Page 122 - Industrial Power Engineering and Applications Handbook

Static controls and braking of motors 611 03

1 10 20 30 40 50 60 70 80 90 Hz
Below the base speed -Above the base speed
0 0 25f1 O.5Of1 0 75f1 1 OOf,
Frequency (Speed) 4
Ns4 Ns3 Ns2 Ns 1
Speed
* Drooping torque at lower speeds. At higher 0 Tst1 = 1 0 Tst2 = 1
speeds too, the torque profile is variable
1 1
0 Ts13= 0.5f 0 Ts,4= 0.25f
(V/f control &,, constant)
Figure 6.2 Actual speed-torque characteristics by a conventional
frequency control (V/f control) Figure 6.3 Speed variation at constant torque

Speed -
s= 1

(Variation in frequency)
Figure 6.4 Speed variation at constant HP

Application during a load variation (see Example 6.1 for more clarity).
Rating, however, is no bar.
In a V/fcontrol generally, only the frequency is vaned to
obtain the required speed control. Based on this frequency,
the switching logistics of the inverter control circuit control 6.4 Phasor (vector) control
the inverter’s output voltage using the PWM technique
to maintain the same ratio of V/f. A Vlfcontrol is, however, A simple V/fcontrol, as discussed above, will have the
not suitable at lower speeds. Their application is limited following limitations:
to fan, pump and compressor-type loads only, where speed
rcgulation need not be accurate, and their low-speed Control at very low speeds is not possible.
performance or transient response is not critical and they 0 Speed control may not be very accurate.
are also not required to operate at very low speeds. They 0 Response time may not be commensurate with the
are primarily used for soft starts and to conserve energy system’s fast-changing needs.