Question 1

There are several ways of controlling the speed of direct current motors. The method employed is generally dictated by the requirements of the load.

Accepted Answer

This statement is true. The speed of a DC motor can be controlled through various methods depending on the requirements of the load.

Question 2

Once horsepower has been converted to a basic unit of power, it can be converted to other power units, such as in the following: ____.

Accepted Answer

1 W = 3.42 BTUs per hour is correct because 1 watt is equivalent to approximately 3.412 BTUs per hour.

Question 3

The armature of a ServoDisc motor does not contain any copper.

Accepted Answer

The armature of a ServoDisc motor typically contains copper windings, which are essential for its operation as they create the electromagnetic fields needed for motion.

Question 4

Most large compound direct current motors have a protective device connected in series with the shunt field called the ____ relay.

Accepted Answer

The protective device connected in series with the shunt field in a large compound direct current motor is called the field loss relay. This relay is designed to disconnect the shunt field in case of an open circuit or field failure, preventing damage to the motor.

Question 5

Permanent magnet direct current motors with iron armatures have a problem with cogging at high speeds.

Accepted Answer

Cogging in permanent magnet direct current motors is more associated with the interaction between the permanent magnets and the slots of the stator as the rotor turns, causing a variation in torque, especially noticeable at low speeds, not high speeds.

Question 6

The efficiency of a motor can be found by using the following formula: (power out/power in) x 100.

Accepted Answer

This formula is used to calculate the efficiency of a motor.

Question 7

The following formula can be used to determine the speed of the rotating magnetic field when the frequency and number of stator poles per phase are known: S = ____.

Accepted Answer

The correct formula to determine the speed of the rotating magnetic field is S = (120 x f) / P, where S is the speed of the rotating magnetic field in revolutions per minute (RPM), f is the frequency of the power source in hertz (Hz), and P is the number of stator poles per phase.

Option A is incorrect because it multiplies the frequency by 120, but does not divide by the number of stator poles per phase.

Option B is incorrect because it multiplies both the frequency and number of stator poles per phase by 120.

Option D is incorrect because it subtracts the frequency from 120, which is not a valid calculation.

Question 8

How much horsepower is being produced by a motor turning a load of 350 lb-in. at a speed of 1375 RPM?

Accepted Answer

The formula to calculate horsepower is:
hp = (torque × RPM) / 5252
Where torque is in pound-inches and RPM is the speed of the motor.

Substituting the given values, we get:
hp = (350 lb-in. × 1375 RPM) / 5252 = 7.648 hp

Rounding off to two significant figures, the answer is 7.65 hp, which is closest to option B.

Question 9

Some converters that supply power to brushless motors produce sine waves, but most produce a ____ waveform.

Accepted Answer

Most converters for brushless motors produce a trapezoidal AC waveform, which is simpler and more cost-effective than generating a pure sine wave.

Question 10

The amount of counter-EMF produced in the armature is proportional to three factors: the number of turns of wire in the armature, the strength of the magnetic field of the pole pieces, and the speed of the armature.

Accepted Answer

According to Faraday's law of electromagnetic induction, the magnitude of the induced EMF is proportional to the rate of change of magnetic flux. The rate of change of magnetic flux depends on the number of turns of wire in the armature, the strength of the magnetic field of the pole pieces and the speed of the armature. Therefore, the counter-EMF produced in the armature is proportional to these three factors.

Question 11

A motor is a device used to convert electrical energy into kinetic potential energy.

Accepted Answer

A motor is a device used to convert electrical energy into mechanical energy (specifically rotational kinetic energy).

Question 12

In the right-hand motor rule, thumb = thrust or the ____.

Accepted Answer

According to the right-hand motor rule, if we point our thumb in the direction of the current flowing through the armature of a motor, then the direction in which our fingers curl would be the direction of rotation of the armature. Therefore, the correct option is D - direction of armature rotation.

Question 13

Series motors have the ability to develop extremely high starting torques. An average of about ____% of full torque is common.

Accepted Answer

Series motors are known for their high starting torque, often reaching about 450% of their full load torque.

Question 14

Brushless DC motors contain a wound armature and commutator but do not contain brushes.

Accepted Answer

Brushless DC motors do not contain a wound armature, commutator, or brushes; instead, they use an electronic controller to switch the current in the motor windings.

Question 15

Compound motors can be connected as short shunt or long shunt just as compound generators can. They can also be cumulative-compounded motors or differential-compounded motors.

Accepted Answer

Compound motors can be connected as short shunt or long shunt and can also be cumulative-compounded or differential-compounded.

Question 16

Small permanent magnet motors are used as ____.

Accepted Answer

Small permanent magnet motors are commonly used as servomotors. They are ideal for applications that require precise control of acceleration, speed, and position, such as robotics, CNC machines, and electric vehicles. The other choices (armamotors, revomotors, and polamotors) are not commonly used terms for motor types.

Question 17

Two factors that determine the amount of torque produced by a direct current motor are the strength of the magnetic field of the pole pieces and the ____.

Accepted Answer

The strength of the magnetic field of the pole pieces determines the strength of the magnetic field that interacts with the armature, which in turn determines the amount of torque produced by the motor. The direction of the magnetic field and the direction of the turning force do not directly affect the amount of torque produced. The strength of the magnetic lines of flux is essentially the same as the strength of the magnetic field, so answer D is not a distinct factor.

Question 18

Many large DC compound motors intended to operate in an overspeed condition will actually contain three separate shunt fields.

Accepted Answer

DC compound motors typically contain two sets of windings: a series winding and a shunt (or parallel) winding, not three separate shunt fields.

Question 19

The speed regulation of a direct current motor is proportional to the resistance of the armature.

Accepted Answer

The speed regulation of a DC motor is affected by the armature resistance because it influences the voltage drop across the armature, which in turn affects the speed of the motor.

Question 20

____ motors are often referred to as constant speed motors.

Accepted Answer

Shunt motors are often referred to as constant speed motors because they have a nearly constant speed regardless of the load on the motor. This is because the field current is constant and independent of the armature current, resulting in a relatively constant speed. Compound and series motors are not constant speed motors, and current motors are not a type of electric motor.

Quiz 15: Direct Current Motors