Motor speed control

Can't you use a controller board culled from an old washing machine ? Might be barking up the wrong tree, but over here, one motor drives all in a washing machine, and its speed is controlled from virtually just turning over to tumble the wash around, to full whack for spinning, all using a simple triac cycle-chopper circuit. I wouldn't have thought that torque was an issue with a fan blade set as a load ?

Arfa

I'm no expert in variable frequency AC drives, but have used a few in some applications . . .

First - if the motor is capacitor run or capacitor start it won't work with a VFD - If you're lucky the capacitor will vent and leak all over. It just won't work - that type of motor is unsuitable for your app.. Capacitor start might work - after the cap is removed.

Straight induction motors can be used with VFD's, but even that isn't a perfect application.

The ideal situation is to drive a three phase motor with a variable frequency drive.

From reading between the lines I get the impression that you just figure on halving the output speed, and figure there's some cheap easy way to do it . . .

A transformer isn't necessary - decrease the voltage by keeping the pulse width low. If you were to use a square wave you would be dissipating more power than the RMS sine wave uses - a 70% duty cycle square wave on each half cycle would be the equivalent of full power.

There will be some extra heat generated and some lost efficiency due to the high frequency component in using a square wave - but it is done all the time.

Pulse width and frequency track each other.

Fans don't require starting windings - but your motor almost undoubtedly has a centrifugal switch in there that will be active whenever the speed is below a rather high threshold (1/2 speed would be too slow for the switch) - so you'd need to do some work inside the motor or connection box if it has one.

I wanted a variable speed fan for my house and after checking the options I settled on an array of 8" "Patriot" brushless DC fans driven with a 6-48 VDC scr controlled power supply. Works well - but a little loud at high speeds.

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A lot easier to just find a two speed motor surplus. :)

If you use a class A amp, anything you save on electricity to the fan will go into the amp transistors. :-)

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some reading material on VFD's

describes method of using pulses to create a simulation of a sine wave for the motor - pretty good article

Each cycle of the sine wave may be chopped into 20 or more on-off pulses with widths simulating the voltage present during those phase angles of the wave . . . Close to zero crossing the pulses are very narrow becoming progressively wider as 90/180 degrees is reached, then becoming more narrow at the next zero crossing. As frequency is lowered all the pulses in each cycle can become narrower - particularly with a fan load since the effort required falls off rapidly as speed is lowered - no need for constant torque

That takes a fair amount of discrete logic or a programmable chip

If I were to attempt to design a vfd for a fan, I think I'd look to an analog design to handle the pwm - be somewhat easier than using all discrete logic.

Wickipedia's dumbed down explanation.

You probably have one of those old Sears fans with the cast iron hubs and steel blades. Built like a tank . . . My very OM was still using the one I remember as a kid . . . . died about three years ago after working for about 40 years with no maintenance.

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I think this one was going for around 40 years, or more, probably more.

I thank you all for the replies but it has become a moot point, the replacement motor burned up today. I think going to 1750 on the drive pulley did it. The load on such fan blades increases pretty much exponentiall with RPM so I think it was just too much, even with the bigger motor. It was noisy. I know the driven shaft bearings are loos as a goose, but so what. We can make new ones.

I observed the fan and if it were the driven shaft bearings locking up one side of the belt would've been tight.

Now that I think of it, this motor was not from a washing machine. Two wires that is it. Even alot of motors from the 70s had at least two speeds, to facilitat the gentle cycle. The old Kemores only switched speeds for this, the transmission took care of the rest. This is more a standard frame machinery motor. Perhaps it was from a VERY old furnace that did not have speed control. The motor control consisted of a temperature sensor in the plenum.

Regardless of what it was, I saw smoke. Brushes generally don't smoke and they usually don't cause the lights to dim. When I heard the sound change and the lights dim I looked at it, and that is how I know the belt was not overstressed. The motor is gone.Unless there is a possibility one of them dislodged and shorted to the frame, however this unit is not grounded so if that caused the smoke the winding insulation must be breached.

We have a pretty good sized fractional HP DC motor with a speed control, but that is slated to go on one of the lathes.

I was just looking at the pitch of the blades on this thing, talk about steep. I don't care about the fanshaft bearings, as long as they are free. In fact, when the first motor went that's what I thought it was. Then with the thing apart I saw that the friction was in the motor shaft, not the driven shaft.

Now it just fired up again. It went off on thermal no doubt. I still have to do something, I can't let this thing run unnattended. Thing that bothers me is how soon it went off on thermal, maybe the driven shaft bearings are locking up ?

