Question 1

A disadvantage of open transition starting is that it does not require accessory voltage reducing equipment.

Accepted Answer

This statement is completely false. Open transition starting often requires accessory voltage reducing equipment in order to prevent damage to the motor and other electrical components. The statement is incorrect. A disadvantage of open transition starting is that it may require accessory voltage reducing equipment to prevent excessive current flow or voltage drop during the switch between the contactors.

Question 2

If the motor nameplate indicates only the delta-connected full-load current, divide this value by to obtain the winding current.&#10;A)1.73&#10;B)3.14&#10;C)6.33&#10;D)10

Accepted Answer

When a motor is delta-connected, the line current is equal to the winding current. However, if the motor nameplate indicates only the delta-connected full-load current, this is the line current value. To obtain the winding current, it is necessary to divide this value by the square root of 3, which is approximately equal to 1.73. Therefore, the correct choice is A. When a motor is delta-connected, the line current is equal to the winding current. However, when a motor is star-connected, the line current is √3 times the winding current. Therefore, to obtain the winding current of a delta-connected motor, we simply divide the full-load current by 1.73.

Question 3

Closed transition starting is accomplished by adding another three pole contactor and resistors to the circuit.

Accepted Answer

Closed transition starting does require the addition of another three pole contactor and resistors to the circuit. Closed transition starting is achieved by using an additional three pole contactor and resistors in the circuit to slowly transfer the motor load from the primary to the secondary power supply without causing any voltage or current spikes.

Question 4

Open transition starting may be objectionable in some applications if the transition causes spikes on the power line when ____.&#10;A)adding another three pole contactor and resistors&#10;B)the motor changes from wye to delta&#10;C)voltage reducing equipment is connected&#10;D)an on-delay timer has been added to the control circuit

Accepted Answer

During open transition starting, when the motor changes from wye (star) to delta configuration, there can be a momentary power interruption. This interruption can cause voltage spikes on the power line, which may be objectionable in sensitive applications.

Question 5

Star-delta motors are similar in construction to standard squirrel cage motors.

Accepted Answer

Star-delta motors have a similar construction to standard squirrel cage motors, but they have additional wiring and connections to enable them to operate in both star and delta configurations. Star-delta motors are indeed similar in construction to standard squirrel cage motors. The main difference is in the winding configuration and how the motor is connected to the power supply.

Question 6

The speed of a star-delta, squirrel cage induction motor depends on the number of poles of the motor and the supply line .&#10;A)resistance&#10;B)capacitance&#10;C)frequency&#10;D)temperature

Accepted Answer

The speed of a star-delta, squirrel cage induction motor mainly depends on the frequency of the supply line. The number of poles also plays a role in determining the motor's speed, but the frequency is the primary factor. The motor will run at a speed directly proportional to the frequency of the supply line, and inversely proportional to the number of poles. Therefore, the correct choice is C) frequency. The speed of a star-delta, squirrel cage induction motor depends on the number of poles of the motor and the supply line frequency. When the frequency of the AC supply to the motor is changed, the motor speed will change proportionally due to the electromagnetic induction principle. The higher the frequency, the higher the motor speed, and vice versa. Therefore, the correct answer is C - frequency.

Question 7

Star-delta motors are widely used on loads having high inertia and a .&#10;A)lagging power factor&#10;B)reversing winding arrangement&#10;C)40C rating&#10;D)long acceleration period

Accepted Answer

Star-delta motors are used on loads having high inertia and long acceleration period. This is because the star-delta starting method allows the motor to start with reduced voltage and current, which limits the initial torque and reduces the mechanical stress on the motor and the load. This reduces the risk of damage to the motor and the load during the starting period, especially when the load has high inertia and requires a long time to reach the rated speed. Star-delta motors are specifically designed for loads with high inertia and long acceleration periods. They are capable of providing a high starting torque while reducing the starting current drawn from the power supply. Therefore, option D is the best choice. The other options do not affect the selection of star-delta motors.

Question 8

Star-delta motor starting is sometimes called double-delta starting.

Accepted Answer

Star-delta motor starting is also known as wye-delta starting, not double-delta starting.

Question 9

Star-delta motors may be used where a reduced starting current is required.

Accepted Answer

Star-delta motors have a lower starting current compared to other types of motors, which makes them suitable for applications where a reduced starting current is necessary. This is because the starting current is reduced when the motor is started in star mode, and then switched to delta mode once it has reached a certain speed. Yes, this statement is true. Star-delta motors have a lower starting current compared to direct-on-line motors, which may be beneficial in systems with limited power supply capacity or when the starting torque requirement is low.

Question 10

Wye-delta starting is accomplished by connecting the in wye during the starting period and then reconnecting them in delta for normal run operation.&#10;A)stator windings&#10;B)overload relays&#10;C)timer contacts&#10;D)motor terminals

Accepted Answer

Wye-delta starting is a method used to reduce starting current and torque in three-phase AC induction motors while still providing full voltage to the motor during normal operation. This is achieved by connecting the stator windings in a wye configuration during the starting period, which reduces the voltage applied to each winding and therefore reduces the current and torque. Once the motor reaches a certain speed, the windings are reconnected in a delta configuration for normal run operation, which provides full voltage and full torque to the motor. Therefore, the stator windings are the components that are connected in a wye configuration during the starting period and reconnected in a delta configuration for normal run operation. During wye-delta starting, the stator windings of the motor are connected in wye during the starting period for reduced voltage, and then reconnected in delta for normal run operation for full voltage. This method is used to reduce the starting current drawn by the motor and prevent damage to the motor and its connected equipment.

Deck 29: Automatic Starters for Star-Delta Motors