BOMA: Motor Controls
What is the maximum voltage drop allowed between the pole transformer and the equipment pole terminals?
10 percent
*Two 50-ohm resistor's in parallel would have the same resistance value as one:
100-ohm resistor/25-ohm resistor
If the potential differences in a three phase motor are 120 degrees apart, how far apart are the currents?
120 degrees
Three phase voltage waveforms are separated by how many degrees:
120 degrees
What is the synchronous speed of a six pole AC motor?
1200 RPM
A motor is operating on 230 volts, drawing 6.5 amps. How many horsepower is the motor operating at?
2
*The maximum distance from a box that nonmetallic-sheathed cable should be stapled will not exceed:
2 feet/1/2 feet
On a WYE-Delta motor, what is the % of voltage across the windings during start up?
58%
How many times greater is motor starting current to F.L.A.?
6
In the U.S.A., all our electricity is _____ cycles.
60
If a motor pulls 5 amps of operating current, a good choice of fuse would be?
7.5 amp slow blow
A machine that converts mechanical power into electrical power is called a?
A generator and alternator
What voltage energizes the potential relay?
Back E.M.F.
What limits the size of the motor that may be started across the line?
Capacity of the power supply
How can a shaded pole motor be reversed?
Disassemble and reassemble the motor
When reading resistance from a motor terminal to the motor shell, what does a very low resistance reading mean?
Grounded winding
The terminals on a compressor measured thusly: between I + II we got 3 ohms, between II + III we got 6 ohms and between I + III we got 9 ohms, therefore:
II is common
What will restricted ventilation cause in a motor?
Increased temperature; premature motor failure; higher motor running amps
A set of normally open contacts are _____ when the relay is_____?
Open, de-energized
A motor starter is a contactor with a _____ built in?
Overload
The result of connecting to large current load is _____?
Overload
In a transformer, the coil to which the induced voltage is applied is the _____?
Primary
The input voltage of a transformer is applied to a winding called the _____?
Primary
Flex?
Provides a good grounding path when used as an equipment grounding conductor
What is the basic difference in single phase motor characteristics?
Starting torque
What is the material typically used in the manufacturing of metallic boxes used in residential wiring?
Steel
What is the abbreviation for heat resistant thermoplastic insulation?
THHN
*Fish tape can be made of?
Tempered steel, nylon, wound steel cable
What difficulty can result from the "wild leg" on an open delta distribution system?
The "wild leg" voltage is higher than the single phase voltage output; A single phase load may be damaged if it is accidentally connected to the "wild leg"; The neutral or center tap still retains it's expected characteristics
The first thing to check when a system is down and won't run at all is?
The AC power supply
In a circuit with a resistive load, will voltage and current stay in phase?
Yes
Three factors which must be considered when selecting a fuse or circuit breaker:
amps, volts, interrupting capacity
Flat conductor cable may be used in:
offices
The definite relationship between current, voltage and resistance is known as:
ohm's law
A _____ is used to measure resistance?
ohmmeter
The meter we would most likely use to check if a relay coil is open would be a:
ohmmeter
Resistance is measured in:
ohms
The primary difference between a motor starter and a motor contactor is the starter has an?
overload protection
Which of the following devices prevent motors from drawing too much current?
overloads
A 3 way switch is similar to a _____?
single pole, double throw switch
The stationary winding in the shell of a motor are called the:
stator
*Synchronous motors have?
stator and magnetic force/rotors and stators
The capacitor is a device that:
stores an electrical charge
The purpose of a UPS system is to:
supply a regulated voltage with the frequency stability to the critical load
A device used to measure the rotating speed of the motor is called?
tachometer
A belt on a motor is too tight if:
the motor will not start; the full load amps are too high; the motor is very hot
What is meant by N.O. or normally open contacts:
they are open when de-energize
Electrical Metallic Tubing (EMT) is sometimes called:
thinwall
The load device used to raise at lower alternating voltage levels is called a:
transformer
Over current protection is provided for an electric motor to prevent damage to the windings from the heating effect of excessive current?
true
Two (2) speed motors require 2 sets of overloads to monitor the motor heating characteristics at each speed?
true
Electromotive force is measured in:
volts
All of the factors listed below have a part in determining the amount of torque developed by a motor except the:
way current is induced in the rotor
Distilled water should be added to a battery:
when level is below plates
Which wire size has the highest current carrying capacity:
#6
What is the microfarad tolerance when replacing a start capacitor?
