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# Torque difference between 1PH and 3PH motor

I am sure the torque variation is there. It's just a matter of looking at a small enough space in the time domain. Many motors these days have slanted armature slots to reduce that effect, there's probably considerably less of a pulsating torque on those.

"I am sure the torque variation is there."

How? I'm sure there's a china teapot orbiting neptune. You cannot possibly prove otherwise.

"I am sure the torque variation is there."

How? I'm sure there's a china teapot orbiting neptune. You cannot possibly prove otherwise.

Because of the nature of the poles on single phase electric motors... They do not provide constant motive force. During the switching there's going to be some drop-off. Whether it's something noticeable in common use, I can't say. I've never noticed it. But on a sensitive enough time scale with a torque sensor that has quick enough response, betcha it shows up. And no, I can't prove otherwise, because I don't have the equipment to measure torque output at a high enough frequency. But I can still make an educated guess.

Capacitor run motors will have continuous torque if the motor is designed for it and the capacitor matches the load.

I still think that the single phase motor problem on many lathes is not because of torque ripple but rather unbalanced magnetic forces in the motor, causing it to have a 60hz vibration.

Jim:

When you can show how there is torque input to the system when current in is zero, then we'll talk. Zero current is zero power input, and thus zero torque input.

There must be zero current, because the current reverses. In reversing, it has to go from maximum down through zero in order to flow in the opposite direction.

Since single phase has zero current twice per cycle, and 140% current twice per cycle, there is by definition going to be a change of torque over the cycle.

During the portions of the cycle where there is less current (and less torque) than the rms value, the load has to be partially or completely supplied by the energy stored in the spinning drive train. That can only happen if the rpm is reduced by the transfer of energy to the load. So there is torque and speed variation.

If you add a properly sized run capacitor, that will still be supplying current (the capacitor produces phase shift) during the current zero of the main winding. Then the variation in torque is reduced to the degree that the capacitor provides run current.

Three phase adds up to be nearly perfect continuous torque, which accounts for it being smoother. The run capacitor with single phase produces a second phase. Balanced two phase and three phase are equivalent and either can be converted to the other with transformers only (Scott connection).

I am sure the torque variation is there. It's just a matter of looking at a small enough space in the time domain. Many motors these days have slanted armature slots to reduce that effect, there's probably considerably less of a pulsating torque on those.
That helps for "cogging", which is s different effect. Nothing as far as rotor design changes the single phase which has no torque when current is zero. You get a sine wave of torque, passing through zero to a maximum and back to zero every half cycle.

The effect of the skewed rotor slots is to skew the rotor magnetic field, so that it "cogs" less passing the stator slots. Nothing to do with single phase, it works with three phase also.

I'm looking at building a nice belt grinder
Assume you are "copying" a sander you've seen somewhere, which is fine.
If they used a 2hp motor then you use a 2hp motor, 1 or 3 ph makes no difference.
If you are worried about torque/power then use a 3 hp motor...1or 3 ph
This ain't rocket science.......

"When you can show how there is torque input to the system when current in is zero, then we'll talk."

Nope. When you can post up actual data showing torque vs rotor angle, THEN we talk. In the meantime this is entirely vaporware and not worth the time to discuss it. Angels/head/of/pin, etc.

Torque measurements, or at least some graphical data of torque ripple:

"When you can show how there is torque input to the system when current in is zero, then we'll talk."

Nope. When you can post up actual data showing torque vs rotor angle, THEN we talk. In the meantime this is entirely vaporware and not worth the time to discuss it. Angels/head/of/pin, etc.
Sorry, Jim, no current, no torque, end of story. Angels dancing are all at your end, eh..... Come back when we are talking about the same thing...

Or is "imaginary current" part of this? maybe the clamp-on ammeter acting wonky?

By the way, rotor angle is pretty much irrelevant to this. It may be important to the "cogging" but not to the single phase torque ripple, which is only determined by electrical phase.

"Cogging" is going to be a tertiary small effect for most. Maybe important sometimes, or they would not bother to try to correct it.

A cap-run or permanent split cap (PSC) motor, tuned properly, doesn't have a torque trough like described. The capacitor is selected so that there's a 90 degree phase shift for the second winding relative to the first (so the main winding might be say 30deg lagging, and the aux winding 60deg leading). This means that when one winding is experiencing zero current, the other is experiencing peak current.

If we wanted to be exceedingly pedantic, it could be debated whether this is a single phase motor, or whether it should be considered a two-phase motor with an onboard phase converter allowing for operation on a single phase supply.

For a classic cap-start or split-phase (resistance/reactance start) motor which has a centrifugal switch completely disconnect the aux winding during operation, I agree with JST. There is no mechanism for torque delivery during zero current, which should deliver 100/120Hz torque ripple - twice mains frequency, as there's a torque pulse on both the positive and negative sine.

It should be possible to calculate the speed ripple expected under full load given the system inertia (rotor inertia as a lower limit) and rated torque, assuming torque is roughly a sine wave.

The "speed ripple" (really a torque ripple under load) results in a "force ripple" on the cutting tool. If the machine has a resonance near to that frequency, or any reasonable harmonic, it can result in wild chatter.

That normally happens only with lighter machines, which are more likely to have resonances in that range, combined with enough flexibility to respond in a more extreme fashion.

Yes, the capacitor would be generally a "static phase converter" set up for a two-phase motor. How effective it is will probably depend on the load, and the capacitor will probably be set up for best performance near full load. At lower loads, the phase shift may be different, and less effective. It still most likely will be noticeably better than a motor with no run capacitor.

That torque is torque and horsepower is horsepower and an inch is an inch and a millimeter is a millimeter and a quart is a quart, in the US or Africa or Ozztralia or the back side of the moon, your assumption is wrong and in fact kind of silly.
Not quite, really, the Americans get short measure on pints, theirs are only 16 fluid ounces versus the Rest Of The World getting twenty (and quarts and gallons follow suit) - elsewhere when folks my age were at primary school we were taught "A Pint of cold water weighs a Pound and a Quarter".

There are also about half a dozen slightly varying definitions of an inch.

### Torque difference between 1PH and 3PH motor?​

apart from lecturing each other how massive his knowledge when it comes to TORQUE developed by AC machine. HAs anyone gave a number??

### Torque difference between 1PH and 3PH motor?​

apart from lecturing each other how massive his knowledge when it comes to TORQUE developed by AC machine. HAs anyone gave a number??
If we say 'there is no difference', the implication is that the number is 0%.

Happy?

nope =)

Peak torque is the same.
Torque curve to the peak is a bit different,
so it may not replace your application perfectly.

There....Summary... Happy?

-D

### Torque difference between 1PH and 3PH motor?​

apart from lecturing each other how massive his knowledge when it comes to TORQUE developed by AC machine. HAs anyone gave a number??
Aside from saying there is no actual difference in average torque between them?

Basically there is no difference in the number , given the same rpm. That was in one (or more) of the first three responses.

Then there was the comment that there is a difference that may or may not MAKE a difference, in the way that torque is delivered (smoothness, etc).

Seems pretty complete, what were you expecting?

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