DC Draft Blower - speed control

[jebatty]

Wondering whether anyone has replaced the AC draft blower on their gasifer with a DC blower and PWM motor speed controller, perhaps controlled by stack temperature. The Tarm AC blower is 150cfm, and DC blower are quite reasonable in price, as are PWM motor speed controllers.

 

[DaveBP]

What would be the advantage of DC over AC, Jim? I thought PWM would vary the speed of any AC motor of the PSC type and maybe some others.

I'm pretty sure that's how Tekmar controllers vary the speed on standard circulators.

 

[jebatty]

A really good answer to your question is beyond my limited areas of knowledge. One aspect of PWM in DC motors is that PWM delivers full power to the motor in pulses, which should work very well with a DC motor to control speed. I think that PWM is used for some AC motors to vary the frequency, but is not appropriate for all AC motors. I used a non-resistive fan speed control (turn a knob) on the Tarm blower in the past, and while it worked to some extent, it did not give much variation in speed. I have a tendency to think that PWM for some AC motors is akin to varying the frequency, and for many AC motors this has the effect of increasing the current and overheating the motor. For an AC motor I think the controller also needs to reduce the voltage when it reduces the frequency to prevent overheating. Not the best answer to your question, but this is about as far as I can go with any hope of coming close to a correct answer.

I suspect that AC motors designed to be variable speed by varying the frequency with PWM are more complicated, hence more costly, then a "normal" AC motor which can be quite inexpensive. I see this when I look at the cost of variable speed circulators (VFD, VFC, other names) as opposed to a fixed speed circulator. Ont the other hand, at the current that a blower motor operates a DC motor is quite inexpensive, no more than an equivalent AC motor, and the PWM speed controller also is quite inexpensive.

Wish I could do better.

 

[dirttracker]

AC inducton motors that are run as "conitnuosly" variable speed are typically 3 phase. With a 3 phase motors it's pretty simple to vary the voltage and frequency in proportion to change speed. This is usually called volts/Hz operation, it does not work for single phase motors due to the way the magnetic field is controlled in a single phase PSC motor. For single phase PSC motors like these fans and pumps, the speed can be varied by reducing the voltage, to a point. If the voltage decreases too far then the motor stalls or may overheat. There are also issues with developing enough starting torque if the initial turn on voltage is set too low. The manual controller you have now probably uses a thyristor to cut down the voltage, but the frequency stays at 60 Hz.

I don't know about these small motors, but 3 phase motors are usually cheaper than single pahse motors in larger sizes. There's no start capacitor and associated circuitry and I think they are easier to wind. There just is not much call for 3 phase 1/20 - 1/8 hp motors like what are used on fans and pumps. In this case it probably would be easier to use a PWM fed DC motor. If you would like soem interesting reading, do a google search for "variable speed control of a single phase induction machine Lippo Benedict" These folks had setup a way to turn a single phase motor into a VS inverter driven motor by removing the satrt capacitor and controlling the start winding with one of the inverter outputs. There's a lot of math involved, but if you wade through it all you start to see why varying the frequency on a PSC motor will not work.