An electric drive consists of
Answer: A
The five parts are the load, the motor, the power modulator that feeds it, the control unit that commands the modulator, and the sensing unit that closes the loop. Treating the motor in isolation misses the fact that its behaviour is set by what surrounds it.
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The function of the power modulator in a drive is to
Answer: B
Correct answer: control the flow of power to the motor
The power modulator sits between the supply and the motor and regulates what reaches it - a converter, chopper or inverter that adjusts voltage, current or frequency to whatever the control unit demands. It also limits the current during transients and, where the drive regenerates, passes power the other way. The sensing belongs to the control unit and the mechanical duty to the load; the modulator handles the power.
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The sensing unit in a closed-loop drive provides
Answer: C
Tachogenerators, encoders and current transducers feed the measured quantity back so that the control unit can act on the error. Without them the drive can only be run open loop, with speed varying as the load changes.
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An individual electric drive is one in which
Answer: D
Correct answer: a single motor drives a single machine
An individual drive gives each machine its own motor, which is the arrangement almost universal today. The group drive, in which one motor turned a line shaft and belted several machines, wasted power, made the shop dangerous and stopped everything when the motor failed. The multi-motor drive is the third case, in which several motors drive different parts of one machine in coordination - a rolling mill or a paper machine.
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A group drive, or line shaft drive, is characterised by
Answer: A
A single large motor turns a shaft from which belts drive individual machines, which was economical when motors were expensive. It is inflexible, wasteful and dangerous, and it has been abandoned in favour of individual drives.
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A multi-motor drive is used where
Answer: B
Rolling mills, paper machines and travelling cranes all have sections whose speeds must be coordinated yet separately adjustable. Each section gets its own motor and the control system holds the required relationship between them.
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The fundamental torque equation of a drive with constant inertia is
Answer: C
Correct answer: \( J\,d\omega/dt = T - T_l \)
The equation of motion for a drive of constant inertia is
\[ J\frac{d\omega}{dt} = T - T_l \]the difference between motor and load torque going into accelerating the inertia. Everything in drive dynamics follows from it: equal torques give steady speed, an excess accelerates, a deficit decelerates, and the run-up time comes from integrating it. Reversing the sign of the right-hand side would have the drive accelerate when the load exceeds the motor.
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In the steady state, the motor torque of a drive is
Answer: D
Correct answer: equal to the load torque
Steady speed means \( d\omega/dt = 0 \), so the torque equation gives \( T = T_l \) exactly. Any excess would be accelerating the inertia and the speed would not be steady. Note that friction and windage are part of the load torque as seen by the motor, not a separate deduction from it.
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A drive accelerates when the motor torque
Answer: A
Correct answer: exceeds the load torque
From \( J\,d\omega/dt = T - T_l \), a positive difference gives a positive acceleration. The magnitude of that acceleration depends on the inertia, but the fact of acceleration depends only on the sign of the difference. A deficit decelerates the drive, and equality holds it at whatever speed it has reached.
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For the speed of a running drive to fall, the torque developed by the motor must be
Answer: B
A deficit of torque leaves the load torque to retard the inertia, and the drive slows until a new balance is found. Deliberately creating such a deficit, by reducing the motor torque or by braking, is how a drive is brought to rest.
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