AC Motor Questions

AC motors, or Alternating Current motors, are common in both homes and industries because they’re dependable and efficient. They run on AC power and come in two main types: synchronous and induction (also called asynchronous) motors. To really get how AC motors work, it helps to understand their design, operation, and how we measure their performance using basics like speed, torque, slip, power, and efficiency. These motors are key to converting electrical energy into mechanical motion, powering everyday devices like fans, pumps, conveyors, and many industrial machines.

1.0Introduction to AC Motor

AC motors are machines that turn electrical energy into mechanical motion using alternating current. They’re popular in industries and homes because they’re durable, efficient, and easy to maintain. The basic idea is electromagnetic induction, where a rotating magnetic field makes the rotor spin.

2.0Types of AC Motors

3.0Construction of AC Motors

Stator: It is the stationary part of the motor and consists of laminated steel cores and insulated coils connected to the AC power supply. The stator creates a rotating magnetic field. 

Rotor

The rotor is the rotating part inside the motor. It can be of two types:

a. Squirrel Cage Rotor (common in induction motors)

b. Wound Rotor (has windings connected through slip rings)

4.0Working Principle of AC Motors

According to Faraday's law, a changing magnetic field induces an electromotive force (EMF) in a conductor. In an AC motor, the AC in the stator windings creates a rotating magnetic field. This changing magnetic field cuts through the rotor conductors, inducing a current in the rotor, which is essential for motor operation.

Rotating Magnetic Field

The three-phase stator windings create a rotating magnetic field at synchronous speed, $N_s$ given by:

$N_s=\frac{120f}{P}$

Ns=synchronous speed (RPM),f=supply frequency (Hz),P=number of poles

5.0Characteristics of AC Motors

1. Starting Torque: Induction motors have moderate starting torque.
2. Speed Regulation: Synchronous motors have constant speed; induction motors vary slightly under load.
3. Efficiency: AC motors have high efficiency, especially three-phase types.
4. Power Factor: Synchronous motors can improve power factor; induction motors usually have lagging power factor.

6.0Advantages and Disadvantages of AC Motor

7.0Applications of AC Motors

• Industrial drives (pumps, fans, compressors)
• Household appliances (washing machines, air conditioners)
• Elevators and conveyors
• Electric vehicles (synchronous motors)
• HVAC systems

8.0Key Formulas in AC Motors

1. Synchronous Speed , $N_s=\frac{120f}{P}$
2. Slip of Induction Motor: Slip s is the difference between synchronous speed and rotor speed$N_s=\frac{N_s-N_r}{N_s}$ Where, $N_r=\text{ rotor speed }(RPM),s=\mathrm{slip}(\text{ fraction or }\%)$
3. Rotor Speed: $N_r=N_s(1-s)$
4. Torque Equation = $\tau =\frac{P_{\text{out }}}{W}$$\tau =\text{ Torque, }{P}_{\text{out }}=\text{ output power, }\omega =\frac{2\pi N_r}{60}=\text{ angular speed }(\mathrm{rad}/\mathrm{s})$
5. Power Factor: It is the cosine of the angle between voltage and current phases.$\text{Power Factor}=cos\text{ Power Factor }=\cos \varphi$