Controlled Rectifiers

Question 1

What does SCR mean?

Answer 1

Silicone Control Rectifier

Question 2

What 2 conditions must be met for a SCR to conduct?

Answer 2

Forward bias A->K
Positive gate pulse G->K
A->K->G->K

Question 3

How long does the Gate pulse need to be applied for?

Answer 3

10u-15u seconds

Question 4

SCRs conduct in how many directions?

Answer 4

One

Question 5

What are 3 types of triggers for a SCR?

Answer 5

-Positive gate pulse (controlled trigger)
-Exceed voltage rating (no gate pulse, no controll)
-Rate of change of voltage(no gate pulse, no controll)

Question 6

How do you prevent large or sudden voltage changes from Anode to Cathod?

Answer 6

A snubber circuit- a resistor and capacitor connected in series, paralleled to the SCR

Question 7

What does a Snubber do?

Answer 7

Limits the rate at which voltage will increase, and prevents voltage spikes at the SCR

Question 8

What is Forward Bias Breakover?

Answer 8

When voltage rating of the SCR is exceeded, current will begin to flow through the SCR.

Question 9

A diode is a _____ terminal,_____ layered,___-directional device

Answer 9

-2
-2
-Uni

Question 10

A SCR is a ____ terminal, ____ layer, ____-directional device.

Answer 10

-3
-4
-uni

Question 11

The firing angle+ the conduction angle =

Answer 11

180°

Question 12

FA+CA is what type of control?

Answer 12

Phase control

Question 13

What are the 3 methods of commutation an SCR?

Answer 13

Natural
Forced
Class F or Frequancy

Question 14

What type of commutation is used the most?

Answer 14

Class F

Question 15

What is Natural commutation?

Answer 15

Happens when a switch is operated in series with the SCR, momentarily interrupts the current,
A switch connected in parallel with the SCR, the current takes the path through the switch, and bypasses the SCR

Question 16

Class F or Frequency

Answer 16

This turns off when the ac voltage drops to zero volts and reverses polarity.

Question 17

Forces commutation

Answer 17

When the current in a SCR is connected to a DC circuit is reversed, the SCR turns off.

Question 18

How do you test a SCR?

Answer 18

You must remove them from the circuit

Question 19

When replacing an SCR, what needs to be considered?

Answer 19

-Voltage
-Power dissipation
-dv/
-gate current
-application

Question 20

What is dv/dt?

Answer 20

The rate of change of voltage with respect to time. Listed in volts per microseconds.
This is the highest rate of change of voltage across the SCR from A->K that can sustain with out turning on incorrectly.

Question 21

How many SCRs can conduct at one time?

Answer 21

Just one, is forward bias at a time.

Question 22

How is phase controlled achieved?

Answer 22

An adjustable resistor

Resistance down= larger conducive angle, smaller firing angle
Resistance up= larger firing angle, smaller firing angle

Question 23

What controls the speed of a DC shunt motor?

Answer 23

Voltage supplied to the motor

Question 24

How is base speed of a DC shunt motor determined?

Answer 24

Rated voltage
Full load

Question 25

What are problems with Rheostats?

Answer 25

I(^2)xR losses, and maintenence

Question 26

What problem may result with a SCR in the field winding circuit?

Answer 26

An open in the field circuit will produce a runaway condition. The motor will accelerate at an unsafe speed

Question 27

What is another name for a controlled rectifier?

Answer 27

Hybrid rectifier

Question 28

What is a MOV? What is it for?

Answer 28

Metal oxide varistor. Surge suppression.

Question 29

What is feedback signal?

Answer 29

Generated Voltage. From a dc motor

Question 30

What happens to a motor when load is added?

Answer 30

The motor slows down Egen decreases, trigger cct senses decrease and decreases firing angle which increases conductive angle. Motor speed picks up. Voltage to armature increases

Question 31

What happens when load is removed from a motor?

Answer 31

The motor speeds up, Egen increases. Trigger cct senses increase and increases the firing angle and decreases conductive angle. Motor slows down. Voltage to armature decreases

Question 32

In a 3 phase motor controller, how many SCR pulse?

Answer 32

All 3 pulse at the same time, but only one will be forward bias.

Question 33

How many MOV are in a 3 phase motor controller.

Answer 33

3

Question 34

What are the parts of a generator unit?

Answer 34

-Alternator field (rotor) -Alternator armature (stator) -Rotation three phase full wave rectifier -Exciting armature (rotor) -Exciting field (stator) -Voltage adjuster (set point voltage)(AVR)

Question 35

Why is rate of change of voltage across an SCR significant?

Answer 35

It can cause anode/cathode circuit to conduct