motor delete in 2 weeks
The maximum torque required to accelerate a synchronous motor into synchronization at the rated voltage frequency is known as ? torque.
pull-in
The torque produced by a motor overload that pulls the rotor out of synchronization is referred to as ? torque.
pull-out
When synchronous torque is at maximum, it is equal to the ? torque.
pull-out
A(n) ? with tapped windings is used to adjust the amount of braking torque applied to a motor.
transformer
In the illustration, a 3PDT switch is used with an electric braking system. When stopping the motor, the 3PDT switch is used to ? to the stator coils and ? to the stator coils.
turn the AC voltage OFF / turn the DC voltage ON
The current in the stator power circuit is at minimum level when a synchronous motor is at ? power factor.
unity
In a Schrage motor, the brushes are placed on different commutator segments in order to change the ? across the primary stator windings.
voltage
The two parts of the rotor of a synchronous motor are an induction-rotor (amortisseur winding) section and a ? section.
wound-rotor
? braking can be used to provide an immediate stop, particularly in an emergency situation.
Electric
Typical synchronous speeds for 60-hertz consequent-pole motors are ? revolutions per minute (rpm) with four poles and ? rpm with eight poles.
1,800 / 900
Using NEC Part XIV of Article 430, match the following motor data to the correct full-load current.
1/2 hp, 208 V, single-phase → 5.4 A, 5 hp, 230 V, single-phase → 28 A, 5 hp, 460 V, 3-phase → 7.6 A, 15 hp, 200 V, 3-phase → 48.3 A
What is the maximum standard rating for a time-delay (dual-element) fuse permitted to be used for motor branch-circuit short-circuit and ground-fault protection for a 10-horsepower, 208-volt, single-phase capacitor motor when starting current is a problem?
110A Table 430.248, single-phase motor 10-hp, 208 V = 55 A Table 430.52(C)(1), Exception No. 2(b) Time-delay dual-element fuse = 225% Rating = 55 × 2.25 = 123.75 A Next lower standard size OCPD 240.6(A) standard size = 110 A
What is the maximum standard rating or setting of the motor branch-circuit short-circuit and ground-fault protective device for a five-horsepower, 240-volt, single-phase capacitor motor using non-time-delay fuses when the starting current is a problem?
110A Table 430.248, single-phase motor 5 hp, 240 V = 28 A 430.52(C)(1), Exception No. 2(a) Non-time-delay fuse = 400% max. OCPD = 28 × 4.00 = 112 A Next lower standard size OCPD = 110 A non-time delay fuse
A single-phase, 10-horsepower, 240-volt, capacitor-start motor is installed. Calculate the maximum rating of the motor branch-circuit short-circuit and ground-fault protective device using a nonadjustable inverse-time circuit breaker for this application.
125A Table 430.248 (10 hp at 240 V = 50 A) Table 430.52, inverse-time circuit breaker = 250% Rating = 50 × 2.50 = 125 A
An adjustable, instantaneous trip circuit breaker that is a part of a listed combination motor controller with coordinated protection in each conductor is used for the branch-circuit short circuit and ground-fault protection for a 50-horsepower, 230-volt, 3-phase Design B energy-efficient motor. What is the maximum instantaneous trip circuit breaker rating permitted for this application?
1430A Table 430.250, 3-phase motor 50 hp, 230 V = 130 A Table 430.52, Design B energy-efficient motor, instantaneous trip circuit breaker = 1,100% Rating = 130 × 11.00 = 1,430 A Note: Table 430.52 contains a separate row for Design B energy-efficient motors.
A single 100-horsepower, 440-volt, 3-phase, continuous-duty, induction type Design B motor is supplied by a motor branch circuit. Calculate the minimum ampacity for the motor branch-circuit conductors.
155A Table 430.250, 3-phase, 100 hp, 440 V = 124 A FLC 430.22(A), branch circuit ampacity = FLC × 125% = 124 × 1.25 = 155 A
A nonadjustable inverse-time circuit breaker is used for the motor branch-circuit short-circuit and ground-fault protection for a 1 1/2-horsepower, 460-volt, 3-phase squirrel-cage motor. What is the maximum rating permitted where an inverse-time circuit breaker is used?
