The main reason variable-speed drives have spread so widely is
Answer: C
Converters that once cost more than the machine now cost a fraction of an installation, which has made variable speed the default rather than the exception. The machines themselves have improved only incrementally over the same period.
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Which of these is NOT a reason for using variable-speed operation?
Answer: D
Correct answer: To increase the losses of the driven machine
The genuine reasons are to save energy on fluid loads, to control a process quantity, and to soften the mechanical and electrical shock of starting. Increasing losses is a consequence to be minimised - a converter itself adds two or three per cent - and never a purpose.
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A soft start achieved by ramping the frequency from zero differs from one achieved by ramping the voltage in that it
Answer: A
Frequency ramping keeps the flux and therefore the slip small, so the machine develops rated torque at rated current from standstill. Voltage ramping reduces the flux and obtains low speed only through large slip, which gives poor torque and heats the rotor.
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The cheapest way to limit the starting current of a large fixed-speed machine is
Answer: B
Correct answer: a soft starter using thyristors
A soft starter is a set of back-to-back thyristors ramping the applied voltage, so it needs no dc link, no inverter and only a fraction of a converter’s components. It limits the starting current and the mechanical shock, and once the machine is up to speed a bypass contactor takes over. What it cannot do is vary the running speed.
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