Speed Control Methods Questions and Answers

Answer: A

Correct answer: a pole-changing machine

A pole-changing winding gives two discrete speeds by reconnection, with no converter, no slip loss and full efficiency at both. Where only two speeds are wanted this is the cheapest and most robust answer; a converter drive would supply a continuous range that the application does not use.

Answer: B

Correct answer: a position loop and a position feedback device

Position control needs an outer loop closed on position, cascaded over the speed and current loops, and it needs a device that actually reports position - an encoder or resolver. A speed loop alone accumulates error indefinitely, since a small speed error integrates into an ever-growing position error.

Answer: C

Estimating the flux and speed from the terminal quantities gives much of the performance of a closed-loop drive without cabling a sensor into a hostile place. Performance falls away close to standstill, where the estimate has least to work with.

Answer: D

The point of a variable-speed drive is that the process, not the supply frequency, decides how fast the machine turns. Once that freedom exists, the speed can be used to control flow, tension, production rate or position as the application requires.

Answer: A

It divides the region where voltage is being raised at full flux from the region where flux is being reduced at full voltage. Every converter drive has such a point, and the torque and power capability differ on either side of it.

Answer: B

The voltage ceiling forces the flux down as the frequency rises, so the torque available for a given current falls in proportion. The product stays roughly constant, which suits a load whose torque requirement falls with speed.

Answer: C

A large workpiece is turned slowly at high torque and a small one quickly at low torque, which is exactly the constant-power envelope. Gearing extends the range at the low-speed end where the converter alone cannot supply the torque.

Answer: D

A drive quoted as 20 to 1 can be run down to five per cent of rated speed while still meeting the torque and accuracy the specification requires. Beyond that ratio the cooling, the feedback resolution or the control stability becomes the limit.

Answer: A

An induction machine slips further and a dc machine drops more voltage across its armature resistance, both of which reduce the speed. Feedback or slip compensation removes the droop by raising the reference as the load grows.

Answer: B

The controller acts on the difference between measured and commanded speed, so the droop disappears and load disturbances are rejected. What the machine can physically deliver is unchanged, and so is everything on the supply side.