Unit 20

What is the approximate output of a 15 hp motor?

11kW

What is the nameplate FLA for a 5 hp, 230V, single-phase motor, with 93 percent power factor and 87 percent efficiency?

20A

For a dual-voltage 230/460V motor, the field windings are connected in parallel for _____ operation and in series for _____ operation.

230v, 460v

What is the approximate output of a 5 hp, 480V, three-phase motor?

3.75kW

What size motor, in horsepower, is required to produce 30 kW of output?

40 hp

What is the nameplate FLA for a 20 hp, 208V, three-phase motor with 90 percent power factor and 80 percent efficiency?

58A

When a motor starts, the current drawn is at least _____ times the motor FLA; this is known as motor "locked-rotor amperes" (LRA).

6

Motors are used to convert electrical energy into mechanical work and the output mechanical work of a motor is rated in horsepower. 1 hp = _____.

746W

DC motors use brushes with springs to hold them in contact with a(n) _____.

Commutator

Motors must be protected against excessive winding heat by a properly sized overload protection device based on the motor _____ current rating.

FLA (Full Load Amperes

True or False: If the speed of a dc motor begins to increase, the armature will cut through the electromagnetic field at an increasing rate. This results in a greater armature CEMF, which acts to cut down on the increased armature current, resulting in the motor speeding up.

False

True or False: Placing a load on a shunt-wound dc motor causes the motor to slow down, which increases the rate at which the field flux lines are cut by the armature. As a result, the armature CEMF increases producing an increase in the applied armature voltage and current. The increase in current results in increased motor speed.

False

True or False: The motor FLA rating is used when sizing motor conductors or circuit protection.

False

If the rotating part of the motor winding is jammed so that it cannot rotate, no CEMF is produced in the motor winding. As a result, the motor operates at _____ and the windings will be destroyed by excessive heat.

LRC (Locked Rotor Current)

Dual-voltage ac motors are made with two field windings. The field windings are connected in _____ for low-voltage operation and in _____ for high-voltage operation.

Parallel, Series

To reverse the rotation of a dc motor, you must reverse the _____.

Rotor field OR Stator field

In a(n) _____ motor, the rotor is actually locked in step with the rotating stator field and is dragged along at the speed of the rotating magnetic field.

Synchronous

True or False: A motor must have two opposing magnetic fields in order to rotate. The stationary field winding is mounted on the stator, and the rotating part is referred to as the armature.

True

True or False: In an ac induction motor, the stator produces a rotating magnetic field that induces current in the rotor windings. The rotor current generates a magnetic field in opposition to the magnetic field of the stator, thereby causing the rotor to turn.

True

True or False: Once a motor begins turning, the rotor windings are increasingly cut by the stationary magnetic field, resulting in an increasing counter-electromotive force.

True

True or False: The actual motor current depends upon the load on the motor and the operating voltage at the motor terminals.

True

Swapping _____ of the line conductors can reverse a three-phase ac motor's rotation.

Two

_____ motors are fractional horsepower motors that operate equally well on ac and dc and are used for vacuum cleaners, electric drills, mixers, and light household appliances.

Universal

The nameplate motor FLA rating describes the motor current rating when it carries its rated horsepower load at its rated _____.

Voltage