Single phase motors are a type of AC motor that uses electromagnetic principles to create useful rotational energy. They operate in much the same way that squirrel cage, wound rotor, and other polyphase motors work, except they are somewhat simplified (more information on these motors can be found in our articles on squirrel cage, wound rotor, and induction motors). “Single-phase” only refers to the input power, so there are many types of motors that use single phase inputs. They are commonly found in induction motors, but can also be synchronous. Single-phase motors contain both stators and rotors like most electric motors, but they only use one winding in their stator which carries only one AC current, and their rotors tend to be more basic than those of other designs. They also require a starter, as using only one phase of input power provides zero starting torque at rest.
Single-phase motors use both stators and rotors like other AC motors, though they work much differently. In three-phase motors, the 120 degrees of phase separation between the three AC currents running through the stator windings produces a rotating magnetic field; however, the magnetic field made by only a single phase “pulsates” between 2 motor poles, as there is only one AC current producing two possible magnetic field states (the AC current has two sinusoidal peaks, where the magnetic fields will be equal but opposite in orientation, or “up-down”). This approximates a rotating field, but not completely. These motors must be given an initial “shove”, or feel a force “out-of-phase” with the stator phase in order for initial movement of the rotor to occur. The stationary rotor will not feel any effects from this pulsating, “up-down” magnetic field if it not already moving, as the up-down magnetic forces cancel each other out perfectly. Motor starters solve this issue by adding an out-of-phase influence (auxiliary windings, capacitors, etc.), which then creates a simulated rotating magnetic field to start the motor. More information on these starters can be found in our article on motor starters.
A single phase motor only refers to the type of input power supply used, and not the specific stator-rotor-starter arrangement. Many of the specifications for other AC motors apply when selecting a single phase motor, and these can be found in our articles on induction motors and AC motors. This article will specify the different types of single phase motors, so that general principles can be applied to these specific designs.
Split-phase motors implement an auxiliary winding outside the stator coil to provide the initial phase difference needed for rotation. The starter winding uses smaller diameter wire and fewer turns than the stator winding, giving it more resistance. It will be out of phase with the main magnetic field because the increased resistance alters the supply phase. This split-phase winding will give the initial push to start rotation, and the main winding will keep the motor running. The starting winding must then be shut off (via usually a centrifugal switch on the output shaft) once the motor has reached a percentage of full speed (around 75% of rated speed). Increasing the resistance of the starting winding also increases the risk of burning out the coil, so these switches are necessary for split-phase motors to work properly and reliably.
In these types of single phase motors, capacitors alongside an auxiliary winding provide the phase difference needed to start rotation in these motors. They are similar to split-phase motors but use capacitance instead of resistance to shift the starter phase. In capacitor start motors, a centrifugal switch disconnects the start capacitor once the motor is at some speed (around 75-80% of full speed). Capacitor start-capacitor run motors use two capacitors (a start capacitor and a run capacitor), where the current flowing through the start capacitor leads the applied voltage and causes a phase shift. The start capacitor then boosts the startup of the motor, and the run capacitor is switched to once the motor is at rated speed.
Permanent-split capacitor motors use a permanent capacitor in series with the starting winding, with no centrifugal switch. The capacitor is in continuous use when the motor is running, meaning it cannot provide the boost that is given by a start capacitor customary in the previous two designs. However, these motors benefit from not needing a starting mechanism (switch, button, etc.), as the run capacitor in series with the auxiliary winding passively changes the phase of the single phase input. Permanent-split capacitor motors are also reversible, and generally more reliable than other single phase motors.
This single phase motor type does not use any windings or starters to get the motor going. Instead, this motor uses a setup such as in Figure 1 below:
This motor is more simplistic than other single phase motors, as it requires no extra starter circuits or switches. The C-core motor housing is made up of magnetically-conducive material (usually iron), which transfers the pulsating magnetic field from the main stator winding to the rotor. The poles of this motor are divided into two unequal halves, where two “shading” poles are created by extending the main stator winding to smaller windings on one of these halves (shown above). When the single phase AC current enters the C-core, it “shades” the winded halves by causing the magnetic field to lag through the shaded portion (the shading coil creates an opposing magnetic field, slowing the magnetic flux). This causes an unequal distribution of inductive forces across the rotor and causes it to rotate.
There are certain applications which call for specific single phase motors. Table 1 shows, qualitatively, the working characteristic of each motor type.
Split phase motors have a relatively simple design that lowers both their cost and performance. However, they have low starting torque and are prone to overheat due to the resistive nature of their starting mechanism. Low torque applications such as handheld grinders, small fans, and other fractional horsepower applications fit split-phase motors best. Do not use this motor if high torque or high cycle rate is desired; split-phase motors will almost certainly burn out when used this way.
Capacitor start motors have improved starting torque over split-phase motors and can withstand high cycle rates. They are more widely applicable as a result and are a mainstay in general-purpose industrial motor applications. These include belt-driven conveyors, large blowers, and geared applications, among numerous others. Their main downside is their cost, as they are more expensive than split phase motors.
Permanent-split capacitor motors, while sporting low starting torque, can perform well under high cycle rates and have excellent efficiency and reliability. They are reversible thanks to the lack of a starting mechanism and can be speed controlled. Their only major downside is that they cannot handle high torques, but otherwise are reliable, highly efficient machines great for garage doors, gate openers, or any low-torque application which needs instant reversing.
Capacitor start-capacitor run motors combine the benefits of both permanent-split capacitor and capacitor start motors, at double the cost. They can power applications that are too difficult for other single phase motors, such as air compressors, high-pressure pumps, vacuum pumps, 1-10hp applications, etc. using their high starting torque. They are efficient at full load current and are reliable due to their simplistic design. If power, reliability, and efficiency are priorities and there are cost is less of a concern, consider this type of single phase motor.
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Shaded pole motors are often considered “disposable” electric motors, as they are simple to produce and cheaper to replace than to repair. Their torque, efficiency, and reliability are nowhere near what other single phase motors can reach, but they are inexpensive and work well in low-horsepower applications. These include domestic uses such as bathroom fans, hairdryers, electric clocks, toys, etc. If the project only needs fractional horsepower and price is of primary concern, the shaded pole motor will function just fine.
This article presented an understanding of what single phase industrial motors are and how they work. For more information on related products, consult our other guides or visit the Thomas Supplier Discovery Platform to locate potential sources of supply or view details on specific products.
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