fan motors

Induction motors, most likely. If there's capacitors they're "regular old" induction motors; if there's no capacitor (not even a hidden one) they're shaded-pole.

Are the capacitors there just to start the motor, or are they permanently-split capacitor motors?

Similarly, are garage door motors capacitor-start, permanently-split, or..?

Thanks,

Michael

Induction if it has a cap. There's a bunch of windings with taps for speeds (if applicable) on a stack of laminated steel as the stator and a cast aluminum rotor. The al casting forms the winding around the laminated steel core of the rotor. The cap causes the lag that causes the surface of the rotor to chase after the poles of the stator to make it spin.

(facepalm) ok let me step back a bit and explain.

I'm putting together a spreadsheet of various motor types, their pros and cons, and where they are typically found (among other things). Call it OCD-therapy if you like :p

Within the class of AC Induction Motor, I see there are four sub-categories: split phase, capacitor start, permanently split capacitor and capacitor start-capacitor run.

Notes

I'm having trouble finding examples of capacitor-start and capacitor start-capacitor run motors though.

I suspect that oscillating fans with caps are permanently-split but I'm not certain; that's why I'm asking here.

Not sure if the garage openers are capacitor-start (capacitor only starts the motor) or permanently-split (capacitor runs during the whole duration of motor operation).

If I'd been aware of the different AC motor types when in high school, that would have been very interesting (I was really only aware of shaded pole, C-frame ones back then).

Thanks,

Michael

Dunno. These would be good questions to ask on rec.crafts.metalworking, if your reader has good killfile support and you don't mind a 1:3 signal- noise ratio even after filtering.

AFAIK garage-door openers have switchable capacitors -- I'm pretty sure that's how they reverse direction.

And three-phase induction motors, but that's an industrial thing so I assume you're not going there.

If your furnace is old enough, look at its blower motor. Just about any single-phase AC machine of 1/4 horsepower or more is going to be one of the two. If you hear it "tick" once as it spins down you're hearing the start capacitor switch engaging.

I suspect you're right, but do not know for sure. Capacitor-start requires a centrifugal switch, which adds cost and impacts reliability.

...

With three phases, there's no problem with starting, right?

Ok! Thanks. I'll listen for a "tick."

Another article I found mentioned the permanently-split caps have relatively low uF ratings.

Thanks!

Michael

An example of cap start-cap run is usually found on single phase motors where more hp is required, like air compressors.

Tim Wescott prodded the keyboard with:

My garage door motor has two identical windings with a capacitor between one end of each winding. The other end of the windings being connected together. The supply goes to the common wire and one end of the capacitor. Or the other end of the capacitor to reverse it.

Essentially a centre tapped winding with a capacitor across it.

Greetings Michael, If the fan is fairly small then the motor is most likely a permanent capacitor run type. These types of motors use the cap for phase shifting one winding in order for the motor to self start. Since single phase motors wired this way have low starting torque they can work well for fans but are not good for uses like garage door openers. If the fan blades are really hard to turn then the cap will be used just for starting and will then be switched out of the circuit. Most motors made this way will have centrifugal starting switches and you can hear them working when the motor is cycled on and off. Eric

Thanks to all who replied.

So, slow startup, like ceiling fans, have a permanent run cap, then?

That reminds me. The 3/4-hp garbage disposal gives a nice click when winding down.

Michael

If there is a cap then yes, most likely permanent run cap. But there are other designs, like shaded pole motors. Shaded pole motors can be useful for fans because even though they are not very effecient they can be speed controlled by changing the voltage, unlike the typical split phase induction motor. Shaded pole motors draw pretty much the same current when stalled as when running, so slowing them down doesn't make them overheat if they don't depend on fan cooling. Split phase motors though will draw much more current, if available, in an attempt to speed up when the voltage is lowered and RPM drops. Which is why they can be overheated by brownout situations. Eric

All the ceiling fans I've tinkered with do have caps and a lot of poles so they can turn slowly. Judging from how long it takes for them to come up to speed, it may be that capacitor phase shifting is quieter and lots more efficient.

And Yeah, that click is probably the centrifugal switch. They put springs and such in there so the switch, when it changes state, does so with alacrity and doesn't dilly dally drawing an arc between the contacts.

As opposed to shaded-pole? Possibly -- or it may be that trying to make a multi-pole shaded-pole motor gets mechanically complex.

I'm guessing, but building the thing so that it'll start up slowly without burning up probably involves winding the stator so that it has a lot of leakage inductance -- that'll give you a more or less constant- torque motor that's not burning up energy in resistance.

correct. There are three phases, out of phase so there something for the rotor to chase around. With single phase there isn't a rotating component between the phases (since there is only one) to get the motor going. The extra cap or shunted windings create the other force to allow rotation.

That reminds me... the fountain pump motor: what kind of motor would this be? A submerged shaded pole..?

Typical rotors look like these. Are they really magnets?

Thanks,

Michael

That could be a 'Potential Relay', instead. They are common on deep well pumps, and I've seen them on power tools with a 1/2HP motor. Emerson loved them, when the built the various models of direct drive Craftsman table saws.

Check out a couple of HVAC books. They have great illustrations. Some might even show schematics of hand dryers that you see in bathrooms.