Single-Phase Induction Motor

---

---

As the name implies, these induction motors are operated on a single-phase AC supply. Single-phase induction motors are the most familiar electric motors because these are commonly used in domestic and commercial appliances like fans, pumps, washing machines, air conditioners, refrigerators, etc.

Although single-phase induction motors are relatively less efficient than three-phase induction motors, but they are extensively used as the substitute for three-phase induction motors in low power applications.

A typical single-phase induction motor consists of two main parts − a stator and a rotor. The stator of a single-phase induction motor carries a single-phase winding, whereas the rotor has the squirrel-cage construction.

Why doesn't a Single-Phase Induction Motor Self-Start?

A single-phase induction motor consists of a squirrel cage rotor and a stator carrying a single-phase winding. But, a single-phase induction motor is not self-starting like a 3-phase induction motor since it requires some starting means.

When a single-phase supply is fed to the stator winding of the single-phase induction motor, it produces a magnetic field that pulsates in strength in a sinusoidal manner. The polarity of the magnetic field reverses after each half cycle but the magnetic field does not rotate in the space.

As a result, the alternating flux cannot produce rotation in a stationary squirrel cage rotor because the magnetic flux can be resolved into two components, each one rotates in the opposite directions at the same speed. Consequently, the net flux is zero, the induced current in the rotor bars is zero, and hence, the resulting torque on the rotor conductors is zero. Therefore, a 1-phase induction motor is not self-starting.

How to Make an Induction Motor Self Start?

The major drawback of a single-phase induction motor is that it is not self-starting, but requires some starting mechanism. In single-phase induction motors, the stator winding produces a pulsating magnetic field which varies in sinusoidal manner. Hence, this magnetic field reverses its polarity after each half-cycle of AC, but does not rotate in space.

As a result, this alternating magnetic field does not produce rotation in a stationary rotor. Although, if the rotor is rotated in one direction by some external mean, it will continue to run in the direction of rotation. However, this method of starting a single-phase induction motor is not convenient in practice.

Therefore, in order to make a single-phase induction motor self-starting, we are somehow required to produce a revolving magnetic field inside the motor. This can be achieved by converting a single-phase AC supply into two-phase AC supply by providing an additional winding. Thus, a single-phase induction motor consists of two windings on its stator, namely main winding and starting winding. These windings are 90 out of phase with each other.

Types of Single-Phase Induction Motors

Depending upon the method employed to make the motor self-starting, single-phase induction motors may be classified into the following three types −

• Split-phase induction motor
• Capacitor-start induction motor
• Capacitor-start capacitor-run induction motor

Let us now discuss each of these Induction Motors in greater detail.

Split-Phase Induction Motor

A split-phase induction motor is a type of single-phase induction motor in which the stator consists of two windings namely, a starting winding and a main winding, where the starting winding is displaced by 90 electrical from the main winding.

The starting winding operates only during the starting period of the motor. The starting and main windings are so designed that the starting winding has high resistance and relatively low reactance, while the main winding has relatively low resistance and high reactance so that currents flowing in the two windings have a reasonable phase difference ($\alpha$) of about 25 to 30.

Now, when the starting winding of the motor is connected to a source of single-phase ac supply, the starting winding carries a current $\mathit{I_{s}}$, while the main winding carries a current $\mathit{I_{m}}$ as shown in Figure-1.

As the starting winding is made highly resistive whereas the main winding highly inductive. Consequently, the currents $\mathit{I_{s}}$ and $\mathit{I_{m}}$ in the two windings have a reasonable phase difference of about 25 to 30. As a result, a weak revolving magnetic field is produced inside the motor which starts it.

Capacitor Start Induction Motor

The type of single-phase induction motor in which a capacitor C is connected in series with the starting winding as shown in Figure-2. This capacitor is called starting capacitor. The value of starting capacitor is so chosen that the starting current $\mathit{I_{s}}$ leads the current $\mathit{I_{m}}$ through the main winding by about 80.

Once the motor attains about 75% of the rated speed, a centrifugal switch isolates the starting winding from the circuit. The motor then operates as a single-phase induction motor and continues to accelerate till it reaches the normal speed. Therefore, in this type of single-phase induction motor, the capacitor in series with the starting winding introduces a phase shift between the two windings so that the motor can start itself.

Capacitor-Start Capacitor Run Induction Motor

This motor is almost identical to a capacitor-start induction motor except that the starting winding is not disconnected from the motor circuit. Therefore, in case of capacitor-start capacitor-run induction motor, both the windings (starting winding & main winding) remain connected to the supply during starting as well as running period.

In this motor, two capacitors C₁ and C₂ are used in the starting winding as show in Figure-3. The capacitor C₁ having small capacitance value is used for optimum running of the motor and hence permanently connected in series with the starting winding, whereas the larger capacitor C₂ is connected in parallel with C₁ and it remains in the circuit during starting period only. The starting capacitor C₂ is isolated from the circuit by a centrifugal switch when the motor reaches about 75% of rated speed. The motor is then runs as a single-phase induction motor.

Applications of Single-Phase Induction Motors

Single-phase induction motors are mainly used in domestic and commercial appliances like fans, ACs, refrigerators, air-cooler, washing machines, etc. Given below are some of the applications of different kinds of single-phase induction motors −

• Split-Phase Induction Motors − These motors are best suited for moderate starting-torque applications like fans, washing machines, oil burners, small machine tools, etc.
• Capacitor-Start Induction Motors − These motors are suitable for applications that require relatively high-starting torque like compressors, large fans, pumps and high-inertia loads, etc.
• Capacitor-Start Capacitor-Run Induction Motors − These motors are suitable for constant torque, and vibration free applications like in hospital appliances, studio equipment and in many other appliances that are used in such areas where silence is important.