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Operation of
an EMPT
The EMPT operates
by storing electrical energy in a large capacitor and the motor
winding inductance while switching the PV array on and off under
control of an electronic circuit that monitors the voltage and temperature
of the PV array. The electronic circuit maintains the PV array at
its maximum power voltage for the existing module temperature. The
PV array is connected to an input energy storage capacitor that
accepts current from the array. The current from the capacitor flows
through an electronic switch (power FET) that is turned on and off
by the control circuit. The percent of the time that the electronic
switch is on (referred to as pulse width) is varied as necessary
to keep the PV array voltage at the desired level. The average voltage
on the motor is the product of the optimum PV array voltage times
the percent of the time that the switch is closed. A "commutating"
diode is placed across the motor to provide a path for the motor
current to recirculate while the switch is open. This is possible
because the winding inductance of the motor stores energy, acting
as an electrical "flywheel". Noise generation due
to the switching circuits has been minimized by employing a capacitor/ferrite
snubber circuit around the commutating diode, the large input filter
capacitor, the motor inductance (the internal inductor in the battery
charging model) and the steel enclosure. Optimal layout design also
helps to reduce both external and internal noise generation.
The circuit operates
as follows: While the switch is turned on, the energy storage capacitor
is directly connected to the motor and causes a rapid build-up of
motor current. If the current required to operate the motor at design
speed is equal to or less than the PV current at optimal array voltage,
the switch remains closed and the motor operates at full speed.
If insufficient power is available from the PV array to operate
the motor at or above the design voltage, the voltage across the
capacitor will drop below the optimum voltage and the control circuit
will turn the switch off. At this time two actions happen, (1) the
commutating diode allows motor current to continue to flow as the
motor coasts for a short period, and (2) the PV array continues
to produce current that charges the capacitor, increasing the capacitor
voltage. When the capacitor voltage increases above the desired
optimum voltage the electronic switch is closed and the cycle repeats.
DC motors operate
at a speed proportional to average applied voltage. For example,
if a PV system is designed to operate a 90 VDC motor, and low sunshine
on the PV array reduces output to 20 percent of the normal motor
current, the electronic switch will be turned on 20 percent of the
time applying 90 VDC to the motor 20 percent of the time. The switch
will be off 80 percent of the time with close to zero volts across
the motor (due to the commutating diode), the average voltage will
then be about 18 volts (20 percent of 90 volts). The motor will
operate at 20 percent of its normal speed. Array current will be
20 percent of the motor current, or looked at the other way, motor
current will be five times array current. This higher current will
produce five times more torque than would be available without the
EMPT. This slower operation of the motor will give about 20 percent
of the full sun output from a "positive displacement"
pump (such as a Jack pump). Direct drive from the array in this
condition would give insignificant torque (current) and would not
operate the pump.
EMPT's are available
for PV power ratings from 100 watts to over 3 kW for use with loads
ranging from 12-volt fractional horsepower DC motors up to 180 VDC,
3 Hp motors. Battery charging models (EMPTB) provide up to 20 or
40 Amps of charging current to a battery from a high voltage PV
array. The EMPTB is also temperature tracking and provides higher
output current as the battery state-of-charge is lower. A specially
designed low-loss inductor is added to the EMPT-B to provide the
voltage transformation. The power inductor also maintains battery
charging current while the electronic switch is in the off position.
This type of circuit can be used with high voltage, lower current
PV arrays and long wire runs to charge low voltage batteries at
high currents, saving wiring costs while operating the PV array
at its optimum power voltage point. By adding an "M" to
the part number (EMPTM) or (EMPTBM) full metering of input and output
voltage and current is provided (four functions). The Temperature
sensor is glued to the back of a cell on the Module. The two leads
from the sensor are connected with spade lugs to spare terminals
in the Module J-Box. Two small gage wires run from the Module J-Box
to the control board Quick Disconnects. Temperature Tracking the
Maximum Power Point can increase output by 20% in the Winter.
When
should I use a Maximum Power Tracker?
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