Single-Phase Single-Phase Induction MotorsInduction , Summaries of Electrical Circuit Analysis

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Chapter 18 Chapter 18

Single-Phase Single-Phase

Induction Motors Induction Motors

Construction of a single-phase induction motor

 Single-phase induction motors are very

similar to 3-phase induction motors. They

are composed of a squirrel-cage rotor

(identical to that in a 3-phase motor) and a

stator.

 The stator carries a main winding, which

creates a set of N, S poles. It also carries a

smaller auxiliary winding that only operates

during the brief period when the motor starts

up.

 The auxiliary winding has the same number

of poles as the main winding has.

Construction

 Each pole of the main

winding consists of a

group of four

concentric coils,

connected in series as

shown below.

 Adjacent poles are

connected so as to

produce alternate N, S

polarities. The empty

slot in the center of

each pole and the

partially filled slots are

used for auxiliary

winding.

Magnetomotive force distribution  (^) In order to optimize the efficiency, the magnetomotive force produced by each stator pole must be distributed sinusoidally.  (^) That is the reason for using the special number of turns (l0, 20, 25, and 30) on the four concentric coils.  (^) Let us examine the mmf created by one of the four poles when the concentric coils carry a peak current of, say, 2 amperes.  (^) For example, the 25-turn coil in slots 2 and 8, produces an mmf of 25 X 2 = 50 amperes between these slots.  (^) the 10-turn coil in slots 4 and 6 produces between these slots an mmf of 20 A.

Torque-speed characteristics  (^) Suppose the rotor is locked in a 2-pole single-phase induction motor. If an ac voltage is applied to the stator.  (^) The resulting current I s produces an ac flux^ s. The flux alternates back and forth but, unlike the flux in a 3-phase stator, no revolving field is produced.  (^) The flux induces an ac voltage in the stationary rotor which, in turn, creates large ac rotor currents.  (^) In effect, the rotor behaves like the short-circuited secondary of a transformer; consequently, the motor has no tendency to start by itself.

Torque-speed characteristics  (^) However, if we spin the rotor in one direction or the other, it will continue to rotate in the direction of spin.  (^) As a matter of fact, the rotor quickly accelerates until it reaches a speed slightly below synchronous speed.  (^) The acceleration indicates that the motor develops a positive torque as soon as it begins to turn.  (^) Following diagram shows the typical torque-speed curve when the main winding is excited. Although the starting torque is zero, the motor develops a powerful torque as it approaches synchronous speed.

Principle of operation

r does not reach its maximum value at the same time as

s. In effect, r lags 90° behind s, due to the inductance

of the rotor.

 Combined action

of s and r

produces a

revolving

magnetic field,

similar to a 3-

phase motor.

 The value of 

r

increases with

increasing speed,

almost equal to s

at synchronous

speed.

Principle of operation

 The diagrams gives a snapshot of the currents and fluxes

created respectively by the rotor and stator, at successive

intervals of time.

 We assume that the motor is running far below

synchronous speed, and so 

r

is much smaller than 

s

By observing the flux in the successive pictures, it is

obvious that the combination of s and r produces a

revolving field.

 The flux is strong

horizontally and

relatively weak vertically.

Thus, the field strength

at low speed follows the

elliptic pattern shown.

Making a Motor  (^) http://sci-toys.com/scitoys/scitoys/electro/electro.html#single  (^) http://sci-toys.com/scitoys/scitoys/electro/electro2.html#double  (^) http://sci-toys.com/scitoys/scitoys/electro/electro3.html#two_coil