15A Table 430.250, 3-phase 11/2 hp, 460 V = 3 A Table 430.52, inverse time circuit breaker = 250% Rating = 3 × 2.5 = 7.5 A 430.52(C)(1), Exception No. 1 Next higher OCPD permitted 240.6(A) standard sizes = 15 A
An adjustable, instantaneous trip circuit breaker that is a part of a listed combination motor controller with coordinated protection in each conductor is used for branch-circuit short-circuit and ground-fault protection for a 15-horsepower, 440-volt, 3-phase squirrel-cage motor. Calculate the maximum setting for the instantaneous trip rating of this circuit breaker.
168A Table 430.250 3-phase 15 hp, 440 V = 21 A Table 430.52, instantaneous trip circuit breaker = 800% Rating = 21 × 8.00 = 168 A
A Schrage motor rotor has ? winding(s).
2
Using NEC Part XIV of Article 430, match the following induction motor data to the correct full-load current.
2 hp, 115 V, 3-phase → 13.6 A, 7-1/2 hp, 480 V, 3-phase → 11 A, 10 hp, 200 V, single-phase → 57.5 A, 60 hp, 575 V, 3-phase → 62 A
Match the motor conductor ampacity to the correct THWN (75°C) copper wire size using Table 310.16. HINT: Section 240.4(D) does not apply to motor circuit conductors.
21 A → 12 AWG, 50 A → 8 AWG, 155 A → 2/0 AWG, 264 A → 300 kcmil
A single 3-horsepower, 240-volt, single-phase, continuous-duty, induction type Design B motor is supplied by a motor branch circuit. Calculate the minimum ampacity for the motor branch-circuit conductors.
21.25A Minimum ampacity BC conductors Table 430.248, single-phase, 3 hp, 240 V = 17 A FLC 430.22(A) branch circuit ampacity = FLC × 125% = 17 × 1.25 = 21.25 A
One 10-horsepower, 208-volt, 3-phase, continuous-duty, induction type Design B motor; one 25-horsepower, 208-volt, 3-phase, continuous-duty, induction type Design B motor; and one 30-horsepower, 208-volt, 3-phase, continuous-duty, induction type Design B motor are supplied by a single motor branch circuit. Calculate the minimum ampacity for the single motor branch-circuit conductors supplying all three motors.
215.6A Table 430.250, 3-phase 10 hp, 208 V = 30.8 A FLC 25 hp, 208 V = 74.8 A FLC 30 hp, 208 V = 88 A FLC 430.24, branch circuit ampacity = (largest FLC × 125%) + other motor(s) = (88 × 1.25) + 74.8 + 30.8 = 110 + 74.8 + 30.8 = 215.6 A
An inverse-time circuit breaker is used for branch-circuit short-circuit and ground-fault protection for a 30-horsepower, 208-volt, 3-phase squirrel-cage motor. Calculate the maximum rating permitted for the circuit breaker.
225A
In the illustration, coil B is a solenoid brake coil that is installed correctly. The voltage rating of the coil is ? .
240 V
A typical solenoid brake coil is rated for ? and can be used in both the low- and high-voltage connections of a dual-voltage motor.
240 VAC
Time-delay fuses are used for motor branch-circuit short-circuit and ground-fault protection for a 125-horsepower, 460-volt, 3-phase wound-rotor motor. What is the maximum rating permitted where time-delay fuses are used for this application?
250A Table 430.250, 3-phase wound rotor motor 125 hp, 460 V = 156 A Table 430.52, wound-rotor motor Time-delay fuse = 150% Rating = 156 × 1.50 = 234 A 430.52(C)(1), Exception No. 1 Next higher OCPD permitted 240.6(A) standard sizes = 250 A
A 40-horsepower, 230-volt, 3-phase synchronous motor uses non-time-delay fuses for motor branch-circuit short-circuit and ground-fault protection. Calculate the maximum rating for a non-time-delay fuse.
250A Table 430.250, 3-phase, synchronous motors (right side of Table) 40 hp, 230 V (sync.) = 83 A Table 430.52, sync. motor Non-time-delay fuse = 300% Rating = 83 × 3.00 = 249 A 430.52(C)(1), Exception No. 1 Next higher OCPD permitted 240.6(A) standard size = 250 A
A single 75-horsepower, 208-volt, 3-phase, continuous-duty, induction type Design B motor is supplied by a motor branch circuit. Calculate the minimum ampacity for the motor branch-circuit conductors.