Either way, driving it that fast is not a good idea. It is beyond design limits. I still need options. The blade pitch on this thing is awesome, it STARTS at about 45 degrees and the output edge is just abnout paralell with the shaft ! When we put this motor in we ran it outside in the yard, it threw a breeze over to the neighbors. It was sitting on a chair and made the grass look like there was a windstorm. Also being around 20" in diameter, with a mask, it moves some serious air. As we have trees around here, I didn't have to use my AC even once last year.

If I were to install the same type of motor, which I might, I really need to reread all your responses. I only skimmed them because of the smoke I saw today, but I will read them in depth and get back to you.

The cheapest way is just get another of what I got. Even if I have to setup my own start relay, start it at full then go to the duty cycle control or whatever. If I don't push it so hard it should last, and, the way it's running now you can't light a match in here. That's nice when it is in the 90s, but right now it is too much.

I have people coming over soon, and I am glad the thing simply works at the moment. Later I will read everything and get back to you.

One respondant mentioned duty cycle. I kind like that, don't bother rectifying anything. Just use an SCR. I could trigger it from a single stage flip flop then to get the 30 Hertz. Heck I could even do 20 Hertz with flip flops, and just cut the duty cycle accordingly.

JURB

It sounds like it's an induction motor from a North American washing machine, unlike the universal motors used in European machines, these are usually two speed with a centrifugal starting switch, they can't be made variable speed easily.

Ok so I guess this was a brush motor, how about a surplus treadmill motor? They're small, quite powerful, and easily controlled.

still

driven

single

Hertz

Okay I'm new here but if anyone knows induction motors here, it seerms to be me. If this is the same kind of fan I am thinking of that is often used as a portable windstorm for low-budget movies, what you are proposing to do is a good way to burn up a brand-new motor. What noone here seems to know is that the stator core contstruction is specifically designed for the frequency response, using a variable frequency power supply, or a variac with such a fan is a sure way to cause a meltdown. You do not "cog" or run an induction motor on a "duty cycle" other than an on and off period. Induction motors do not respond to SCR switching like a DC motor does.

The thermal is blowing on the motor because you are overloading it. The formula for fans is as follows:

Cu. ft. air per minute x water guage pressure (in.) HP = ________________________________________ 6,350 x % efficiency

where approx efficiency of a propeller-type fans is as low as 35%

Points about driving fans to remember: The volume of air delivered by a fan varies directly with the fan speed The pressure produced by a fan varies as a square of the fan speed The HP of a fan varies as the cube of the fan speed

To replace an 1100 rpm motor with a 1750 rpm motor will require twice the horsepower. If this fan is pulley-driven, you need to adjust pulley sizes to accomodate your speed requirements. a 20" fan blade sounds like nothing more than the typical box fan.

Alternately you can try to be a bit more creative and try to repair the fault in the motor, or even rewind it. If you can't manage this, just get a new motor. I like the idea of using the treadmill motor, as if you are brilliant enough to strip it's speed control as well, you will have a fully-variable fan out of it, and treadmill motors are rated at least 1 horsepower. Just be sure that if you get the opportunity to strip a treadmill, that you take ALL of it's electronics and electrical, most importantly all parts of the speed control including the slider pot on it's console for speed adjustment. If you can remove the flywheel from the motor (if not set-screwed it is threaded on), do so and use a pulley that will fit. I would suggest grinding a flat or keyway into the shaft (keep metal dust out of a permanent magnet motor), and then being sure not to exceed 1800 RPM on a 20" fan blade or you will have quite a mess on your hands...

Anytime you want to replace a motor that has burnt out, always check for binding in the driven side first, then for binding in the motor itself. Belt pressure should ideally be at just barely above the point where slippage occurs, but it seems lack of actual care for the motor is what caused the burnout in the first place because either a bearing went dry and gummed-up, or the motor got loaded up with dust and debris and overheated as a result of the insulating layer of crud. Try a bi-annual maintenance schedule next time.....

For this application, the ideal motor is a Permanent Split-Capacitor motor (capacitor start/capacitor run with the same capacitor). This is the same type of motor used in 14" or so desktop fans and is always accompanied by a low-valued capacitor that is constantly energized. The PSC motor is essentially a two-phase motor adapted for single-phase, and the ideal as a fan motor. This motor is also switchless so there is no switch to maintain. Find yourself a japanese-made washing-machine and you will likely have a perfect motor for it....

