Design Considerations
Application of Thyristor Power Controls
To ensure the most cost effective use of thyristor controls it is important to have a basic understanding of the products. The notes below describe key features of the technology and critical aspects of the application which must be considered in the design process.
What is a Thyristor Power Controller?
The essence of a power controller is a triac or thyristor and a firing circuit (triacs are used up to currents of about 40A and thyristors to 500+A). The simplest form of a power controller will utilise these two components but in the final installation other components such as; heatsinks, enclosures, fuses, minature circuit breaker's, EMC filters and various control devices may be required.
What is a Thyristor?
A thyristor is a semiconductor device also known as a silicon controlled rectifier (SCR). It has three electrodes, an anode, cathode and gate (control electrode). When the anode is positively charged with respect to the cathode no current will flow until a pulse, provided by a firing circuit, is applied to the gate. The thyristor will remain conductive until the voltage in the device drops to zero. To control an alternating current two thyristors are connected in inverse parallel.
What is a Firing Circuit?
A firing circuit is a device which converts an input signal; current, voltage or potentiometer into an output signal which will fire a triac or thyristor.
What is Burst Firing?
Burst firing is the rapid on-off voltage switching (typically on a one second time base) through the thyristor thus controlling the power to the load.
What is Phase Angle Firing?
Phase angle firing is controlling of the power to the load by chopping the supply voltage before it completes the full cycle.
What is Dual Mode Firing?
The dual control feature includes both burst fire and phase angle firing. Either mode may be used independently or phase angle mode can be used for soft starting and automatically switched over to burst mode when the control signal has reached a pre-set level. The control will then remain in burst fire mode even if the input signal drops below the pre-set level.
What is Logic Control?
Logic control of a thyristor enables the unit to operate like a solid state relay or contactor i.e. when the logic signal is low the thyristor is off and when high the thyristor is on. To minimise the radio frequency interference (RFI) the thyristor unit switches on at the zero crossing of the mains voltage after the logic input goes high and when the logic input goes low the unit switches off at the next zero crossing point.
Design Considerations
The following considerations must be applied to ensure the most effective use of power controllers.
Standards
UAL's products are designed to the latest standards to ensure optimum performance and compliance. Standards covered include:
Health and Safety
In the design and use of power controllers consideration should be given to the requirements of the Health and Safety at Work Act 1974 (HSW 1974), the EC "Provision and Use of Work Equipment Regulations 1992" (PUWER) and the "Functional safety of electrical/electronic/programmable electronic safety-related systems" IEC61508 all available from the Health and Safety Executive (HSE) publications, within the UK.
CE Directives and Guidance Notes
These are European regulations, which apply to our industry. They affect the equipment emissions and immunity to Radio Frequency Interference (RFI) and various elements of safety for electrical equipment. For a complete listing of these standards refer to UAL's standards reference document.
The European Community 'CE' Directives that mainly concern United Automation Limited (UAL) are, the Low Voltage Directive (LVD) and the Electromagnetic Compliance Directive (EMC). With further reference to appropriate European Harmonised Standards, the Company opted for the self certification method of assessment to address the wide range and variety of products supplied by United Automation Limited.
All equipment, unless otherwise stated, is rated to CLASS II Insulation (Over-voltage category) and CLASS I (Protection category). For specific insulation test voltage ratings see the individual product datasheet.
Declaration of Conformity may be supplied with the product or supplied on request
Installation and Maintenance
We recommend that installation and maintenance of all United Automation Limited equipment should be done with reference to the current edition of the I.E.E. wiring regulations (BS7671), by suitably qualified/trained personnel. The regulations contain important requirements regarding safety of electrical equipment within the UK. For International Standards refer to I.E.C Directive IEC 950.
Design
Cooling Requirements
The ambient temperature surrounding the power controller should not exceed 40°C according to the requirements of EN 61010-1 therefore local cooling may be required. Unless otherwise stated the components themselves must be limited to a maximum of 65°C through the use of an appropriate heatsink and/or forced cooling. Care should be taken to use suitable thermal bonding compound between all heat transmission surfaces.
Overload and Short Circuit Protection
Power controllers must be protected against current overload and short circuit through the use of an minature circuit breaker (MCB) or semiconductor fuse. We recommend that semiconductor, fast acting to BS88 IEC 269, type fuses or circuit breakers (semiconductor MCB) should be used for unit and/or device protection. The appropriate maximum load current should be known to select the required SCR fuse or mcb and the protective device should be selected with an appropriate safety factor. The I² t (A² s) rating of the selected fuse, must be less than that of the equipment so as to protect the equipment's discrete device. Further appropriate fusing may be required for protection of the unit supply using standard fuse links and holders.
Inductive Loads
Where phase angle controllers are used on inductive loads e.g. transformers, motors and induction heaters a variety of problems may be experienced as detailed below.
EMC Filtering
Phase angle power controllers and firing circuits require the use of an EMC filter to comply with IEC 1992.
High Surge Inrush
When the power controller is used to control certain typess of inductive or heating loads relatively high inrish current surges can occur which may result in damage to the semiconductor or protective device. To prevent this occurring it is recommended to use a controller with a "soft start" function to protect the device on start-up.
Transients
Remote voltage spikes may cause damage or failure of the power controller components, to prevent this occurring it is recommended to fit a suitable Voltage Dependent Resistor (VDR). In other cases transients can cause misfiring of the thyristors, to prevent this a Resistance-Capacitance Network (snubber) should be fitted in parallel with the load or power controller.
Minimum Set Speed
Where power controllers are used for proportional motor control there is a speed below which stabilized control is not possible i.e. hunting, therefore a minimum speed needs to be set. With potentiometer control a typical solution would be to fix a minimum set-point.
Installation
Polarity
Polarity sensitive equipment - care should be taken to ensure that any positive and negative connections are correctly connected prior to switching on the equipment. Failure to do so may cause damage to internal circuitry.
Phase Sensitivity
Phase sensitive equipment - care should be taken to ensure that the supply is correctly connected. For single phase this is LIVE/NEUTRAL, in three phase and phase-to-phase supplies this is RED/YELLOW/BLUE by rotation (some firing circuit equipment detects and corrects this error automatically).
Earthing
The protective conductor terminal of the equipment must be utilised at all times and bonded to a 'good' Earth (ground). The earth bonding (strapping) leads of any combined equipment should be as short as possible and be substantial, i.e. at least rated higher than the equipment's load.
Torque Settings
Torque settings for safe electrical power connections are critical and specified settings, found in the product data sheet, should be observed at all times. For most signal terminal connections we would recommend a typical torque setting of 1 to 5Nm. For mechanical torque settings refer to the appropriate Harmonised Standard.
Maintenance
Electronic equipment has few mechanical moving parts and is therefore, inherently, very reliable. Before any servicing is carried out, reference should be made to the appropriate installation instructions, drawings and labelling which may come with the equipment. Personnel should switch off, isolate and lock the supply before accessing or removing any safety covers.
General checks (weekly/monthly/annually)
Periodic checks should be built in to a maintenance programme. We would recommend that this is undertaken at least annually, as a minimum requirement. Checks should include the following:-
Moving parts (e.g. fans) - ensure good working order and free movement.
Vents/louvers - clean, free blockages.
Signs of corrosion - inspect areas that may be susceptible.
Electrical connections - Ensure all connections are suitably tightened.