263.75A Table 430.250, 3-phase, 75 hp, 208 V = 211 A FLC 430.22(A), branch circuit ampacity = FLC × 125% = 211 × 1.25 = 263.75 A
An adjustable, instantaneous trip circuit breaker that is a part of a listed combination motor controller with coordinated protection in each conductor is used for branch-circuit short-circuit and ground-fault protection for a 15-horsepower, 440-volt, 3-phase squirrel-cage motor. Calculate the maximum instantaneous trip circuit breaker rating permitted when starting current is a problem and where the need for further adjustment has been demonstrated by engineering evaluation.
273A Table 430.250 3-phase 15 hp 440 V = 21 A 430.52(C)(3), Exception No.1 Max. rating instantaneous trip circuit breaker = 1,300% Max. rating = 21 × 13.00 = 273 A
Two 10-horsepower, 440-volt, 3-phase, continuous-duty, induction type Design B motors are supplied by a single motor branch circuit. Calculate the minimum ampacity for the single motor branch-circuit conductors supplying both motors.
31.5A Table 430.250, 3-phase, 10 hp, 440 V = 14 A FLC 430.24, branch circuit ampacity = (largest FLC × 125%) + other motor(s) = (14 × 1.25) + 14 = 17.5 + 14 = 31.5 A
Match the motor conductor ampacity to the correct THWN (75°C) copper wire size using Table 310.16. HINT: Section 240.4(D) does not apply to motor circuit conductors.
32 A → 10 AWG, 64 A → 6 AWG, 69 A → 4 AWG, 216 A → 4/0 AWG
An inverse-time circuit breaker is used for branch-circuit short-circuit and ground-fault protection for a 30-horsepower, 208-volt, 3-phase squirrel-cage motor. Calculate the maximum standard rating permitted for the inverse-time circuit breaker when starting current is a problem.
350A Table 430.250, 3-phase motor 30 hp, 208 V = 88 A 430.52(C)(1), Exception No. 2(c) Max. inverse-time circuit breaker = 400% Max. rating = 88 × 4.00 = 352 A Next lower standard size OCPD240.6(A) standard size = 350 A
A consequent-pole motor with two poles when wired in series has ? poles when wired in parallel.
4
What is the maximum rating of the motor branch-circuit short-circuit and ground-fault protective device for a 7 1/2-horsepower, 208-volt, 3-phase squirrel-cage induction motor using time-delay fuses?
45A Table 430.250, 3-phase motor71/2 hp, 208 V = 24.2 A 430.52(C)(1), Table 430.52 Squirrel-cage motor with time-delay fuse = 175% Rating = 24.2 × 1.75 = 42.35 A 430.52(C)(1), Exception No. 1 Next higher OCPD permitted 240.6(A) standard sizes = 45 A
A single 30-horsepower, 460-volt, 3-phase, continuous-duty, induction type Design B motor is supplied by a motor branch circuit. Calculate the minimum ampacity for the motor branch-circuit conductors.
50AMinimum ampacity branch-circuit conductors Table 430.250, 3-phase, 30 hp 460 V = 40 A FLC 430.22(A), branch circuit ampacity = FLC × 125% = 40 × 1.25 = 50 A
What is the maximum standard rating for a time-delay fuse used as the motor branch-circuit short-circuit and ground-fault protection for a 230-volt, 3-phase hermetically-sealed compressor motor with a branch-circuit selection current (BCSC) of 38 amperes?
60A 440.22(A), not to exceed OCPD rating = BCSC × 175% = 38 × 1.75 = 66.5 A Next lower OCPD 240.6(A) standard sizes = 60 A
Match the following motor branch-circuit conductor ampacity to the correct copper wire size using 75°C insulation type THWN.
62 A → 6 AWG, 83 A → 4 AWG, 156 A → 2/0 AWG, 211 A → 4/0 AWG
One 15-horsepower, 440-volt, 3-phase, continuous-duty, induction type Design B motor and one 25-horsepower, 440-volt, 3-phase, continuous-duty, induction type Design B motor are supplied by a single motor branch circuit. Calculate the minimum ampacity for the single motor branch-circuit conductors supplying both motors.
63.5A Table 430.250, 3-phase15 hp, 440 V = 21 A FLC 25 hp, 440 V = 34 A FLC 430.24, branch circuit ampacity = (largest FLC × 125%) + other motor(s) = (34 × 1.25) + 21 = 42.5 + 21 = 63.5 A
A 10-horsepower, 208-volt, single-phase, continuous-duty, induction type Design B motor is supplied by a motor branch circuit. Calculate the minimum ampacity for the motor branch-circuit conductors.
