SOLID STATE SWITCHING FOR TEMPERATURE CONTROL APPLICATIONS-OR IS IT TIME TO THROW AWAY YOUR CONTACTORS?

Solid state switching of AC currents has been a practical proposition since 1950s when the Silicon Controlled Rectifier (SCR) and its cousin the Triac were developed.

SCRs (or triacs) are the basic switching unit used in solid state relays and phase angle or burst power controllers. When used correctly this type of switch can operate reliably over millions of cycles, making it ideal for applications that involve high switching rates, particularly temperature control.

Temperature control loops usually involve a large amount of repetitive switching. Typical applications that can benefit from solid state switching techniques are kilns, ovens, furnaces, plastic extruders and moulding machines, packaging equipment, environmental test chambers and incubators.

Despite the benefits that these devices can offer, many who could profit from their use are not doing so. This may be due to lack of awareness of the advantages they offer, or a belief that their implementation is too complicated. The aim of this article is to briefly examine the operation of solid state switches, the types of switches available, their benefits and drawbacks, and the criteria that must be met for a successful installation.

It will also look at the economics of solid state switches over contactors and perhaps shed some light on the question, ‘Should throw away your contactors?’.

BENEFITS

Besides the obvious benefit of high reliability, solid state switches offer some other advantages.

The fast switching rates available compared to electro mechanical devices can often improve control and as a result, product quality. For example many heating applications use temperature controllers with a time proportioning output with a cycle time that is programmable by the user. When this type of control is used with contactors the cycle time must be set for a relatively long duration, usually around thirty seconds, otherwise the contactor life will be diminished. If the control output of the temperature controller is say 50% the contactors will turn on for 15 seconds and off for 15 seconds. On a ‘fast’ process such as heating air flowing through a duct the result will be cyclic bursts of hot and cold air, If the same temperature controller is used with solid state relays the proportional cycle time may be reduced to one second or so. With the output of the temperature controller at 50% the solid state relays would be on for half a second then of for half a second. A person standing at the end of the air duct would sense the air leaving the duct at a uniform temperature, not in hot and cold bursts.

This improvement in control is often reflected in more consistent product quality.

Another result of the higher switching rates available with SCRs is that electric heating elements remain at a more constant temperature. This improves element life by minimising stress and fatigue bought on by thermal expansion and contraction.

DRAWBACKS

Due to the inherent voltage leakage of the SCR and of the RC networks often used to protect them from voltage transients solid state relays cannot be relied upon for isolation. In practice all solid state switching installations require a mechanical circuit breaking device upstream for full isolation from the mains.

Like all solid state devices, solid state relays generate heat. Cooling is normally accomplished with extruded aluminium heat sink. For higher currents considerable heat is generated (approximately 1 watt/amp) and large space consuming heat sinks are required.

 OPERATION

Most AC solid state switches in use today are based on the SCR or the triac.

The SCR has three terminals, an anode, a cathode, and a gate. It functions as a diode that can be turned on by a pulse of current to its gate. Once turned on the SCR remains on until the current conducted through it becomes zero. When used to switch an AC phase two SCRs are arranged in inverse parallel. One device conducts over the positive AC half cycle, the other over the negative, each device automatically turning off as the AC current wave passes through zero.

Control electronics supply the firing pulses to the SCR gate and provide isolation between the SCRs and the control signals. Isolation is normally provided by an optocoupler or a pulse transformer.

Triacs work in a similar manner but only one device is required to switch AC due to their bi-directional mode of operation.

TYPES OF SOLID STATE SWITCHES

Solid state relays

Solid state relays incorporate SCRs (or triacs) and their isolation/control electronics in a convenient modular package. They are available in single phase and three phase versions with low voltage DC or line voltage AC control voltages.

The majority of solid state relays sold are zero voltage turn on types. These relays have internal circuitry that detects the zero voltage point of the switched AC and as a result cause minimum electromagnetic interference. Random turn on types are also available which are normally used with inductive loads.

Be aware that to run these devices reliably at their full rated current they must be installed on a correctly installed heat sink. Manufacturer’s current ratings are meaningless unless the devices are correctly mounted on adequate heat sink. PCS offers complete relay assemblies that incorporate solid state relays, heat sink, fuses and voltage transient protection in an integrated package.

The largest solid state relays are rated at around 120 amps, however the size of their terminals limits their practical use to around 80 amps.

Power Controllers

While solid state relays integrate the control and the switching electronics into one package, power controllers generally utilise discrete SCRs and control circuitry.

