26309 - motor calcuations
A three-phase motor with four stator poles has a synchronous speed of _____. 3,600 rpm 1,200 rpm 900 rpm 1,800 rpm
1,800 rpm Four poles equal 1,800 rpm in synchronous speed
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 _____. 8 AWG 6 AWG 10 AWG 12 AWG
10 AWG
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? 100 amperes 200 amperes 115 amperes 125 amperes
125 amperes 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.
What is the minimum feeder conductor ampacity for four motors: 50hp, 40hp, and two at 10hp? (Round off to a whole number). 100 amperes 145 amperes 161 amperes
161 amperes 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. Now add the others: 52A + 14A + 14A = 80A. Multiply the largest motor (65A) by 1.25 = 81.25A. Then add this to the total. 81.25A + 80A = 161.25A. Rounded off to 161A.
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? 20.4 amperes 10.2 amperes 5.1 amperes
20.4 amperes When a motor is operated at 240V, the current draw of the motor is double the current draw of a 480V connection
What is the maximum size nontime-delay fuse that can be used to protect a motor that has an FLA of 65 amperes? 200 amperes 60 amperes 70 amperes
200 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).
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. 250 volts 125 volts 480 volts 300 volts
250 volts
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? 5 9 4
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? 7 to 6 and 1, 8 to 4 and 2, 9 to 5 and 3 4 to 7, 5 to 8, 6 to 9
4 to 7, 5 to 8, 6 to 9 For higher voltages, leads 4 and 7, 5 and 8, and 6 and 9 are connected together.
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 68.75 62.70 55.00 38.50
62.70 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
Rating
A designated limit of operating characteristics based on definite conditions.Such operating characteristics as load, voltage, frequency, etc., may be given in therating.
Torque
A force that produces or tends to produce rotation. Common units ofmeasurement of torque are foot-pounds and inch-pounds.
Circuit interrupter
A non-automatic, manually operated device designed to open acurrent-carrying circuit without injury to itself
Choose the correct statement. Three-phase motors are able to start turning because of a start winding. A synchronous motor is a type of AC motor. The rotor is the stationary winding in an AC motor.
A synchronous motor is a type of AC motor.
Under which of the following conditions can three 5-horsepower motors be supplied by one branch circuit that has short circuit protection? The short circuit protection in the branch circuit does not exceed 20A. Each motor starter is equipped with heaters. The trip setting of the circuit breaker is no greater than 15A in a 120V circuit.
Each motor starter is equipped with heaters. Two or more motors of any rating, each having individual overload protection, may be connected to a single branch circuit that is protected by a short circuit protective device.
True or False. Branch circuit conductors are sized at 100% of the FLA for motors rated as continuous duty.
False NEC Section 430.22 requires that the ampacity of branch circuit conductors supplying a single continuous-duty motor must not be less than 125% of the motor full-load current rating.
True Or False. A dual-voltage, delta-connected motor always has nine leads.
False.
True or False. MSCPs can be installed in any fuse holder.
False. MSCPs are designed for use in a special type of fusible-switch combination motor starter.
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? Auxiliary contacts Heaters Dual-element time-delay fuses
Heaters Heaters in motor starters protect the entire circuit and all equipment against overloads up to and including locked-rotor current.
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 Yes Not enough information given to answer
No 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.
Terminals
Points at which an electrical component may be connected to anotherelectrical component.
Service factor
The number by which the horsepower rating is multiplied to determinethe maximum safe load that a motor may be expected to carry continuously at its ratedvoltage and frequency
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 No Not enough information available
Yes In this situation, the sizing is not required to be larger than the circuit conductors supplying the equipment.
A motor overload device functions in direct response to _____. ambient temperature temperature rise of the motor motor current voltage
motor current
All of the following provide motor overload protection except _____. circuit breakers time-delay relays fuses overload relays
time-delay relays A time-delay relay does not provide motor overload protection.
