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Solid State Relays (SSR's) and SCR's (Silicon Controlled Rectifiers, also called Thyristors) are becoming more popular all the time to replace mechanical relays and contactors and mercury relays due to their relatively low cost, lack of moving parts, and their lessened wear and tear on heating elements. Zero fired devices (SSR's and some SCR's) do not send a large spike down the electrical line every time they turn on, like mechanical relays do, and the contacts do not erode due to arcing (no arcing occurs with SSR's and SCR's). Phase angle fired SCR's do cause spikes, however, as well as possible harmonic distortion on the line, so care must be given with their placement.

Zero Fired Devices

1. Solid State Relays (SSR's)

 

 

For single phase resistance (electrical heating element) heating, solid state relays with ratings of 10, 25 and up to 100 amps @ 120-240V (480V also available) are commonly utilized. These incorporate two Zero Fired SCR's which provide zero voltage turn-on and zero current turn-off, which practically eliminates any spike generation when the load is turned on or off. Since these are solid state relays, they do not wear out like a mechanical relay. However, their amperage capability must be derated considerably if the environmental temperature increases beyond normal ambient (i.e. if they are mounted in a confined space without convection "heat sinks" or external cooling). Also, during the "off" state, some leakage, say 1 milliamp AC, usually takes place, which might trigger some sensitive devices. This is never a problem with resistive heaters; however, with a coil of a contactor or a motor, this could be a problem (some SSR's are 120V prime power and thus switch like a relay with a 120VAC coil). One other caveat: when these devices fail, they tend to fail closed, keeping the circuit operating. For processes where this is unacceptable, an FM approved Hi-Limit Alarm and thermocouple should be included to shut down an overheating situation. The standard input to a SSR is a 3-32 VDC pulse train from a SS output (gating) controller.
 

2. SCR Power Controller

 

 

SCR Power Controllers are available either as Zero Fired (for resistive loads) or Phase Angle Fired (for transformer coupled loads and some heating loads which change resistance dramatically with temperature or time), and single or 3 phase loads.
 

1. Zero-Fired

 
1. General

Most power control applications involve simple resistive heating loads which require little in the way of sophistication. They undergo negligible resistance change as they heat, and they operate at or near available line voltages. Modern controllers often offer a line voltage variation feedback correction to offset this. Precise temperature control is the main reason for selecting an SCR, as it modulates the power flow into the elements as needed. By electrically switching an SCR on at the AC sine wave zero crossing point, it remains on through the half cycle of the sine wave and commutates off at the next zero crossing. This duplicates the action of a contactor, but at a much faster rate and without the electrical noise and mechanical contacts wear and tear. The important fact to be noted is that since the SCR is turned on at or nearly at the zero crossing point, no power is being switched under load. This results in virtually no EMI generation. Also, the fast rate of fire lessens thermal shock and maintains temperature much better than mechanical relays. Standard Zero-Fired SCR's take a 4-20 mA DC control input; 0-10 VDC and DC pulse trains are other possibilities.
 
2. Single Phase Control

By controlling the number of cycles of a fixed time period (microseconds in some units), the control signal to the SCR will cause the SCR to be gated on for a certain time period and then off for a given time period. For example, at 50% power, it will be on for 10 cycles and off for 10 cycles. At 10% power, it will be on for 2 cycles and off for 18, etc. It is important to note that zero fired SCR's control average power to the load. There is no voltage or current control as in phase control (although this is available in some controllers). It is, therefore, not practical to current limit zero fired SCR's. Of course, with purely resistive loads, there is no need for current limiting. More exotic SCR's offer Fast Cycle, Half Cycle or Single Cycle firing.
 
3. Three Phase Control

Confusion occasionally results when users notice that three phase zero fired SCR's often control only two legs of the three phases. This is standard practice throughout the industry and results from the fact that in common wye or delta connected loads, current flow through the third leg must also flow through the other controlled legs. This effectively controls all power to the load, and results in some cost saving.
 