Unlike solid state relays that turn either on or off in response to a control input, power controllers give a continuously variable analogue output in response to an analogue control input. They can be divided into two types, phase angle firing mode and burst firing mode.

Because the power controller output is virtually continuous power input can be exactly matched to the power required to maintain the process at setpoint. The result is accurate control.

Phase angle mode power controllers

The phase angle power controller works in the same manner as the domestic light dimmer. The control electronics turn on the SCRs over a portion of the AC sine wave in proportion to the control input. The result is a continuously variable voltage.

Often phase angle controllers have a current limit function that sets a maximum operating current by modulating the output voltage. This is particularly useful with variable resistance heating loads such as Molybdenum or Silicon Carbide.

Some controllers, such as the PCS T3000, also compensate for Silicon Carbide ‘ageing’ (resistance increase over time) by adjusting output voltage to maintain the set power output. In some cases this eliminates the need for the tapped transformers traditionally used with these heaters.

Burst mode power controllers

Burst mode power controllers are very fast cycle on off switches. Their output is modulated on a time proportional basis in response to a control input. Because the SCRs are switched over a few AC cycles the load sees the output as continuous.

Burst mode power controllers turn on at the zero volt crossover and off at the zero current crossover, making them inherently low generators of electromagnetic interference.

 Which firing mode?

As a general rule only use phase angle control on variable resistance loads or for control of transformer coupled loads where fast current limiting action is required. The low electromagnetic interference and harmonics generation of the burst fire mode makes them the first choice for the majority of applications.

Some power controllers, such as the PCS T3000, have user selectable firing modes.

 CRITERIA FOR RELIABILITY

Cooling

Adequate cooling of the switch is vital for reliable operation. When conducting, SCRs generate heat that must be dissipated, normally through aluminium heat sink. For operating currents above 10 amps careful consideration must be given to cooling.

Solid state relay manufacturers simplify heat sink selection by publishing current versus power graphs and ambient operating temperature versus case temperature graphs. For example a 70 amp solid state relay that is to run in a 60 degrees C ambient operating temperature requires a heat sink with a maximum thermal resistance of around 0.5 degrees C/W.

PCS offers relay assemblies complete with heat sink designed to operate at the full rated current at the specified ambient temperature.

Power controllers are usually supplied as an integrated assembly complete with heat sink.

Adequate enclosure ventilation is also important. Heat dissipated by the heat sink must be allowed to escape the enclosure. In some cases forced ventilation may be required.

Fusing

One of the most misunderstood areas of semiconductor switch application is fusing. Fuses designed for semiconductor protection are extremely fast acting and are selected for their I²t rating, as well as their current rating. Conventional fuses and circuit breakers are too slow to be effective.

The fuse I²t rating is an indication of how much energy the fuse will let through to the device under protection. Fuses are selected to have a lower I²t let through than the device under protection can absorb.

Note that semiconductor protection fuses are intended only to protect semiconductors. Given the right environmental conditions they can operate for long periods above their nominal current rating, causing damage to wiring and insulation. Conventional fuses or circuit breakers should be installed in series to the semiconductor protection fuses to prevent this.

High quality fuses tested to international standards should always be used.

Surge current

Excessive surge current can drastically reduce the life of semiconductor switches. Solid state relays used on loads with high inrush currents such as incandescent heating lamps should be oversized to compensate. Most manufacturers supply surge current versus number of operating cycles charts to aid selection.

The fast acting current limit function available with some phase angle power controllers effectively limits destructive surge currents.

 Voltage transients

Metal Oxide Varistors (MOVs) are often placed on switch outputs as a cheap and effective over voltage clamp.

Rapid voltage rises, or high dV/dt, can also damage semiconductor switches. RC filter networks or ‘snubbers’ protect against high dV/dt by absorbing these transients.

ECONOMY

The price of a typical solid state relay assembly is three to four times that of an electro mechanical contactor.

A typical 40 amp 3 phase contactor would cost around $250. A solid state relay based contactor assembly of the same current rating complete with heat sink and fast fuses would cost around $650.

If we assume the contactors have a service life of six months while the solid state alternative will last the life of the machine, and that an electrician must be called in for two hours at $50 an hour to change the conventional contactors, simple mathematics show us that the solid state switch pays for itself rapidly and in the long run is the cheaper option.

CONCLUSION

Referring back to our initial question, ‘Is it time to throw out your contactors?’, the answer is a definite maybe. Where electrical isolation is required, switching rates are low and space is at a premium, conventional contactors and relays fit the bill.