Chapter 9 Motor Calculations
What are the two factors that determine the synchronous speed of a three-phase motor?
# of strator poles and frequency of AC line in hertz
The synchronous speed of a three-phase, 60Hz motor with six poles is
1,200rpm
A three-phase motor with four stator poles has a synchronous speed of
1,800 rpm
Where an air conditioning unit is supplied with size 6 AWG CU conductors and protected by a 60 ampere circuit breaker, where required, the MINIMUM wire-type size CU equipment grounding conductor permitted for this installation is _____.
10 AWG, This answer can be found in NEC Table 250.12
If a motor has a current draw of 20A at 240V, what will the amperage be if 480V is applied?
10A (Double = half amount)
Dual motor drawing 5A at 480V will draw
10A at 240V
If the total resistance of two motor windings connected in parallel is 24ohms and 240V is applied to the circuit, how much current will flow?
10A, (240V/ 24ohms) = 10A
A circuit breaker suitable for use as a motor disconnecting means must have a current rating of at least
115% of the motor full-load current
NEC Section 430.22 states that for a single continuous duty motor, the conductors must have an ampacity of not less than ________ of the motor's full-load current.
125%
The maximum recommended size of a dual-element, time-delay fuse for a 50hp, 460V, three-phase motor rated at 65 FLA is....
125A
If the total resistance of two motor windings connected in parallel is 20ohms and 240V is applied to the circuit, how much current will flow?
12A, (240V / 20ohm) = 12A
When calculating the power factor correction for a motor with a nominal speed of 1,200 rpm and a rating of 10hp, the line current reduction should be
14%
If a 480V, three-phase motor control center feeds a 50hp motor (65A), a 40hp motor (52A), and a 10hp motor (14A), the minimum feeder ampacity is....
147.25A
When sizing a dual-element, time-delay fuse for a DC motor, do NOT exceed _______ of the motor's full-load current rating
150%
The maximum size of a dual-element, time-delay fuse protecting a wound-rotor motor is
150% of the motor's full-load current
If a squirrel cage induction motor draws 2A of current at 240V and is connected for use on 480V, the amperage will be ____
1A
In order for two or more motors to be fed from the same branch circuit, each motor must be rated at no more than
1hp
Of the following motors, the one most likely to have 15 or 18 motor leads is
2,400V three-phase motor
If two 10 ohm windings are connected in series for operation on 480V, the total impedance is _____.
20 ohm, 10 + 10 = 20
If the current drawn by a three-phase motor operating at 480 volts is 10.2 amperes, what will the current most likely be if the motor is reconnected to run and is operated at 240 volts, with all conditions remaining the same?
20.4A
A circuit breaker used as the disconnecting means for a 20A motor must have a minimum size of
23A, (20 x 1.15) = 23A
If 480V is applied to 20 ohm, what current will flow
24A, 480V/20ohm = 24A
What is the maximum value for short circuit protection for multi-motor branch circuits?
250% of the smallest motor
For circuits of OVER _____ to ground, the electrical continuity of rigid metal conduit (RMC) or intermediate metal conduit (IMC) that contain any conductor other than service conductors shall be ensured with two (2) locknuts, one inside and one outside the box or cabinet.
250V, This answer can be found in NEC Section 250.97(2)
A motor control center serves three motors, each with a rating of 10hp and an FLA of 14A. What is the minimum feeder size?
3(14) = 42A x .80 = 33.6A
The synchronous speed of a 60Hz, three-phase induction motor with two poles is
3,600 rpm
For ordinary motors, the short circuit ground for protection for non-time delay fuses are sized at what percentage of the motor full-load current?
300%
For ordinary motors, the short circuit/ground fault protection for non-time-delay fuses are sized at what percentage of the motor full current?
300%
Ordinary motor must have short circuit / ground fault protections sized ______ of the motor FLC, while DC and wound rotor motors can have protection at ______
300% and 150%
What is the maximum kVAR capacitor rating that should be applied as a power factor correction in a 15-horsepower induction motor with a nominal speed of 1,800 rpm?
4
When connecting a three-phase, dual-voltage, delta-wound motor for 480-volt operation, which of the following combinations of lead connections is correct?
4 to 7, 5 to 8, 6 to 9
The total resistance of the stator windings in a three-phase motor, if the windings are connected in parallel and the resistance of each winding is 96 ohm, is
48 ohm, (96 ohm / 2) = 48 ohm
With 240V applied to a total parallel impedance of 5 ohms, the current is ____.
