FLOMATCHER/MARATHON
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The Wattmiser is an adjustable speed drive system that varies the speed of a wound rotor induction motor steplessly and efficiently. The Wattmiser system has three major parts: a wound rotor motor, a motor starter, and a slip recovery section, which is the Wattmiser control. A wound rotor induction motor is similar to a squirrel cage induction motor except that in place of the bare rotor bars found in the squirrel cage motor, the wound rotor conductors are insulated and brought out to slip rings. Carbon brushes riding on these slip rings allow the rotor to be connected to external impedance elements such a resistors. Thus, it has been common practice to control the speed of a wound rotor motor by switching resistors in and out of the rotor circuit in steps. Two major problems are encountered when using resistors to vary the speed of a wound rotor motor. A resistor and contactor are required for each speed step, so only a limited number of different speeds are practical. Secondly, the resistors produce wasteful energy loss in the form of heat. The Wattmiser control avoids this energy loss by returning the energy to the power line. In addition, the speed can be controlled in a stepless manner rather than in a limited number of steps available with the stepped resistor control. The Wattmiser system accomplishes these functions as shown in Fig. 1. The three phase power from the slip rings of the rotor is rectified from AC to DC by a solid state full wave rectifier bridge. After a smoothing inductor reduces current ripple, the power is inverted to AC at power line frequency by a silicon controlled rectifier (SCR) full wave bridge which functions as a line commutated inverter. A line commuttated inverter is the simplest and most reliable type available. A starting resistor is supplied to bring the motor up to the minimum point in the operating speed range. After a few seconds for starting, this resistor is shorted out of the circuit by the RUN contactor. The isolation transformer provides voltage matching between the inverter and the power line and provides additional protection by limiting fault current to the inverter section. Power factor capacitors provide correction for the normally lagging power factor of the motor and control and thus reduce the current flow through the power system conductors and transformers which supply the load. |
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