Motor self reversing

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garisilver

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Our HVAC mechanic showed me an exhaust fan with a115v 1/4 hp motor that changes direction each time he turns the switch on. Never saw that before and I'm at a loss to explain it. My guess is that something happened to the start winding, but that's all it is, a guess. Anyone have an explanation? Thanks
 
Fuzzy memory, but I believe I have seen such when there was air flow with the fan shut down which caused the blades to be turning backwards. When the motor was tuned on it adapted to the direction of the turning blades. Someone with more motor knowledge hopefully will chime in, but I think this is possible with single-phase motors.
 
I'm pretty sure it must be an alternating control relay logic,each time the relay is energized by the start switch closure the output change change state

dick
 
garisilver said:
Our HVAC mechanic showed me an exhaust fan with a115v 1/4 hp motor that changes direction each time he turns the switch on. Never saw that before and I'm at a loss to explain it. My guess is that something happened to the start winding, but that's all it is, a guess. Anyone have an explanation? Thanks
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If a split phase motor, and it is "windmilling" because of air movement, it will continue to run the direction it is turning - especially if spinning fast enough that start winding switch has opened.

If a capacitor start motor same thing if spinning fast enough the start switch has opened, otherwise the capacitor start likely has enough torque to reverse it if spinning wrong direction.

If a PSC motor it should reverse if spinning wrong direction.

Outside of that, there about has to be a control that reverses it, but don't know why anyone would want that on an exhaust fan in most cases.
 
dicklaxt said:
I'm pretty sure it must be an alternating control relay logic,each time the relay is energized by the start switch closure the output change change state

dick
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Yes It can be done with a alternating relay that has two Form -C output contacts,,,I sketched up a schematic if you would like ,I'll post it later.

dick
 
kwired said:
If a split phase motor, and it is "windmilling" because of air movement, it will continue to run the direction it is turning - especially if spinning fast enough that start winding switch has opened.

If a capacitor start motor same thing if spinning fast enough the start switch has opened, otherwise the capacitor start likely has enough torque to reverse it if spinning wrong direction.

If a PSC motor it should reverse if spinning wrong direction.

Outside of that, there about has to be a control that reverses it, but don't know why anyone would want that on an exhaust fan in most cases.
Click to expand...

Ditto
 
dicklaxt said:
Yes It can be done with a alternating relay that has two Form -C output contacts,,,I sketched up a schematic if you would like ,I'll post it later.

dick
Click to expand...


Sorry I need to read these posts closer,I thought you wanted to do that.

dick
 
There's no control logic relay, no alternating relay, no capacitor. It starts and stops with a single pole switch. I watched it come to a complete stop each time I turned the switch off, and each time when I turned it back on, it went in the opposite direction. We're going to replace the motor, but we both would really like to know what's happening here.
 
garisilver said:
There's no control logic relay, no alternating relay, no capacitor. It starts and stops with a single pole switch. I watched it come to a complete stop each time I turned the switch off, and each time when I turned it back on, it went in the opposite direction. We're going to replace the motor, but we both would really like to know what's happening here.
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I'm still not clear on whether this is SUPPOSED to do this, or you consider it an anomaly. It sounds more like you think it's an anomaly. But without knowing more about the motor, it's hard to imagine how that would happen. Reversing a standard single phase motor requires swapping wires in the motor connection box. If nothing is making that swap, the only other possible explanation is the one put forth earlier, that the motor direction is being pre-determined by the air flow (it's called "windmilling"). In a single phase cap-start, split phase or shaded pole motor, it is definitely possible for a windmilling motor to continue running in the wrong direction if started that way.

But... if it is one of the new EC (Electronically Commutated) motors being used a lot in fans now for energy efficiency, it is possible that there is some control logic inside of the fan that is making it reverse alternately. No idea why you would want that on a fan, but that may be where the glitch lies. These are essentially brush-less DC, synchronous or even switched reluctance motors, but the issue is that they all will have an electronic controller built-in to the motor housing. Something may be wrong inside that controller that is causing this to happen.
 
No, the motor is only supposed to turn in one direction. It's an exhaust fan.
Nothing is making a wire swap.
It's a split phase motor.
The fan is not windmilling. It starts from a complete stop each time, and it changes direction each time it starts. It did not used to do this.
It's an old motor. No electronics or other internal control.
I wouldn't believe it if I wasn't watching it with my own eyes.
We're changing it out, but are really curious what's going on.
 
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garisilver said:
No, the motor is only supposed to turn in one direction. It's an exhaust fan.
Nothing is making a wire swap.
It's a split phase motor.
The fan is not windmilling. It starts from a complete stop each time, and it changes direction each time it starts. It did not used to do this.
It's an old motor. No electronics or other internal control.
I wouldn't believe it if I wasn't watching it with my own eyes.
We're changing it out, but are really curious what's going on.
Click to expand...
I would have to guess the start winding cut out switch is stuck open, allowing random direction, and for some reason something is giving it a push to get it rolling otherwise it would not rotate at all. Seems funny that it would alternate every time but maybe there is something having that effect for some reason. I would check the start winding cutout either way, if working properly the motor should have enough starting torque in proper direction to overcome any minor rotation that may be imposed on it.
 
I have found very small single phase AC motors with a random rotation direction each time started.
Nightclubs etc. often use a rotating mirror ball onto which are directed spotlights, the small AC motors used for the rotation have a random direction of rotation each time started.
I have also seen desk fans that have a rotating grille in front of the fan blades, so as to better distribute the airflow. The rotation of this grille is random, though the fan blades rotate the same way each time.
 
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