Question 1

A 15-lead motor has ____ coils per phase.&#10;A) one&#10;B) two&#10;C) three&#10;D) four

Accepted Answer

A 15-lead motor is likely a three-phase motor where each phase has five coils. Therefore, each phase has three phases at the motor's start, three phases in the middle, and three phases at the end of the motor windings, totaling nine coils. Additionally, there are six coils that connect the three phases together. Therefore, a 15-lead motor has three coils per phase.

Question 2

Three-phase motors with dual-voltage stators are designed to be connected to ____.&#10;A) 90 volts or 120 volts&#10;B) 120 volts or 240 volts&#10;C) 90 volts or 480 volts&#10;D) 240 volts or 480 volts

Accepted Answer

Three-phase motors with dual-voltage stators are designed to be connected to either 240 volts or 480 volts. This allows for flexibility in the voltage supply and can be useful in different applications. Connecting the motor to 120 volts or 90 volts, as listed in choices A and C, would not be an option with a dual-voltage stator. Choice B is also incorrect as it only lists 120 volts and 240 volts as options.

Question 3

The stator windings of three-phase motors are always connected in a wye.

Accepted Answer

The stator windings of three-phase motors can be connected in either a wye (star) configuration or a delta configuration, depending on the specific requirements of the application.

Question 4

When the motor is to be operated on 240 volts, the stator windings are connected in series.

Accepted Answer

When the motor is to be operated on a higher voltage like 240 volts, the stator windings are typically connected in parallel to accommodate the higher voltage without increasing the current excessively. In contrast, for lower voltages, the windings might be connected in series to increase the effective resistance and reduce the current draw.

Question 5

The type of three-phase motor is determined by the rotor or rotating member.

Accepted Answer

The type of a three-phase motor is primarily determined by the design of its rotor, which can be either a squirrel cage rotor or a wound rotor, leading to the classification into squirrel cage induction motors and wound rotor motors.

Question 6

The direction of rotation of a three-phase motor can be reversed by changing any two stator leads.

Accepted Answer

Reversing any two stator leads changes the direction of the rotating magnetic field produced by the motor, resulting in a reverse direction of rotation of the motor.

Question 7

Which of the following is a factor that causes the magnetic field of a three-phase motor to rotate?&#10;A) The voltages of a three-phase system are 90 degrees out of phase with each other.&#10;B) The three voltages change polarity at regular intervals.&#10;C) The pole pieces are wound such that they will always have the same magnetic polarities.&#10;D) The stator windings are arranged around the outside of the motor.

Accepted Answer

The three voltages in a three-phase system change polarity at regular intervals, creating a rotating magnetic field. This is fundamental to the operation of three-phase motors, as the sequential change in polarity across the phases generates a magnetic field that rotates around the stator, inducing rotation in the rotor.

Question 8

A motor with 15 or 18 leads can be found in high-voltage installations.

Accepted Answer

Motors with 15 or 18 leads are commonly used in high-voltage installations as they are designed to be connected to various voltage and configuration options.

Question 9

The speed at which the magnetic field rotates is known as the ____.&#10;A) synchronous speed&#10;B) broadband speed&#10;C) asynchronous speed&#10;D) download speed

Accepted Answer

The speed at which the magnetic field rotates in a synchronous motor is known as synchronous speed. Option B, C, and D are not related to the concept of magnetic field rotation in a motor.

Question 10

Which of the following is a basic type of three-phase motors?&#10;A) The squirrel-cage induction motor&#10;B) The split phase motor&#10;C) The asynchronous motor&#10;D) The servomotor

Accepted Answer

The squirrel-cage induction motor is a basic type of three-phase motor that is commonly used in industrial and commercial applications due to its simplicity, reliability, and low cost. The other options (B, C, D) are either single-phase or more complex types of three-phase motors that are not as widely used as the squirrel-cage induction motor.

Deck 14: Three-Phase Motor Principles