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true/false Three-phase motors typically cost more to operate per horsepower than single-phase motors or DC motors.
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If the total number of coils wound in a 3-phase stator is 24, then each phase contains ? sets of coils.
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motors

lesson 4 Three-Phase Motors

QuestionAnswer
true/false Three-phase motors typically cost more to operate per horsepower than single-phase motors or DC motors. false three-phase motors cost less to operate per horsepower than single-phase motors or DC motors.
If the total number of coils wound in a 3-phase stator is 24, then each phase contains ? sets of coils. 8 each phase must have the same number of coils, so the total number of coils must be divisible by the number 3.
The ? is a cylindrical bar used to carry the revolving rotor and to transfer power from the motor to the load. motor shaft
true/false It takes 360 electrical degrees, or one electrical cycle, to complete one revolution in a 2-pole motor. true
The synchronous speed of a 60-hertz, 4-pole motor is ? . 1800 rpm
? increases with increasing frequency and decreases with decreasing frequency. Inductive reactance
The ? of 3-phase motors can be reversed by interchanging any two of the 3-phase power lines to the motor. direction of rotation
? is the torque required to produce the rated power at full speed of the motor. Full-load torque
Torque is usually measured in ? . pound-feet (lb-ft)
A(n) ? is the base unit of electrical power. watt (W)
? is also referred to as breakaway torque or starting torque. Locked-rotor torque
Motor power is rated in ? or ? . horsepower / watts
true/false High-inertia loads are always easy to accelerate and decelerate with a motor that has sufficient torque to drive the load at full speed. false These types of loads are hard to accelerate and decelerate, even with a motor that has sufficient torque to drive the load at full speed.
When the speed of a motor changes, the horsepower and torque required at the new speed may be ? . higher, lower, or the same
? loads require increase in torque and horsepower with an increase in speed. Variable-torque
? loads require a constant torque output at different speeds. Constant-torque
true/false With a variable-torque load, the horsepower varies with the square of the speed of change. false horsepower varies with the cube of the speed change
? is the power drawn by a motor that produces useful work. true power
Apparent power is measured in ? or ? . VA or kVA
What is the power factor on a 480-volt, 60-horsepower, 3-phase motor with an efficiency of 85% when the full load current is 70 amperes? 91%
true/false A low power factor indicates that the circuit is drawing more current than is required by the load; utility companies penalize customers with low power factors. true
? is calculated by dividing output power by input power. Motor efficiency
? is the loss of flux in the air gap between the stator and rotor because the air gap has increased reluctance compared to the cores. Flux-linkage loss
true/false Flux-linkage loss is energy lost by blowing air past a motor to remove heat. false windage loss is energy lost by blowing air past a motor to remove heat.
A motor operates with the following phase currents: Phase A = 73.8 amperes, Phase B = 68.5 amperes, and Phase C = 64.4 amperes. What is the maximum current difference from the average? (Round the FINAL answer to one decimal place.) 4.9 A (73.8+68.5+64.4)/3=avg 73.8- avg=4.9
A motor operates with Phase A = 73.8 amperes, Phase B = 68.5 amperes, and Phase C = 64.4 amperes. Motors should not be operated above a 10% current unbalance. Calculate the maximum percentage phase unbalance for this motor. 6.64% 4.9/73.8 X 100= 6.64
true/false A motor with an 8.3% current unbalance can be operated, but with caution to avoid overheating the motor. true
Created by: Athomp705
 

 



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