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Re: (ET) Deck Motor Alternate Use
David,
Your explanation is very accurate and spot on. Speed (torque) on an
induction motor is volts per hertz which is why drives will very both to
try to mimic a DC motor performance. Higher load at low speed by reducing
voltage (Variac) is not a good idea on an induction motor. It will
probably work on a small blower, for example, because the load is a cubic
relation so it is low at low voltage/speed.
I put a small DC motor with controller on my band saw and it works well
(being partial to DC motors). I would avoid using a mower motor as it can
probably be sold/traded for another DC motor and then go find a small DC
controller.
Good forum....
...Walt Erie PA
-----Original Message-----
From: David Roden [mailto:etpost drmm net]
Sent: Wednesday, October 09, 2013 2:41 PM
To: Elec-Trak
Subject: Re: (ET) Deck Motor Alternate Use
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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