Page 154 - Industrial Power Engineering and Applications Handbook

6/134 Industrial Power Engineering and Applications Handbook
chapter). Their severity increases when they cause a With the availability of phasor control technology, by
reflection (doubling phenomenon) at the motor terminals. which one can separate out the active and magnetizing
The amplitude of the reflected wave will depend upon components of the motor’s stator current and vary
the length of the interconnecting cables, between the them individually, it is now possible to achieve higher
inverter and the machine and the switching frequency of dynamic performance and accuracy of speed control
the inverter unit. The amplitude of the wave after reflection in an a.c. machine similar to and even better than a
may exceed the BIL (basic insulation level, Table 1 1.4 separately excited d.c. machine.
for LT and Table 11.6 for HT motors) of the machine, With this technology it is now possible to achieve
particularly when the machine is old and its insulation extremely accurate speed control of the order of
has deteriorated (Section 9.2), or when it is required to f 0.01% to f 0.001%. To achieve such high accuracy
perform frequent switching operations. In IGBTs in speed control, closed-loop feedback control systems
particularly, the rise time is too short and the surges and microprocessor-based control logistics can be
behave like an FOW (front of wave, Section 17.3.3), introduced into the inverter control scheme to sense,
which are all the more dangerous for the end turns of the monitor and control the variable parameters of the
connected machine. The length of cable from the inverter motor to very precise limits.
to the machine plays a vital role. The longer the cable, A very wide range of speed controls is available through
the higher will be the amplitude of the voltage surge at this technology as it is possible to vary frequency on
the motor terminals. This aspect is discussed in greater both sides (k) of the rated frequency.
detail in Section 18.6.2. The leading manufacturers of Controls are available in the range: IGBTs 1600 V,
static drives specify, as a standard practice, the amplitude 2000 A and thyristors 10 kV, 3000 A (ratings are only
and the rise time of the switching surges of their devices indicative) and can cover the entire voltage ranges
at a particular voltage and switching frequency of their and ratings of a motor.
inverter unit (normally 2-4 kHz) and the maximum safe
cable lengths. While most installations may not need
separate surge protection, it is advisable to take precautions 6.15 Energy conservation through
against any contingency during actual operation to protect solid-state technology
the machine against these surges under the most onerous
operating conditions. The remedies are the same as those
discussed in Section 17.10 on the protection of electric While the motor is operating under loaded
motors. For example, surge capacitors for most of the
motors will prove sufficient and economical to protect In the various types of static drives discussed so far, the
the machine by taming the steepness of fast rising surges at supply voltage would adjust automatically at a level just
the motor terminals. (See curves 1 and 2 of Figure 17.21 .) sufficient to drive the motor to meet its load requirements.
The surge capacitor may be provided with a discharge Hence it is not necessary for the motor to be applied
resistance as standard practice, as also noted in the snubber with full voltage at all times: the voltage adjusts with the
circuits. The resistance would help the capacitor to load. This is an in-built ability of a static drive that would
discharge quickly and prepare for the next operation. save energy and losses. One would appreciate that most
In addition, it would also help to dampen the amplitude of the industrial applications consider a number of
of the arriving surge. The use of inductor on the load deratings and safety margins while selecting the size of
side to provide an impedance to the arriving surges with the motor to cope with a number of unforeseen unfavour-
a view to suppressing them is not good practice, for it able operating conditions occurring at the same time.
may diminish the p.f. of the circuit and also cause a This is discussed in Chapter 7 (see also Example 7.1).
voltage drop across it, which may affect the machine’s The size of the motor is therefore chosen a little larger
performance. than actually required. As a consequence even when the
motor may be performing its optimum duty, it may hardly
Note A manufacturer of static drives would normally give an be loaded by 60-80% of its actual rating causing energy
option to the user to operate their inverter circuit fIGBTs normally) waste by extra iron and copper losses and operating at a
at high PWM carrier frequencies (typical 2-8 kHz) to smooth the reduced p.f. Static drives are therefore tangible means to
output (load side) voltage. But at high frequencies, the propagation conserve on such an energy waste.
of surges becomes faster and may cause quick reflections, which
would require either a shorter cable or the use of a surge suppressor.
High-frequency operations also raise the noise level in the ground While performing a speed control
path and can cause sensitive devices like PLCs, sensors and analogue
circuits to behave erratically, as they are all connected through the A very important feature of solid-state technology is energy
ground circuit. It is therefore desirable to operate the IGBTs or conservation in the process of speed control. The slip
GTOs at lower frequencies, preferably 24 kHz, as this will cause losses that appear in the rotor circuit are now totally
low ripples as well as a low noise level. A moderate carrier frequency eliminated. With the application of this technology, we
will also help in taming the aniving surges at motor terminals with can change the characteristics of the motor so that the
only moderate steepness.
voltage and frequency are set at values just sufficient to
Conclusion meet the speed and power requirements of the load. The
power drawn from the mains is completely utilized in
1 Induction motors, both, squirrel cage and slip-ring, doing useful work rather than appearing as stator losses,
can be easily controlled to achieve the required charac- rotor slip losses or external resistance losses of the rotor
teristics by applying solid-state technology. circuit.