68.75A Table 430.248, single-phase, 10 hp 208 V = 55 A FLC Branch circuit ampacity = FLC × 125% = 55 × 1.25 = 68.75 A
Depending on the motor design, an ammeter may read up to ? of full-load current at startup in locked rotor.
700%
Synchronous motors are typically used for NEMA ? design applications.
Class B
? circuit logic requires the operator to start the motor at low speed.
Compelling
? circuit logic is used in applications where a motor or load cannot withstand the stress of changing quickly from high speed to low speed.
Decelerating
The illustration shows the starting circuit for a synchronous motor. Which component is used to discharge any AC potential that builds up in the rotor field windings?
Discharge resistor
When a motor is installed and the starting circuit is connected, the motor should be tested at the highest speed.
False Note: Always test at the low speed first; if the motor operates properly at the low speed, then proceed to testing it at the high speed.
A single-phase shaded-pole motor can be used in a plugging circuit because it can be reversed at full speed.
False Note: Most single-phase shaded-pole, split-phase, and capacitor-start motors cannot be plugged because their centrifugal switches remove the starting windings when the motor accelerates. Without the starting winding in the circuit, the motor cannot be reversed.
A consequent-pole motor only operates at one speed.
False Note: Motor speed can be changed between low and high as the number of poles on the winding changes.
In friction braking, the braking action is applied directly to the shaft.
False Note: The breaking action is applied to a wheel that is mounted on the shaft of the motor.
Braking a motor one time by plugging puts about the same amount of heat into the rotor as starting the motor at full voltage.
False Note: The current may be three or more times higher during plugging than during normal starting.
In a Schrage motor, the number of control coils is half the number of commutator segments because each end of the control coil is connected to adjacent commutator segments.
False Note: The number of control coils matches the number of commutator segments.
Pull-out torque is the same as break-down torque of an induction motor.
False Note: The pull-out torque is not the same thing as the break-down torque of an induction motor. There is no speed change as the load increases. See the "Pull-Out Torque" section in Chapter 7 of the Motors textbook for a full explanation.
The rotor of a synchronous motor has only induction windings.
False Note: Unlike an induction motor, a DC field winding is placed in the rotor of a synchronous motor.
? braking requires frequent inspection, maintenance, and replacement of brake shoes.
Friction
? braking requires more maintenance than any other method of braking.
Friction
If a synchronous motor rotor slips out of synchronous speed for a period of time, ? are needed to shut it down.
protective relays
The illustration shows the starting circuit for a synchronous motor. Which component is used to protect a motor when the rotor falls out of step with the rotating stator field?
Out-of-step relay (OSR)
Match the following motor circuit conductors to the correct size of the raceway specified.
Three 2/0 AWG THWN in PVC Schedule 80 Conduit → 1 1/2", Three 2/0 AWG THWN in Rigid Metal Conduit → 1 1/2", Three 4/0 AWG THWN in PVC Schedule 80 Conduit → 2", Three 4/0 AWG THWN in a Rigid Metal Conduit → 2"
The illustration shows the starting circuit for a synchronous motor. Which component is used to apply current to the DC field windings and to remove the discharge resistor from the circuit?
Polarized field frequency relay (PFFR)
? torque develops during transition as the motor speeds up to the synchronous speed from the maximum speed running as an induction motor.
Pull-in
? braking is a method of dynamic braking that reuses the braking energy to the AC source instead of dissipating the energy as heat.
Regenerative
A(n) ? is a 3-phase AC motor with a rotor fed by a commutator and a set of brushes.
Schrage motor
In the illustration, a load (shunt) resistor is used with a dynamic braking system. What is the purpose of the resistor?
The resistor provides a path for the current
Match the following motor circuit conductors to the correct size of the raceway specified.
Three 4 AWG THWN in PVC Schedule 80 Conduit → 1", Three 4 AWG THWN in Rigid Metal Conduit → 1", Three 6 AWG THWN in PVC Schedule 80 Conduit → 3/4", Three 6 AWG THWN in a Rigid Metal Conduit → 3/4"
A common type of design for an out-of-step relay (OSR) is a current-type relay in series with the discharge resistor.
True
A motor drive can control the time it takes for a motor to stop.
True
A motor-generator set is a motor and a generator with shafts connected used to convert one form of power to another form.
