612 ROTATING MACHINES3 phase, 50-Hz supplyDC linkConverter 1 Converter 2
(both fully controlled)sfsfsInduction
generatorFigure 13.6.3Double-output induc-
tion-generator scheme.generation over a wide range of shaft speeds, in the slip range of−1to+1, is possible. The net
output power is maximized by varying the firing angles of both converters.
Induction generates are also employed nowadays for power generation in conjunction with
helical water turbines developed for operating in water streams.Problems
13.1.1A three-phase, 50-Hz induction motor has a full-
load speed of 700 r/min and a no-load speed of
740 r/min.
(a) How many poles does the machine have?
(b) Find the slip and the rotor frequency at full
load.
(c) What is the speed of the rotor field at full
load (i) with respect to the rotor? and (ii)
with respect to the stator?
13.1.2A three-phase, 60-Hz induction motor runs at
almost 1800 r/min at no load, and at 1710 r/min
at full load.
(a) How many poles does the motor have?
(b) What is the per-unit slip at full load?
(c) What is the frequency of rotor voltages at full
load?
(d) At full load, find the speed of (i) the rotor
field with respect to the rotor, (ii) the rotor
field with respect to the stator, and (iii) the
rotor field with respect to the stator field.*13.1.3A four-pole, three-phase induction motor is ener-
gized from a 60-Hz supply. It is running at a load
condition for which the slip is 0.03. Determine:
(a) The rotor speed in r/min.
(b) The rotor current frequency in Hz.
(c) The speed of the rotor rotating magnetic field
with respect to the stator frame in r/min.
(d) The speed of the rotor rotating magnetic field
with respect to the stator rotating magnetic
field in r/min.
13.1.4Consider a three-phase induction motor with a
normal torque–speed characteristic. Neglecting
the effects of stator resistance and leakage react-
ance, discuss the approximate effect on the char-
acteristic, if:
(a) The applied voltage and frequency are
halved.
(b) Only the applied voltage is halved, but the
frequency is at its normal value.
13.1.5Induction motors are often braked rapidly by
a technique known asplugging, which is the
reversal of the phase sequence of the voltage
supplying the motor. Assume that a motor with
four poles is operating at 1750 r/min from an
infinite bus (a load-independent voltage supply)
at 60 Hz. Two of the stator supply leads are
suddenly interchanged.
(a) Find the new slip.
(b) Calculate the new rotor current frequency.
13.1.6A four-pole, three-phase, wound-rotor induction
machine is to be used as a variable-frequency