7 Signs of Single-phase Motor Induction Motors


Motors come in two varieties. They are either AC motors which run on alternating current or  DC Motors & Controls which run on direct current. Direct current motors are typically what you find in your household appliance ad small applications. Alternating current motors are used in households for larger applications like air conditioners or refrigerators for industrial applications such as powering machinery. Of the Alternating current types of motors, you typically have either a single-phase motor or a three-phase motor. Each has its advantages and disadvantages. This topic will cover single-phase motors.

How Do They Work?

A single-phase motor is a motor that runs on alternating current. It is an AC motor.  A single-phase AC motor produces an alternating magnetic field as opposed to a rotating magnetic field as is produced by a three-phase induction motor. This alternating magnetic field is what produces the power for the motor. 


Single-phase induction motors are used for smaller applications than three-phase induction motors. You will find single=phase induction motors powering things like washing machines, fans, refrigerators, and air conditioners. Part of the reason for this is that the copper wire that conducts the current would have to be quite large since it is the only conductor. With a three-phase inductor motor, the current is split between three wires so that each can be smaller but attain higher voltage. 


The single-phase induction motor comes in several types. There are split-phase induction motors, capacitor-start induction motors, Permanent Split-Capacitor Motors, capacitor start-capacitor run motors, and shaded-pole motors. 

Starting Mechanism

Unlike three-phase induction motors, single-phase induction motors are not self-starting. This type of induction motor requires some type of starting mechanism. The starting mechanism that is used is dependent upon the type of single-phase motor you are running i.e. split-phase, capacitor-run, shaded-pole, etc.


The single-phase inductor motor is not as efficient as its three-phase cousin. With the three-phase design if one phase fails, though you have less power capacity, you still have power. With the single-phase induction motor if the single-phase fails you have no power. On the bright side, since it does only require one wire it is cheaper to produce than the three-phase motor which requires three copper wires, one for each phase. 


The single-phase induction motor has a low starting torque compared to the three-phase induction motor. How low the starting torque depends on the type of single-phase motor. For example, the capacitor-start induction motors are sized to have a high starting torque, sometimes as much as 300% of its rating. A centrifugal switch in the motor removes the auxiliary winding after start-up to prevent the capacitor from continuously operating. The split-phase motor, on the other hand, has a relatively low starting torque of only 100-150% of its rating. 

Miscellaneous Information

Compared to three-phase induction motors, single-phase induction motors do have some advantages. Since they have fewer components, they are easier to manufacture since they have a simpler structure. This, of course, makes them easier to repair and less costly to manufacture. They are also more reliable because they have less potential points of failure. 

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