Function of Motor Start Capacitor

[Hawkas]

I have a small 240VAC motor which I believe has a faulty start cap. When the motor is first started it runs fine, but when it it stopped via a separate control limit switch and the contactor drops out, if the limit switch is closed again the contactor will re-energise but the motor will not start again in that direction. It can however be switched in the reverse direction. This occurs no matter which direction is engaged initially, once the limit switch has stopped the motor, it will not re-start in the same direction. The installing electrician has wired the motor with no protective earth, is it possible that this has in some way affected the operation of the capacitor or is that unrelated (just dangerous!!!!)
thanks

 

[itsmoked]

It should certainly have a protective earth.

That said you have in order of likeliness:

1) A bad centrifugal switch.

2) Screwed up wiring.

3) A failed starting cap.

4) Open(failed) start winding.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Likeliness?

 

[itsmoked]

Ahh common jraef! I fell down a sail boat companionway head first today. I swear nothing in my years of existence has left me this sore. You have to cut me some slack here..

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ccjersey]

It has to be in the control/reversing scheme. The motor would not function at all, just sit and hum if any of the other things mentioned were the problem.

My bet is the mechanism of the reversing relay/switch is sticky. Somehow, making it change directions is making it fully close the start winding into the circuit or allowing another relay that's being held open to drop out before restarting in the other direction.

You might try completely cutting power to the controls and motor after it stops on the limit and then see if the motor will start again in the same direction when power is restored. This would indicate to me that there is a relay held open by the control logic that's disconnecting the start winding until the motor is reversed. If it makes no difference to cut the power, then look for a sticky/binding reversing relay.

 

[Hawkas]

The limit switches drop out the forward or reverse contactors, so when the limit switch is activated all power is removed from the motor. Its just when the limit switch is de-activated and the contactor comes back in, the motor will not re-start in the same direction. It will however run in the opposite direction if the manual switch is changed over. The manual switch is operating okay, the control system also has an auto function where the forward/reverse contactors are operated by relay contacts instead of the switch. The motor operates exactly the same way in auto or manual conditions. The motor is a 240VAC three wire, there is a neutral point and a forward and reverse terminal. The capacitor is connected across the forward and reverse motor terminals, which also causes some confusion, but also leads me to question whether the capacitor is not charging/discharging correctly causing the operating problem ?????

 

[waross]

Duh..... Once the limit switch stops the motor it can not be started in the same direction. What's the problem? When the limit switch stops a motor, it is not supposed to be able to be restarted in the same direction. Like, when the gate closes and the limit switch stops the motor, if you restart the motor in the same direction, you may damage something. This is a standard circuit doing exactly what it was designed to do.
The capacitor is not a problem. Both windings are identical. By connecting power to the common terminal and one end of the capacitor, a phase shift is created between the windings. This creates a rotating magnetic field which develops the torque to start and run the motor. When the power is applied to the common terminal and the other end of the capacitor, the rotating magnetic field will rotate in the opposite direction and the motor will run in reverse.
If you care to share with us what type of machine this is we may be able to provide more help.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Hawkas]

No I'm talking about manually operating the limit switch while it is moving in one direction or another to simulate the standard operation. The motor is controlling a shade screen in a greenhouse. At night the screen covers the house to keep the heat inside. In the morning the control system causes the screen motor to open up to approx 10% opening to allow outside air to mix with inside air slowly. However, as described when the system tries to continue opening the screen for daytime position the motor will not move in the open direction again, but it can be moved in the close direction. If left in auto mode, once the initial move has been made and the motor stops (not by limit switch but by time) then the motor will not re-start in the same direction when dictated by the control system, and will eventually trip out the overload. The time between initial 10% opening and then getting the signal to move to 100% open could be as much as five minutes. Whatever is controlling the forward and reverse contactors, once the motor has moved in a particular direction, it cannot be started again in that same direction it has to be moved in the other direction first.

 

[ccjersey]

From the description, sounds like a PSC motor without a centrifugal start switch and a dedicated start winding. I never saw a motor wired exactly like that, but see how it could work as Bill says with either of two identical winding put in series with the capacitor, one for clockwise and the other for counter clockwise rotation. More commonly the two ends of a series circuit of an auxillary winding and a run capacitor would be swapped between the terminals of the run winding by a switch or relay.

Maybe it's a load problem, meaning it has something to do with a brake or other characteristic of the winch or whatever moves the shade cloth. Any chance of running the system with no load? What about winding the mechanism by hand to feel if there is a point were the shade cloth stops each time that has damage which causes the mechanism to bind there.

 

[itsmoked]

That's what I'm thinking too. If, say, the shade is spring loaded once it's extended some percentage the load on the motor is going to be higher as are the binding forces. Trying to turn the motor shaft by hand at the initial position and at the partially opened position would be informative.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Compositepro]

I've seen this problem before. In my case it was caused by the fact that a relay is used to control direction. The motor will always start in the same direction unless the one limit switch was closed that energized the direction relay, which latches closed until the other limit switch de-energizes the relay. Whenever the motor was stopped that also unlatched the direction relay so it would always start in the same direction. I could manually trip the limit switch to reverse the motor when it started the wrong way.

 

[RPM3450]

The motor is not an Instant Reverse design is it?

 

[Hawkas]

The motor isn't an instant reverse type, the direction of the motor is controlled by separate forward and reverse contactors, which are electrically interlocked via the N/C auxiliary on the opposite contactor.
Whatever direction the motor happens to be travelling in, when the control system tells it to stop by de-energising the contactor,the motor will not start again in that same direction when the control system asks it to. The contactor does come in again, and power is applied to the motor terminals but it will not move. If we force the system to send a signal for the motor to operate in the other direction then it will start. But again when that direction is stopped for whatever reason it will not start again in that same direction. It has to be moved in the opposite direction first.