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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 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.
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.
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.