48A, 240/5 = 48A
if the stator windings of a dual-voltage motor both have an impedance of 10Ω and they are connected in parallel, the total impedance is _____.
5 ohm, (10 ohm / 2) = 5 ohm
The locked-rotor current of a motor is typically
6 times the normal running current
The most common number of motor leads found on a three-phase, wye-wound motor is
9 leads
You are connecting a 10hp motor rated at 14A and a 50hp motor rated at 65A to a 480V motor control center. The minimum feeder ampacity is
95.25A, (65A x 1.25) + 14A = 95.25A
Choose correct statement A. The rotor is the stationary winding in an AC motor. B. Three-phase motors are able to start turning because of a start winding. C. A synchronous motor is a type of AC motor.
A synchronous motor is a type of AC motor.
The most effective method of power factor correction is the installation of
Capacitors
The most effective method of power factor correction is the installation of....
Capacitors
When sizing equipment grounding conductors for branch circuits of duty motors,
Combine the rating of each branch circuit OCPD and, when applicable, the rating of each overload protective device
In addition to short circuit protection MCPs may also serve as
Disconnecting means or overload protection and more control
If we were installing dual-element time-delay fuses to protect a motor that has an FLA of 65 amperes, what is the maximum fuse size we could use without increasing the size to 225% because of high starting current?
Dual-element, time-delay fuses may be sized up to 175% of the motor's full-load current for all standard motors: 65 ´ 1.75 = 113.75 amperes. The closest fuse size to that is 125 amperes, because 115 amperes is not a standard fuse size.
Under which of the following conditions can three 5-horsepower motors be supplied by one branch circuit that has short circuit protection? A. The short circuit protection in the branch circuit does not exceed 20A. B. Each motor starter is equipped with heaters. C. The trip setting of the circuit breaker is no greater than 15A in a 120V circuit.
Each motor starter is equipped with heaters
True or False, A dual-voltage, delta-connected motor always has nine leads.
False
True or False, Branch circuit conductors are sized at 100% of the FLA for motors rated as continuous duty.
False
True or False, MSCPs can be installed in any fuse holder.
False
The synchronous speed of an induction motor is determined by the number of stator poles and the.....
Frequency in hertz
Which of the following describes MSCPs?
Fuse-like devices designed for use only in a special type of fusible-switch combination motor starter
To calculate the maximum safe load that a motor may expected to carry continuously and it's rated voltage and frequency the service factor is two motors multiplied by.....
Horsepower rating
Used for short circuit protection of a motors branch circuits may be adjustable
Instantaneous Trip circuit breakers
When use for short circuit protection of the motor branch circuits what type of breaker must be adjustable and can be used in conjuction with combination motor starters in.....
Instantaneous trip circuit breakers
When used for short circuit protection of motor branch circuits, which of the following must be adjustable and used in conjunction with combination motor starters?
Instantaneous trip circuit breakers
Fuse like devices design for use only and special type of fusible switch combination motor starters are also known as
Motor short circuit protectors
The requirements for torque motors shaded motors permanent split motors and easy adjustable voltage motors can be found in
NEC Article 430
Information on calculating conductor ampacities for shaded-pole motors can be found in
NEC Section 427.45
The requirements for torque motors, shaded-pole motors, permanent split capacitor motors, and AC adjustable-voltage motors can be found in.....
NEC Section 430
List FLC for three-phase motors
NEC Section 430.250
The minimum size equipment grounding conductor for a motor circuit can be found in
NEC Table 250.122
The primary full-load current ratings for wound-rotor motors are listed in
NEC Table 430.250
Which of the following lists the full-load currents for three-phase motors with a known winding type, horsepower and voltage?
NEC Table 430.250
What NEC article can you find load current for three face motors with known winding type horsepower and voltage?
NEC articles 430.250
Can a circuit breaker with a current rating of 90A be used as a motor disconnecting means on a motor with an FLA of 80A?
NO
Proper overload protection includes both.....
Overload and current limiting short circuit protection
What is the minimum feeder conductor ampacity for four motors: 50hp, 40hp, and two at 10hp? (Round off to a whole number).
