Speed control in 1 ph induction motors, non-capacitor

Awl --
How to do?
TRIACs? http://en.wikipedia.org/wiki/TRIAC In that article they mention "alternistor". Useful? Never heard of it.
How bout PWM? http://en.wikipedia.org/wiki/Pulse-width_modulation good article, overview, incl. rheostats and variacs. Not clear, tho, if pwm acts to change the effective input frequency.
I'm guessing that, in general, speed control of 1 ph induction motors is not so swift, given that people here are swapping out 1 ph induction for 3ph + VFDs.
But if it IS doable on smaller motors, it would be a big help. The blower motor at hand (kitchen hood) is surely no more than 2-3 A, and I'll bet about 1.5 . I've used a variac on this motor, with very middling results.
If 1 ph induction is freq dependent, why can't a VFD be used on a 1 ph (220 V) induction motor?
If I need to go 3ph+VFD, Dealerselectric.com has fractional hp 3 ph motors, down to 1/4 hp, prices not on web. Graingers has no 3 ph motors under 1 hp, and none of those under $250... wow....
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EA, PV'd



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Would http://www.grainger.com/Grainger/items/5JJ60?cm_mmc=Google%20Base-_-Motors-_-Motor%20Supplies-_-5JJ60
work? $36. What type of controller is this?
Higher amp controllers: http://www.grainger.com/Grainger/ac-speed-controls/motor-supplies/motors/ecatalog/N-9yj?op=search Also higher priced -- $200 range.
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EA

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Heh.... just looked at the plate.... 1/20 hp!! mebbe 1/2 A?? Hellified 1/20 hp blower.... wow..... With oil cups. :)
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EA



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> If 1 ph induction is freq dependent, why can't a VFD be used on a 1 ph
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wrote: [speed control of 1ph induction motors]

A stepped pulley on the motor, and another stepped pulley on the load, with provision for changing the belt position...
That example is THE most difficult type of motor to speed-control. With 3ph, you at least can rectify the input to get DC power (but with 1ph, your DC power is pulsed; you have to filter it to begin with).

... are a kind of switch that are easily turned ON, hard to turn OFF; not right for this problem.

... requires DC input power and a controllable ON/OFF switch; yes, this can work, but usually it's done with DC motors for simplicity. For AC you need to solve the problem twice...

The small induction motors that DO work with speed controls (chem lab stirrer, some multispeed fans) are very low power because they're inefficient and would burn up if scaled to 1/8 hp or larger.
Universal motors (with brushes) can be speed-controlled with a triac or SCR, with some care; that's how the variable-speed Dremel works. Most such motors are designed for intermittent duty, the power efficiency is not usually high. You lose torque as well as speed at low drive levels (the stepped pulleys are better in this regard).
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wrote: [speed control of 1ph induction motors]

A stepped pulley on the motor, and another stepped pulley on the load, with provision for changing the belt position...
That example is THE most difficult type of motor to speed-control. With 3ph, you at least can rectify the input to get DC power (but with 1ph, your DC power is pulsed; you have to filter it to begin with).

... are a kind of switch that are easily turned ON, hard to turn OFF; not right for this problem.

... requires DC input power and a controllable ON/OFF switch; yes, this can work, but usually it's done with DC motors for simplicity. For AC you need to solve the problem twice...

The small induction motors that DO work with speed controls (chem lab stirrer, some multispeed fans) are very low power because they're inefficient and would burn up if scaled to 1/8 hp or larger.
Universal motors (with brushes) can be speed-controlled with a triac or SCR, with some care; that's how the variable-speed Dremel works. Most such motors are designed for intermittent duty, the power efficiency is not usually high. You lose torque as well as speed at low drive levels (the stepped pulleys are better in this regard). ================================================ What do you think of the Dart controller in the graingers link? What method does it use? It's sposedly good up to 2.5 A, which on paper is about 1/3 hp.
I put the variac back on this blower, on my bench. The variac actually does an OK job, but speed regulation is not very stable, as the motor tries to get in synch with the 60 Hz, and thus it's full rpm. Once at full rpm, more V does not increase speed, just increases the hum! Probably more load would call for more voltage.
The Q is: will a Variac at 60 V or so make the motor hotter than usual, at reduced rpm? Not enough back-EMF to limit current?
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EA, PV'd



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wrote:

Intended for universal motors; this is a triac or SCR type, it always puts out the same as line frequency but at lower average voltage. The high end of the current range would only apply to motors of the universal type, with brushes, and it loses torque at low speed. It's just a Variac-substitute (as the Variac functions, so will this).

The variac and the Dart controller both work on a shaded-pole motor by running the voltage low enough to partially stall the motor; that means the rotor is demagnetizing and generating eddy currents. The rotor gets HOT when that happens, it's only recommended if the motor was designed for it (like the multispeed range hood fan motors one often sees). No high power motors are designed for that kind of use.
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stable,
at
Yes you will toast the motor in short order running in at 1/2 voltage. When the voltage goes down, the amperage goes up and the wires in the motor are not designed to carry that many amps and they will overheat.
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Roger Shoaf

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acts
3ph
(220
motors,
What are you trying to accomplish in the end? You can buy cheaper multi speed motors or you can possibly make a change to the fan itself a whole lot easier than reinventing the wheel.
--

Roger Shoaf

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Well, speaking of wheels, I am basically going from a blower like http://www.grainger.com/Grainger/items/1TDU8?Pid=search , oriented along the horizontal *within* the hood, to http://www.grainger.com/Grainger/items/1TDV4?Pid=search , where the blower wheel diameter (suction side) is facing straight down, and re-located *above* the hood, blowing directly out to/through the wall vent. This greatly simplifies overall air flow.
Now, I didn't actually buy this second blower, but by removing one of the squirrel cages on the old double blower, I basically converted the old blower into the second grainger blower.
Now, try finding a mutli-speed small blower! I spose I could find a multi-wound motor and mount it on a blower, but it seems like speed control is the easier route. And, from the grainger link I posted on the Dart controller, it appears these small motors are somewhat amenable to speed control.
Any add'l links would be appreciated.
I will, however, monitor my old modified blower (oil cups and all) for heat, at reduced voltage. Given that the original speed controller was just a crappy tapped transformer on a rotary switch, my Variac is really just doing the same thing, except better.
I am shocked by some of the prices of hoods -- the cheapest HD stuff is $400, and it quickly goes into the thousands, $4,000 in one showroom, Broan "Best" series. The few that turned on did not seem all that quiet or impressive. They were all cosmetically done well, but most are bulky, wierd, or incompatible with my long-ish 6 burner cooktop.
So I'll be finagling away, with compound angles and air flow, for some time, it appears. :) :(
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