Electrical Motor Control Midterm Review

NEMA-rated motors are designed to operate at plus or minus ____ percent of their rated voltage.

10

Magnetic control systems generally operate on ____________________ volts.

120

A metal bellows-type switch can sense pressures up to ____ psi.

2,000

Electrical ratings for the contacts of the basic micro switch are generally in the range of ____ volts AC.

250

____ proximity detectors will sense ferrous metals better than nonferrous.

All-metal

____________________ timers use a small AC synchronous motor to provide the time measurement for the timer.

Clock

____________________ pressure is the difference in pressure between the cut-in or turn-on pressure and the cut-out or turn-off pressure.

Differential

Connecting eleven-pin timers into a circuit is generally even simpler than connecting the coil to power.

False

Flow switches are used to detect the movement of air, but not liquid, through a duct or pipe.

False

If two of the lines supplying power to a three-phase motor are reversed, the motor will not reverse the direction of rotation.

False

In industrial applications, manually operated valves are generally used.

False

One advantage of the turbine-type sensor is that the turbine blade offers no resistance to the flow of the liquid.

False

Overloads are designed to protect the circuit from a short-circuit condition.

False

Pressure switches are the only pressure sensing devices that an electrician is likely to encounter on the job.

False

Push buttons are always shown as normally open.

False

Relays are electromechanical switches that lack auxiliary contacts.

False

Sensors targeting specific types of metal should be mounted as close to other metals as possible.

False

The Hall effect was discovered by Edwin H. Hall at Johns Hopkins University in 1979.

False

The Hall generator cannot be used in a manner similar to a limit switch.

False

The most complicated method of starting a motor is across-the-line starting.

False

The off-delay relay is often referred to as DOE, which stands for "Delay On Energize."

False

Three-phase motors must have an overload sensor (heaters or magnetic coils) in two of the three-phase lines.

False

Chapter 9

Float Switches

Chapter 10

Flow Switches and Sensors

Chapter 1

General Principles of Motor Control

Chapter 15

Hall Effect Sensors

Chapter 11

Limit Switches

____ are designed to produce an output voltage that is proportional to the strength of a magnetic field.

Linear transducers

Chapter 3

Manual Starters

The conductor size, fuse or circuit breaker size, and overload size are generally determined using the ____.

National Electrical Code

Chapter 4

Overload Relays

Chapter 12

Phase Failure Relays

____ timers operate by restricting the flow of air through an orifice to a rubber bellows or diaphragm.

Pneumatic

Chapter 8

Pressure Switches and Sensors

Chapter 16

Proximity Detectors

Chapter 5

Relays, Contractors, and Motor Starters

Chapter 13

Solenoid and Motor-Operated Valves

Chapter 2

Symbols and Schematic Diagrams

Chapter 14

Temperature-Sensing Devices

Chapter 6

The Control Transformer

Chapter 7

Timing Relays

A float switch is used when a pump motor must be started and stopped according to changes in the water (or other liquid) level in a tank or sump.

True

Integral horsepower manual starters with low voltage release automatically restart after a power failure and don't have to be manually reset.

True

Limit switches are drawn by adding a wedge to one of the four basic switches.

True

Limit switches are used to detect when an object is present or absent from a particular location.

True

Manual starters are characterized by the fact that the operator must go to the location of the starter to initiate any change of action.

True

Micro switches are used in small spaces that would never be accessible to a larger device.

True

Most industrial motors operate on voltages that range from 240 to 480 volts.

True

Motors are intended to operate on systems with balanced voltage.

True

Pressure switches are found throughout industry in applications where it is necessary to sense the pressure of pneumatic or hydraulic systems.

True

Proximity detectors are designed to detect the presence or absence of some object.

True

Standard temperature transmitters generally send a 4-20 ma signal to indicate the temperature range and are calibrated for a specific range.

True

The greatest difference between relays and contactors is the fact that contactors are equipped with large contacts that are intended to connect high-current loads to the power line.

True

The mercury bulb float switch does not depend on a float rod or chain to operate.

True

When a machine is installed, the motor, machine, and controls are all interrelated and must be considered as a unit.

True

When two dissimilar metals are joined at one end, on a thermocouple, and that junction is heated, a voltage is produced.

