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Introduction to Pulse Width Modulation (PWM)
In this technique several pulses are produced in each half – cycle but the width of the pulses
is not the same as in case of multiple – pulse width modulation, however the width of each pulse is
varied in accordance with the amplitude of the sine wave reference voltage. The width of the pulse
at the center of the half – cycle is maximum and decreases on either side. The figure 6(a) shows the
generation of the output signal by comparing a sinusoidal reference signal fr with a triangular carrier
wave of frequency fc. The carrier and reference waves are mixed in a comparator and when
sinusoidal wave of has a higher magnitude than the triangular wave the comparator output is high,
otherwise it is low. This output of comparator is used to turn on the MOSFETs in the bridge
configuration of Figure 6(b), which generates the output voltage. The reference signal frequency fr
determines the output frequency fo of the inverter, and its peak amplitude Ar, controls the
modulation index M, and thereby the rms output voltage vo. Thus varying the amplitude of the sine
wave within the range of zero to Vp, where Vp is the peak of the triangular wave, controls the output
voltage. The number of pulses in each half – cycle depends on the carrier frequency. If the ratio of
these two signals (reference and carrier) is equal to m, then the number of pulses in each half –
cycle is (m - 1).
1. Objective :
To vary the speed of a single phase squirrel-cage induction motor by varying supple
frequency with the help of Pulse Width Modulator (PWM) based Inverter.
(Note: to change the frequency we change the resistance of controlling circuit.)
2. Speed Control of Induction Motors :
Induction motors are of two types - Squirrel-cage motor and Wound-rotor motor. There are
various types of speed control methods of induction motor. These are –
(i) Pole Changing,
(ii) Stator Voltage Control,
(iii) Supply Frequency Control,
(iv) Eddy-current Coupling,
Load Vs
(v) Rotor Resistance Control,
(vi) Slip Power Recovery.
(i) is applicable for squirrel-cage motor, (ii) to (iv) is applicable for both wound-rotor and
squirrel-cage motor and (v) and (vi) is applicable for wound-rotor.
For squirrel-cage type motor, here pole changing, stator voltage control and supply
frequency control methods are discussed.
2.1 Pole Changing :
For a given frequency speed is inversely proportional to number of poles. Synchronous
speed, and therefore, motor speed can be changed by changing the number of poles. Provision for
changing of number of poles has to be incorporated at the manufacturing stage and such a machine
is called “pole changing motor” or “multi-speed motor”.
In squirrel cage motor the number of poles are same as the Stator winding. So there is no
provision for changing the number of poles. But for wound rotor arrangement for changing the
number of poles in rotor is required, which complicates the machine. So it is only used for Squirrel
cage induction motor.
A simple but expensive arrangement for changing number of stator poles is to use two
separate winding which are wound for two different pole numbers. An economical and common
alternative is to use single stator winding divided into few coil groups. Changing the connections of
these coil groups change number of poles. Theoretically by dividing winding into a number of coil
group and bringing out terminals of these group a number of arrangements of different pole
numbers is obtained.
Fig. – 2 Stator phase connection for 6-poles
Figure 2(a) above shows a phase winding consisting of six coils divided into two groups –
a-b consisting of odd number coils (1, 3,5) connected in series and c-d consisting even numbered
coils (2,4,6)
connected in series. The coils can be made to carry currents in the given directions by connecting
coil groups either in series or parallel as shown in figure B and C. With this connection machine has
six poles. If the current through the coil group a-b is reversed [Fig. 3(a)], then all coils will produce
north poles. Fluxes coming out of the north poles will now find paths through Interpol spaces for
going out consequently producing south poles in Interpol spaces. The machine will now have 12
poles. Here again the direction of current through coils can be obtained by connecting two sections
a-b and c-d either in series or parallel for both pole numbers 6 and 12.