Any questions adress me directly or PM me

still

driven

single

Hertz

Okay I'm new here but if anyone knows induction motors here, it seerms to be me. If this is the same kind of fan I am thinking of that is often used as a portable windstorm for low-budget movies, what you are proposing to do is a good way to burn up a brand-new motor. What noone here seems to know is that the stator core contstruction is specifically designed for the frequency response, using a variable frequency power supply, or a variac with such a fan is a sure way to cause a meltdown. You do not "cog" or run an induction motor on a "duty cycle" other than an on and off period. Induction motors do not respond to SCR switching like a DC motor does.

The thermal is blowing on the motor because you are overloading it. The formula for fans is as follows:

Cu. ft. air per minute x water guage pressure (in.) HP = ________________________________________ 6,350 x % efficiency

where approx efficiency of a propeller-type fans is as low as 35%

Points about driving fans to remember: The volume of air delivered by a fan varies directly with the fan speed The pressure produced by a fan varies as a square of the fan speed The HP of a fan varies as the cube of the fan speed

To replace an 1100 rpm motor with a 1750 rpm motor will require twice the horsepower. If this fan is pulley-driven, you need to adjust pulley sizes to accomodate your speed requirements. a 20" fan blade sounds like nothing more than the typical box fan.

Alternately you can try to be a bit more creative and try to repair the fault in the motor, or even rewind it. If you can't manage this, just get a new motor. I like the idea of using the treadmill motor, as if you are brilliant enough to strip it's speed control as well, you will have a fully-variable fan out of it, and treadmill motors are rated at least 1 horsepower. Just be sure that if you get the opportunity to strip a treadmill, that you take ALL of it's electronics and electrical, most importantly all parts of the speed control including the slider pot on it's console for speed adjustment. If you can remove the flywheel from the motor (if not set-screwed it is threaded on), do so and use a pulley that will fit. I would suggest grinding a flat or keyway into the shaft (keep metal dust out of a permanent magnet motor), and then being sure not to exceed 1800 RPM on a 20" fan blade or you will have quite a mess on your hands...

Anytime you want to replace a motor that has burnt out, always check for binding in the driven side first, then for binding in the motor itself. Belt pressure should ideally be at just barely above the point where slippage occurs, but it seems lack of actual care for the motor is what caused the burnout in the first place because either a bearing went dry and gummed-up, or the motor got loaded up with dust and debris and overheated as a result of the insulating layer of crud. Try a bi-annual maintenance schedule next time.....

For this application, the ideal motor is a Permanent Split-Capacitor motor (capacitor start/capacitor run with the same capacitor). This is the same type of motor used in 14" or so desktop fans and is always accompanied by a low-valued capacitor that is constantly energized. The PSC motor is essentially a two-phase motor adapted for single-phase, and the ideal as a fan motor. This motor is also switchless so there is no switch to maintain. Find yourself a japanese-made washing-machine and you will likely have a perfect motor for it....

Any questions adress me directly or PM me

A BELT drive fan? The sears I was talking about was direct drive . . . Why not just change the size of the pulley, or put an adjustable speed pulley in - that's a low cost easy solution even if you to change the belt size.

I used the words duty cycle, but as an analogy to make a distinction between RMS and square wave power consumption, and again in making an ersatz sine wave for a VFD.

You can't just cut down the duty cycle or use SCR/Triac type controllers for FHP Induction motors. They overheat, unless the pulses approximate a sine wave - the two efficient ways are with stepped voltages (usually six levels, plus and minus - and relatively complicated to pull off) or varying the widths of several pulses for each half cycle of the sine wave (still relatively complicated IMHO - but better than dealing with voltage levels)

Then there's that pesky starting winding - you're driving a fan. A fan (unless it is seriously broken) turns freely with no real resistance (unlike wash machines with transmissions seals and gears to get moving, compressors, or most anything else . . .) until the fan gets up to speed - torque required tracks shaft speed.

Run the motor at half speed and the starting winding will kick back in even if you start it at full speed.

DC brush, or "universal" AC/DC motors are ideal for speed control applications - but the longevity of the brushes is a limiting factor and one reason they are seldom used to drive fans. (also noisy, create sparks and ozone, radio interference)

We had this centrifuge at work - big one horse brush type motor with a variable transformer to change the speed. Probably dated back to the

40's. The transformer burned out one day and they wanted $400 for a new one. I went to Grainger's and got a high power lamp dimmer for $20 - no problems.

A VFD drive that uses single phase power and drives a three phase motor is probably the ideal if cost is less important than longevity, excellent speed control, and efficiency.

Plenty of appliance repair/junk dealers around with two speed wash machine motors - and a new one might cost $50- $70.

