Wattmiser

Theory of Operation

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 commutated 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.

Wattmiser Speed Regulator

The rotor circuit voltage in a wound rotor motor is inversely proportional to speed, and torque produced is proportional to rotor current. Therefore, the speed of the motor can be held at a set point by regulating the rotor current (torque) so that it equals the torque required by the load. Regulation of the rotor current in the Wattmiser is accomplished by control of the firing angle of the inverter SCR's. Because the firing angle is continuously variable, the speed of the wound rotor motor is stepless and continuously variable.

The Wattmiser regulator compares a speed reference signal with a rotor derived voltage feedback signal representing actual motor speed. The resulting error signal is fed into a current regulator which adjusts the firing angle of the SCR's to reduce the error signal to zero. This process balances motor torque with load torque at the set speed point. In addition, the current regulator is supplied with an inverter current feedback signal to limit the inverter current to a safe value.

Control protective features include under-voltage, phase loss, instantaneous overcurrent, and inverse time overcurrent. The control is insensitive to incoming line phase rotation.

Wattmiser Benefits

ENERGY SAVINGS - The Wattmiser recovers the slip power normally lost as heat in other slip drives and returns this power back into the incoming line. This results in substantially reduced power costs.

INCREASED EFFICIENCY - The high efficiency of the Wattmiser drive offers dramatic savings in energy consumption over slip type drives such as the wound rotor motor with resistor control, eddy current drives and hydraulic couplings. (FIG 4) The Wattmiser offers high efficiency with far less complexity than adjustable frequency drives because it uses a simple line commutated inverter.

VARIABLE SPEED CONTROL - By regulating the rotor voltage and current, the Wattmiser provides infinitely variable motor speed matched exactly to the system speed requirements.

REDUCES PEAK DEMAND - The Wattmiser limits the inrush current to a minimum level thereby reducing utility peak demand charges.

LONGER MOTOR LIFE - By soft start and infinitely variable speed control, the Wattmiser reduces motor heating, bearing wear, and coil flexing since the rotor torque is limited to that required by the load.

FLEXIBILITY - In pumping applications duplex, triplex or quadraplex systems may be provided so that pumps are started, stopped and speed controlled automatically to meet the system flow requirements. Provision is made to alternate the lead pump and thus provide even wear.

RELIABILITY - All printed circuit boards are burned-in to eliminate infant mortality. The complete Wattmiser controller is thoroughly tested and run at the factory before shipment to the site. All components are conservatively rated.

MAINTAINABILITY - The Wattmiser is a solid state control requiring minimum maintenance. Printed circuit boards are plug-in design for quick removal and ease of maintenance.

EXPERIENCE - Marathon Electronics is the industry leader in application and design of wound rotor slip recovery systems. Hundreds of Wattmiser have been installed providing owners with years of trouble-free, energy conserving operation.

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Additional Items

• Adjustable Speed Range.
• Lower Operating Cost
• Higher Efficiency
• Positive Heat Control
• Longer Life of Controller