single phase motors

6738 words 27 pages
Chapter (9)

Single-Phase Motors
Introduction
As the name suggests, these motors are used on single-phase supply. Singlephase motors are the most familiar of all electric motors because they are extensively used in home appliances, shops, offices etc. It is true that singlephase motors are less efficient substitute for 3-phase motors but 3-phase power is normally not available except in large commercial and industrial establishments. Since electric power was originally generated and distributed for lighting only, millions of homes were given single-phase supply. This led to the development of single-phase motors. Even where 3-phase mains are present, the single-phase supply may be obtained by using one of the three lines and the
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Therefore, single-phase induction motor is not self-starting. This fact is illustrated in Fig. (9.4 (ii)).
Note that each rotating field tends to drive the rotor in the direction in which the field rotates. Thus the point of zero slip for one field corresponds to 200% slip for the other as explained later. The value of 100% slip (standstill condition) is the same for both the fields.

229

Fig.(9.4)

(ii) Rotor running
Now assume that the rotor is started by spinning the rotor or by using auxiliary circuit, in say clockwise direction. The flux rotating in the clockwise direction is the forward rotating flux (φf) and that in the other direction is the backward rotating flux (φb). The slip w.r.t. the forward flux will be

sf = where Ns − N
=s
Ns

Ns = synchronous speed
N = speed of rotor in the direction of forward flux

The rotor rotates opposite to the rotation of the backward flux. Therefore, the slip w.r.t. the backward flux will be

sb =
=


N s − ( − N) N s + N 2 N s − N s + N
=
=
Ns
Ns
Ns
2 N s ( N s − N)

=2−s
Ns
Ns

sb = 2 − s

Thus fur forward rotating flux, slip is s (less than unity) and for backward rotating flux, the slip is 2 − s (greater than unity). Since for usual rotor resistance/reactance ratios, the torques at slips of less than unity arc greater than those at slips of more than unity, the resultant torque will be in the direction of the rotation of the

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