Motors Final Study Guide

Threshold for Tingling Current

1.2mA

Where do the current to resistance power losses come from in power transmissions?

100(squared) x R

Threshold for Fatal Current

120mA

By how much should a motor branch circuit conductor be oversized?

125%

How do two-wire and three-wire circuits differ excluding the number of wires?

2 wire circuits only have FWRD and REV. 3 wire circuits have a stop command as well as a FWRD and REV.

What is the typical efficiency of a transformer running at full load?

98.5-95%

Difference between NO PB and NC PB

A NO PB will have a open contact until actuated, a NC PB has a closed contact until actuated

What are auxillary contacts and how are they actuated?

A contact that acts as a secondary switching device; They are actuated manually

What is the difference between a contactor and relay?

A contactor joins two different poles together. A relay has a common contact at a neutral position

Definition of Actuators

A device that causes a machine or other device to operate

Definition of sensor

A device that measures or detects a physical property

How does a GFCI function?

A device that senses small ground fault currents and will shut off the circuit when detecting a leakage of 5mA

What is the difference between pilot and primary control devices?

A primary device can start or begin the circuit, a pilot device can control the functions within the circuit

Describe a squirrel cage motor

A rotor with a cylindrical framework around a core that resembles a wheel for animals

What is a motor starter?

A way to supply power to a motor

What type of power is mainly supplied to commercial customers?

AC

What two types are motors broken into. Which is used more and why?

AC and DC motors. DC is used more often because they use energy more efficiently than AC

What 2 categories are motors classified in?

AC or DC

Factors in electric shock intensity

Amount of electric current in the body Voltage and Resistance of the path

What is a motor control center?

An area where most or all motors can be controlled from

What normally causes a motor overload condition?

An excessive amount of current by too much load

What is Plugging?

Braking using reverse rotation

Type of Extinguisher needed for Electrical Fires

Class C

NEC Article 430

Covers the application and installation of motor circuits and motor control connections

What type of power is mainly supplied to residential customers?

DC

What state are components shown on a diagram?

De-energized

How is DC Motor direction determined?

Direction of the magnetic field and direction of current flow.

How does body resistance affect electric shock hazard?

Dry Skin is a good insulator

What is the most common method of stopping a motor?

Dynamic braking

What type of energy conversion takes place in a motor?

Electrical to Mechanical

Describe electromechanical dynamic braking

Electromechanical braking uses magnetic energy to counteract the motor's current polarity.

How is a power circuit differentiated in a diagram?

Heavy conductor lines indicate high current

Why is a capacitor used with a split-phase motor?

High starting torque

What variable does a soft starter modify to make a motor start smoothly?

Initial voltage

Grounding

Intentional connection of a current-carrying conductor to the Earth

What is the purpose of overload protection with a motor?

It prevents motor damage

What is jogging and when is it used? How does it effect power contacts?

Jogging is the repeated closure of a circuit to allow a motor to do small movements.

What is the different between light and dark photoelectric sensors?

Light PE only activates when light is reflected and vice versa

How is a control circuit differentiated in a diagram?

Light conductor lines indicate low current

Applications of limit switches in a motor control circuit

Limit switches allow manual control over the function of a circuit

What does logic as applied to motor control circuits

Logic is determined in the type of function used such as an OR logic circuit

Types of primary control devices

MS, AC/DC drives

2 basic elements of a motor starter

Magnetic contactor and an overload relay

What materials are detected by capacitive proximity sensors?

Metal, plastic, non-metals, and wood

What materials are detected by inductive sensors?

Metals only

How does moisture affect body resistance?

Moisture lowers resistance of skin

How much conductor ampacity is required in a motor installation?

No less than 125%

How is AC (induction) motor speed determined?

Number of poles and Frequency

What is the operation of on and off timers

On and Off timers set the amount of time a motor or circuit will run before turning on or off

Difference between open and closed loop systems

Open loop runs as planned while closed loop can be modified

Types of pilot control devices

PB, Limit switch

Bonding

Permanent joining of metal parts

Describe dynamic braking

Reconnecting a running motor to act as a generator

What is Dynamic Braking

Regenerative braking

What is the most common maintenance operation on friction brakes?

Replacement of shoes or pads

Rules for Ladder Diagrams

Start at the top left Use Rungs Read from left to right then, Read from Top to Bottom

What do the three adjustment potentiometers set?

Start ramp, stop ramp, and initial voltage

What is the difference between starting and full load current?

Starting current

What does Inductive Load mean?

The conversion of current into a magnetic field

What is single-phasing?

The loss of one phase of a three-phase circuit. This may also cause damage to the motor or circuit

Differences between a thermal and electronic overload relay

The requisite to shut off the circuit is different as one monitors current and the other, heat.

What is synchronous speed?

The speed of the rotating magnetic field

What is inrush current?

The sudden rush of current that comes from starting a motor

How does a motor thermal protector work?

The thermal protector will shut off the circuit if the heat within the circuitry becomes too high

Similarities between Thermal and Electronic overload relays

They both cut the circuit if a determined amount of current is exceeded

Why are Capacitors dangerous

They can store a lethal charge for a considerable amount of time after disconnecting

Why should a motor have disconnecting means? Where should it be located?

To prevent damage to the circuit It should located where anyone can hit the button safely

How is power effectively transferred over long distances?

Transformers and Transformers Substations

Describe the operation of an electromechanical relay and name some applications

Using electromagnetism, physical contacts are actuated which turns the load on or off

How is transformer power rated?

VA or KVA

How do bimetallic overload relays operate?

When the bimetallic strip is heated up, it cuts the power of the circuit.

Can a motors nameplate determine overload protection? Ampacity

Yes; No

What are the 2 basic parts of a motor circuit?

control and power circuits

What is hysteresis in sensors?

maximum difference in sensor output

3 types of photoelectric sensors

retro reflective, thru beam, diffused

Ground-fault condition

unintentional, electrical conducting connection between an ungrounded conductor and a normal conductor