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8 : V/F Control Of VSI Fed Three-Phase Induction Motor

 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 applications.


  • ·        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 following features:

(a) Speed control and braking operation are possible from zero to above base speed.

(b) During transients (starting, braking and speed reversal), the operation can be carried out at the maximum torque with reduced current giving good dynamic response.

(c) Copper losses are reduced, efficiency and power factor are high as the operation is in between synch. speed and max. torque point at  all frequencies.

(d) Drop 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  K .

Sinusoidal-Pulse-Width-Modulation (SPWM)

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 voltage.

The amplitude modulation index is defined as


where, Vc  = peak magnitude of control signal (modulating sine wave).

             Vcar  = peak magnitude of carrier signal (triangular signal).

The frequency modulation ratio is defined as


where, fc = frequency of control signal (sine signal).

             fcar = frequency of carrier signal (triangular signal).



  Fig. 8 (e). SPWM generation



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