Fractional or low horsepower 3 phase motors are more expensive
than single phase in my experience. Very typical from what I have
seen in the industry. If the condenser fan motor gets above say 3 hp
then they would have used 3 phase
I wonder why this is so? It would seem the capacitor and extra
connections (7 leads vs. three) would make the single phase motor
more costly to manufacture and implement.
3-phase motors have several poles, and at least 3 power leads.
Except for that, they are fairly simple and quite efficient.
1-phase capacitor-start motors have quadrature windings, which
require at least 1 extra pole pair; also starting capacitors; also
(usually) centrifugal switches to switch the starting capacitors in
and out; also usually extra leads to reverse the rotation direction.
The quadrature windings produce a rotating magnetic field, which
starts the motor. Starting torque can be fairly high.
Shaded-pole motors run on 1 phase, but do not require starting
windings and capacitors and all that goes with them; therefore such
motors are relatively simple and inexpensive. The magnetic
pole-pieces are bifurcated, with copper rings around diagonally
opposite sub-pole-pieces. The alternating magnetic field induces
current in the copper rings, and that current delays the magnetic
field through those sub-pole-pieces, and thus creates a rotating
magnetic field for starting the motor. For small motors that do not
require much starting torque, as for fans, shaded-pole motors work
well. The copper rings waste some power, but usually that is not
significant for small motors.
I don't know why the subject condenser fan motors are 1-phase
capacitor start, rather than 3 phase. Maybe replacement 1-phase
motors are more readily available. What is their power rating?
alvarez at alumni dot caltech dot edu