I have written previously about three-phase induction motors which are ubiquitous in industrial applications. There is however another class of induction motors you are more likely to encounter in a domestic setting – the single-phase induction motor.
We have seen previously that the stator windings in a three-phase motor produce a smoothly rotating magnetic flux vector. This induces currents in the rotor windings which create their own magnetic field that interacts with the stator field to produce a torque.
In a single-phase motor the stator windings can only produce a field which pulsates along a single axis as shown in Figure 1. In the positive half-cycle, the field points to the right and increases from zero to some peak, before reducing again to zero. In the negative half-cycle, the same thing happens but in the opposite direction. No torque is produced on the stationary rotor, so the motor cannot start on its own. Once the rotor is moving a torque is produced, and the motor will continue to rotate. Several methods are available to provide some initial torque to get things started as we shall see. These motors can run in either direction, depending on the direction of the initial “kick” that gets them started.
One of the simplest starting methods is used in the shaded pole motor. Figure 2 shows a typical example. A shorted turn or two on one (or both) of the poles, visible at the top of the picture, creates enough distortion in the magnetic field to create a small starting torque that is enough to get the rotor moving.
This type of motor is typically less than 30% inefficient and therefore limited to quite small motors, usually not more than a couple of hundred Watts.
Another common type of single-phase induction motor is the Permanent Split Capacitor (PSC) motor. This motor uses an auxiliary winding which is physically offset from the main winding. This winding connected to the motor terminals via a capacitor, as shown in Figure 3. The auxiliary winding produces a weak field that creates the staring torque. This torque is relatively low, as the current through the auxiliary winding needs to be small to keep the size and cost of the capacitor down, and to maximize efficiency. PSC motors are typically used in applications requiring low starting torque such as fans and centrifugal pumps up to about 2kW in size. They can generally be used with speed controllers if required.
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For better starting torque a capacitor-start motor should be used. Figure 4 shows that like the PSC motor an auxiliary winding is present, but in this case the capacitor is larger and the auxiliary winding draws significant current. The auxiliary winding and start capacitor are not rated for continuous operation, so they are switched out by a centrifugal switch when the motor reaches about 70% of its rated speed. Figure 5 shows a typical centrifugal switch.
Capacitor-start motors are used for conveyers, geared applications or wherever a high starting torque is required. They are not suitable for use with speed control since at low speeds the start winding will be switched in for long periods of time and may burn out.
A variation of this theme is the capacitor-start/run motor shown in Figure 6. In this case there is a start capacitor in series with a centrifugal switch and a smaller run capacitor that is permanently connected to the auxiliary winding. This arrangement provides the best torque characteristic across the widest speed range, and good overload performance. Of course, this comes at the expense of complexity and cost. These motors are used for really demanding with frequent starting applications such as compressors, cement mixers, brick saws and the like.
References
Circuit Globe. “What Is a Capacitor Start Capacitor Run Motor? – Its Phasor Diagram & Characteristics,” February 13, 2016. https://circuitglobe.com/capacitor-start-capacitor-run-motor.html.
“Electrical Machines – Single Phase Induction Motors.” Accessed November 9, 2021. https://people.ucalgary.ca/~aknigh/electrical_machines/other/spim.html.
“An Introduction To PSC Motors | Beckett Corp.” Accessed November 9, 2021. https://www.beckettcorp.com/support/tech-bulletins/an-introduction-to-psc-motors/.
“Permanent Split-Capacitor Motors.” Accessed November 9, 2021. https://www.industrial-electronics.com/AC-DC-motors/54_Permanent-Split-Capacitor-Motors.html.
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Andrew Levido (andrew.levido@gmail.com) earned a bachelor’s degree in Electrical Engineering in Sydney, Australia, in 1986. He worked for several years in R&D for power electronics and telecommunication companies before moving into management roles. Andrew has maintained a hands-on interest in electronics, particularly embedded systems, power electronics, and control theory in his free time. Over the years he has written a number of articles for various electronics publications and occasionally provides consulting services as time allows.
