True or false: a drive must be able to brake electrically in order to stop.
Answer: C
Removing the supply lets friction and windage bring the drive to rest, and a mechanical brake speeds that up. Electrical braking is chosen when the stop must be quick, controlled, or when the energy is worth recovering.
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True or false: the load torque of a drive always opposes the motion.
Answer: D
Correct answer: False, since active loads such as gravity can drive the motor round
Passive loads - friction, windage, a fan - always oppose the motion and reverse sign when it reverses. An active load carries its own energy: a suspended weight pulls in the same direction whichever way the drum turns, so it opposes the hoist and drives the lower. That is what makes the fourth quadrant necessary on a hoist drive, and what makes regeneration worth having there.
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Which of these determines the quadrant in which a drive is operating?
Answer: A
The pair of signs locates the operating point in the torque-speed plane, and their product gives the direction of power flow. Magnitudes decide how far out along the axes the point lies but not which quadrant it is in.
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A conveyor carrying a uniform load along a horizontal path presents
Answer: B
The friction to be overcome depends on the weight carried rather than the speed, so the torque is essentially flat and the power rises with speed. An inclined conveyor adds a gravity component, which is an active load.
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A traction drive above its base speed operates as
Answer: C
Field weakening holds the power at the converter's limit while the available torque falls with speed, which suits a vehicle that needs high torque to start and high speed to cruise. Below base speed the same drive is constant torque.
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The torque required to accelerate a drive is called the
Answer: D
It exists only while the speed is changing and vanishes in the steady state, which is why the machine must be rated for a starting current in excess of its running value. Its magnitude is the inertia times the acceleration.
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The static torque of a drive is the torque required
Answer: A
Correct answer: to keep the load moving at constant speed
The static or steady-state torque is what the load demands once the drive is running at constant speed, with nothing left over for acceleration. It is what determines the continuous rating of the motor. The additional torque needed to accelerate the inertia is the dynamic torque, and the two together make up the motor torque during a start.
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Breakaway torque is the torque needed to
Answer: B
Static friction exceeds the running value, so the initial torque demand can be well above the steady figure, particularly in a machine that has stood idle. Drives are checked against it as well as against the running load.
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A drive selected on running torque alone may fail to
Answer: C
Breakaway friction and the inertia of the load both demand torque in excess of the running value, and a machine chosen without allowance for them may stall on starting. Starting duty is therefore assessed separately from running duty.
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Cogging in an induction motor drive refers to
Answer: D
Interaction between the stator and rotor slot harmonics produces a locking torque at particular alignments, so the machine hums without turning. Skewing the rotor slots and choosing the slot numbers carefully avoid it.
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