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