jsburger wrote: Sun May 29, 2022 8:38 pm
As Beeg said "it is" as are all SS motors. There are no electronic speed controls for a capacitor start induction motor (SS motors). If there was we all would probably have one and SS would have incorporated it and not have developed the Power Pro head stock.
I am not sure that is an accurate statement. At least it is not my understanding so for example
I am not sure about the specifics of the Power Pro head stock but there are many reasons why Shopsmith might have opted to develop it. Generally speaking by the 21st century a DC brushless motor is considered superior under roughly 2kW than an induction motor. The basic reason is that power electronics required for speed control are now sufficiently cheap and much better. Historically maintaining constant torque while varying speed on a DC motor required expensive "delicate" electronics. This is no longer the case.
Note I cannot get my head around quoting an electric motor's power in anything other than Watts. SI units for the win as electrical power is just volts times amps, so a 2kW motor at 120V draws just under 17A. I guess Americans and their units.
I am not sure about the specifics of the Power Pro head stock but there are many reasons why Shopsmith might have opted to develop it. Generally speaking by the 21st century a DC brushless motor is considered superior under roughly 2kW than an induction motor. The basic reason is that power electronics required for speed control are now sufficiently cheap and much better. Historically maintaining constant torque while varying speed on a DC motor required expensive "delicate" electronics. This is no longer the case.
Note I cannot get my head around quoting an electric motor's power in anything other than Watts. SI units for the win as electrical power is just volts times amps, so a 2kW motor at 120V draws just under 17A. I guess Americans and their units.
I hadn't heard of that type of speed control being used for single phase motors. So far I've only had a quick look at that link but it sounds like an expensive approach even if feasible. I also recall that controlling AC frequency for variable speed is bad for the motor, but I don't recall why. However, those motors are optimized to work with the limited conditions of common line frequency and voltage. I want to go over that article in detail but it is from 2004 and the links it has to details on the 3 options it discusses aren't working.
Single Phase Induction Motor Speed control is possible, but not for every type of motor. I believe it can be done on PSC, shaded pole and AC synchronous induction motors, but won't work for example on motors with a centrifugal starting switch, capacitor start, etc., etc. Even though it is possible, I have never seen these used in any applications anywhere so if anyone has I would appreciate them posting about it showing a real world example. Some of the key reasons are detailed below. IF you just change the supply voltage then a large change in supply voltage is needed to get a relatively small change in speed, but doing so will greatly alter the magnetic field inside the motor. Changing the frequency has torque implications and other issues. In my opinion, this is more of an academic exercise, i.e. sure it can be done but you likely won't get the speed control range and/or torque that you want out of it and it won't be cheap to generate the V & F control circuitry. Likely cheaper to buy a suitable 3 phase induction motor and a corresponding VFD by comparison. Hence, StriaTech reinvented the switched reluctance motor and sold it to Shopsmith to put in the PowerPro.
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RF Guy