Power switching devices

About Australian power

The power supplied in Australia is alternating current, called a.c. It has a frequency of 50 cycles per second, called 50 Hertz. The current (movement of electrons within the conductor) oscillates forward and backward, reversing direction 100 times per second. The voltage swings continuously from being positive to negative to positive again, as in Fig 1. Each time it crosses the base line the voltage is zero, so for that instant there will be no current flowing.

Fig 55-1.
In some overseas countries the power is supplied at 60Hertz. This different frequency will have no adverse effect on heating elements but can sometimes adversely affect other electric components such as motors and transformers.

All domestic power in Australia is supplied at 240Volts. Other countries may use different voltages. For example, both 110Volts and 220Volts 60 Hertz are used in USA and kilns are offered there to operate on either of the voltages; neither of which suit our supply.

240Volts applied to an element designed for 220Volts will cause the current to increase by about 9%. This will make the kiln heat faster but may overload the circuit breaker. 240Volts applied to an element designed for 110Volts will cause the current to more than double, overheat the kiln wiring and create a fire hazard.

Art Glass industry importers hopefully make special arrangements with overseas kiln builders to customise shipments to meet Australian electrical standards.

Care should be taken by individuals contemplating overseas purchase of equipment to ensure that voltage frequency and other items such as electrical plugs meet our requirements. There are two main types of power switching device used for controlling the  a.c. power in electric kilns:

• Contactors (two or three phase) or power relays (single phase) which use mechanical contacts to turn the power on or off.
• Solid state devices which use semi-conductors to switch the power, without any moving parts. Most commonly used is the Solid State Relay (SSR), and less frequently the Silicon Controlled Rectifier (SCR).

Methods of switching power to control temperature.

Before looking at these devices it is best that we look at the various control methods available.As discussed in 53. Simple kiln controls, there are two ways to control the power to a kiln;

• On-Off control, and
• Proportional or PID control.

Any power switching device can be used for On-Off control.There are two main methods for proportional control;

• Burst firing, and
• Phase angle firing.

Burst firing

Fig 55-2
This is where the power is applied in one continuous burst for the proportion of the cycle time needed to best follow the pattern See Cycle time in 68. Programmable controllers for more. The Solid State Relay (SSR) is ideally suited for this method of control; the contactor is also used. Assuming a cycle time of three seconds, or 300 half cycles, with burst control it is possible for the power to be applied for zero (Off) , full On, or anywhere in between. This gives extremely precise control.

Phase angle firing

Fig 55-3
This is where power is applied for a proportion of each half cycle. The Silicon Controlled Rectifier is suitable here. One disadvantage of this method is the generation of electrical interference. See below for more.

Contactors & power relays

These consist essentially of sets of fixed and movable contacts and a coil to pull the contacts closed when a current is applied to it.

Fig 55-4
A contactor is designed for switching three phase power and may have three or four sets of contacts; one for each phase and maybe an ‘auxiliary’ set. A power relay is an equivalent device used for switching single phase power, so will have only one or two sets of contacts.

Each time the points open to interrupt the power to the elements an arc is created, because the current wants to continue to flow. This is like a welding current and heats up the metal contact points. Unless they are given time to cool down before being again closed, the metal could be sufficiently hot that they may stick together; resulting in catastrophic over-heating and a possible melt-down of elements and kiln contents. It is desirable that time is allowed for the heat to dissipate after the points open before they close again. 

There are times early in any firing where the power is turned on to the elements for only a short part of the cycle, and this proportion increases as the kiln heats up; until sometimes the power is on all of the time. There will usually be some point in the firing when power is on ‘most-of-the-time’; with only a very brief interval for the contacts to cool before they are closed again. It is during this period that damage to contacts is most likely to occur.

Even though modern contact design allows for hundreds of thousands of operations rather than the tens of thousands previously, there is still a practical limit to their life before problems may occur. Sometimes ‘chattering’ or rapid switching of a contactor or power relay will occur. This can be quite damaging to contacts, giving rise to numerous possible faults;

• They may be ‘burnt’ so that they don’t make good contact.
• Poor contact may produce excessive heat at the contact point whilst the power is flowing, causing them to stick together and the kiln to overheat, or
• Burnt surfaces can prevent current flowing the next time the points close, causing the kiln to cool down even though things appear normal.

The arc created on point opening will be less with resistive loads such as heating elements than with inductive loads such as motors and transformers, so a contactor will often have two current ratings,

• AC3 rating for inductive loads and
• AC1 which is a higher current safe for resistive loads. This is the one used with kilns using wire wound elements.