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I agree much with Default, yet there is one general rule for any induction motor: They are not variable speed. You can select different speeds based upon the wound poles, but to cause the motor to slip excessively is not an operable condition. The only way to safely adjust the speed of any induction motor (aside from shaded-pole) is to reduce frequency and voltage amplitude on a measured scale. Shaded-pole motrs are horribly inefficient, but more tolerant of moderate overloads.

Once the stator field loses grip on the squirrel-cage, the losses are converted to heat in the stator as a result of decreased inductive-reluctance in the stator, which results in increased current. The heat will be turned on the resistive value of the windings rather than the inductive properties that normally control the current, and hence it will overheat, meaning the motor will lose it's inductive properties and become closer to an unwound length of wire to the line current....

If you are intending on varying the speed other than preset values for an induction motor that are avaliable, you best either use a DC power motor driven by SCR's such as a treadmill motor, or a brushless DC motor, which is phenomenally-more expensive and rarely worth the project.

My enthusiastic advice here is to either repair or replace the motor, or convert yourself to DC. Then and only then will a duty-cycle/amplitude modulation be useful for adjusting the speed.

In any case I advise anyone to get speed controllers from discarded treadmills, they are the diamonds in the rough for anyone working with anything DC-compatible, replacing the lab variac for DC applications. Just be sure to salvage or at least know the pot (varistor, not weed ya goofball) value for it, as guessing can be hazardous to the controller. I now have one rated 28 amps DC

145+V-out, detuned to work on a standard 115VAC circuit at 15 amps...The motors couldn't be any more ideal for any application anyway, so grab those too!

Gold I tells ya....gold....grab them like they are the last power supply on Earth, because most of the cost is in these parts anyway...

"Electromotive Guru" wrote in message news:449b44e7$1$70924$ snipped-for-privacy@authen.yellow.readfreenews.net...

Am I right in thinking the RMS voltage should be proportional to the frequency when this is done?

Yes, the voltage and frequency are usually reduced. To vary the speed of AC motors a device called an inverter is used. Internally it will convert the AC to a DC voltage and then convert it back to an AC type of voltage that the frequency and voltage are matched to the motor load. I say AC type because most of them do not put out a sine wave but odd looking waveforms. Some use several steps in the output to step up to the maximum voltage and then back down to 0 and then the same for the negative half cycle. We use some where I work that put out a very odd waveform and normal volt and amp meters will not work correctly on them. They use something called a carrier wave that is operating at from around 5 to 15 kiloHz. No where the normal 60 Hz or less you would normally think of in the US.

Here is a link to the type I am thinking of.

Am I right in thinking the RMS voltage should be proportional to the

Yes, it should be, however the formula for this varies by the motor itself and it's physical construction. What you need to do if you intend to try this is to carefully watch current as you alter the frequency, however there is only so far you can go before the reactance of the stator core breaks down and the stator fails to respond proportionally because magnetic saturation has a sort-of flywheel effect....yes, electrons have mass too....

As mentioned before, only a switchless motor will respond to any resonable degree because most induction motors are not meant to vary their speeds. When you lower the frequency, you affect the magnetic flux of each laminate in the stator and rotor cores, as well as overall efficiency and it's power factor. Lower-frequency motors have thicker plates because they can respond to the lower frequency and saturate more slowly.

The best way to perform this is to find yourself a 400Hz induction motor (see aircraft service) made to run at a comparable speed as what you are replacing, and then work from there. Be sure to find the highest service factor for this motor that you can find, 1.35 is an ideal, with class H insulation (preferably class B rise with class F-H insulation)...

While it may be entertaining to try varying the speed of an induction motor, it is far from preferred practice. If you really want to vary speed while taking advantage of the non-contact design of an induction motor, go find a synchronous/stepper motor to run from, as this will respond exactly as you would want it to up to a much wider range than the standard induction motor, the more phases the better. If you really wanna get trick, use a Hall-Effect motor/circuit, alhough a bit excessive for a simple fan....

Still above and beyond is the use of a DC power motor. Brush replacement is not nearly the factor that many think it is, as proper maintenance/operation can provide over a decade of service. If brushes are that much a biog deal, opt for a brushless DC motor...

If ya wanna try something fun, take an alternator and short it's rotor's slip-rings and run it as a low-impedance 3-phase induction motor after you remove all the DC components from the circuit. Over

80 amps is usually wound delta, so keep his in mind...

Whatever you do, best of luck to ya, but I still strongly recommend the DC power motor, like the treadmill motor.