Silicon Carbide Heaters controlled by Thyristor (Part 6)

Use of Transformers with Silicon Carbide Heaters

If the starting voltage of the silicon carbide heaters is significantly below half the available line voltage it is usually beneficial to use a transformer to supply the load. The transformer is usually specified so that the voltage output on the secondary is equal to the finishing voltage of the silicon carbide heaters. Unlike a thyristor, a transformer won't draw more current on the input side to give a reduced output on the output side.
For a transformer VpIp=VsIs

Where Vp = primary voltage, Ip = primary current, Vs = secondary voltage, Is = secondary current 

Consider the example of a 1000W silicon carbide heating load with a 220V mains supply, 70V starting voltage, 120V finishing voltage.
With a tapped transformer set at 70V for a new heater element the secondary current Is is
1000W/70V = 14.29 amps

with a corresponding primary current of
70V/220V x 14.29 amps = 4.54 amps

At the end of the heater's life with the transformer secondary tapping set to 120V the secondary current Is is
1000W/120V = 8.33 amps

with a corresponding primary (mains)current of
120V/220V x 8.33A = 4.54A

From this it can be seen that if we change the transformer tapping to match the load resistance for any given power output the primary current draw remains constant 

Now consider the case where we have no transformer and use a thyristor only to regulate voltage to the load. 
Mains current draw with a new silicon carbide heater with thyristor output set to 70V will be
1000W/70V = 14.29 amps

Mains current draw with the heater at the end of its life with thyristor output set to 120V will be
1000W/120V = 8.33 amps

It is clear that a transformer great reduces the mains current draw by matching the required load voltage to the load impedance. 

Typically a transformer and thyristor are combined to control low resistance silicon carbide heaters. This gives the benefit of reduced current mains current draw due to the transformer as well as the benefit of power regulation and automatic compensation for element ageing offered by the thyristor. If we use a transformer with a 120V secondary, the finishing voltage of the heaters, and combine it with a thyristor on the transformer primary we get a starting secondary current of
1000W/70V = 14.29A

with a primary (mains) current of
120V/220V x 14.29A = 7.79 amps