True
A synchronous motor operating a blower is used to move air as well as correct the system power factor. The table in the image gives the measured currents. The currents are plotted on the graph. A rotor current of less than 18 amperes results in an underexcited rotor.
True
Dynamic braking CANNOT brake a motor to a complete stop or be used as a holding brake.
True
In a consequent-pole motor, the current can be made to travel through windings connected in series or parallel with each other.
True
In a dual-voltage delta-configured motor wired for high-voltage operation, the solenoid brake coil is wired across only one coil.
True
In a simple two-speed consequent-pole motor control circuit, the motor may be started from either the low speed or high speed connection.
True
In some control circuits, a motor may be placed directly in low speed from high speed without stopping the motor.
True
In the illustration, the circuit for a two-speed consequent-pole motor requires the motor to be de-energized with the stop pushbutton before changing speed.
True
One starter is required for each speed of a consequent-pole motor.
True
Plugging a motor to a stop results in high current and heat.
True
The bearings for high-speed rotors are commonly mounted in endbells that are bolted to the motor.
True
Total interlocking is the use of mechanical, electrical, and pushbutton interlocking.
True
A 60-horsepower, 3-phase, 208-volt squirrel-cage motor is installed with a motor nameplate full-load current rating of 165 amperes. The minimum ampacity of the branch-circuit conductors is calculated to be 211 amperes. The branch-circuit conductors are required to be not less than 4/0 AWG THWN copper, and they are installed in a two-inch rigid metal conduit. The maximum rating of the motor branch-circuit short-circuit and ground-fault protective device using time-delay fuses has been calculated to be not greater than a 350-ampere time-delay fuse when starting current is a problem. Therefore, this motor with 4/0 AWG THWN copper conductors and a full-load current of 169 amperes can be protected by a 350-ampere time-delay fuse.
True 430.52(C)(1), Exception No. 2(b) Max. time-delay fuse = 225% Max. rating = 169 × 2.25 = 380.25 A Next lower standard size OCPD 240.6(A) standard size = 350 A
The motor connections are reversed with ? so that the motor develops a reversed torque to brake the motor.
plugging
A(n) ? is a relay that removes the discharge resistor from the starting circuit of a synchronous motor and applies current to the DC field windings.
polarized field frequency relay (PFFR)
A ? may be used to start the rotor rotation and speed it up to almost synchronous speed.
pony motor
Which of the following parts are included in the starting circuit for a synchronous motor?
a. Control circuit b. Exciter circuit c. Power circuit
A circuit with ? circuit logic allows the operator to select the desired speed by pressing either the low or high pushbutton.
accelerating
The relative position of the ? in Schrage AC motors is altered to change the speed of the motor.
brushes
A(n) ? circuit logic function requires an operator to start and operate a motor in a predetermined order.
compelling
A synchronous ? is a synchronous motor that provides power factor correction when operated at no load.
condenser
A(n) ? motor has stator windings that can be connected in two or more different ways so that the number of stator poles can be changed.
consequent-pole
In an application of ? circuit logic, a motor must be allowed to decelerate by coasting or braking before being changed to a low speed.
decelerating
Power resistors are used with ? braking to dissipate the heat of braking.
dynamic
The discharge resistor is wired into the ? circuit to provide a complete path for the current induced in the field windings.
exciter
A solenoid is used to activate the brake shoes with ? brakes.
friction
In the event of a power failure, ? is the only method that can stop a motor.
friction braking
Stator windings in a consequent-pole motor usually provide two speeds, one of which is ? the speed of the other.
half
The ? in the control circuit prevents the motor starter and braking contactor from being energized at the same time.
interlock system
A ? power factor is the normal operating condition of a synchronous motor; although it is not the most efficient operating condition for the motor, it is the most efficient operating condition for the entire distribution system.
leading
For torque motors, the rated current shall be ? current, and this nameplate current shall be used to determine the ampacity of the branch-circuit conductors covered in 430.22 and 430.24, the ampere rating of the motor overload protection, and the ampere rating of motor branch-circuit short-circuit and ground-fault protection in accordance with 430.52(B).
locked-rotor 430.6(B)
Synchronous motors can be protected from damage caused by the loss of excitation in the DC winding by a(n) ? .
loss-of-excitation relay
When a load is placed on the motor, the torque angle increases and the poles of the rotor begin to slip behind the stator and ? synchronous speed.
maintain
A brake solenoid should always be connected directly into the ? circuit.
motor
