Induction motor, Electrical Engineering

Assignment Help:

A very widely used alternative form of synchronous motor is the 'induction motor'. This has the advantage that it does not require an auxiliary motor to run the rotor up to synchronous speed.

895_induction motor.png

The rotor consists of stout copper (or aluminium) conductors arranged in the form of a cylindrical cage (commonly known as a 'squirrel cage' rotor). These are laid in slots in a soft iron core that focuses the magnetic flux produced by the stator across the bars of the cage and all the bars are electrically connected together at each end by copper (or aluminium) rings.

Three stator windings arranged at 120° to each other around the rotor are energised by the three phases of an ac supply and this creates a magnetic field that rotates at the frequency of the supply.

1943_induction motor1.png

With the rotor stationary, the rotating magnetic field induces an emf in the cage that in turn drives a current through its conductors (an 'eddy' current). This current reacts against the magnetic field to produce a torque that causes the rotor to turn in the direction of the rotating magnetic field.

If the rotor were to rotate at the same speed as the rotating magnetic field, then it would not experience any  change in the magnetic field and no emf would be induced in the rotor.

No current (and therefore no torque either) would then be induced in the rotor.

Some torque will always be needed to overcome mechanical losses (friction, air resistance etc) in addition to any mechanical load applied to the motor, so in practice the rotor always turns more slowly than the rotating magnetic field.

The fractional difference in speed between the rotational speed of the magnetic field (the synchronous speed) and that of the rotor is called the 'slip'.

Slip = Synchronous speed - Rotor speed

                Synchronous speed

NOTE:

(1)  The larger the torque applied to the motor, the greater the slip required to produce the torque needed.

(2)  Because of the slip, the frequency of the induced currents in the rotor is  less than that of the applied stator voltage. The induced voltage is proportional to the rate of change of the magnetic field strength as 'seen' by the rotating armature.

(3)  If the slip is small, the frequency of the currents flowing in the rotor is low and so the effect of any inductance of the rotor is negligible. (Z=j  L). In this case, only the resistance of the rotor limits the current in the rotor (and hence the torque produced by the motor).

So: Torque  =    K.S/R

where K is a constant for a given machine.

Advantages:  no brushes or slip rings are required - relatively easy and cheap to make. Reliable (no sliding electrical contacts). Smooth torque output.

Disadvantages:  operates at one speed (determined by the frequency of the three-phase ac supply used). Needs electronic controllers to produce variable frequency supplies if required to operate at variable speeds.

Normally needs three-phase supplies (it is possible to use single phase supplies from which other phases can be derived by phase-shifting circuits).

Applications: aircraft fuel pumps, (that are immersed in fuel to aid cooling), fans, conveyer belt drives, pumps etc.


Related Discussions:- Induction motor

Define causal system - discrete time systems, Define Causal System - Discre...

Define Causal System - Discrete Time Systems? A causal system is one which produces an output only when there is an input. All physical systems are causal. In general, a causal

Draw timing diagram of synchronous counter, Q. Consider the synchronous cou...

Q. Consider the synchronous counter shown in Figure of the text. (a) Draw its timing diagram. (b) Show the implementation of the same synchronous counter using D flip-flops.

Example of binary subtraction of negative number , Example of binary subtra...

Example of binary subtraction of Negative number Case 2.   Negative number is more than  the positive number ( i e, answer  is negative)  Example : Subtract 92 10 ( 101

SOLID MENSURATION, the base of isosceles triangle and the altitude drawn fr...

the base of isosceles triangle and the altitude drawn from one of the congruent sides are equal to 18cm and 15cm, respectively. Find the lengths of the sides of the triangle.

Objectives of national electricity policies, OBJECTIVES OF NEP: Ac...

OBJECTIVES OF NEP: Access to Electricity for whole households within the next five years. Availability of Power to be ensured to meet the demand through 2012. Pe

Simulink help, I want simulink model for carrier based pwm method

I want simulink model for carrier based pwm method

Determine the current and voltage for n-channel jfet, Q. An n-channel JFET ...

Q. An n-channel JFET with A = 300 V, P = 2 V, and I DSS = 10 mA is to be operated in the active mode. Determine i D when v DS = 10 V and v GS =-0.5V.

Met, AC and DC distribution onboard ships

AC and DC distribution onboard ships

Armature winding - motor control , Armature Winding In this  winding t...

Armature Winding In this  winding the working  emf is induced by the working  flux. The current  which is  dependent on the load  called current.  The windings  that handle

Write Your Message!

Captcha
Free Assignment Quote

Assured A++ Grade

Get guaranteed satisfaction & time on delivery in every assignment order you paid with us! We ensure premium quality solution document along with free turntin report!

All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd