Induction Motors and Synchronous Speed

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Questions and Answers

Why can't an induction motor (IM) run precisely at synchronous speed?

The motor's control system limits the speed to prevent damage at synchronous speed.

At synchronous speed, the rotating magnetic field does not cut the rotor, leading to zero induced current and torque. (correct)

Frictional losses within the motor increase exponentially as synchronous speed is approached.

The rotor's inertia always prevents it from reaching synchronous speed.

What is the consequence of the rotor rotating at the synchronous speed?

Maximum torque is generated, leading to potential motor damage.

The motor operates at peak efficiency.

No torque is generated, causing the rotor speed to decrease. (correct)

Induced current flows in the rotor, producing a strong magnetic flux.

What does the slip speed ($n_{slip}$) represent in an induction motor?

The speed of the magnetic field relative to the stator windings.

The difference between the synchronous speed and the mechanical shaft speed of the motor. (correct)

The absolute speed of the rotor.

The rate at which the motor accelerates from standstill.

If an induction motor has a synchronous speed of 1800 RPM and a rotor speed of 1750 RPM, what is the slip speed?

50 RPM (B) Signup and view all the answers

Besides slip speed, what other metric describes the relative motion between the rotor and the rotating magnetic field in an induction motor?

Slip (C) Signup and view all the answers

Which of the following isn't an advantage of a squirrel cage rotor?

Ability to insert resistance in the rotor circuit. (D) Signup and view all the answers

What is a primary disadvantage of a squirrel cage rotor regarding starting torque?

It is impossible to insert resistance in the rotor circuit to increase starting torque. (C) Signup and view all the answers

According to Faraday's law of electromagnetic induction, the magnitude of the induced EMF in the rotor windings is proportional to which factor?

The relative speed between the rotating flux and the rotor conductors. (A) Signup and view all the answers

What happens when a 3-phase supply feeds the stator windings?

A magnetic flux of constant magnitude rotating at synchronous speed is set up. (C) Signup and view all the answers

Which of the factors affect the synchronous speed of an induction motor?

Number of poles and frequency of the supply. (C) Signup and view all the answers

What operational characteristic is improved by smaller rotor overhang leakage in squirrel cage rotors?

Better power factor and greater pull out torque. (B) Signup and view all the answers

What rule determines the direction of the induced EMF in the rotor conductors?

Fleming’s Right-hand rule. (C) Signup and view all the answers

Compared to a wound rotor motor, which is true of a cage rotor motor?

Smaller starting torque and larger starting current. (B) Signup and view all the answers

What is the value of slip (S) when the rotor of an induction motor runs at synchronous speed?

S = 0 (C) Signup and view all the answers

An induction motor is operating with a slip of $S = 0.05$. If the synchronous speed ($n_{sync}$) is 1200 RPM, what is the mechanical speed ($n_m$) of the rotor?

$n_m = 1140$ RPM (C) Signup and view all the answers

In an induction motor, if the rotor is stationary, what is the value of the slip (S)?

S = 1 (A) Signup and view all the answers

How does an induction motor (IM) differ from a transformer in terms of operation?

In an IM, the secondary windings (rotor) can move, while in a transformer, both primary and secondary windings are stationary (B) Signup and view all the answers

If the synchronous angular velocity of an induction motor is $\omega_{sync} = 100$ rad/s and the rotor angular velocity is $\omega_m = 95$ rad/s, what is the slip (S) in percentage?

S = 5% (A) Signup and view all the answers

During the starting of a large induction motor, what is one of the primary concerns regarding the high starting current?

Excessive line voltage drops due to the large current drawn (A) Signup and view all the answers

An induction motor is directly connected to the power line during startup. Approximately how many times the rated current can flow through the stator windings?

5 to 8 times (A) Signup and view all the answers

Which principle is common to both induction motors and transformers?

Production of induced voltage via electromagnetic induction (D) Signup and view all the answers

What is one of the main advantages of a squirrel cage induction motor compared to a wound rotor motor?

No need for slip rings (D) Signup and view all the answers

What is a key feature of the wound rotor induction motor during the starting process?

It connects to a three-phase star connected rheostat (C) Signup and view all the answers

Which of the following statements about squirrel cage motors is true?

They have no slip rings or brush gear. (D) Signup and view all the answers

In wound rotor induction motors, what happens to the slip rings during normal operation?

They are automatically short-circuited. (B) Signup and view all the answers

What is a characteristic of squirrel cage induction motors in terms of maintenance?

They have low maintenance requirements. (D) Signup and view all the answers

What is the purpose of connecting additional resistance in the rotor circuit of a wound rotor motor during start-up?

To increase the starting torque (D) Signup and view all the answers

Which rotor type typically allows for easier control of operating speeds?

Wound rotor (D) Signup and view all the answers

Which of the following statements best describes the construction of squirrel cage induction motors?

They feature simpler construction with low cost. (D) Signup and view all the answers

What is the primary use of induction machines in industry?

As motors for various kinds of industrial drives (D) Signup and view all the answers

Which of the following is an advantage of induction machines?

Low cost and high reliability (B) Signup and view all the answers

What is a significant disadvantage of induction machines?

Their speed cannot be varied without efficiency loss. (C) Signup and view all the answers

Which feature characterizes the construction of a squirrel cage induction motor?

It has a simple and rugged construction. (A) Signup and view all the answers

How does an induction machine start up?

It starts from rest without needing an extra starting motor. (B) Signup and view all the answers

Which statement is true about the rotor winding in induction machines?

