Controll of AC loads Flashcards
What are the 2 types of control for AC loads
Burst control and Phase control
Why is it important to control AC
Some loads only work with AC and if a load is Connected to AC mains it is allot easier to control the AC directly
What should switching be synchronised to
Mains period
What is burst control
Load is turned of for integer number of cycles then on for an integer number of cycles
What is phase control
Load is turned on part way through each half cycle
What are examples of electronic power switches used for AC Controls
TRIACs and SCRs
What sort of switch is SCR
A semi controllable switch
How can a SCR be turned on
By firing a pulse positive gate current when in forward position (firing pulse)
What happens to the gate current when the SCR has started conducting
It can be removed
Once conducting can the SCR be turned of by the gate
No
When will the SCR turn off
When the anode current iA =0
What is the time before current flow called for a SCR
delay or firing angle
For a SRC what is the conduction angle
The remainder of 180 - firing angle
To find Vavg of a SCR what are the limits of the integral
Firing angle and pi
What is the equation for the average voltage for half wave SCR phase control
Vavg= (Vpk/2pi) x (1+cosa)
What is the equation for Vr.m.s for half wave SCR control
V^2r.m.s= (V^2pk/4pi) x (pi- a + 1/2x sin 2a)
How does the size of the firing angle effect the power in a SCR
Smaller angles produce higher power
What are TRIACS often used for
Dimmer switches or heater control
What is a TRIAC
A pair of back to back SCRs sharing a common gate
For full wave SCR control in a TRIAC what are the equations for Vave and V^2r.m.s
Vavg=(Vpk/pi) x (1 +cosa) between 0 and pi
V^2r.m.s=(V^2pk/2pi) x (pi-a+sin2a)
Would it be good practice to use Burst control for a light bulb
No as this will cause bulb to flicker as the filament cools and heats
For inductive loads is it better to switch loads on or off
better to switch on and allow them to switch off naturally when current falls to 0
Essentially what is a SCR
A diode that has to be turned on
Describe how a SCR is semi controlled
It is turned on after a delay angle and turns off when current drops to 0
What does a single SCR act as
A half wave rectifier
What is a solid state relay
Like a TRIAC with insolation in between control and load
