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Single Phase Motors

On this page we will attempt to explain and identify the different types of Single Phase AC Electric Motors.

Single-Phase motors are identified primarily by the way they are "started" and the "torque" they develop.

The types of Single-Phase motors are:

Shaded Pole
PSC (Permanent Split Capacitor)
Split Phase
Capacitor Start
Capacitor Start/Capacitor Run
Repulsion Start/Induction Run

Shaded Pole - These motors have low starting torque, low efficiency, medium high running current, low power characteristics, no capacitors, no starting switch and are low cost. Motors of this type are found on small direct drive furnace blowers, window fans and other fan applications found in the residential community. Shaded pole motors should NOT be used to replace OTHER TYPES of single-phase motors, mostly due to the low torque and efficiency. Motors of this type are also used in small appliances and items such as bathroom exhaust fans, clock motors, evaporator fans on refrigerators and freezers. Although low in efficiency, with low starting torque, because of the inherent LOW COST, these motors abound in the proper residential applications. The output capability of a shaded pole motor will range from "sub-fractional HP" i.e. 1/30 HP to a maximum of 1/4 or 1/3 HP. Speeds will be 2 pole (3000 RPM), 4 pole (1550 RPM) and 6 pole (1050 RPM). 

PSC (Permanent Split Capacitor) - Motors of this type are used in many of the same applications as the shaded pole motor. The major differences are that the PSC motor has much higher efficiency, lower running current (50% - 60% less), and higher output power capability. The PSC motor gets it's name from the fact that there is a "RUN" capacitor connected in the motor circuit at all times. This device assists in maintaining a high efficiency and power factor, and decreasing the amount of power consumed for the same power output. These motors can be used to replace ANY shaded pole motor except those where the physical size of the PSC will not fit.... i.e. a clock motor or small evaporator cooling fan. The output power of a PSC motor will be in the range of "sub-fractional HP" i.e. 1/20 HP to a maximum of 3/4 HP. Single speed or multi-speed motors can be designed with the top speed being 1625 RPM and the 1075 RPM being the most popular speed. Multiple speeds in one motor are accomplished either by "tapping" the winding or a "choke" coil.  Starting torque on this motor type is also very LOW.

Split Phase - Split phase motors have a starting switch but no capacitor or additional starting mechanism. Their starting winding is simply electrically offset from the running winding by an amount sufficient to begin the rotating member in a specific direction. Because there is no "additional" starting assistance, this motor has moderate to low starting torque.... in the range of 100% to 125% of full load torque. In addition, the starting current will be rather high. Motors of this type are used on applications that are relatively easy to start but may build power requirements as the rotational speed increases.

Typical applications are belt driven fan loads and some pump applications.

Capacitor Start - This is a real "work horse" of the single phase line of motors. These motors incorporate a starting winding, starting switch and electrolytic capacitor. When the motor is asked to start, the capacitor discharges into the start winding giving it a "shot in the arm" to get it going. Then, as in other single phase motors with starting switches, when the rotor reaches approximately 75% to 80% of full speed, the starting switch OPENS, removing the capacitor and start winding from the circuit and allows the MAIN or running winding to complete the ramp up of speed to full operational RPM.

These motors can be manufactured with moderate to high starting torque, based heavily on the capacitor rating and start winding design. The motor will also have high breakdown torque which keeps the motor "locked" into it's running speed even with high overloads. These motors with MODERATE starting torques of 175% or less are typically used on fans, blowers, and pumps. The high starting torque motors.... used on loads requiring up to and above 300% full load torque to start, can be used on compressors, and industrial, commercial and farm equipment. In the agricultural market, applications such as silo un-loaders and other "hard to start" loads are a natural for these devices.

Capacitor Start/Capacitor Run - These motors parallel the design and application of the Capacitor Start motor identified above, except that they have an oil filled, RUNNING capacitor in the circuit with the MAIN or running winding. This capacitor stays in the circuit ALL THE TIME and helps improve the running efficiency and lowers the full load running current. These motors are typically in the higher single phase horsepower ratings... above 2HP. The agricultural industry is a major user of these motors.

Repulsion Start/Induction Run - This motor is an OLDER design that uses a commutator (like a DC motor) and brushes to start the motor. The other major difference from it's single phase counterparts, is that this motor has an actual "winding" in the rotating member. Other single phase motors have a rotor of cast aluminum alloy but the Repulsion Start motor has a winding with the ends of the coils attached to the commutator. In this way, a device known as a "short-circuiter", can short out the coils at the appropriate time and turn this winding into a rotating member that acts like the cast aluminum alloy rotors or the other "induction" motors.

When the motor reaches a pre-set speed .... in the range of 75% or so, the "short-circuiter" automatically "shorts out" the brushes and commutator so that the winding in the rotating member acts as a standard induction motor.

These motors have starting torques equivalent to the capacitor and many can be built with VERY HIGH starting torques in the range of 400% to 600% full load torque! Such high starting torques are sometimes required in the agricultural market on silo unloaders.

The RI technology still exists but is slowly and systematically being replaced with the Capacitor Start/Capacitor Run higher horsepower single phase motors. The major draw back of the RI motor is the maintenance required on the commutator and brushes. This, of course, is eliminated by the capacitor start motors.

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Copyright 2002 - 2018 Apparatus Repair & Engineering, Inc. All rights reserved.
Revised: November 10, 2018.