Microcontroller-based multi-point true RMS current measurement system

This work describes the design and implementation of a multi-point measurement system. The system is applied to monitor the true RMS values of currents in a heat treatment furnace in the aluminium industry in Egypt. Electric power is used as a source of heating. The whole system consists of a 3-phase balanced supply feeding balanced 3-phase resistors. Silicon controlled rectifiers are used to control the flow of power from the supply to the heating elements. The integral cycle control principle with zero crossing instant switching is applied for the sake of temperature control in the homogeneous furnace. The idea behind that method is to allow the current to flow in the heaters for an integer number of supply cycles and to prevent the current flow throughout other integral number of supply cycles. As a result, line current is not a continuous sine wave in shape. Conventional ammeters can not give an accurate indication for such a current wave. The use of an 8-bit microcontroller Intel 8031 facilitates the on line calculations of the true RMS values of the measured currents. The system is applied in the field to monitor 30 points of measurements and updates their readings every 15 sec. The system can be modified to pick up any other quantities (electrical or nonelectrical) with minor modifications in the interface circuits. The implemented system deals with power of a maximum 2000 kW. The system is simple in design and cheap in cost.