1.
It is possible for CEMF to be produced if the armature is not turning.
Correct Answer
B. False
Explanation
CEMF, or counter electromotive force, is a voltage that opposes the applied voltage in an electric circuit. It is generated when there is relative motion between the magnetic field and the conductors in the circuit. In the case of an armature, which is a rotating part of an electric machine, CEMF is produced when it is turning. Therefore, if the armature is not turning, there will be no relative motion and no CEMF will be produced. Hence, the statement is false.
2.
The direction of rotation by the shaft of a universal motor can be changed by reversing the AC power leads.
Correct Answer
B. False
Explanation
The direction of rotation by the shaft of a universal motor cannot be changed by reversing the AC power leads. This is because a universal motor is designed to operate on both AC and DC power, and reversing the AC power leads will not affect the direction of rotation. To change the direction of rotation, the polarity of the DC power supply needs to be reversed.
3.
The direction of the wound rotor induction motor can be reversed by interchanging two of the three rotor terminals.
Correct Answer
B. False
Explanation
The direction of the wound rotor induction motor cannot be reversed by simply interchanging two of the three rotor terminals. The direction of rotation in a wound rotor induction motor is determined by the phase sequence of the stator windings, not the rotor terminals. To reverse the direction of rotation, the phase sequence of the stator windings needs to be changed.
4.
Interchanging the rotor leads of a wound rotor motor reverses the direction of the motor.
Correct Answer
B. False
Explanation
Interchanging the rotor leads of a wound rotor motor does not reverse the direction of the motor. The direction of rotation in a wound rotor motor is determined by the stator winding configuration and the phase sequence of the applied voltage. Interchanging the rotor leads may cause the motor to operate in an abnormal or inefficient manner, but it does not reverse the direction of rotation.
5.
BDCMs have low maintenance requirements because they do not have brushes.
Correct Answer
A. True
Explanation
Brushless DC motors (BDCMs) do not have brushes, unlike brushed DC motors. Brushes in motors are used to transfer electrical current between the stationary and rotating parts of the motor. However, brushes are prone to wear and tear, requiring regular maintenance and replacement. Since BDCMs do not have brushes, they have lower maintenance requirements compared to brushed DC motors. Therefore, the statement that BDCMs have low maintenance requirements because they do not have brushes is true.
6.
In the shading coil of the shaded-pole motor, the direction of the induced current_______the change in magnetic field that induced it.
Correct Answer
B. Opposes
Explanation
The shading coil in a shaded-pole motor is designed to create a phase shift in the magnetic field, which helps to start the motor and maintain rotation. The direction of the induced current in the shading coil opposes the change in magnetic field that induced it. This opposition creates a rotating magnetic field, which in turn creates torque and allows the motor to start and continue rotating.
7.
When the DC excitation increases to cause an over-excited synchronous motor, a_________is developed.
Correct Answer
B. Leading power factor
Explanation
When the DC excitation increases to cause an over-excited synchronous motor, the armature current lags behind the excitation voltage, resulting in a leading power factor. This means that the current leads the voltage in the circuit, which is characteristic of an over-excited synchronous motor.
8.
Which winding of a split-phase motor has the largest inductance?
Correct Answer
B. Run
Explanation
The run winding of a split-phase motor has the largest inductance. This is because the run winding is designed to provide the main torque for the motor to operate at normal speed. It is typically made with more turns of wire, resulting in a higher inductance compared to the start winding. The higher inductance helps to create a phase shift between the two windings, allowing the motor to start and run smoothly.
9.
The speed of a moving coil motor (MCM) is varied by _______.
Correct Answer
B. Changing the width of DC pulses applied to the armature
Explanation
The speed of a moving coil motor (MCM) is varied by changing the width of DC pulses applied to the armature. This means that by adjusting the duration of the pulses, the motor's speed can be controlled. A wider pulse will result in a higher speed, while a narrower pulse will result in a lower speed. This method of speed control is commonly used in various applications where precise speed adjustments are required.
10.
If the sequence in which the stator poles are magnetized moves in a CCW direction, the rotor direction of a VR stepper motor will be_______.
Correct Answer
A. Clockwise
Explanation
If the sequence in which the stator poles are magnetized moves in a counterclockwise (CCW) direction, the rotor direction of a VR stepper motor will be clockwise. This means that as the stator poles are magnetized in a counterclockwise direction, the rotor will rotate in a clockwise direction.
11.
The ________-hand rule is used to describe the direction of motion in a motor.
Correct Answer
right
Explanation
The right-hand rule is used to describe the direction of motion in a motor. This rule states that if the thumb of the right hand points in the direction of the current flow, then the fingers will curl in the direction of the magnetic field. In the context of a motor, the right-hand rule is used to determine the direction of the force experienced by a current-carrying wire in a magnetic field, which ultimately determines the direction of motion of the motor.
