Chapter 35-36 understanding motor controls

What is the synchronous speed of a six pole motor operated with an applied voltage of 20 Hz?

400 RPM.

After an SCR has been turned on, what must be done to permit it to turn off again?

A capacitor being charged in the opposite polarity of the SCR.

What device is often used to sense motor speed?

A electrotachometer.

What is a freewheeling or kickback diode?

A large diode connected across the output of the rectifier used to kill inductive spike voltages produced in the armature.

What is a thyrister?

A solid state device with two modes, ON or OFF.

What unit controls the voltage applied to the armature?

Armature power supply.

If an alternator is used to provide variable frequency, how is the output voltage of the alternator control?

By the amount of DC excitation applied to the rotor.

Name two methods of sensing current flow through the shunt field.

Current relay, sense resistor.

if the motor speed decreases, does the output voltage of the electrotachometer increase or decrease?

Decreases.

What is the advantage of an IGBT over a junction transistor?

IGBT is a voltage control device which gives them the ability to turn on and off very fast.

How are junction transistors driven into saturation and what is the advantage of driving a transistor into saturation?

Junction transistors are driven into saturation by supplying them with excess current. It allows them to control large amounts of current.

What is meant by ramping and why is it used

Ramping is either used to accelerate or decelerate a motor over a period of time. It allows for the motor to be brought up to speed without connecting it directly it directly to the line.

Why is this component used instead of a diode?

SCR, because to control the output voltage.

What is the main disadvantage of using SCRs in a VFD?

SCRs cause harmonics in the line.

Name two methods of sensing armature current.

Sense resistor, current transformer.

What electronic component is generally used to change AC voltage to DC voltage in large DC motor controllers.

The SCR (silicon controlled rectifier)

What is the primary difference between a GTO and an SCR?

The condition through the GTO can be stopped by applying a negative, with respect to the cathode, to the gate.

What is the disadvantage of driving a junction transistor into saturation?

They cannot recovery or turn off as fast which limits the frequency response of the transistor.

What type of motor is generally used with IGBT drives?

Three phase motors that are inverted rated.

Why are SCRs used to construct a bridge rectifier in many solid state variable frequency drives?

To permit the output voltage of the rectifier to be controlled.

Why is it necessary to reduce the voltage to a motor when the frequency is reduced?

To prevent overheating the windings due to excessive current.

What solid state device is generally used to produce variable frequency in drives designed to control motors under 500 HP?

Transistor.

In variable frequency drives that employ IGBTs, how is the output voltage to the motor controlled?

With Pulse Width Moderation (PWM).