4. Prohibited Load

Transformers head the list as prohibited for most Zero Fired devices. As a power transformer is energized, a magnetizing current flows into the primary winding to set up flux lines within the transformer so that power may be taken from the secondary. This inrush of current may be several times the surge current rating of the SCR and fusing. This is why "soft start" or "ramping on startup" features are specified with controllers dealing with phase controlled SCR's. The voltage is started at a small percentage of the line voltage and is slowly brought up to the line value, so that inrush currents can never exceed ratings. Units turn fully on at zero, and therefore, cannot be prevented from impressing the entire inrush current upon the transformer. The inevitable result is a blown fuse or a damaged SCR and transformer.

Four wire wye loads also present problems. Think of them as three separate and independent loads. A four wire wye (or "star") load has each phase connected to a neutral point; therefore, the voltage across the leg is the line voltage divided by 1.732. That is, on a 480V 3 Phase system connected 4 wire Wye, three 277 volt units must be used. They will be connected from line to neutral. Some SCR's, however, are designed for use with 3 or 4 wire Wye, open or closed Delta, 3 wire Delta and Star loads. Just make extremely sure that they are designed for this purpose.

Any heating element which requires current limiting may not be controlled by standard zero fired devices. Such elements include: molybdenum, tantalum, silicon carbide (globars), super kanthal, and platinum. All of these exhibit a significant resistance change over time and temperature, usually from 2 or 3:1 to 20:1. These nearly always have an interposing transformer in the system and as such will be eliminated from zero fired consideration.

Any load, such as a tungsten lamp, which has a minimal resistance change, but which has a very fast response time, may not be zero fired.

Generally speaking, any three phase load which tends to be unbalanced, or any resistance load with a high temperature coefficient and significant thermal mass, cannot be operated zero fired.
 

2. Phase Angle Fired

Basically, motors or transformer-coupled loads must be fired by a phase angle gating device because at start-up, lots of power is necessary to energize the coil before any work can be done. The inrush of current for this would blow the fuses of a zero-fired device, or even blow it up. Certain electrical connections and heating elements also must be phase angle fired. Any prohibited load for a zero fired device is a candidate for phase angle firing.

Whereas zero fired devices turn on at each zero crossing point in the AC sine wave, and control by regulating bursts of whole sine waves to the load, phase angle fired devices may be turned on at any point within the half cycle. This controls the RMS voltage out of the SCR, and thus the power. See Figure 1. The ability to choose any of an infinite number of firing points is sometimes called stepless control. This allows Phase Angle Fired SCR's to replace Saturable Reactors, Variacs, Stepper Switches, Contactors, Ignatrons, and Thyratrons with a much more stable, reliable, efficient and acoustically quieter device.

When an SCR is fired at some conduction angle other than zero, let's say 90°, large amounts of power are being switched under load. If the SCR is driving a transformer, this steep waveform will permit a very large inrush current to flow into the transformer primary. This may be several times the surge current rating of the SCR, fuse and transformer. Similarly, if under steady state operation, the power line drops out for a few cycles, the power may be reapplied to the transformer when the flux is at an extreme in the hysteresis curve; this may drive a transformer into saturation. Again, this saturation will cause very large and potentially damaging currents to flow.

Instead of impressing the full line voltage on the transformer, the SOFT START feature of a phase angle SCR will slowly increase its output from zero to full line voltage over a 12 cycle period, so that the transformer core has time to magnetize before taking full power from the secondary (See Figure 2). This occurs each time the power control is turned on, each time there is a momentary outage on the power line, say a soft start reset, or even after a very large step change in the control signal.

To protect the load, current limiting should be considered with a phase fired power control. This feature overrides any other control signal over a selectable range and shuts the system down if the watt density rating of the heater load is exceeded by the power flowing into it. Fast gating limit monitors currents above a prescribed level and limits gate pulses to the SCR to prevent damaging currents.