+ 20 percent
What is the microfarad tolerance when replacing a run capacitor?
+ or - 10 percent
Where can we read back electro motive force (EFM)?
Across the C and S terminals
When does an internal temperature overload device reset?
After the motor has cooled off
Why is the run capacitor left in the motor circuit after the motor starts?
To improve the running efficiency of the motor
Why is the start capacitator quickly switched out of the motor circuit?
To prevent damage to the start widing from higher current
Why is proper motor lubrication important?
To protect bearings; to prevent grease from collecting on bearings; to help maintain motor running efficiency
What precautions must be taken when installing overloads on a 30 motor?
To stop motor no matter how many legs are pulling excessive current
Ohm is a unit of electrical resistance.
True
Which meter may be used without any electrical connection?
Ammeter
Return overload current in reference to motor circuits refers to?
Any load current that exceeds rated motor current
A generator?
Changes mechanical power into electrical power
The speed of an AC squirrel cage motor can be changed by?
Changing the number of poles
A combination motor starter always includes?
Circuit breaker, fuse and disconnecting device
Which of the following devices does not use a motor speed control?
Clocks
A normally closed relay contact will be _____ when the relay is _____?
Closed, de-energized
What is a standard conductor?
Conductor composed of a group of wires
what is meant by a "pilot duty" overload?
Controls the contactor circuit
The most difficult arcs to extinguish on a set of contacts are?
DC arcs
Which of the following would not be the cause of excessive motor vibration or noise?
Defective starter
What does the bleed resistor on a start capacitor do?
Discharges the capacitor when it is switched out of the circuit
What is the advantage of using an automatic reset overload?
Does not require personal attention following a nuisance trip
Thinwall conduit is another name for _____?
EMT
The current rating of a contactor or starter is rated for?
Each individual contact in the contactor or starter
What is meant by EP switch?
Electric Pneumatic operated switch
*Which is required in all computer rooms:
Emergency electrical shutoff\Halon system
A "day tank" is used with:
Emergency generator
What is the advantage when external overloads are used instead of internal overload?
Failed external overloads are easily replace; internal overload failures often require motor replacement & troubleshooting is easier on an external overload
A series circuit is a circuit that has more than one path for current through the components.
False
It would be practical to use just one type of single phase motor in all applications.
False
The polarity of the stator pole in an A.C. motor is fixed.
False
The symbol of normally closed contacts are two parallel vertical lines?
False
What determines the speed of an AC motor?
Frequency, slip & number of poles
What determines the speed of an induction motor?
Frequency; slip; number of poles
What advantage is obtained by adding a start capacitor to the motor circuit?
Higher starting torque will result
What resistance reading indicates an open set up relay contacts?
Infinite ohms
*A motor control circuit that allows the motor to automatically start after power is restored provides low voltage release?
Is said to be wired across the line is provided/is a 2 wire control circuit
As the motor speed increases from the instant of start-up, what happens to the motor back E.M.F.?
It increases
What is the advantage of using the thermotector motor thermostat?
It will follow the rapid rate of temperature rise caused by lock rotor amperage: it will follow the slow rate of temperature rise clearing normal motor operation & not overly effect by ambient conditions
The difference between jogging and inching circuits is that?
Jogging circuits remain energized for a specific amount of time, while inching circuits are energized only until the inch button his released.
A 4 S box is the most frequently used in one of the situations shown below. Which is it?
Junction box
When using a fuse jumper for troubleshooting, be sure never to connect the jumper between?
L1 and L2
In a circuit with and inductive load, the voltage will:
Lead the current through the load
The counter E.M.F. opposes the applied voltage and _____ the stator current.
Limits
To indicate a good diode, an ohm meter should show?
Low resistance in one/working direction and high resistance in the other direction
What electrical problem can result in using extremely long lines to run power to a machine?
Low voltage at the equipment terminals resulting from high line losses; difficulty in equipment starting; excessive heat generated in inductive loads
What is (or are) the major results of overloading the distribution system transformer?
Low voltage operation; failure of the distribution system transformer; power loss at equipment
Is the back E.M.F. induced in the stator higher or lower than the applied voltage?
