FAQ Power controller

Power controllers are used for positioning and regulating electric power. They are based on the need for a device which allows a fast, exact and wear-free regulation of electricity. Before, electrical energy was mostly provided by electromagnetic relays, contactors or variable transformers. However, these solutions were unsatisfactory concerning wear, switching speed and costs.

The following parameters determine the matching power controller:

• Switching mode (1-phase, 2-phase, 3-phase)
• Current
• Voltage
• Type of control
• Control
• Special functions

• HP-series: universal, single phase, ohmic loads, up to 75A
• ID-series: compact, single phase, ohmic loads, up to 75A
• HS-series: universal, single phase, ohmic and inductive loads, up to 75A
• IP-series: universal, single phasie, ohmic and inductive loads, partial load fault detection, current control, current measurement, current limiting, up to 50A
• UP-series: universal, single phase, ohmic and inductive loads, voltage control, up to 75A
• PSS-series: universal, single phase, ohmic and inductive loads, 300 to 700A
• PST-series: universal, single phase, ohmic and inductive loads, 300 to 700A
• Customer-specific design by request

Four procedures have been established for switching-on the electric power: On/Off, pulse width modulation, burst firing and phase-angle control. Each has its advantages and disadvantages and is differently suited for the various load types. Therefore the selection of the ideal type of control is depending on the environmental conditions as well as on the load type.

Four procedures have been established for the control of power controllers. The most widely used of these is the analogue control with 0-10VDC via a SPS. Furthermore it can be controlled analogue with 4.20mA, via a serial data telegram or via an industrial bus such as ProfibusDP.

The value in the range of 0-10VDC is sended via an analogue output to the power controller (the value can also be adjusted by a potentiometer instead of a SPS). 0VDC = 0% value, 5VDC = 50% value and 10VDC = 100% value. The power controller sets the value according to the selected type of control e.g. in a phase angle (at the phase control) or the duty cycle (at the burst firing). With this variant an easy and fast control is available. For each power controller an analogue output and for the whole system a digital release-output is needed.

In this case control current (serial current) is used instead of control voltage. The insensitivity to electromagnetic disturbances (e.g. when switching on neighboring users) and to voltage losses due to long wires and the consequently line resistance. Lengths up to 1,000m can be realized. Serial current is mainly used in very big plants as they can be found in the chemical industry.

Instead of transferring the value via an analogue signal it is transmitted digitally. For each power controller a digital output for the data and for the whole system a digital output for the cycle is required. In contrast to the analogue control significant savings in the peripheral are possible. For the control the use of a SPS or a PC is recommended. The value is transmitted via protocol SYSTRANS.

The most effortful variant of control is the use of an industrial bus as e.g. Profibus DP, CANopen or Modbus. Therewith the power controllers can be perfectly integrated in an existing infrastructure. Moreover, further possibilities of reading in and out of parameters and values is possible. For power controllers which are based on solid state relays a bus control is seldom realized due to cost reasons. In the case of power controllers which are based on thyristors the bus control is directly integrated in the power controller as an optional module.

One of the most important requirements for a power controller is the reliable and maintenance-free operation. Furthermore, the power controller should report failures on the power controller itself as well as on the loads and on the supply so that influence can be taken on the controlling level.

• Safety case: The load circuit has to be secured with a safety device. The power controllers recognize the safeguarding case and report the fault either via the signal relays or/and via the bus.
• Wire breakage: A wire breakage is also reported in the supply or load line.
• Load failure: A load failue (e.g. burning out of a spiral-wound filament) is reported as well.
• Undervoltage: In case of undervoltage it can happen that the required energy cannot be added anymore to the process so that the quality of the process is going down. Therefore this condition is also reported as an alarm or pre-alarm.
• Failure load voltage or auxiliary voltage: Failures in load voltage or auxiliary voltage are reported as well.
• Self-diagnosis: Power controllers also often monitor themselves concering failures or partial failures. In addition, parameters as heat sink temperature or fan functioning can be reported.

The easiest variant is that the thyristor controller sets the value irregularly. The regulation is done by the controller. Beside there are subordinated regulation methods as the voltage control (U, U²), current control (I, I²) and the power control (P). Depending on the load the ideal method has to be chosen.

To be able to operate multiple loads with only one controller and without endangering the process reliability there are power controllers with partial load failure recognition. However, this requires the control type phase angle control as otherwise the partial load failure recognition would start in the time offs of the burst firing. The nominal current is set by teach-function of the controller while the failure of one or more loads – when falling below the nominal value – is reported. Thereby steps of 10% can be realized which means that up to ten loads can be run with one controller. By parallel connection all loads are run with the same value therefore an individual regulation is not possible in this case. The big advantage in the partial load failure recognition is the reduced need of power controllers and the required peripheral.

A current limiting is used for the protection of the load. Thereby the maximum permitted True RMS load current is set at the power controller or rather learned in operation. A typical application is the operation of loads with a high starting current, i.e. Rcold < Rwarm.
In this case either a phase control or a burst firing with soft-starting is possible as a type of control. If the controller is reaching the current limiting the phase angle control is taken back until the current falls again below the permissible value. Through the warming of the heating element the resistance is changing and therefore the heating element can slowly be brought to the desired value. Examples are molybdenum disilicide heating values or infrared radiators.

For some applications a minimum value is required, e.g. to preheat the spiral-wound filament although usable energy is not emitted yet into the process. Therefore a minimum value, the so called offset in the power controller is set. This can be done from the hardware (e.g. via a potentiometer) or the software (from the manufacturer in accordance with customer specifications) perspective.

Each ohmic load can be easily set with power controllers. The spectrum ranges from simple heating elements through high temperature heating elements (e.g. SiC or MoSi2) to infrared radiators. The special characteristics of certain heating elements (e.g. high starting current, aging) should be noted when selecting the suitable power controller. Furthermore, inductive loads (e.g. inductive heating) and some engines (e.g. fan motors) can be operated with power controllers as well.