Solid State Devices

These have no moving parts, no contacts, so can switch the power on and off extremely rapidly.

Silicon Controlled Rectifier (SCR).

Fig 55-5
Essentially large transistors capable of conducting high current in one direction only. They are turned on by a trigger signal at a particular point in any power half cycle when power is required and turn off automatically when the voltage across them falls to zero.

Fig 55-6
To switch the power in both positive and negative half cycles as shown in fig 3, two devices are arranged ‘back-to-back’ so that each will conduct in one direction.  

Solid State Relay (SSR)

Essentially, consists of two SCRs mounted on a base plate and packaged for ease of installation. Unlike a contactor which has a number of contacts in the one package, each SSR switches only one phase.

Fig 55-7
SSRs develop heat whilst in operation so must be fitted to some form of heat dissipating surface to allow it to be dissipated. For small SSR’s this can be a metal shell or case, but should preferably be a finned aluminium extrusion called a ‘heat sink’. Proper design is essential to ensure long life of the components. Any instrument supplier should be able to offer the components and the design expertise. To ensure proper heat transfer from the SSR to the heat sink, a heat conducting paste should be applied to the mating faces of SSR and heat sink. Many electricians are unaware of this need and failure to use it accounts for the bulk of failures which do occur.

Leakage current.

When a solid state device is not conducting there remains a high impedance (resistance to a.c current)  between the power terminals. This allows an extremely small current to flow even when no trigger voltage is present and is called a “leakage current”. In a kiln this leakage current would flow through the element. Fortunately, in this situation the voltage at the element would be so low as to be harmless and insufficient to light even a torch globe.

It is the practice of some kiln builders (including Riley Glass Kilns) to install a small neon lamp in parallel with an element to show when power is actually going to the element. This as a diagnostic tool and a check on the proper functioning of the door or hood interlock switch.
Because the tiny leakage current will be conducted to earth by the element it will not cause the lamp to light other than when full voltage is applied.
However, should the element fail then the leakage current is sufficient to light a neon indicator lamp. This would cause the lamp to be lit at times when no power should be going to the elements.

A lit “power to elements” lamp at times when it should not be lit could indicate a faulty element.

Varistor

The small blue device on one SSR in Fig 7 is a ‘varistor’ or ‘variable resistor’. It protects the SSR against damage from a high voltage surge should one occur, by rupturing and causing the circuit breaker to trip. One of the appropriate voltage rating should be fitted to all SSR’s. The one shown is rated at 275Volts. It has an extremely high impedance (resistance to a,c, power) to 240Volts but will rupture and short if voltage goes much higher. When it’ shorts’ it conducts high current and trips the circuit breaker. A few dollars can avoid costly damage.

Zero Voltage firing.

The SSRs generally used in kilns contain special circuitry to ensure that, irrespective of the instant the trigger signal arrives at the device, it will not conduct power until the start of the next half cycle.   When power is turned on or interrupted part way through a half cycle it can create radio frequency (RF) or harmonic interference which can travel through the air or down the power lines to affect TV’s radio and computer equipment in your or neighbouring properties. Similarly when electrical contacts open mid cycle. If the power is switched at the instant the voltage is zero then there can be no arc and no interference.

Fig 55-8
A 3 phase contactor switches all poles at the same instant. With 3 phase power the zero voltage points occur at different times, so it’s impossible to switch power when all voltages are zero. With SSR’s, a separate device is used for each phase. Even if the trigger signals all arrive at the same time, the zero crossing capability ensures that it delays switching until the voltage applied to it is zero. RF interference is avoided.

Trigger signal

The trigger signal normally used with contactors is 240Volts a.c. whilst that used with SSR's is 12Volts d.c. This low voltage is not dangerous, simplifies the placing of interlock switches and does not involve complex wiring. In addition, the d.c. voltage is present every instant,  without the  zero voltage gaps which occur with a.c trigger signals.

Expected life

Whilst taking up more space and costing slightly more to initially install, SSR's allow greater flexibility in control as well as providing more precise conformity to firing patterns than do contactor switching. In a properly designed control system, failure of an SSR is a rare event, irrespective of how long the kiln has been in service. Nevertheless, both contactors and SSR’s can fail, so it is wise to install Overheat Protection unless the kiln is to be monitored continuously. Another instance of a small outlay avoiding costly damage.