Lower
What force created by current will create rotor movement?
Magnetism
Interrupting the circuit of one of the supply lines to a three-phase motor would?
Make the motor single phase
What unit is used to measure or define a capacitors electrical size?
Micro forad
Will an ohmmeter accurately indicate the microfarad rating of a capacitor?
No
The speed of an induction motor can be changed by changing the?
None of the above
A sudden current surge that occurs when a motor driven appliance is turned off is _____?
Normal overcurrent.
Which of the following meter must we use with the electricity turned off?
Ohmmeter
What ohmmeter scale should be used when reading resistance between two motor terminals?
RX1
What resistance range on a V.O.M. should be used to field test a capacitor?
RX1,000
What ohmmeter scale should be used when reading resistance from a motor terminal to ground.
RX10,000
*What are the three main components of an uninterruptible power supply?
Rectifier, batteries, invertors/inverter
Which of the following is not a load?
Relay contacts
Replacing overload heaters or fuses on a three-phase circuit you should?
Replace all three
What type of motor enclosure is common and an AC motor?
Reproof; splash proof & totally enclosed
What side of the power circuit or motor terminal should the marked terminal on a run capacitor be connected to?
Run
How can indications of eventual motor failure be detected?
Sight; sound & feel
What are the two general classes of AC motors?
Single phase and polyphaser
A 3 way switch is essentially a _____?
Single pole double throw switch
If a switch can be closed by moving a contact to only one position, it is a _____ switch?
Single throw
The most practical switch used when the neutral connection must not be broken is a _____ switch?
Single throw
What is the best conductor of a magnetic field?
Soft iron
*When soldering conductors are used in electrical construction, the most suitable flux is?
Soldering paste/rosin/paste
*Rheostats are generally used in a motor controls for _____?
Specially designed motors/small motors
What type of capacitor is used in a single phase motor circuit?
Start
Which capacitor is switched out of the starting circuit after a short period of time?
Start
A holding contact or maintaining contact in a magnetic motor starter is normally connected in parallel with the _____?
Stop push button, start push button, and starter coil
To indicate a good capacitor, the needle on an ohmmeter should?
Swing to 0 and slowly move to ¼, 1/3, ½ to way across the scale to infinity
Which type of overload device uses a bimetallic strip?
Thermal
What happens to motors when the voltage fails?
They will lose speed; they will lose power & they will overheat
On what type of motor do we use a wye-delta start?
Three phase
What is the purpose of oil in a capacitor?
To assist in dissipation of internal heat from the capacitor plates
What is the function of the potential relay?
To disconnect the start capacitor
What is the purpose of a centrifugal switch in a split phase motor?
To drop the start winding from the circuit when the motor reaches 75% of it's speed
Why is a capacitor used in a single phase motor circuit?
To generate a second phase of motor current
Why does the run capacitor have a lower microfarad rating than the start capacitor?
To limit the amount of current through the start winding, preventing winding failure
Rotary motion is produced by a motor. The twisting force causing the motion is called?
Torque
What does variations of applied voltage affect?
Torque
What word describes the force required to start a motor turning?
Torque
A double pole switch is used when it is necessary to open both conductors of a circuit.
True
A parallel circuit is a circuit in which the voltage across each branch is the same.
True
A relay is a device that can transfer an action from one circuit to another without an electrical connection between the two circuits?
True
A small size motor starter is given the designation of 00?
True
A starter that has 3 electrical separate sets of a power contacts would be classified as a 3 pole motor starter?
True
An ammeter is connected in series with the load?
True
Combination starters designed to have disconnect and the motor starter include in one single enclosure?
True
Improper alignment of the pole faces can cause excessive hum in a AC electromagnetic or solenoid?
True
Most magnetic starters use double break contacts because the arc voltage is reduced to half (1/2) its total value at each break.
True
When a conductor passes through a magnetic field, a current flow is always produced?
