Induction Machines, the most widely
used motor in industry, have been traditionally used in open-loop control
applications, for reasons of cost, size, reliability, ruggedness, simplicity, efficiency,
less maintenance, ease of manufacture and its ability to operate in dirty or
explosive conditions. However, because the induction machine requires more
complex control methods, the dc machine has predominated in high performance
applications. With developments in Micro-processors/DSPs, power electronics and control theory, the
induction machine can now be used in high performance variable-speed
- · heating,
- · ventilation,
- · air conditioning systems,
- · waste water treatment plants,
- · blowers,
- · fans,
- · textile mills,
- · rolling mills, etc.
The induction motor speed variation can be easily achieved
for a short range by either stator voltage control or rotor resistance control.
But both of these schemes result in very low efficiencies at lower speeds. The
most efficient scheme for speed control of induction motor is by varying supply
frequency. This not only results in scheme with wide speed range but also
improves the starting performance.
If the machine is operating at speed below base speed, then
v/f ratio is to be kept constant so that flux remains constant. This retains
the torque capability of the machine at the same value. But at lower
frequencies, the torque capability decrease and this drop in
torque has to be compensated for increasing the applied voltage.
Fig. 8 (a). Speed Torque Characteristics
of Induction Motor with frequency variation
The above curve suggests that the speed control and braking operation
are available from nearly zero speed to above synchronous speed.
Fig. 8 (b). voltage and frequency variation in VSI fed Induction motor
In Fig. 8 (b) it is noted that V is kept
constant above base speed and freq. is increasing. The variable frequency
control provides good running and transient performance because of the
(a) Speed control and braking operation are
possible from zero to above base speed.
transients (starting, braking and speed reversal), the operation can be carried
out at the maximum torque with reduced current giving good dynamic response.
losses are reduced, efficiency and power factor are high as the operation is in
between synch. speed and max. torque point at
in speed from no load to full load is small.
Fig. 8 (c) shows the block diagram
of a V/f control of VSI fed three phase induction motor drive. In this
according to the reference speed input command (Nr*) the
reference frequency (f*) and reference voltage
(V*) commands are calculated such that V/f ratio maintained to be constant.
The reference commands V* and f* are given to the SPWM
generator to generate 6-PWM pulses to the three-phase voltage source inverter
which drives the three-phase induction motor.
Fig: 8 (c). Block Diagram Schematic of V/f
control of VSI fed 3-phase Induction Motor drive
Fig.8 (d). Modes of operation and variation of is, ωsl,,
T and Pm with per unit frequency
In sinusoidal pulse width modulation there are multiple pulses per
half-cycle and the width of the each pulse is varied with respect to the sine
wave magnitude corresponding to that duration. Fig 4(c) shows the gating
signals and output voltage of SPWM with unipolar switching. In this scheme, the
switches in the two legs of the full-bridge inverter are not switched
simultaneously, as in the bi-polar scheme. In this unipolar scheme the legs R,
Y and B of the full-bridge inverter are controlled separately by comparing
carrier triangular wave vcar with the three control sinusoidal
signals vc_R, vc_Y and vc_B respectively which
are displaced by 120o. This SPWM is generally used in industrial applications.
The number of pulses per half-cycle depends upon the ratio of the frequency of
carrier signal (fc) to the modulating sinusoidal signal. The
frequency of control signal or the modulating signal sets the inverter output
frequency fo and the peak magnitude of control signal controls
the modulation index ma which in turn controls the rms output
The amplitude modulation index is defined as