PSC motor reversing at stall

[Barry1961]

A customer has an 1/8 hp, 115 vac PSC motor that is reversing rotation at stall. The stall only happens when something goes wrong but the effect of motor running in reverse is costly.

I have read that increasing the capacitor size will help this problem but increase the starting current. Does increasing the capacitor just make the motor harder to stall or does it bias the direction of rotation?

Increasing capacitor size will reduce number of starts and stops per hour and may require a larger circuit breaker. Is there anything else I should worry about?

I am fuzzy on this so any help would be appreciated.

Barry

 

[Barry1961]

A little more information may help.

The motor is used to open and close an overhead door. If for some reason the door jams and motor stalls without tripping it starts running in reverse. I can see how a larger capacitor would boost starting torque but I don't know the effect it would have while running.

Does a PSC motor run just as well in reverse when connected to run forward once it is forced to start turning in reverse? Will a larger capacitor help this problem?

The end solution will probably be to detect a jam quicker and turn off the motor but the larger capacitor idea has been suggested by a key player.

Help please!!

Barry

 

[Skogsgurra]

Go test it! You cannot destroy anything.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[sreid]

I've just got to ask.

To open and close overhead doors, doesn't the motor have to reverse?

Some (Most) doors automatically reverse if stalled for safety.

 

[Barry1961]

The problem is that the motor reverses without the starter reversing. This causes the door to go back up and hit the up end of travel limit switch which does not turn off the down relay. When this happens expensive things break.

Most doors reverse when they sense an obstruction on the leading edge of door with a fail safe strip pressure switch. When this problem happens there is no signal of a jam except the current spike as the door jams and quickly starts running in reverse. They may have the current trip set up high to handle worst case low line voltage during start.

I am not sure what exactly causes the jam but it could be an obstruction in the guide rails like a hoe handle. This would stop the travel quickly without tripping the safety pressure switch. The motor current could then spike and reverse before the overload has time to trip.

I am trying to find some information on the effect of putting a larger capacitor on a PSC motor but am not finding much besides higher starting torque/current.

Thanks!!

Barry

 

[itsmoked]

It sounds like the current sense for normal door turn around should be adjusted! You may be adding all sorts of Rube-Goldberg stuff to just get around a mis-adjusted current sensor.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Laplacian]

I am also interested in a technical response to the OP's question. I have an electrician asking me why a PSC motor for a condenser fan has reversed rotation by changing out the capacitor. He has "confirmed" the replacement capacitor is the same value as the old unit, but it is a different manufacturer. A new capacitor from the same manufacturer results in correct rotation.

 

[Barry1961]

It may be that the current spike is fast enough to be in the range of normal start up current/time over the expected power range. PSC motor are more sensitive to low voltage that other types.

PSC motors have a fairly low starting torque/current and are often used in high cycle reversing applications for this reason. I have not found much information on stall torque and current, let alone the effect of a larger capacitor on it.

A larger capacitor may effect start current but not stall, which is what needs to be sensed better. This may be a job for a limit switch on a pointer that trips if the motor bracket flexes either direction.

I never have liked single phase motors.....

Thanks

Barry

 

[sreid]

A single phase motor reversing on stall is a new one for me. To reverse a motor, one has to let the motor stop first before switching the capacitor, otherwise it keeps running the same direction.

A stalled motor to me would be the same as starting the motor (both conditions = 0 RPM). Maybe when the stall occurs, something "Winds up" that turns the motor shaft backwards momentarilly.

I know I'm not being very helpful but all the motors I've stalled tripped the circuit breaker.

 

[itsmoked]

sreid; It's a roll up door! The definition of "Wind Up".

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Barry1961]

There will be force trying to back drive the motor when it stalls. If this is enough to overcome the positive locked rotor torque I don't know. But the motor does reverse and runs well enough to do damage.

Thanks!!

Barry

 

[waross]

I had a problem with a pair of sliding garage door openers.
1/2 HP openers were purchased and they did not have enough break-away torque to start the doors moving. Before ordering two 3/4 HP motors and discarding the existing motors I tried adding extra capacitors. This increased the starting torque and both motors have been working well for over 10 years now.
In your case, when there is a solid jam, the motor will probably still reverse and do even more damage.
Try arranging the control circuit so that the end of travel limit stops travel in either direction. Bypass the end of limit switch with the start button for the opposite direction.
respectfully