NCCER Electrical Level 2 Module 2 Motors: Theory and Application

If a motor has 4 poles and is operating at a frequency of 60Hz, its synchronous speed is _____. 120(60Hz)/4= 1,800 rpm

1,800 rpm

The most common service factor appearing on a motor nameplate is ___.

1.15

A motor with a NEMA kVA code letter of M has a locked-rotor value of ____.

10.0-11.9 kVA/hp

Per NEC Section 430.32, a motor with a service factor of 1.0 must trip at ____.

115% of the full-load current

The rpm of a 60Hz motor with two poles is ___.

3,600rpm

The code requirements for motor disconnects are covered in ___.

430.102

All motors must have a thermal protector if their voltage rating is greater than​ _____.

600V-1,000V

The electrical energy required to produce 1hp of mechanical energy is _____.

746W

When variable-frequency speed control is used on a motor with a nameplate rating of 480V at 60Hz, the voltage-to-frequency ratio is ___. 480v/60hz=8

8

magnetic field

A _____ is produced by the fields poles, and a loop of wire in the armature turns and cuts this magnetic field.

Brush

A conductor between the stationary and rotating parts of a machine. It is usually made of carbon.

Circuit breaker

A device designed to open and close a circuit by nonautomatic means and to open the circuit automatically on a predetermined overcurrent without injury to itself when properly applied within its rating.

Controller

A device that serves to govern, in some predetermined manner, the electric power delivered to the apparatus to which it is connected.

Commutator

A device used on electric motors or generators to maintain a unidirectional current.

Equipment

A general term including material, fittings, devices, appliances, fixtures, apparatus, and the like used as a part of, or in connection with, an electrical installation.

wye-connected motor

A type of motor terminal arrangement in which three coils are connected together and three coils are isolated

The four letters that designate insulation class are __, __, __, and __.

A, B, F, and H

overcurrent

Any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short circuit, or ground fault.

Most industrial motors are NEMA Design ____.

B

How is the speed normally changed in a multiple-speed induction motor?

Change number of pole connections

Using NEMA frame designations, the distance from the motor feet to the shaft center line is known as ___.

D dimension

Name three conditions that can cause a synchronous motor to become out of step.

Excessive load applied to the shaft, Supply voltage reduced excessively, Motor excitation lost or too low

Rotation

For single-phase motors, the standard rotation, unless otherwise noted, is counterclockwise facing the lead or opposite shaft end. All motors can be reconnected at the terminal board for opposite rotation unless other wise indicated.

___ use dynamic braking.

Forklifts

The NEMA frame designation for gasoline pump motors is ____.

G

Periodic duty: (Definition)

Intermittent operation at a substantially constant load for a short and definitely specified time.

periodic duty

Intermittent operation at a substantially constant load for a short and definitely specified time.

How DC motors operate?

Magnetic lines of force are continuous and always form closed loops; Magnetic lines of force do not cross.

Continuous duty

Operation at a substantially constant load for an indefinitely long time.

varying duty

Operation at varying loads and/or intervals of time

intermittent duty

Operation for alternate intervals of (1) load and no load; or (2) load and rest; or (3) load, no load, and rest.

Types of DC motors

Shunt Series Compound

The difference between the synchronous speed and the rotor speed is known as ____.

Slip

What type of rotor is most commonly used in an induction motor?

Squirrel cage motor

Types of AC motors

Synchronous and Induction (Single and Polyphase)

Which type of motor requires brushes, slip rings, and an amortisseur winding?

Synchronous motor

Revolutions per minute (rpm)

The approximate full-load speed at the rated power line frequency. The speed of a motor is determined by the number of poles in the winding.

Branch circuits

The circuit conductors between the final over current device protecting the circuit and the outlets

horsepower

The rated output capacity of the motor. It is based on breakdown torque, which is the maximum torque a motor will develop without an abrupt drop in speed.

