# SIngle phase ac motor question

• posted

My understanding is that you must have a rotating magnet field to drive an ac motor. Hence often a capacitor start system is used with a second winding (for single phase) which is switched off once the motor is started. Essentially this is a two-phase motor for starting.Am I therefore wrong with the idea that a single phase on its own will not rotate a motor since there is a single phase and one coil only after the thing has started? How does a single phase give a rotating field?

Hardy

• posted

"HardySpicer"

** No Google where you live ????

Most commonly, single phase motors use a capacitor to shift phase and make the field rotate.

Low powered ( sub 100 watt input ) examples often use a "shaded pole" to achieve a similar result.

Plenty of good, basic AC motor stuff here:

..... Phil

• posted

I know that much. I was just asking as to whether without the capacitor (we are back to single phase then and not simulated two- phase) do we still have a rotating field.

Hardy

• posted

A capacitor-start induction motor uses two coils and the phase-shift cap to make a rotating field at startup. Once the rotor gets up to near synchronous speed, the auxiliary winding and cap are usually cut out by a centrifugal switch. The motor will continue to run in single-phase induction mode. It's similar to a single-phase synchronous (pm rotor) motor in that it runs on a single-phase (non rotating) field as long as it's at speed.

Most of the caps and start windings would fry if you left them on for long.

As Phil so delicately points out, details are on the web.

John

• posted

Obviously you have no idea either along with the otehr chappie who is rude.

When the motor is at standstill, the induced voltages are equal and opposite resulting in two equal and opposite torques which cancel each other out. So the net torque will be zero. However, if the rotor is given an initial rotation by auxilliary means in either direction the torque due to the rotating field acting in the direction of initial rotation will be more than the torque due to the other rotating field. Therefore the motor will develop a net positive torque in the same direction as the original rotation. This follows from cross-field theory or double revolving field theory for a pulsating magnetic field. A pulsating magnetic field is the type you get with a single phase coil.(and no starter coil)

I wasn't after a superficial understanding as was given in that link. If you are not a professional engineer please don't bother answering.

Thanks Hardy

• posted

"HardySpicer" schreef in bericht news: snipped-for-privacy@d2g2000pra.googlegroups.com...

Phil is always rude, but he knows quite a lot about electronics.

John Larkin knows even more.

Both of them answered your question a level that was appropriate to the quality of your question, which didn't suggest that you had a particularly firm grasp of the subject.

This is good - as far as it goes. It talks about voltages rather than currents, which is a bit odd, since force and torque are generated by currents flowing through conductors in a magnetic fields. Voltages are only interesting as far as they drive currents.

"Cross-field theory" and "double rotating field theory" don't ring any bells with me. I imagine that some academic somewhere swears by them, but practical enegineers tend to think more in terms of magnetic fields and currents flowing through conductors immersed in the relevant magnetic fields.

Professional engineers don't waste time getting a deep understanding if a superficial understanding works. Studying takes time, and time is money.

```--
Bill Sloman, Nijmegen```
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Jerk.

John

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That's rubbish Bill abd you know it. Why the hell do you think Wermher Von Braun managed to stabiise a rocket when others could not? He had a deep understanding.

Hardy

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make

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
• posted

make

Don't give me that superiority bullshit - show me anywhere in the link that was given by that other idiot

that answers the question? Nowhere. It gives the standard stuff about how a capacitor start works with TWO phases!! I wanted info on a single phase and not about a single phase that was split into two. Surely he could understand that. I needed info on the pulsating magnetic field essentially but I didn't know the terminology to use until I read the book. So he didn't know he was just pretending to know or miss-read my question.

Hardy

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OK, which direction would it rotate?

John

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With the ac single field and no capacitor it does not rotate at all. There is zero net torque. However, if you spin it in one direction it stays there and starts rotating and continues in that direction. My simple question was why? With a capacitor you create two vectors of flux which add and give a rotating magnetic field. That much is everywhere on the net and was discovered well over 100 years ago. And that idiot also mentioned shaded pole ac motors which also has a net rotating flux. It's my first point that is not well explained. Actually even the book explanation that I gave earlier isn't exactly compelling.

Hardy

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Like I said "Professional engineers don't waste time getting a deep understanding if a superficial understanding works". Superficial understanding didn't work in von Braun's application, so he had to put the time in to develop an understanding that was deep enough to work. And he's a poor example of a professional engineer - his primary interest was in space flight, rather than rockets as such, and he did a Ph.D. on rocket design

In many ways he's better seen as an obsessed hobbyist, who managed to con the German and American governments into funding his hobby, by selling them space rockets as ballistic missiles.

Some engineers don't always understand that their superficial over- simplified models can break down if you try to use them in situations where the simplifying assumptions are invalid, but that's another story.

-- Bill Sloman, Nijmegen

• posted

You've probably gathered by now that intruders are not welcomed by the senile and abusive members of the gruesome glee club that s.e.d. has become.

Pommy B

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Another method is the Shaded-pole motor

There is a phase difference between the fields from the shaded and unshaded pole faces.

Robert H.

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Ah, finally you ask a coherent question.

Imagine that the rotor was a bar magnet, perpendictular to the shaft. Apply the AC current to the single stator coil, making a single-phase field. The magnet will just sort of buzz, with no usable torque applied to the shaft, unless you want a very weak vibrator. [1]

But spin the shaft up to 3600 RPM. The magnet now aligns with the field at both the positive and the negative peak of the 60 Hz AC cycle. If it gets a little ahead of or behind the time peaks, a torque develops that tries to pull it back into alignment. That's a synchronous motor. It won't start by itself, but it does fine once you're up to synchronous speed. Think of the field as a stroboscope that's illuminating the rotor only at its current peaks; that image looks like a DC field forcing the bar magnet into one position.

Now replace the magnet with a cylinder of moderately conductive stuff. At 3600 RPM, the field essentially magnetizes the rotor every time it peaks, and the magnetization makes it act like the magnet in the synchronous motor. The "magnetization" is actually eddy currents induced into the rotor, and the best rotor isn't solid, it's shorted turns of copper over a laminated steel core. Since the "magnetization" isn't fixed like the PM rotor, it sort of slides around, so the induction motor doesn't, and doesn't have to, run at synchronous speed.

More detail tends to be mathematical.

Oh, don't thank me, I'm going to write a book on electronics for beginners, and I need to practice explaining simple stuff to idiots.

John

[1] One interesting sort-of-countercase being the single-pole induction motors that spin microwave oven turntables. They self-start in a random direction.
• posted

Shaded pole motors aside (the shading coil always provides a secondary flux field physically separate from the primary and lagging it in time, creating a rotating component):

Once a single phase induction motor reaches some RPM, the slip between the rotor and stator causes the rotor currents to lag the air gap flux such that the average torque produced is greater in the direction of rotation. From that point on, the start capacitor is no longer needed and is switched out by a centrifugal switch.

I've had portable equipment with cap start induction motors where the cap (or centrifugal switch) died. When turned on, the motor would just sit there and growl, not turning. So I'd pick it up, flip the switch and give the unit a little twist along the motor's axis. Often this was enough to get the thing spinning and then it ran fine. Or backwards.

```--
Paul Hovnanian  paul@hovnanian.com
----------------------------------------------------------------------```
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Thank you for your long reply. That's good. I am still confused by this however because to me you need a field that rotates in order to get a motor and the single phase version does not. In fact it just pulsates and as you said gives zero torque. Now I was aware of synchronous machines and induction motors (not the microwave example which was interesting). They have fields that rotate however. Now that book which I references earlier had another explanation - if you look back in this thread. Neither explanation seems that good to me and there are no diagrams anywhere on the net which describe this effect (other than the book which splits the torque into two parts). Of course there are many animations showing a rotating magnetic field and the induction motor.

Hardy

• posted

make

The field doesn't have to rotate is the rotor is already spinning at close to synchronous speed. I tried to explain that.

The induction motor delivers usable torque when the rotor is spinning at somewhat below synchronous speed. At zero speed, it has no average torque (again, no rotational direction is preferred to any other, so it gets confused and dithers) which is why you need to apply some amount of rotating field to get it started. My dad's old table saw had a dead start capacitor, so he'd grab the belt and fling it hard to get it started, and he still has all his fingers.

If you connect an induction motor to the ac line and drive it from an engine, above synchronous speed, it becomes a generator and pumps power upstream into the ac power lines.

John

• posted

No you don't.

To get a motor, you need torque; how you get that torque ultimately doesn't matter.

Now, the torque needs to have a given orientation relative to the rotor, and the rotor is rotating. Making the field rotate with the rotor will maintain the correct relative orientation, but so will a fixed field orientation if you vary the magnitude so that it has the right sense at the right times.

A polyphase motor has the advantage of a specific direction of rotation, and the ability to start regardless of rotor position. A single-phase motor may rotate either way, or not at all if the rotor is exactly aligned, hence the need for some phasing "tricks" when starting.

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