Page 120 - Industrial Power Engineering and Applications Handbook

Static controls and braking of motors 6/101
cheap, robust and is devoid of any such operating 6.3 Vlfcontrol (speed control at
limitations and, has an obvious advantage over d.c. constant torque)
machines. It alone can provide an immediate answer to
such limitations. With the advent of static technology as
discussed later, it has now become possible to make use This is also known as variable frequency control. Consider
of cage motors with the same ease and accuracy of speed the following equations from Chapter 1 :
control and that are even better than d.c. machines. Static
drives response extremely fast as they can be micro-
processor based. They can compute process data and
provide system corrections almost instantly (called ‘real-
time processing’) as fast as within 1-2 ms and even less.
In Table 6.5 we show a broad comparison between a d.c. and el = 4.44 KCL’ . $,,, . Z, ‘,f, (1 3)
machine and a static drive using cage motors. It gives an
idea of applying static technology to all process :. for the same supply voltage VI
requirements with more ease and even better accuracy.
With the advent of this technology, the demand for d.c.
machines is now in decline as noted in Section 6.19.

6.2.1 Theory of application

The application of solid-state technology for the speed i.e. for the same design parameters ($,,, remaining the
control of a.c. motors is based on the fact that the same) and ratio ezlf,, the torque of the motor, T, will
characteristics and performance of an induction motor remain constant. Since both e2 andji are functions of the
can now be varied, which until a few years ago were supply system, a variation in V, and f can alter the
considered fixed and uncontrollable. This concept is now performance and the speed-torque characteristics of a
a matter of the past. With the advent of solid-state motor as required, at constant torque. By varying the
technology, which was introduced around 1970 for frequency smoothly from a higher value to a lower one
industrial application, the motor’s parameters and therefore or vice versa (within zero to rated). an almost straight
its performance can now be varied by varying the supply line torque can be achieved (Fig. 6.3). This type of a
parameters of the system, for example the voltage and control is termed variable voltage, variable frequency
frequency in cage motors, and rotor resistance or rotor (v.v.v.f or Vu) control. At speeds lower than rated, the
current in slip-ring motors, as discussed in Chapter 1.
This technology can also provide a varying resistance in natural cooling may be affected, more so at very low
speeds, and may require an appropriate derating of the
the rotor circuit of a slip-ring motor by varying the rotor machine or provision of an external or forced cooling.
current as discussed in Section 6.16.3 without the loss of The practice of a few manufacturers, up to medium-
power in the external resistance. It is thus also suitable sized motors, is to provide a cooling fan with separate
to provide speed control in a slip-ring motor. Speed control power connections so that the cooling is not affected at
of slip-ring motors with the use of solid-state technology lower speeds.
is popularly known as dip recovery systems, as the slip
power can also be fed back to the source of supply through Note The speed of the motor can be varied by varying the frequency
a solid-state feedback converter bridge, discussed later. alone but this does not provide satisfactory performance. A variation
in frequency causes an inverse variation in the flux. $I,,,, for the
same system voltage. The strength of magnetic field, $I,,,, develops,
6.2.2 Effects of variable supply parameters on the torque and moves the rotor, but at lower speeds, f would be
the performance of an induction motor reduced, which would raise $I,,, and lead the magnetic circuit to
saturation. For higher speeds, f would be raised, but that would
Here we analyse the effect of variation in the incoming reduce @,,,, which would adversely diminish the torque. Hence
supply parameters (voltage and frequency) on the charac- frequency variation alone is not recommended practice for speed
teristics and performance of an induction motor (such as control. The recommended practice is to keep Vlf as constant, to
its flux density, speed, torque, h.p., etc). We also assess maintain the motor’s vital operating parameters, i.e. it\ torque and
&,, within acceptable limits.
the effect of variation of one parameter on the other, and
then choose the most appropriate solid-state scheme to The above is valid for speed variation from zero to the
achieve a required performance. We generally discuss rated speed. For speed variations beyond the rated speed,
the following schemes: the theory of VI’’ will not work. Because to maintain the
same ratio of Vlfwould mean a rise in the applied voltage
1 Vlfcontrol (speed control at constant torque) which is not permissible beyond the rated voltage, and
2 Phasor (vector) control which has already been attained by reaching the rated
~ Single-phasor (vector) control speed. The speed beyond the rated is therefore obtained
- Field-oriented control (FOC), commonly known as by raising the supply frequency alone (in other words,
double-phasor or phasor (vector) control and by weakening the field, $,,,) and maintaining the voltage
- Direct torque control (DTC) as constant at its rated value. We can thus achieve a