Lecture 23 Electro Mechanical System 1

Effect of Rotor resistance

Lecture 23 Electro Mechanical System 2

Summary:A high rotor resistance is desirable because it produces a high starting torque and a relatively low starting current.

Unfortunately it produces a rapid fall in speed with increasing load.

Slip at rated torque is high, the motor I2 R losses are high. Efficiency is low and motor tends to overheat.

Under running conditions it is preferable to have low resistance.

The speed decreases very less with increasing load. Slip at rated torque is small. The efficiency is high and the motor tends to run cool.

Effect of Rotor resistance

Lecture 23 Electro Mechanical System 3

We can obtain both high starting and low running resistance by using a wound rotor induction motor. Such a rotor allows us to vary the rotor resistance as desired by means of external rheostat.

Wound Rotor Motor

Lecture 23 Electro Mechanical System 4

Advantages:The locked rotor current can be drastically reduced by inserting three external resistors in series with the rotor. The locked rotor torque will be high.The speed can be varied by varying the external rotor resistance.The motor is ideally suited to accelerate high inertia loads, which require long time to bring up to speed.Under running conditions it is preferable to have a low rotor resistance.

Speed decreases much less with increasing load. Slip is small at rated torque. Efficiency is high.

Wound Rotor Motor

Lecture 23 Electro Mechanical System 5

Consider a standard 3-phase, 4-pole, wye-connected motor having a synchronous speed of 1800 rpm. Let us cut the stator in half so that:

Sector motor

Half of the winding is removed Two complete N and S poles left

(per-phase) Mount original rotor to this sector

stator, leaving air-gap If we connect the stator terminal to

3-phase, 60Hz source Rotor will move at close to 1800 rpm Voltage can be reduce to half, because now stator winding has

half the original number of turns This sector motor still deliver 20% of its rated power The sector motor produces the same revolving field as the flux in

the original 3-phase motor

Lecture 23 Electro Mechanical System 6

Sector motor can be laid out flat, without effecting shape or speed of magnetic field to make linear induction motor

Linear induction motor

The flux travel at a linear synchronous speed given by:

vs = 2 w fWhere: vs = linear synchronous speed[m/s]

w = width of one pole-pitch[m] f = frequency [Hz]Linear speed does not depend upon no. of poles but pole pitch Distance between adjacent poles is called the pole pitch

If flat squirrel cage rotor is brought near, the field drags it along Practically we use simple aluminum or copper plate as rotor Practical applications, the rotor is stationary while stator moves Like in high-speed trains

Lecture 23 Electro Mechanical System 7

Travelling Waves We usually get the impression that

when the flux reaches the end of a linear stator, there is a delay before it restarts.

This is not true, the linear induction motor produces a travel wave of flux which moves continuously and smoothly.

Diagram shows that the flux moves from left to right in a linear motor.

The flux cuts off sharply at the extremities A and B of the stator.

As fast as N and S poles disappears at the end, it builds up again at the left.

Lecture 23 Electro Mechanical System 8

Magnetic levitation Moving permanent magnet,

sweeping across a conducting ladder, tends to drag the ladder along with it

Horizontal forces is also accompanied by a vertical force which tends to push the magnet away from the ladder

Let center of N-pole is sweeping across the top of conductor 2

Maximum voltage will induce due to maximum flux density

If the magnet moves slowly current reaches its maximum when the magnet is at the top

Returning currents from 1 and 3 creates nnn and sss as shown

Front half is repelled upward while rear half is attracted downwards Due to slow motion nnn & sss poles are symmetrical with respect to

centre of the magnet, so resulting vertical force is nil

Lecture 23 Electro Mechanical System 9

Magnetic levitation Magnet moves very rapidly Current in 2 reaches max. after

the voltage attained max. Maximum voltage will induce

due to maximum flux density When current in 2 in maximum

centre of magnet is ahead of it Returning currents from 1 and

3 again creates nnn and sss The N-pole is now directly

above an nnn pole Resulting in a strong verticle

force tends to push the magnet upwards

This is called principle of magnetic levitation

This principle is used in some ultra high speed trains that glide on a magnetic cushion rather than on wheels

Lecture 23 Electro Mechanical System 10

High-speed Train

This 17 t electric train is driven by a linear motor. Motor consists of a stationary rotor and a flat stator fixed to the undercarriage of the train.

Rotor is the vertical aluminum plate mounted in the center of the track. The 3-tonne stator is energized by a 4.7 MVA electronic dc to ac inverter

whose frequency can be varied from zero to 105 Hz.

Lecture 23 Electro Mechanical System 11

Home Work

Home WorkPage 279, Example 13.5Page 280, Example 13.6Page 281, Example 13.7