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
Bob, what Eric described is an AC PC or 2-phase induction motor.
I don't believe there are any other motor types that have 2 identical
windings, connected, with an attached (external) capacitor, and reversed by
moving the AC line connection from one capacitor/winding connection to the
other capacitor/winding connection.
If there are, let us know
Not sure about your definition of AC PC motor ? What does PC mean ?
I believe you are "seeing" a capacitor start, single-phase induction motor;
maybe even a capacitor-run single-phase induction motor. There would be no
need for a capacitor in a real (very old fashioned) 2-phase motor. As
someone else mentioned, genuine 2-phase motors are rare, nowadays.
Bob, you and I seem to have been referring to two different types of motors,
but both of them could/would be called 2-phase motors. I'm not at all
familiar with 2-phase AC power available from a utility company. I've read
that it has existed, and may still be available in some locations.
I didn't mean to imply that their motors required a 2-phase power source to
In the two instances of both Eric and Don's motor descriptions, the motor
types are AC Permanent Capacitor induction motors that operate as 2-phase
motors, with the capacitor providing/creating the phase shift.
These presently produced (not antique) types of motors operate on single
phase AC, and don't require a 2-phase AC supply for operation.
There are old versions of the PC motors around.. the design isn't new, but
they're still being manufactured for many uses.
By design, the capacitor used with the single phase PC motor creates the
"second phase", much in the same way that capacitors permit a 3-phase motor
to be operated with single phase AC.
The characteristics of the single phase PC motors are that there are no
separate start and run windings, and no centrifugal switch.
Additionally, they have 2 identical windings connected together to form 3
stator connections. The capacitor is connected to 2 specific leads, and the
single phase is connected to 2 of the 3 leads.
These motors are typically labeled as single phase induction motors, and
nearly always have a capacitor value (uF) shown on the label or plate. The
capacitor is always an externally connected component AFAIK, generally not
mounted under a cover attached to the motor case (like split-phase or
capacitor run motors are).
The capacitor value is usually in the range of 2 to 30uF for small
fractional HP motors up to about 1/8 HP and has a voltage rating of 250VAC
Numerous manufacturers rate and/or label their small motors in watts instead
of HP, and indicate if the motor is intended for use with 50 or 60Hz.
Very Well, Wild Bill. Your description of a capacitor start/capacitor run
motor is well taken. However it is incorrect to call a motor a "2-phase
motor" unless it is designed to run on 2-phase power. I think you are
describing an ordinary, capacitor run, single-phase induction motor that is
designed to run on single-phase power only.
Bob, how would I identify a motor that only runs on 2-phase power by the
internal characteristics, in case I would happen to encounter one?
I used to have a multi-volume set of books from the early 1900s on
electrical engineering, but don't have them now. That was a fascinating
resource, if anyone would happen to encounter something like that at a yard
sale or flea market.
Perhaps it would be more technically accurate for me to stop referring to PC
motors as 2-phase motors, but instead say that they "operate as a 2-phase
The stator windings of permanent capacitor motors are both considered to be
As there is no start winding (and no combination of start/run windings with
a centrifugal switch), these motors have low starting torque.
They are indeed single-phase only motors, as you stated.
The capacitor start/capacitor run motors that I'm familiar with are motors
with 2 separate capacitors, separate start/run windings, and a centrifugal
switch. These types are designed for high starting torque and heavy duty use
(compressor-rated duty, for example).
They have a large value capacitor for starting, and a low value capacitor
for run, typically secured to the outside of the motor case under covers.
While the 2 capacitor CS/CR motor design also incorporates 2 identical
stator windings, it differs from the PC motor in that the PC motor has only
one capacitor, no start winding and no centrifugal switch.
"> Perhaps it would be more technically accurate for me to stop referring to
Bill, that would be absolutely correct if PC motors were not referred to as
2-phase motors. Even though operated with single-phase power, such motors
do run as 2-phase machines, as you said. I'm sorry, if earlier I seemed to
dispute you on this fact. In a PC motor, or a split-phase motor for that
matter, the "2nd phase" is derived by virtue of the auxiliary winding and
capacitor; except the ordinary split-phase machine omits the cap. The
windings of PC motors are displaced from each other by 90 degrees, the same
as in the very old fashioned 2-phase motors designed to run on 2 phases as
delivered from the power lines. There are several variations of this scheme
re. the aux. winding and capacitor. AFAIK, motors of this type have
identical windings if they are designed to deliver any appreciable power -
such as in AC compressors and dishwashers. One obvious advantage in
appliances is the ease of reversing via external contacts.
I have to side with bill, Technically it is a two phase motor. It is
designed to run on two phase power, with the capacitor providing the 2
To say you are correct, would make a (3 ph) motor run off of a static
phase converter also a single phase motor.
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