Refer to NEC Table 430.250 to find the FLA of each motor: 50hp = 65A, 40hp = 52A, and the two 10hp motors each draw 14A. The largest motor has an FLA of 65A. We add the others: 52A + 14A + 14A = 80A. We multiply the largest motor (65A) by 1.25 = 81.25A. We add this to the total of the others: 81.25A + 80A = 161.25A, rounded off to 161A.
Good electrical protection for a motor includes both proper overload protection and
Short circuit protection and current limiting
Name the three basic type of three-phase motors
Squirrel cage induction, wound-rotor induction, synchronous motor
which of the following is true concerning the starting current of an induction motor at the moment the circuit is energized?
The current rises rapidly
The service factor multiplied by the HP rating calculates
The maximum safe load a motor can carry continuously
The synchronous speed of an induction motor is determined by the number of.....
The number of stator poles and frequency in Hertz
One of the three factors that will cause at three phase motor magnetic field to rotate is that.....
The state or windings around the inside of the motor or arrange any specific manner to induce rotation
A feeder supplying two (2) continuous-duty, 208-volt, 3-phase motors, one 10 hp and one 7½ hp shall have a MINIMUM ampacity of _____ amperes.
This answer can be found in NEC Section 430.24(1)&(2), Table 430.250. 10 hp - FLC = 30.8 amps x 125% = 38.5 amperes 7½ hp - FLC = 24.2 amps x 100% = 24.2 amperes TOTAL = 62.7 amperes
All of the following provide motor overload protection except _____. A. time-delay relays B. overload relays C. circuit breakers D. fuses
Time-delay relays
True or False, An MSCP is designed for use only in a special type of fusible-switch combination motor starter.
True
What three factors cause the magnetic field to rotate
Voltage of 3 phase 120* out of phase from each other, 3 voltages change polarity intervals, stator windings induced
Branch circuit power is supplied to a continuous duty motor using No. 12 AWG wire. Can No. 12 wire be used as the grounding conductor in this application?
Yes
Stator windings can best described as
a structure of copper or aluminum wire coils
To size overcurrent protection for an existing oversized motor, determine the full-load running current of the motor using an
ammeter
When it occurs at the starting current of induction motor at the moment the circuit is energize
current rises rapidly
Good electrical protection for a motor includes both proper overload protection and......
current-limiting short-circuit protection
When a stator is connected for both starting and running, the running connection is a
delta configuration
In addition to short circuit protection, MSCPs may also serve as a
disconnecting means
When a motor is operated at 240V, the current draw of the motor is_______ the current draw of a 480V
double
Starting current of an induction motor rises rapidly the moment the motor is.....
energized
The main purpose of motor overload protection is to protect the motor
from damaging overload currents
When a motor is oversized, a higher degree of overload protection can be provided by sizing the fuses based on the
full-load current
When a motor is oversized, a higher degree of overload protection can be provided by sizing the fuses based on the....
full-load current
Where a central vacuum assembly is located in a storage closet adjacent to the laundry room of a dwelling, accessible non-current-carrying metal parts of the assembly likely to be energized shall be _____.
grounding
Doubling the voltage on a dual-voltage motor
halves the full-load current
If an instantaneous trip circuit breaker is used in a three-phase motor circuit, what must be installed in the motor starter to protect the motor from overload?
heaters
To calculate the maximum safe load that a motor may be expected to carry continuously at its rated voltage and frequency, the service factor is multiplied by the........
horsepower rating
Stator windings are arranged to
induced rotation
A motor overload device functions in direct response to _____.
motor current, This answer can be found in NEC Section 430.32(A)(1)
It may be necessary to increase the size of a dual-element fuse for an oversized motor when the
motor is frequently reversed
The type of a three-phase motor is determined by
rotor
The synchronous speed of a motor is the
speed at which the magnetic field rotates
One of the three factors that will cause a three-phase motor's magnetic field to rotate is that the.....
stator windings around the inside of the motor are arranged in a specific manner to induce rotation
The two most common three-phase motor configurations are
wye and delta
What is the maximum size nontime-delay fuse that can be used to protect a motor that has an FLA of 65 amperes?
Nontime-delay fuses can be sized at 300% of the FLA: 65 ´ 3.00 = 195. Because 195 is not a standard size fuse, NEC Section 430.52(C)(1), Exception No. 1 allows us to increase the fuse size up to the next standard size, which is 200 amperes.
