RME Review - Autotransformer Motor Starter

What is the primary function of an autotransformer starter in motor control?

• To increase the voltage applied to the motor

• To reduce the inrush current during motor startup

• To reverse the direction of the motor

• To control the motor speed

Answer explanation

The primary function of an autotransformer starter is to reduce the inrush current during motor startup. This helps prevent damage to the motor and electrical system by limiting the initial surge of current.

How does an autotransformer starter reduce the starting current?

• By introducing resistance into the rotor circuit

• By supplying a reduced voltage to the motor through transformer taps

• By supplying a reduced voltage to the motor changing transformer power rating

• By using a timer to delay the application of full voltage

Answer explanation

An autotransformer starter reduces starting current by supplying a reduced voltage to the motor through transformer taps, which limits the initial current draw and helps prevent damage during startup.

What is the typical percentage of voltage reduction provided by an autotransformer starter?

• 50%, 65%, 80%

• 10%, 20%,30%

• 25%, 50%, 75%

• 70%, 85%, 90%

Answer explanation

An autotransformer starter typically provides a voltage reduction of 50%, 65%, or 80%. This helps in reducing the starting current of motors, making the correct choice 50%, 65%, 80%.

What is the key difference between an autotransformer starter and a wye-delta starter in motor control?

• Autotransformer starters are simpler and cheaper than wye-delta starters.

• Wye-delta starters provide higher starting torque than autotransformer starters.

• Autotransformer starters allow more precise voltage reduction and provide higher starting torque than wye-delta starters.

• Wye-delta starters use transformer taps for voltage reduction during startup.

Answer explanation

Autotransformer starters allow for more precise voltage reduction, which helps in providing higher starting torque compared to wye-delta starters, making them more effective for certain motor control applications.

In an autotransformer starter, how does the torque change if the voltage is reduced to 80% of the line voltage?

• Torque is reduced to 64% of full-load torque.

• Torque is reduced to 33% of full-load torque.

• Torque is reduced to 80% of full-load torque.

• Torque is unaffected by the voltage reduction.

Answer explanation

In an autotransformer, torque is proportional to the square of the voltage. Reducing voltage to 80% results in torque = (0.8)^2 = 0.64, or 64% of full-load torque.

Which type of motor is most suitable for starting with an autotransformer starter?

• Single-phase motor

• Small three-phase induction motor

• Large three-phase induction motor with high starting torque requirements

• Stepper motor

Answer explanation

An autotransformer starter is ideal for large three-phase induction motors with high starting torque requirements, as it reduces the voltage during startup, minimizing inrush current and mechanical stress.

What happens when the motor transitions from the reduced voltage to full voltage in an autotransformer starter?

• The motor stops momentarily

• The motor operates at reduced torque

• The autotransformer is bypassed, and full voltage is applied to the motor

• The motor reverses its direction

Answer explanation

When the motor transitions from reduced voltage to full voltage in an autotransformer starter, the autotransformer is bypassed, allowing full voltage to be applied to the motor for normal operation.

What is the main advantage of using an autotransformer starter over a wye-delta starter?

• It is simpler to install

• It provides better control of starting torque

• It is more cost-effective for small motors

• It eliminates the need for overload protection

Answer explanation

The main advantage of an autotransformer starter is that it provides better control of starting torque compared to a wye-delta starter, making it more suitable for applications requiring precise torque management.

In which industrial application is an autotransformer starter commonly used?

• ) Lighting circuits

• Conveyor systems with heavy starting loads

• Fans and blowers

• Low-power domestic appliances

Answer explanation

An autotransformer starter is ideal for conveyor systems with heavy starting loads because it provides a reduced voltage at startup, minimizing inrush current and mechanical stress on the motor.

What is a major disadvantage of an autotransformer starter?

• It is not suitable for high-torque applications

• It causes high inrush current

• It is more expensive and complex compared to other starters

• It does not reduce starting voltage

Answer explanation

The major disadvantage of an autotransformer starter is that it is more expensive and complex compared to other starters, making it less favorable for some applications despite its benefits.