How do synchronous motors differ from induction motors?

The key difference is rotor speed: a synchronous motor’s rotor spins at synchronous speed with no slip, while an induction motor’s rotor spins slower than synchronous speed, creating slip needed for torque.

Why can't a synchronous motor start by itself?

A synchronous motor cannot start on its own because the rotor needs to be at synchronous speed to lock in with the stator's rotating magnetic field. At standstill, there is no torque. Therefore, an external starting mechanism is required to bring the rotor close to synchronous speed.

What causes torque in an induction motor?

In an induction motor, torque is generated when the stator’s rotating magnetic field induces currents in the rotor due to slip. These currents engage with the stator field, producing torque.

AC Motor Questions: Practice Examples with detailed Solutions

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JEE Physics

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

Feature	Synchronous Motor	Induction Motor
Speed	Constant and equal to synchronous speed	Less than synchronous speed (slip exists)
Slip	Zero slip	Always has some slip
Starting	Requires external means (e.g., damper winding or separate motor)	Self-starting (most types)
Rotor Current Source	Supplied from external source or permanent magnets	Induced by stator's magnetic field
Torque Production	Only when running at synchronous speed	Due to relative motion between stator field and rotor
Efficiency	High at constant load	Generally lower than synchronous motor
Applications	Precision timing devices, synchronous clocks, power factor correction	Fans, pumps, compressors, conveyors

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:

$N_{s} = \frac{120 f}{P}$

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

5.0Characteristics of AC Motors

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

6.0Advantages and Disadvantages of AC Motor

Advantages	Disadvantages
Simple and rugged construction	Starting torque may be low (for some types)
Low maintenance	Slip causes energy loss (induction motors)
High efficiency and reliability	Complex starting mechanism for synchronous motors
Wide range of power and sizes	Speed control is more complex

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

Synchronous Speed , $N_{s} = \frac{120 f}{P}$
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, $N_{r} = rotor speed (RPM), s = slip (fraction or %)$
Rotor Speed: $N_{r} = N_{s} (1 - s)$
Torque Equation = $τ = \frac{P _{out}}{W}$ $τ = Torque, P_{out} = output power, ω = \frac{2 π N _{r}}{60} = angular speed (rad / s)$
Power Factor: It is the cosine of the angle between voltage and current phases. $Power Factor = cos Power Factor = cos ϕ$

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