Both stator and rotor windings carry alternating current. (D) Signup and view all the answers

What is a common characteristic of the power factor of induction machines?

It is reasonably good. (A) Signup and view all the answers

What is typically not a satisfactory performance capability for induction machines when used as generators?

Supplying electrical power to a load (D) Signup and view all the answers

What happens to the speed of a shunt motor as the load increases?

The speed decreases. (C) Signup and view all the answers

Which characteristic is true about a lightly loaded induction motor?

Runs at low and lagging power factor. (B) Signup and view all the answers

What is one disadvantage of shunt motors in terms of starting torque?

It is somewhat inferior to that of a d.c shunt motor. (D) Signup and view all the answers

What construction feature allows the stator in an induction motor to produce a magnetic flux?

A 3-phase winding supplied with 3-phase currents. (B) Signup and view all the answers

Which of the following statements about the stator's pole count and speed is accurate?

Greater number of poles, lesser speed. (D) Signup and view all the answers

What is a common use for large three-phase induction motors?

Industrial applications like pumps and compressors. (D) Signup and view all the answers

What construction aspect helps reduce eddy currents in the induction motor's stator?

Stacked lamination sheets of 0.4 - 0.5 mm thickness. (D) Signup and view all the answers

What is a typical issue associated with shunt motors in terms of speed control?

Their speed is not easily controlled. (A) Signup and view all the answers

Study Notes

Induction Machines

Induction machines are the most rugged and widely used in industry.
They consist of a stator and a rotor.
Both stator and rotor windings carry alternating current (AC).
The machine can operate as both a motor and a generator.
However, it is not usually used as a generator for supplying electrical power.
The induction machine's generator characteristic is not satisfactory for most applications.
Poly-phase induction motors are extensively used in industrial drives.
They offer several advantages and disadvantages.

Advantages

Simple and extremely rugged construction (especially squirrel cage type).
Low cost and highly reliable.
High efficiency in normal running, reducing frictional losses (no brushes needed).
Reasonably good power factor.
Requires minimal maintenance.
Starts from rest without external starting. Simple starting for squirrel cage type motors.

Disadvantages

Speed cannot be easily changed without sacrificing efficiency.
Speed decreases with increased load (similar to DC shunt motor).
Speed control is not easily achieved.
High starting current.
Low power factor during light loads.
Lower starting torque compared to DC shunt motors.

Classification of Electrical Machines

Electrical machines are classified as static (e.g. transformers) and rotating (e.g. DC and AC motors).
Rotating machines are further grouped as DC and AC machines.
AC machines are categorized as synchronous and asynchronous machines (induction machines).
Induction machines come in single-phase and three-phase varieties.

Construction

Induction motors have two main components: stator and rotor.
The stator is a stationary steel frame supporting a hollow, cylindrical core built from stacked laminations(0.4-0.5mm thick).
Slots are evenly spaced in the laminated core to house the stator winding.
The laminations are insulated from each other (e.g varnish, oxide).
The stator houses three-phase winding fed from a three-phase AC supply.
The number of poles determines the speed—more poles, slower speed.
The stator windings generate a magnetic field that rotates at synchronous speed.

Rotor

The rotor is composed of stacked laminations producing slots to house the rotor winding.
It can be of two types: wound-rotor or squirrel-cage.
Squirrel-cage rotor: consists of copper or aluminum bars shorted at both ends by conducting rings. These bars are permanently short-circuited, so no external resistance is required for starting. The rotor slots are skewed to reduce magnetic hum.
Wound-rotor: similar winding structure as the stator, but the rotor winding terminals are connected to slip rings, allowing for external resistance addition during starting, for enhanced starting torque, lower starting current and speed adjustment.

Principle of Operation

Three-phase stator windings, when energized by a three-phase supply, create a rotating magnetic field of constant magnitude.
This rotating field interacts with the rotor, inducing an emf in the rotor windings.
The relative speed between the rotating field and the rotor windings creates the induced current.
The direction of this induced current is determined by Fleming's right-hand rule.

Induction Motor Speed and Slip

The rotor never rotates at the synchronous speed.
Slip: the difference between synchronous speed and the rotor speed.
Slip is defined in terms of percentage (the difference between synchronous speed and motor shaft speed, expressed as a percentage of synchronous speed).
Slip = [(synchronous speed - shaft speed)/synchronous speed] x 100%
If the rotor is stationary, slip = 1

Starting of Induction Motors

Most induction motors start directly across the line, but large motors may experience excessive voltage drops and low starting torque.
Reduced voltage starting methods (e.g. stator resistance, autotransformer, star-delta) are employed for large motors for improved performance during startup.
Wound-rotor motors allow for external resistance addition during starting by means of slip rings (adjustable starting torque and current). Once running at rated speed the resistance are shorted out.
Different starting methods are employed depending on the motor size, power line capacity, and load type.

Transformer

Induction motors and transformers both operate on the principle of induced voltage.
A transformer's induced voltage in the secondary winding is directly related to that of the primary winding.
The induction motor's rotor winding voltage varies due to its rotation, unlike the transformer's secondary winding.

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Description

This quiz explores the operational characteristics of induction motors, focusing on synchronous speed and slip speed. It covers fundamental aspects such as the consequences of rotor speed, advantages and disadvantages of squirrel cage rotors, and the effects of a 3-phase supply on motor performance. Test your knowledge on these key concepts in electrical engineering!