Controll of AC loads

Question 1

What are the 2 types of control for AC loads

Answer 1

Burst control and Phase control

Question 2

Why is it important to control AC

Answer 2

Some loads only work with AC and if a load is Connected to AC mains it is allot easier to control the AC directly

Question 3

What should switching be synchronised to

Answer 3

Mains period

Question 4

What is burst control

Answer 4

Load is turned of for integer number of cycles then on for an integer number of cycles

Question 5

What is phase control

Answer 5

Load is turned on part way through each half cycle

Question 6

What are examples of electronic power switches used for AC Controls

Answer 6

TRIACs and SCRs

Question 7

What sort of switch is SCR

Answer 7

A semi controllable switch

Question 8

How can a SCR be turned on

Answer 8

By firing a pulse positive gate current when in forward position (firing pulse)

Question 9

What happens to the gate current when the SCR has started conducting

Answer 9

It can be removed

Question 10

Once conducting can the SCR be turned of by the gate

Answer 10

No

Question 11

When will the SCR turn off

Answer 11

When the anode current iA =0

Question 12

What is the time before current flow called for a SCR

Answer 12

delay or firing angle

Question 13

For a SRC what is the conduction angle

Answer 13

The remainder of 180 - firing angle

Question 14

To find Vavg of a SCR what are the limits of the integral

Answer 14

Firing angle and pi

Question 15

What is the equation for the average voltage for half wave SCR phase control

Answer 15

Vavg= (Vpk/2pi) x (1+cosa)

Question 16

What is the equation for Vr.m.s for half wave SCR control

Answer 16

V^2r.m.s= (V^2pk/4pi) x (pi- a + 1/2x sin 2a)

Question 17

How does the size of the firing angle effect the power in a SCR

Answer 17

Smaller angles produce higher power

Question 18

What are TRIACS often used for

Answer 18

Dimmer switches or heater control

Question 19

What is a TRIAC

Answer 19

A pair of back to back SCRs sharing a common gate

Question 20

For full wave SCR control in a TRIAC what are the equations for Vave and V^2r.m.s

Answer 20

Vavg=(Vpk/pi) x (1 +cosa) between 0 and pi

V^2r.m.s=(V^2pk/2pi) x (pi-a+sin2a)

Question 21

Would it be good practice to use Burst control for a light bulb

Answer 21

No as this will cause bulb to flicker as the filament cools and heats

Question 22

For inductive loads is it better to switch loads on or off

Answer 22

better to switch on and allow them to switch off naturally when current falls to 0

Question 23

Essentially what is a SCR

Answer 23

A diode that has to be turned on

Question 24

Describe how a SCR is semi controlled

Answer 24

It is turned on after a delay angle and turns off when current drops to 0

Question 25

What does a single SCR act as

Answer 25

A half wave rectifier

Question 26

What is a solid state relay

Answer 26

Like a TRIAC with insolation in between control and load