Why Does Foot Pedal Reduce Top Speed of Universal Motor?

[Roy Batty]

So long, and thanks for all the fish.

[Benta]

I think you need to provide a bit more detail. I have difficulty visualizing your setup.

Cheers.

[rollatorwieltje]

Is it a carbon pile rheostat? Those often have up to several ohms of resistance even when fully pressed. If it's a wiper based unit, it could be that it doesn't use the entire range.

[IanB]

The pedal certainly has a problem. It reduces the top speed because when fully depressed it still limits the power to the motor. It may be faulty, or it may have been designed that way. Most speed controllers will provide the full, unrestricted AC voltage on the output when on the maximum setting.

If you want to troubleshoot it you will need to take it apart and see how it is constructed inside, and then you will need to reverse engineer the circuit to find out why does not provide unrestricted power on the max setting.

[james_s]

Are you sure it's a simple rheostat and not a triac or SCR speed control? 1/6HP seems awfully large for a rheostat, it would be burning off a lot of excess power as heat at lower speeds.

[Brumby]

So maybe the wiper isn't traveling far enough to kill the resistance?

That's one of the options already mentioned above - BUT - there is more than one way to implement motor speed control.

To offer any real help here, we would need to know what system is used.  The easiest way to start down that path is to take a cover off and take a picture of what's inside.

[Brumby]

The other thing to keep in mind is that - if the speed controller works well for the device that it came with, then the sub-maximum power limit may be a real, deliberate design feature to ensure the original device operates within it's intended capabilities.


As a result, if you modify the controller to give full speed to other equipment and then connect it to the original device, you might drive it to destruction when you put your foot all the way down.

[johnwa]

Are you sure it's a simple rheostat and not a triac or SCR speed control? 1/6HP seems awfully large for a rheostat, it would be burning off a lot of excess power as heat at lower speeds.

Yes, this may have something to do with it. I think some power tools with an inbuilt speed control use an SCR, that can only pass positive half cycles, and therefore the maximum effective output voltage is only 1/sqrt(2) that of the line voltage. The universal motors are then wound for full speed operation at e.g. 170V (for a 240V system). Running a motor designed for 240V from such a speed control would mean that it would not be able to reach full speed.