12.
The armature is placed inside the__________field.
Correct Answer
mainfield
Explanation
The armature is placed inside the mainfield. This suggests that the armature, which is a component of an electric motor or generator, is positioned within the main magnetic field. The mainfield refers to the primary magnetic field that is generated by the stator or field magnets. By placing the armature inside the mainfield, it allows for the interaction between the magnetic field and the armature conductors, resulting in the generation of electrical energy or the conversion of electrical energy into mechanical energy.
13.
Motors that operate by using AC or DC power are called __________motors.
Correct Answer
universal
Explanation
Motors that operate by using both AC and DC power are called universal motors. These motors are designed to work with either type of power source, allowing them to be versatile and used in a wide range of applications. Universal motors are commonly found in household appliances such as blenders, vacuum cleaners, and power tools, where they can easily switch between AC and DC power depending on the needs of the device.
14.
When a WRIM is started, the ohm setting of the resistor network is _________, and the torque is maximum because____________________________________________.
Correct Answer
set to maximum
the flux strength of both fields are at their maximum valuse
Explanation
When a WRIM (Wound Rotor Induction Motor) is started, the ohm setting of the resistor network is set to maximum. This is done to limit the initial current drawn by the motor and prevent excessive torque during startup. At the same time, the flux strength of both the stator and rotor fields are at their maximum values. This allows for a higher torque production, enabling the motor to overcome the inertia and start rotating efficiently. By gradually reducing the resistance in the rotor circuit, the motor can smoothly transition from startup to normal running conditions.
15.
Suppose that the rotating speed of the magnetic field in the stator of an induction motor is 2,700 rpm. If the rotor speed is 2,670, the difference in speed between these two rpm values is called_______, and is measured in _________, of which the value is ______%.
Correct Answer
slip
percentage
1.11
Explanation
The difference in speed between the rotating magnetic field in the stator and the rotor speed is called slip. Slip is measured in percentage, which represents the relative difference between the two speeds. In this case, the slip is 1.11%, indicating that the rotor speed is 1.11% lower than the speed of the rotating magnetic field in the stator.
16.
How much work does a DC motor accomplish when it moves a 400-pound weight a distance of 20 feet?
Correct Answer
8000 ft-lbs
17.
A 4,000-pound sack of grain is lifted to a height of 25 feet in 45 seconds. How many horsepower of work is performed?
Correct Answer
4.04 hp
Explanation
In order to find the amount of work performed, we need to calculate the amount of energy used to lift the sack of grain. The formula for work is work = force x distance. The force can be calculated by multiplying the weight of the sack (4,000 pounds) by the acceleration due to gravity (32.2 ft/s^2). The distance is given as 25 feet. By substituting these values into the formula, we can find the work done. To convert the work into horsepower, we divide the work by the time taken (45 seconds) and then multiply by a conversion factor. The resulting value is 4.04 hp.
18.
Find the armature current of a shunt motor with an armature resistance of 6ohms and 120 volts applied. Then find the amount of torque produced if the motor has a Kt rating of 1 and a field flux of 2.
Correct Answer
20 A
40 lb-ft
Explanation
The armature current of a shunt motor can be calculated using Ohm's law, which states that current (I) is equal to voltage (V) divided by resistance (R). In this case, the armature resistance is given as 6 ohms and the applied voltage is 120 volts. Therefore, the armature current can be calculated as 120 volts divided by 6 ohms, which equals 20 A.
The torque produced by the motor can be determined using the torque constant (Kt) and the field flux. The torque produced is equal to the product of the torque constant and the field flux. In this case, the Kt rating is given as 1 and the field flux is given as 2. Therefore, the torque produced is 1 multiplied by 2, which equals 2 lb-ft.
19.
How much power in watts is consumed to move a 30-pound sack 100 feet in 2 seconds?
Correct Answer
2085.2 W
Explanation
To calculate the power consumed, we need to use the formula: power = work/time. In this case, the work done is equal to the force applied multiplied by the distance moved. The force applied can be calculated using the weight of the sack (30 pounds) multiplied by the acceleration due to gravity (32.2 ft/s^2). The distance moved is given as 100 feet. The time taken is 2 seconds. By substituting these values into the formula, we can calculate that the power consumed is 2085.2 watts.
20.
Find the armature current of a series DC motor with an armature resistance of 7 ohms and a field coil resistance of 15 ohms when 220 volts are applied.
Correct Answer
10 A
Explanation
The armature current of a series DC motor can be calculated using Ohm's Law, which states that current (I) is equal to voltage (V) divided by resistance (R). In this case, the total resistance is the sum of the armature resistance and the field coil resistance. Therefore, the total resistance is 7 ohms + 15 ohms = 22 ohms. The voltage applied is 220 volts. Plugging these values into the formula, we get I = 220 V / 22 ohms = 10 A.