Chapter 6

_____ can be reduced by dividing the core into many flat sections or laminations.

Eddy currents

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

FLA

Autotransformers use separate windings for the primary and secondary.

False

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

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

Because of conductor resistance, flux leakage, eddy currents, and hysteresis losses, not all of the input power is transferred to the secondary winding for useful purposes.

True

Current flow in a secondary transformer winding creates an electromagnetic field that opposes the primary electromagnetic field resulting in less primary CEMF. The primary current automatically increases in direct proportion to the secondary current.

True

For most practical purposes, transformer efficiency can be ignored.

True

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

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

True

The actual motor current depends upon the load on the motor and the operating voltage at the motor terminals.

True

The energy transfer ability of a transformer is accomplished because the primary electromagnetic lines of force induce a voltage in the secondary winding.

True

The heating from harmonic currents is proportional to the square of the harmonic current.

True

The leakage of electromagnetic flux lines between the primary and secondary windings of a transformer represents wasted energy.

True

The primary electromagnetic field induces a voltage in the secondary. As the secondary current flows, it produces an electromagnetic field that reduces the strength of the primary flux lines. This results in an increase in primary current.

True

Voltage induced in the secondary winding of a transformer is dependent on the number of secondary turns as compared to the number of primary turns.

True

_____ 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

Wasteful circulating _____ in the iron core cause(s) the core to heat up without any useful purpose.

eddy currents

As current flows through the transformer, the iron core is temporarily magnetized. The energy required to realign the core molecules to the changing electromagnetic field is called "_____ loss."

hysteresis

Transformers are rated in

kVA

The ____ of an ac generator contains the electromagnetic field, which cuts through the stationary conductor coils.

rotor

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

A _____ is used to raise or lower voltage and it has the ability to transfer electrical energy from one system to another with no physical connection between the two systems.

transformer

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

two

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

voltage

Three- Phase ac generators have three equally spaced winding, ____ out of phase with each other.

120 degrees

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

20 A

If the primary phase voltage is 480V and the secondary phase voltage is 240V, the turns ratio is _____.

2:1

Three-phase, _____ wye-connected systems can overheat because of circulating odd triplen harmonic currents.

4-wire

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

58 A

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 = _____.

746

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

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.

Parrell, series