Re: (ET) Deck Motor Alternate Use

["David Roden" <etpost drmm net>]

On 9 Oct 2013 at 13:18, Charlie wrote:

> My forge blower is a 120vac motor, and when I plug it into my ancient
> variac I get continuously variable speed on the motor just by turning
> the knob. 

I'm not a motor expert.  Motor guys, correct me if I'm wrong on this.  But 
here is the way I understand it.

A universal motor has brushes and a series field.  It will run on either 
AC 
or DC of the proper votlage.  It has good low speed torque, but not very 
good speed regulation.  My electric drill uses a universal motor.  So does 
my vacuum cleaner.  

What's good about a universal motor is that it's a cinch to control. You 
can 
use an electronic pwm controller to simply reduce its voltage. 

For a DC input, the controller works like a golf car controller.  This 
controls the effective voltage applied to the motor by varying the duty 
cycle at a high frequency (typically 2kHz or more).  

For AC input, you can use an even simpler triac controller.  This works 
like 
a light dimmer, chopping up the incoming sine wave so that it's not as 
tall, 
so to speak.  (It switches on later in the rise of the sine wave to 
effectively reduce the voltage.)

Or, as you do, you can use a Variac.  This produces a much cleaner output 
waveform, but costs a lot more for a given current capacity.  (BTW, 
"Variac" 
is a trademark for "variable autotransformer," in case you care.)

Now, I don't know what you have there for your forge, but most blowers 
I've 
met use AC induction motors,  This includes my furnace blower and 
woodstove 
blower.    All the fans in my house use induction motors, as do my washing 
machine and dryer and water pump.  IIRC my power saw also has an AC 
induction motor, and I think my bench grinder does. 

Induction motors are very common round the house and shop.  Their speed 
regulation tends to be pretty good.  They're cheap to make, and they don't 
have brushes to wear out, but they work only on AC.

An induction motor is tougher for speed contorl.  You can sort of control 
it 
by varying the input voltage, but the efficiency goes straight to hell.  

If I understand aright, what you're doing is causing the "slip" to vastly 
increase.  What does that mean?  Well, the rotor is following a rotating 
magnetic field; that's what makes it turn. You have to have a little 
"slip," 
where the rotor is a little behind that rotating field, to create torque.  
Too much slip, though, and things get out of whack.  

When you reduce the voltage, you weaken the field, so the slip increases 
too 
much.  This can cause overheating and/or poor performance.  At least 
that's 
how I understand it.  (In some applicatiions that may not matter.  A 
ceiling 
fan is one example where PWM is often used to vary the speed of an 
induction 
motor.)

To properly control an induction motor where you need real work done, real 
torque, you have to change both the voltage and the frequency presented to 
the motor.  Changing the frequency actually slows down the rotation of the 
magnetic field.  Then you can keep the slip within the design range, but 
slow down the motor.

This is what your variable drives do, if I understand them aright.  It's a 
lot more complex and expensive.

To (finally) cut to the chase, what you probably want for your lathe is a 
120 volt universal motor, and a nice foot-pedal operated controller.


David Roden - Akron, Ohio, USA

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