Page 118 - Industrial Power Engineering and Applications Handbook

Static controls and braking of motors 6/99
6.1 Speed control in squirrel The method for speed control as discussed earlier are
cage motors only conventional and can only provide a fixed speed
variation say, from 3000 r.p.m. to 1500 r.p.m. to 750
r.p.m. or vice versa. They cannot provide a smoother
Speed control in slip-ring motors has been discussed in speed variation between any two speeds. The application
the previous chapter. Squirrel cage motors have limitations of variable voltage is also not practical nor advisable, for
in their speed control in view of their fixed rotor para- it means a poor performance by the machine at lower
meters. Speed variation, in fixed steps, however, is possible voltages. whereas a higher voltage (more than 5% of the
in such motors if the stator is wound for multipoles and rated) is not permissible. Moreover, through this method
such motors are known as pole changing motors. Up to the speed can be varied only within a very limited span
four different speeds can be achieved in such motors due to very unstable conditions below the T,,, region,
economically, in combinations of 1/4,4/6,4/X, 6/8,6/1’.?, and a more than proportionate reduction in the h.p.
1/4/6,4/6/X, 1/4/6/12 and 4/6/8/13 poles etc. or any other developed. The torque also reduces in square proportion
similar combination. For limitation in the motor size and of the voltage. For such applications, therefore. which
flux distribution, winding sets of more than two are not required smoother speed variation and over a wide range,
recommended. The two windings can be arranged for one had no choice but to select d.c. drives. These drives
two. three or (maximum) four different speeds. were costly and needed higher maintenance because
commutators, sliprings and brushes etc. caused continuous
6.1.1 One winding arcing and required constant checks and maintenance as
well as more downtime, which a process industry could
The hgle winding can be connected in delta/double least afford. Systems were used that required very elaborate
star (NYV) to give two combinations of poles in the ratio arrangements, using two or more a.c. machines, rendering
of 2: I. i.e. 4/2. 8/4 or 1Y6 poles etc. as shown in Table the whole system very cumbersome, vulnerable and yet
6.1. more expensive. Since the speed was normally changed
through the variation in frequency (Nm.0, these systems
6.1.2 Two windings were basically frequency changers (converters) and were
known as
When more than two or non-multiple speeds are required
(2.g. 46 or 6/8 etc.) then two windings are necessary.
Each can further be connected in NYYas noted above to Cascade connections (concatenation)
give one additional speed for each winding and can thus Use of two motors and the prime mover was a slip-
Le arranged Tor three or four different speeds as shown ring motor.
in Table 6. I. Frequency converters
Schrage type motor (commutator brush shifting
arrangement)
6.2 Speed control through Leblanc or Scherbius Advancers.
solid-state technology
These systems were evolved to provide a variable -
frequency supply source to feed directly the stator
In the following text we have discussed how, with the terminals of the a.c. motor or its rotor through the slip-
application of varying supply parameters (V andf), one
can alter the characteristics of a fixed parameter induction rings. The motors had to be invariably a combination of
two or more slip-ring motors to receive the rotor frequency
motor in any desired way. We then deal with the application
of solid-state technology to obtain the variations in the voltage from the other machine or feed back the rotor
fixed supply parameters to achieve the required controls frequency voltage to another machine. The easiest method
in an a.c. machine. was to have a variable-frequency supply source, which
The static drives also provide a few more advantages was not possible, unless the supply source itself was
such as captive and specified for this drive alone or a combination
of these drives on the same bus.
There was thus a practical limitation in employing an
I They transform an unbalanced supply system auto- a.c. motor for all such applications that required frequent
matically to a balanced supply system through the speed variation. Since these drives are no longer in practice.
switching logistics of the ICBTs* or the SCRs*. The we have not considered it relevant to provide more details
feature is termed dynamic phase balancing. of these systems.
2 Since the starting inrush current is kept moderate for The above methods provide speed variation in steps,
all types of drives. it can economize not only on ratings as in squirrel cage motors or in two machines or more,
of the switchgears and cables but also on the size of as in frequency converters, and cannot be used for a
the generator when a captive power is required to process line, which requires frequent precise speed
feed the load. controls. Until a few years ago there was no other option
with all such applications and they had to be fitted with
d.c. motors only. D.C. motors possess the remarkable
*IGBTs - Insulated gate bipolar transistors
SCRs - Silicon-controlled rectifiers (thyristors) ability of precise speed control through their separate
Both are discussed in the subsequent text. armature and field controls. In d.c. motors the speed