Cascade connection has fallen out of use because it
Answer: A
Two wound rotor machines, their slip rings and their switching are far dearer than one cage machine and a converter, and they give only a handful of fixed speeds. It survives in textbooks as an illustration of the slip relation rather than in practice.
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An open-loop drive holds its speed
Answer: B
With no feedback, the speed droops as the load is applied by whatever the machine's characteristic dictates, typically a few per cent for an induction machine. That is perfectly adequate for a fan or a pump and quite inadequate for a machine tool.
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Slip compensation in an open-loop V/f drive works by
Answer: C
Correct answer: raising the frequency in proportion to the measured current
An induction machine slips more as it is loaded, so an open-loop drive holding a fixed frequency lets the shaft slow down. Slip compensation infers the load from the measured current and raises the frequency to offset the expected slip, recovering much of the regulation of a closed loop without a feedback device.
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A closed-loop speed drive differs from an open-loop one in that it
Answer: D
Comparing the measured speed with the reference and acting on the difference removes the droop and rejects load disturbances. The cost is the feedback device, its cabling and the risk of failure that the extra component brings.
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The standard structure of a closed-loop drive is
Answer: A
Current responds in milliseconds and speed in tens or hundreds, so the loops are nested with the faster inside. That arrangement also gives a natural current limit, since saturating the speed controller's output caps the torque demand.
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A position loop in a servo drive is placed
Answer: B
The three loops nest with position outermost, speed in the middle and current innermost, each an order of magnitude faster than the one enclosing it. Feedforward of velocity and acceleration is added to reduce the following error during a commanded move.
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An incremental encoder provides
Answer: C
Two channels in quadrature give direction as well as count, and a reference mark establishes a datum once per revolution. Because the count is relative, the axis must be homed after every power-up unless an absolute device is fitted.
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An absolute encoder differs from an incremental one in that it
Answer: D
Correct answer: reports its position without needing to be homed
An absolute encoder gives a unique code for each position, so the drive knows where the shaft is the instant it powers up. An incremental encoder counts pulses from wherever it happened to start, so the axis must be driven to a reference before its position means anything - which on some machines is neither quick nor safe.
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A resolver is preferred to an optical encoder where
Answer: A
Being a small rotating transformer with no optics and no electronics at the shaft, it tolerates conditions that would destroy an encoder. The signals need conversion electronics in the drive, which is where the cost sits.
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A drive quoted as having a 1,000 to 1 speed range with an encoder of 1,024 pulses per revolution will at the bottom of that range
Answer: B
Estimating speed by counting pulses in a fixed window becomes coarse when few arrive, so very low speeds need either a finer encoder or a period-measurement method. It is a common reason for poor low-speed performance in an otherwise sound drive.
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