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

Plc - programmable logical controller , PLC ( Programmable Logical  Contr...

PLC ( Programmable Logical  Controller ) PLC  stand for programmable logical  controller.  PLC   is an  industrial computer  used to monitor inputs and depending upon  their  m

Determine the node voltage by using kcl, Determine the node voltage by usin...

Determine the node voltage by using KCL: Determine the node voltage V and then current flowing through each element by using KCL. Solution Apply KCL at the node whose

Realize the following boolean expression, Q. Using a minimum number of NAND...

Q. Using a minimum number of NAND gates, realize the following Boolean expression: F(A,B,C) = ∑ mi (0, 3, 4, 5, 7).

What is resistivity, What is resistivity? Resistivity of a material ma...

What is resistivity? Resistivity of a material may be explained as the resistance of the material having unit length and also unit cross sectional area. Unit of it is ohm-m.

Draw and explain the r-2r ladder dac, Question 01 Convert the following bi...

Question 01 Convert the following binary numbers to its specified number system 111.101 (2) to octal 10111 (2) to decimal 0.1010 (2) to hexadecimal 1011.11 (2)

Show npn common emitter amplifier, Q. Show NPN Common Emitter Amplifier? ...

Q. Show NPN Common Emitter Amplifier? The common emitter configuration lends itself to voltage amplification and is the most common configuration for transistor amplifiers.

Meter field testing - sealing points, Meter Field Testing - Sealing Points ...

Meter Field Testing - Sealing Points The meter seal should be tamper proof. The consumer should be briefed about seals. The sealing of all metering systems should be completed

Scr, What is finger voltage in a scr?

What is finger voltage in a scr?

Explain how memory paging is used for memory addressing, Explain how memory...

Explain how memory paging is used for memory addressing. The memory paging mechanism placed inside the 80386 and above permits any physical memory location to be assigned to se

Statics, a mass of 20 kg is lifted u p to the roof of a cliff 100 m high.de...

a mass of 20 kg is lifted u p to the roof of a cliff 100 m high.determie Ep of the mass while it is at the top

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