Single phase motors vs multiphase vibration

Well, it gets TWO impulses every revolution. One impulse from each half cycle, when the current peaks. It should be fairly clear that the rotor is not getting any torque when the current in the windings passes through zero. Now, this applies to a straight single-phase induction motor, not a capacitor run motor.

The windings may appear to be spread around the stator, but in fact, there really are only two poles on a 3450 RPM motor, and 4 on a 1725 RPM motor. The windings are broken up into a number of overlapping coils to spread out the pole and to make the motor more convenient to assemble.

So, a 2-pole (3450 RPM) single phase motor gets 2 pulses of torque every revolution, while a 2-pole 3-phase motor gets 6 pulses (two pulses from each of the 3 phases). Therefore, each of those 6 pulses only needs to be 1/3rd the strength (speaking very roughly). Another way to look at it is the rotor only slows down 1/3rd as much between accelerations.

There is actually more to it, though, as the induced current in the rotor has less time to decay between pulses where the stator flux restores it.

Jon

Neither motor has pulses per se.

The field in a polyphase induction motor is a rotating vector of constant amplitude. Therefore, a polyphase motor has constant torque (at given speed) with no torque ripple. If there is slop in the bearings then the rotor might shake, but that isn't usually the case.

One artifice for looking at a single-phase motor is to consider the field as two vectors of equal amplitude but rotating in opposite directions. This is mathematically equivalent to a sinusoidally-varying non-rotating field which clearly is the case. Once the motor is spinning (in either direction), the counter-rotating component has less influence on the rotor because the frequency it induces is higher -- so rotor inductance limits the current it induces in the rotor and the countertorque it produces. Now you have a rotating field vector with a smaller counter-rotating field vector. The result is ripple in the torque, which is converted to vibration by gears or a belt.

The way I think of it is to consider the delivered power to the motor. This power is the product of the voltage times the current. This power if you think of it mathematically, goes to zero twice per period, or 120 times per second. So does each phase of a 3-phase motor, of course, but when one phase's instantaneous power is zero, the other two aren't. The net result is that 3-phase motors run more smoothly.

GWE

Thank you Mr Tesla:)

So even a two phase motor would eliminate the zero torque situation twice per second. Thanks, Eric

Thanks Don, I understand about the two counterrotating. I just used the single pulse per rev to illustrate why I didn't understand vibration when coils are wound all around the stator-not just in one place. So when a capacitor run and start motor which only uses one cap and no starting winding, but instead uses two identical windings with the cap shifting the phase of one winding, eliminates the ripple (at least most of it) because of the phase shift. While one winding is crossing zero the other is not. Thanks Again, Eric R Snow

that's twice per revolution, not per second...

Here's a few pages I scanned from a reference book:

Whats a 2 phase motor and where do you get two phase power??

Eric,

Slight correction. A polyphase motor that is to be run on single-phase current will have to be started either by external mechanical means or a phase shifting network, usu. a capacitor, momentarily connected in series with 1 of the windings. In some designs the capacitor is not used, the necessary phase shift being derived from the start winding only. After startup the poly-phase machine will continue to run on single-phase power, as is well known, and the basis for all the so-called "static" phase converters. Any motor running on a single-phase supply is subject to the torque pulsations described in this thread. In essence, a 3-phase motor (idler motor for example) is a single-phase machine when operated on single-phase current.

A capacitor "start and run motor" does, in fact, have a start winding. Generally, "cap. start and/or cap. run" implies a single-phase motor with a start winding. The start winding is usu. wound with smaller gauge wire than the main winding and is used only temporarily during the starting interval. The start winding is separated by mechanical space upon the stator and thus is responsible for "phase shifted" sets of poles on the stator. A single-phase motor starts as a 2-phase machine. A 3-phase motor started on single phase is also temporarily connected "2-phase" during start time.

Bob Swinney

Bob, Eric isn't thinking about starting a 3-phase motor on single phase. Eric, you are confused by the term "two-phase" which the person who wrote it was too. There do exist two-phase electrical systems but NOT in the US and here motors are either single phase or three phase (outside of some lab or special function, of course).

GWE

Robert Sw>

jk

They used to be fairly common in certain parts of the US -- and probably other places as well.

It can be generated from three-phase with a Scott-T transformer setup -- and vice versa.

Agreed.

That could easily be so. Old technology which would cost more to upgrade than to continue to support it -- because the power company can turn three phase to two phase with the above-mentioned Scott-T transformer setup.

Enjoy, DoN.

Capacitor run and shaded pole motors are two-phase.

Not necessarily, Bob, if "start winding" means a winding that is switched out once the motor is started. I have a 2 HP cap-run buffer that operates as a two-phase motor with no switching from start to run. It has only one cap, a continuous duty run cap. There is no intermittent-duty electrolytic start cap. It does gronk the line on startup! I observed start transient current to be about 190 amps, but it gets going quick enough not to trip a 20-am QD breaker.

Actually Bob, the motor I was thinking about does not have a start winding or a centrifigul switch. It has two identical windings. The windings are connected to each other at one end and this end is connected to the line. The other ends are connected to each other through a capacitor. Connecting the other line wire to one side of the cap or the other determines the rotation. So I think the motor acts like a two phase machine because of the phase shift provided by the capacitor. Eric

But it still has a 2nd winding that acts to "2-phase" it for starting doesn't it? Caps don't necessarily have to be switched out after start-up. Correct me if I'm wrong, but a permanent, unswitched, run cap is standard mode of operation for capacitor-run motors (as opposed to cap-start, cap-run), I think.

How do it know what direction to start?

Some common 2-phase motors are the fractional HP AC permanent capacitor types that have 3 leads to the stator. The capacitor provides/creates the out-of-phase souce for the second stator winding. The 2 windings of the stator are wound with 2 ends of the 2 windings tied to a common point, and the other 2 ends free (providing 3 stator connections).

The types of applications for these types motors is fairly wide. Many low power applications such as fans, pumps, blowers, gear head torque slow speeds, and others.

The stators can have several different pole configurations, and this allows motors that attain specific speeds from an AC source. Speeds of these motors can vary from about 600 RPM versions to 3200 RPM models.

Many manufacturers make numerous versions of the PC motors for a wide variety of applications.. Bodine, Dayton, Oriental Motor and many others.

WB ...............