True

____ flow sensors operate on the principle that when a moving liquid strikes an object, a swirling current is created.

Vortex

The type of bumper arm used is determined by the ____ of the limit switch

application

The most common coil symbol used in schematic diagrams is the ____________________.

circle

The symbol for a float switch illustrates a ball float. It is drawn by adding a ____ to a line.

circle

All overload relays contain a set of normally ____________________ contacts.

closed

Regardless of the method used to sense pressure, all pressure switches activate a set of ____________________.

contacts

Anytime a motor has tripped on overload, the electrician should check the motor and circuit to determine why the overload tripped. The first step is generally to ____.

determine if the motor is actually overloaded

Diodes are used to suppress the voltage spikes produced by coils that operate on ____________________.

direct-current or DC

Many ultrasonic sensors have an analog output of voltage or current, the value of which is determined by the ____.

distance to the object

The continuous change of both amplitude and polarity of the magnetic field causes currents to be induced into the metal core material. These currents are called ____ currents.

eddy

It is a common practice to not ground one side of the control transformer. This is generally referred to as a ____ system.

floating

A(n) ____________________ valve is used to adjust the maximum air flow in a bubbler system.

hand

The ____ relay is so named because it uses a length of resistive wire connected in series with the motor to sense motor current.

hot-wire

Programmable controllers contain ____ electronic timers.

internal

Mechanically held contactors and relays are often referred to as ____ contactors or relays.

latching

When using a sensing pilot device to directly control the operation of a motor, you must make sure that the type of pilot device is equipped with contacts that can handle the rated current of the motor. These devices are generally referred to as "____________________" devices

line voltage

There are two major types of overload relays: thermal and ____.

magnetic

Some limit switches are momentary contact (spring returned) and others are ____________________ contact.

maintained

Motor control systems can be divided into three major types: ____________________, semiautomatic, and automatic.

manual

The ____ switch appears as a rubber bulb connected to a conductor.

mercury bulb

The ____________________ level gauge operates by emitting a high frequency signal of approximately 24 gigahertz into a tank and then measuring the frequency difference of the return signal that bounces off the product.

microwave

When a process has a definite on and off operation, or a sequence of successive operations, a(n) ____________________ timer is generally used.

motor driven

Schematics are always drawn to show components in their de-energized, or ____________________, state.

off

Fuses and circuit breakers are generally employed for circuit protection, and ____ are used to protect the motor.

overload relays

Generally, limit switches are used as ____________________ devices to control the coil of relays and motor starters in control circuits.

pilot

The number of ____________________ refers to the load contacts and does not include the number of control or auxiliary contacts.

poles

Solder melting-type overloads are often called solder ____________________ overloads.

pot

The ____________________ rating of control transformers generally ranges from 0.75 kilovolt-amperes, or 75 volt-amperes, to 1 kilovolt-ampere, or 1000 volt-ampere.

power

The liquid level in a bubbler system is sensed by ____ switches.

pressure

The term ____ means that the output voltage will be proportional to the supply voltage.

ratiometric

Air flow switches are often called ____________________ switches.

sail

Control transformers generally have ____ terminals connected to the primary and secondary leads.

screw

Vortex sensors insert a(n) ____________________ bar in the line to produce a vortex.

shedder

Push buttons that contain multiple contacts are often called ____ push buttons.

stacked

Two factors determine the polarity of the voltage produced by the Hall generator: the direction of current flow and the ____ of the ____ field.

strength, magnetic

Manual starters that are installed in a(n) ____________________ inside a wall are especially susceptible to ambient temperature problems.

switch box

Since thermocouples produce such low voltages, they are often connected in series. This connection is referred to as a(n) _____.

thermopile

In a MOV, the ____ switch is generally used to stop the motor when the valve is closed.

torque

To detect the amount of liquid or air flow, a ____ must be used.

transducer

Electronic overload relays employ a current ____ to sense the motor current.

transformer

Solid-state relays that are intended to connect alternating current loads to the line use a device called a(n) ____________________.

triac

Starters that are intended to protect motors that operate on 240 volts should contain ____ load contacts.

two

The percentage of heat rise in the motor caused by the voltage unbalance is equal to ____ times the percent squared.

two