If you want a nice cheap DC motor, try a treadmill motor. There are plenty on ebay:
elec-trak cosmos phy tufts edu
and you can probably get some even cheaper with the whole treadmill from Craigslist.
Harry Landis
> From: Walter Konstanty ge com
> To: noaddress drmm net; elec-trak cosmos phy tufts edu
> Date: Fri, 11 Oct 2013 13:44:05 +0000
> Subject: 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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