Solid State Relay Used for Three Phase Resistive Loads control

Solid State Power Control 101;

I'm Sister Mary Elephant. Class is in session, shut up, sit down and pay attention.

There are two ways to control power going into a resistive element with solid state relays (SSRs).

1. Zero Cross, which comes in two styles:
• Either a solid state on-off contactor / relay
• Or a Proportional / Variable Time Base analog controller
2. Phase Angle Control

Starting backwards in order to explain;

The Phase Angle control method uses the SCRs to provide an infinitely variable RMS output by delaying the turn-on (Gate) signal in each and every sine wave cycle. By delaying the gating, the total RMS that gets to the load in each cycle is reduced and by moving the gate timing backwards, it's possible to smoothly ramp the RMS voltage into the load. This is important for some types of heating elements where the of temperature coefficient of resistance is positive, meaning that the cooler the element, the less resistance it has and the resistance increases as the element heats up. So if an element has a PTC of resistance, as soon as you turn it on the power will experience little resistance and the current will surge until the element heats up. This can be very undesirable, so by ramping the voltage to it slowly, one can avoid this. In addition, some people like to use transformers on the element side of a circuit in order to reduce the current used at a higher voltage on the primary side (and thus a smaller SSR) but also reduce watts density (hot spots) on the element itself by using a lower voltage. Phase angle firing is OK to use on the primary side of a transformer. The down side is, every SCR is firing in every cycle and at any point in the cycle as necessary, so a lot of electrical noise is created, which may also have some negative consequences.

If however the electrical noise created by the firing of SCRs in Phase Angle control is untenable, as it is in may high precision control systems, then you must use Zero Cross. What that means is that the gating of the SCRs takes place precisely when the sine wave is at the Zero Cross, therefore there can be no noise created because nothing is happening at that moment. So if all you are doing is using the SSR as a solid state contactor, i.e. turning it on and leaving it on until you turn it off, Zero Cross allows that to happen with no adverse side effects. If you need variable power output and Zero cross, then you use proportional Control / Variable Time Base gate firing where you fire the SCRs in bursts of complete cycles, but for only a portion of time. That means that if you want 75% power going to the elements, you would fire the SCRs on for 6 cycles, off for 2 cycles, then on for 6, off for 2 etc. etc. etc. This method however does not lend itself to being used with transformers because of the magnetic field collapsing and expanding over and over, and it is less conducive to ramping, so it is not as good for PTC resistive elements.