CSCR TEFC 1hp motor

[wb4jtb]

My lathe has a 1hp 120/240v TEFC taiwan made Cap Start Cap Run motor that I would like to be able to reverse. One end of the start winding is internally bonded to one end of one of the run windings and one side of the caps is connected to the other side of the same run winding.

 

If I were running it on 240v, I believe that it could be wired so that the end of the start winding (bonded internally) was at the center of the combined run windings. I could then connect the caps externally to the other side of the line to reverse the phase. However, I need to keep running it on 120v.

 

Along the same lines, I believe that I can use a transformer with a 240v center tapped winding to generate an additional phase to connect to the start caps. The transformer CT would be connected to the same side of the line as the start winding and one end would be connected to the other side of the line. A simple SPDT switch to move the caps from one end to the other on the transformer would reverse the phase of the start winding.

 

Please correct me if my logic is flawed.

 

Now my question: How much current flows through the start winding during run relative to the main windings? Startup isn't much of an issue as a momentary overload on a transformer won't damage it. But I need an idea how to rate the transformer for running.

 

Thanks,

---Scott.

[marke]

Hello Scott

 

Welcome to the forum.

 

To reverse the motor, you need to reverse the polarity of the start winding relative to the run winding. This means that you need to separate the end of the start winding from the run winding and then use a double pole double throw switch to connect the winding in either polarity.

You could use the circuit that you describe, but there are two issues here. a) the cost as the transformer would need to be big enough to have a low enough impedance to carry the start current and there would be significant leakage reactance in the starnsformer which would move the phase angle of the current in the start winding. You would need to be sure that the effective reactance of the transfomer was very small compared to the impedance of the capacitors.

I would go for separating the windings and using a dpdt switch!!

 

Best regards,

[wb4jtb]

Thanks for replying. I agree that reversing the start winding is the correct way. It's just that it may not be possible and to find out will require removing the motor from the lathe and performing exploratory surgery. It would be nice to avoid that.

 

I came across this http://www.homemetalshopclub.org/news/sep01/sep01.html last night. The author describes a method of reversing where he replaces the cap with an inductor. That should work.

 

For my application I don't need full power in reverse. It's a wood lathe and it's very useful to be able to sand in reverse, against the direction that the wood was cut. Consequently, I don't need full torque. I would actually be quite happy disconnecting the start winding completely, giving the lathe a spin backwards and switching it on. Part of my concern is the capicator run aspect of the motor. I just don't have a good grasp on the issues involved.

 

Thanks again,

---Scott.

[marke]

Hello Scott

 

An interesting idea, but I am not convinced. The current flowing in the run winding is an inductive current. The current flowing in the start winding will now also be inductive. there may not be sufficient phase shift to create a rotating field effect. The answer is to try it and see.

Please report back how you get on. I suspect that if it works, the starrt torque will be very low and it will only work in some situations. - I could be wrong here!! My expectation is that where the resistance of the run winding is high, (particularly small motors), this may work.

Interested to see how you get on!!

 

Best regards,

[wb4jtb]

I disconnected both the run and start caps. The no load current goes from 2A to 3A without the run cap. Loaded current is about the same. Substituted the output side of a variac (as an inductor only, no power applied) for the start cap. There was a fair range of inductance that would start the motor in reverse. I was able to dial in a value that started the motor just as fast as the start cap. Measureing the current in the start winding at this optimal setting showed it to be about the same as with the cap. So, it does work. Further tests could be be done with the run side of the circuit, but for my needs I think I can just leave that disconnected when using the motor in reverse.

[marke]

Hello Scott

Thanks very much for the update. It would be interesting to know if the reactance of the inductance is equal to the reactance of the start capacitance, or was the value quite different.

The results suggest that the impedance of the run winding must be quite resistive rather than inductive. This may restrict this concept on larger motors.

 

A great result and than you for sharing it with us.

Best regards,

[Carl]

Hello Scott

 

that is an interestng result and a bit confusing there has to had to be a phase shift between the starting winding and the running winding ..hence the use of a cap. but to use an inductor and get a sufficient phase shift. Did you say that you run the motor without any starting winding...becauce the motor only needs a very small movement in either direction to get it started in that direction.

 

Could it be the direction of the windings of the introduceed inductor that is giving a phase shift??

 

Carl

[wb4jtb]

Assuming that the start winding is wound on 2 of the same 4 poles as the main, then I think it’s just the phase shift of the combined LRC vs. LRL circuits that reverses the motor. The motor runs equally well in either direction without the run/start winding, it just needs a little push to get going.

 

Throwing some arbitrary numbers at it and ignoring the effect of R. If the phase shifts are -5 for the start winding, +20 for the start cap, then the net phase shift is +15 and the motor spins in that direction. If we substitute an inductor with a phase shift of -10 for the cap, then we get a net phase shift of -15 and it runs in the other direction. Actually, we can’t really ignore the effects of R. R is the reason that the start winding has any phase shift at all.

 

So, my guess is that the reactance of the substituted inductor is a actually less than the reactance of the original start cap. Unfortunately, I have no way to measure. If I actually build a permanent setup, I’ll use a home made bridge circuit to match up my variac with a fixed transformer/inductor.

 

Thanks for your input, the comments and questions have helped me explore this problem.

 

---Scott.