I've read Electric Motors and Control Techniques. More than once. And
I can't figure out why, once it's rotating, a single phase motor will
run with more vibration than a three phase motor. I could understand
it if there was only 1 coil on one side of the motor and it only got
an impulse one every revolution but the windings are spread all
around. Could someone please explain it simply enough for the typical
non- AC knowledgeable types?
Eric R Snow,
E T Precision Machine,
not E T Precision Motors
Well, it gets TWO impulses every revolution. One impulse from each
half cycle, when the current peaks. It should be fairly clear that the
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,
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.
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.
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.
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
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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.
It know dis way: The motor has two identical windings. One end of
each winding is connected to the other winding. This connection is
also connected to one of the two wires providing power. The other ends
of the two windings are connected together with a capacitor. The
remaining power wire is then connected to one side or the other of the
capacitor. This determines the direction of rotation.
Another way to explain it is like this: There are two windings and
each winding has two ends. We'll call these W1, W2, W3,and W4. W1 and
W2 are the ends of one winding, W3 and W4 the ends of the other
winding. The capacitor ends we'll call C1, and C2. The power wires P1,
and P2. Connect W1, W3, and P1 together. Connect C1 to W2 and C2 to
W4. Now, connect P2 to the C1,W2 junction and the motor will spin one
way. If P2 is connected to the C2, W4 junction the motor will spin the
other way. Motors built this way run smooth but have much less
starting torque. Since there is no centrifigul starting switch they
can be made to run slower with lower voltage. Higher voltage will only
increase torque. The motor can not attain
Eric R Snow
The capacitor is connected between 2 of the 3 motor leads, and these
connections stay connected that way.
One of the AC line connections is made to either of the same 2 connections,
one for CW, or the other for CCW rotation.
This leaves one of the capacitor/stator connections floating (not connected
to the AC line connections, which would be L1-L2 for 240VAC or L-N for
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.
Eric R Snow
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
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, 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).
Robert Swinney wrote:
On Tue, 17 May 2005 15:14:25 -0500, "Robert Swinney"
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.
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