Armature

The rotating windings of a DC motor.

Synchronous speed

The speed of the revolving field of the stator, which is dependent on the supply frequency and number of poles in a stator winding.

synchronous speed

The speed of the revolving field of the stator, which is dependent on the supply frequency and number of poles in a stator winding.

Field poles

The stationary portion of a DC motor that produces the magnetic field.

A wye-connected motor has ____.

Three wire in one group and three sets of two wires grouped together.

Horsepower: (Equation)

Watts/746

Torque

_____ is produced by mounting a loop in a fixed magnetic field and as long as the fields of the magnet and armature interact.

thermal protector

a protective device for assembly as an integral pat of a motor or motor compressor that, when properly applied, protects the motor against dangerous overheating due to overload or failure to start.

delta-connected motor

a type of motor terminal arrangement in which there are 3 sets of two coils connected together

A three-phase synchronous motor is started with a(n) _____.

amortisseur winding

To determine the horsepower rating of a motor that is already installed and in place, ____.

check the nameplate data

The most common class of insulation used in manufacturing and rewinding is NEMA ______.

class B

A(n) ______ is used to provide the switching action required for continuous DC motor rotation.

commutator

duty

describes the length of operation

When studying the operation of electric motors, it is important to remember that magnetic lines of force ____.

do not cross

Reconnecting a running motor to act as a generator is known as ____.

dynamic braking

When using a motor with dynamic braking, ____.

electromechanical friction brakes may be required with large loads

The principle reason for developing the brushless DC motor was to ___.

eliminate commutator problems

A NEMA designation for a type of enclosed motor is _____.

explosion-proof

The letters FLA on a motor nameplate stand for ____.

full-load amps

The method of braking in which DC is applied to the motor windings after AC is removed is known as ____.

injection braking

When a single motor is supplied from a branch circuit, the ampacity of the branch circuit must be _______ of the motor full-load current as determined by NEC Section 430.6(A)(1)

no less than 12.5%

The speed of an induction motor depends on the power supply frequency and the ___.

numbers of poles

In a splash-proof motor enclosure is open or totally enclosed.

open

The two primary types of motor enclosures classified by NEMA are ____.

open and totally enclosed

Any exposed live parts of motors must be guarded if the motor ____.

operates at 50V or more between terminals

overload

operation of equipment in excess of the normal, full-load rating, or of a conductor in excess of rated ampacity.

A ____ motor has three stator windings that are 120° apart.

polyphase

The starting torque characteristics of a wound-rotor motor are defined by _____.

rotor resistance

An open AC squirrel cage motor that makes use of​ form-wound coils and an insulation system is known as​ a(n) _____.

sealed windings(type)

When there is a wide variation in both torque and speed requirements, such as a blower or hoist, use a _____.

series DC motor

The multiplier to the nameplate horsepower rating is called the _____.

service factor

A motor with the field coils connected in parallel with the rotor winding is a _____.

shunt motor

A heating system is most likely to use a _____.

single-phase induction motor

The type of motor most likely to be used in a location with flammable dust and torque fluctuations is a ____.

squirrel cage induction motor

In three-phase motors, the rotor most commonly used is the _____.

squirrel cage rotor

The name of the stationary motor part that produces the magnetic field during operation is the ____.

stator

The speed on a multiple-winding motor is normally changed by ____.

switching contacts in the motor controller

A type of motor used in precision applications that require constant speed with zero slip is a _____.

synchronous motor

A delta-connected motor has ____.

the coils connected in parallel

A wye-connected motor has _____.

the coils pointed in series

hours

the duty cycle of a motor

To protect the motor from overloads and starting failures, use a ____.

thermal protector

A delta-connected motor has ___ leads

three sets of three

A consequent-pole motor has _____.

two speeds

With a constant-torque load, the horsepower ____.

varies in direct proportion to the speed

An insulation resistance test can be used to determine _____.

whether a motor is damp