Power control of platinum draw-plate: glass wire manufacture
Glass wire manufacturing requires a high temperature stability of the draw-plate to ensure a good quality of the wire, one important parameter being a constant diameter. To obtain this stability, the temperature control is made with a Eurotherm controller giving very high stability and a thyristor stack which ensures that the right electrical power is injected into the load. This stack is a true power controller giving a constant power for a constant demand even if the resistance or the supply voltage varies.
The physical configuration of the control loop is very classical:
The final load, whose temperature must be kept constant, is the draw-plate itself -made in platinum -which has a high positive temperature coefficient for its resistance: 55 x 10-4 per °C
The cold resistance is about 8 times less than the resistance when at the correct temperature. In the cold condition the load is nearly a short-circuit. The SCR controller current limit must handle this characteristic without any damage both for the stack and the load. The stack will allow the right level of current to flow through itself and through the load requiring a fast current limit action.
When hot, the load resistance is still very low. The load supply voltage is a few volts, but the current needed to get the right power is very high: a few thousand amperes, so a steep down transformer is needed to draw the power from the 380V supply. The thyristor stack controls the transformer primary. The stack current rating needed is then only 150 Amps for up to 57 KW maximum load power, depending on draw-plate size.
To control the exact active power transferred to the load, the SCR is a Eurotherm EPower controller using a true power measurement method: Product of instantaneous values of current (I) and voltage (u).
Due to the temperature coefficient of the platinum, the transformer primary control and the high temperature stability required, the thyristor is working in phase angle firing mode. The control loop is closed by a temperature measurement on the draw-plate sent to the 2704 controller. The achieved temperature stability is 0.1°C at working temperature.
The Eurotherm controller with Digital Communication provides the facility to remotely control and supervise 75 couples of controllers/471.
To have access from the control room, where the supervisory system is installed, to the level of power delivered by each EPower controller stack, the 0-10V output power signal available on each EPower unit is sent to each associated controller on its external set point input, calibrated to 0 – 57 for 0-10V which gives a direct reading of power. The EPower instrument offers communication (TCP/IP, Modbus or Profibus): it can be connected on the communication bus. All EPower controller parameters can be available to operators in the control room; power, current, unit status, alarms etc.
