Page 124 - Industrial Power Engineering and Applications Handbook

Static controls and braking of motors 6/105
-
- Consant torque region Constant HP region f

Voltage
I 1 @ =Torque curve
2 = Output curve
Armature voltage fixed I, =Armature current
field current reduced I0
Current

Speed (= f) -
Torque
N
Base meed
Base speed - It is normally the rated speed at which the rated
parameters are referred (T,, HP and V,)

Figure 6.7(a) Approximate Figure 6.7(b) Variation of torque with speed in a d.c. machine (same for an a.c.
characteristics of vital parameters machine)
after pulse width modulation

where, mathematical model. The motor is calibration* run on
no-load at different voltages and speeds and the drive is
I,- T = V, and able to establish near-accurate vital parameters of the
machine in terms of the equivalent circuit diagram shown
in Figure 1.15 earlier. With these parameters known, it is
Therefore a normal Vlfsan also be transformed into a now possible to achieve the required speed controls noted
phasor control, I, and I, being torque- and flux-pro- above. Since these parameters are fixed for a motor, the
ducing components respectively. These components are motor has to be selected according to the load duty and
represented only theoretically. In fact they are not separate may require a pre-matching with the load.
and hence the difficulty in controlling each of them more
precisely. Application
Such a control is good for machines that are required to
Variation of speed beyond the base (rated) speed operate at low speeds with a high accuracy. Now the
See Figure 6.7(b). The machine now operates in a constant phasor I,, in terms off,,,, is varied according to the speed
h.p. region. The frequency is raised but the voltage is required. Figure 6.2 now changes to Figure 6.8, which is
kept constant at its rated value (as it should not be raised a marked improvement on the earlier characteristics. The
beyond rated). The flux will diminish while I, and also torque variation with speed is now almost constant, except
I,, will remain almost the same. The torque therefore at very low speeds. The reason for poor torque at low
reduces so that the h.p. developed remains almost a speeds is the method of speed variation which is still
constant (h.p. 0~ T.W. This is also known as the field- based on V/f Now a motor’s mathematical model is used
weakening region.
In the above schemes the two quantities (I, and I,) are
not separated. Initially it was not easy to separate them ______
and the whole phasor I, was varied to achieve a speed *1 Calibration run or autotuning is a feature of the motor’s
variation. Yet close speed control was possible but the mathematical model that can establish the motor parameterb
motor’s basic parameters were essential to achieve more with its test run.
accurate speed control. Since it may not be practical to *2 If the actual motor data are available frnm the motor manufacturer,
a calibration run will not be necessary. The motor’s mathematical
obtain all the parameters of each motor promptly, the model can now be fed with the explicit data to achieve more
drive software is designed so that the name plate particulars precise speed control.
of a motor (V, I,, Nr, and kW) alone are enough to determine *3 All the motor models are implemented by mlcrocomputer
the machine’s required parameters through the motor’s software.