True
What automatically starts a stand-by generator:
a voltage monitor relay
The common method for starting a three phase squirrel cage induction motor is:
across the line
The most common method for starting a squirrel cage three phase motor is:
across the line
The common method of correcting power factor is:
add a capacitor
The armatures of both AC and DC contactors must move freely or?
all the contacts may weld together
The action of a diode is to:
allow current flow only in one direction
The purpose of a bypass isolation switch:
allows testing of the emergency generator (ATS)
The electrical energy that a battery can deliver is measured in?
amp hours
Current is measured in:
amperes
A _____ is used to measure current?
ampmeter
Current flow is measured in:
amps
A short circuit will result in:
an increased in current
Unbound resistance readings between the windings of a 3-phase motor indicates the motor windings may be _____?
breaking down
A squirrel cage motor is the simplest kind of multispeed motor because it requires no:
brushes
To get 240 volts from a 480-volt source, we:
can use a transformer
How do you change the rotation of the three-phase motor:
change two feed wires or change any 2 of the 3 wires
Voltage can be produced by:
chemical action, thermal action and magnetic induction
The cross-sectional area of electrical wire is measured in:
circular mils
Material that will readily support current flow are called:
conductors
*To convert DC to AC we would need:
converter or inverter
*The resistance of a copper conductor is inversely proportional to the:
current through the conductor
If you increase the load on a motor beyond it's full load point, the greatest effect of the increase in slip is in:
decreasing the power factor
In a process control, the time it takes for a change in the process to be felt at the measuring device is called?
delay
The major cause of most motor failures is?
deterioration of the insulation
A cause of the AC motor failure is:
dust; moisture; friction and overload
When charging a battery at a high rate, the rate must be reduced to the normal rate if the:
electrolyte temperature exceeds 100°F.
The voltage of a replacement capacitor must be at least:
equal to the original
A "jockey pump" is used with:
fire sprinkler system
Maximum number of bends allowed any conduit between pull boxes is:
four 90 degree bends
If you throttled the discharge side of a centrifugal pump into closed the amps would:
go up
The color green on a wire means:
ground
What does isolated ground mean:
ground that goes to nearest ground
To which of the following wires is the rounded prong of a three prong plug connected?
grounding wire
Load connected in series will:
have a lesser applied voltage than the voltage source
Loads connected in parallel will:
have different currents because of their different resistances
As current flow through loads such as nichrome, what is created:
heat
The mortal enemy of the motor is:
heat
The difference between the squirrel cage rotor and the wound rotor is in:
how the conductors are connected.
The state of charge the battery is measured with:
hydrometer
The electrical term used to indicate effective alternating current resistance is:
impedance
In dual voltage 30 motor, on low voltage the windings for each phase are wired:
in parallel
On high voltage, the phase windings are wired:
in series
The cause of increased temperature in a mortar winding is:
increase load on motor shaft; rapid cycling on locked rotor; improper voltage and voltage imbalance
Why are high voltage transmission systems used:
increasing voltage decreases line current.. decreased current results in lower voltage drops in the transmission lines.. smaller conductors can be used
In non-synchronous motors, the current in the rotor is produced by:
induction
In the squirrel cage motor, the potential difference across the rotor conductors is supplied by:
induction
The size of the rigid metal conduit is determined by:
inside diameter (ID)
If a 30 motor "single phases":
it cannot start if it's stopped
The current through the windings in series with a capacitor:
leads the current through the winding with no capacitor
The resistance of a good motor winding will be very:
low
Fewer turns in the secondary winding will result in a secondary voltage that is:
lower
As current flow through a coiled conductor, what is created around the conductor:
magnetic field
After replacing any motor, we must check:
motor full load amps
The most common motor control is the:
motor starter
A transformer works on the principle of:
mutual inductance
The actual full load amps of a motor are found?
nameplate of the motor
Electrons flow from:
negative to positive
On a duplex receptacle, the white is connected to the:
nickel screw
In a transformer, to which the input voltage is applied is the?
primary
The connections on the transformer marked "H" are the:
primary
To convert AC to DC you would need:
rectifier
The purpose of a reduced motor is to?
reduce voltage on motor starter
Reduced voltage starting is used as a means of _____?
reducing the starting current
The electrical term used to state the ability of material to hold back current flow is:
resistance
conduit fittings are use for:
rigid metal conduit
Generators make electricity by _____?
