PSC motor reversing at stall
PSC motor reversing at stall
(OP)
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
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





RE: PSC motor reversing at stall
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
RE: PSC motor reversing at stall
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: PSC motor reversing at stall
To open and close overhead doors, doesn't the motor have to reverse?
Some (Most) doors automatically reverse if stalled for safety.
RE: PSC motor reversing at stall
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
RE: PSC motor reversing at stall
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: PSC motor reversing at stall
RE: PSC motor reversing at stall
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
RE: PSC motor reversing at stall
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.
RE: PSC motor reversing at stall
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: PSC motor reversing at stall
Thanks!!
Barry
RE: PSC motor reversing at stall
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