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2 Engineering
2.9 Single-phase AC motors
DC1-S…20…, DC1-S…OE1 Variable Frequency Drives 05/21 MN040058EN www.eaton.com
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2.9 Single-phase AC motors
Single-phase induction / AC motors are frequently used for applications in
which three-phase power is either not available or not viable, which applies
above all to cases in which low outputs are required. In the German market,
single-phase motors with an output of up to 2.2 kW (230 V) can be run on the
public mains. Meanwhile, outputs of over 3 HP are common in countries in
which powerful single-phase power is also available (Middle East, USA).
When it comes to their configuration and the way they basically work, single-
phase AC motors are similar to asynchronous motors. A squirrel-cage rotor is
used, and the windings are split into two windings (main winding: 2/3,
auxiliary winding: 1/3) displaced in space (by approximately 90°) inside the
stator slots in the laminated core. Shaded-pole motors are an exception here,
as their stator instead consists of a laminated core with salient poles.
The rotating field required in order for the rotor to rotate requires not only for
the main and auxiliary windings to be displaced in space, but also for the
corresponding alternating fields to be produced with a phase (time)
displacement. The required phase difference between the currents of the
main winding and auxiliary winding is achieved by means of the following:
• Capacities (capacitors),
• The auxiliary winding having a higher inductance
• Self-induction (shaded-pole motors)
Together, the main winding field and the auxiliary winding field produce an
elliptic rotating field. This field enables the single-phase AC motor to start by
itself, but is heavily load-dependent and results in a smaller starting torque
when compared to three-phase motors with the same output. This means
that it may be necessary to oversize the motor in certain cases in order to
ensure that there will be a sufficiently large starting torque. In addition to
this, the motor’s output will decrease at low speeds, which is why it is not
recommended to reduce the speed below 50% of the rated motor speed.
The fact that a single-phase alternating field is used means that the operating
direction of AC motors cannot be changed electrically. The only available
option is to reverse the connections for the individual stator windings,
meaning that the windings must be routed separately into the terminal box
for this purpose. In addition, in the case of shaded-pole motors, the shaded
poles (four total) must be salient.
When it comes to AC motors, speed n is the ratio of mains frequency f to the
number of pole pairs p (main winding):
n = f/p.
With their special function for single-phase AC motors, DC1-S… variable
frequency drives make it possible to achieve reliable starting with an
increased starting torque and stable operating performance even at reduced
speeds.