rotating armature in a magnetic field
The rotating part of a motor is called the:
rotor
If the motor is drawing excessive amps you would expect to find it:
run hot
The transformer connections marked with and "X" are:
secondary
The difference between the speed of the magnetic field and the speed of the rotor is called the:
slip
All single phase motors in order to start, require:
split phase
The centrifugal switch of a single phase induction motor is connected in series with the ______ windings?
start
A motor overload is:
temperature sensitive; current sensitive and safety device
The current interrupting capability of a fuse breaker is to _____?
the maximum amperage it should conduct during normal operation
What is the purpose of grounding?
to insure safety
What is the purpose of a circuit breaker:
to protect equipment
A 3600 RPM motor has:
two poles
If the light is to be controlled from three different locations, the following switches must be used:
two three-way and one four-way
When manually switching from normal to auxiliary power can be accomplished by _____?
using an automatic transfer switch
Storage batteries are rated for ampere-hour capacity and _____.
voltage
A _____ is used to measure electrical potential?
voltmeter
How many watts of power are required to do one horsepower of work:
746 watts
The actual speed of a two pole motor was measured at 3250 rpm, what is its slip?
9.7%
What resistance reading indicates a closed set of relay contacts?
0 ohms
A circuit with 230 volts applied is drawing 10 amps. A wattmeter connected to the circuit is reading 2070 watts. What is the power factor in this circuit?
0.9
Almost all residential electricity is _____ phase.
1
A fluorescent tube F40 has a diameter of:
1 1/2 inches
Three capacitors are connected in parallel. Their voltage ratings are 120, 370 and 410 respectively. The maximum safe voltage that can be tolerated by circuit is?
120
On a plan "scale 1/8" = 1', then 2 1/2 inches in a ruler equals:
20 feet
*What would be the wattage of a D.C. circuit drawing a two amp load with a resistance of five hundred ohms?:
2000 watts/1000 watts
How many watts of power are being used by a resistance heater operating at 460 volts drawing 5 amps:
2300 watts of power
Most modern homes have _____ volts available.
240
How much temperature rise can be expected by a motor with a 3.5% voltage unbalance?
25%
Most electrical plants in the U.S.A. generate _____ phase electricity.
3
The most practical type of switch used to control lights from two locations is a _____?
3 way switch
How many BTU's of heat per hour will one watt of power generate?
3.414
What is the slip for a 4 pole motor that runs at 1735 R.P.M.?
3.6%
One horsepower of work equals how many foot pounds per minute:
33,000 foot pounds per minute
How many BTU's of heat are generated by a 10 horsepower motor that is 88% efficient?
3460
When a run capacitor was charged with a 120 volts it had a current of 1.58 amps, what is its microfarad rating?
35
Most newer commercial buildings have _____ supplied by the power company.
480V 30
A 20 MFD and a 50 MFD capacitor are connected in parallel, what is their total capacitance?
70
What result can be can be obtained by passing a coil of wire through a magnetic field.
A current will be induced into wire
What is meant by torque?
A force producing rotation around an axis; force times radial distance & turning moment
A short circuit ground fault device may consist of?
A fuse and a circuit breaker
What is made when an electric current flow through the turns of a coil?
A magnetic field
What is the advantage of using internal overload?
A much more accurate sensing of winding temperature results
All the terminal measurements on a compressor measured 2 ohms, therefore:
The compressor is three phase
What does a voltage rating of a capacitor mean?
The maximum voltage that can be applied across the capacitor without damaging the capacitor
What is the effect of passing motor winding current through the bi-metal element of an overload?
The motor current will heat the bimetal, making the overload device temperature and current sensitive
The overload protection devices protect _____?
The motor windings
Describe the ohmmeter needle movement obtained when a capacitor is tested okay.
The needle quickly swings towards zero ohms and then gradually swings back toward a very high resistance.
What is magnetism?
The physical phenomenon that includes the attraction of an object for iron
What makes the rotor turn in an induction motor?
The rotating magnetic field
Metering of utility company services to a building is usually done at?
The service entrance point
Why can the run winding withstand a higher continuous current than the start windings?
The start winding is constructed of lighter wire
What is torque?
The strength that a motor produces by winding
Which of the following are not required on high-rise buildings:
U.P.S. system
Potential difference is measured in:
Volts
When replacing a 30 one horse power motor:
We can use a 10 motor if no 30 motors available
To reverse rotation of a 30 motor:
We reverse any two power leads
