July
2005, Issue 180
Solar-Powered
Water Pump Controller
Cypress
PSoC High Integration Challenge 2004 Contest Winner
Lindsay’s
CY8C27443-based Solar Pump Controller operates a submersible
water pump directly from solar panels. It’s ideal
for remote areas that lack conventional power supplies.
The powerful PSoC provides three-phase PWM generation,
signal conditioning, temperature monitoring, constant
voltage-to-frequency control, and over-current and
fault protection.
by
Lindsay Meek
Solar-powered
pumps are used to extract ground water in remote areas
where conventional power may not be readily available.
The low-cost submersible pumps operate from three-phase
power, so the DC power from the solar panel must be
converted to AC with an inverter. The inverter, which
generates a three-phase composite waveform, controls
the pump’s motor using the fixed-ratio, voltage-to-frequency
control method.[1] This method maintains a constant
AC output voltage that’s directly proportional to the
inverter’s AC frequency. Therefore, lower AC frequencies
result in lower AC output voltages. This maintains a
constant flux in the pump motor and maximizes its efficiency.
The
output frequency is controlled to maximize the power
drawn from the solar array. By increasing the output
frequency, the power drawn by the pump motor also increases,
thus increasing the current drawn from the solar array.
A nonlinear relationship exists between the output voltage
and current in a solar panel (see Figure 1). A maximum
power operating point can be set by varying the pump
frequency.
|
(Click
here to enlarge)
|
Figure
1—Compare the current and power to the voltage curves
for a string of solar cells. The maximum power-point
knee occurs when the current times voltage gives
the highest possible value.[2] |
The
inverter’s main system components are the control section
and the power stage (see Photo 1). Figure 2 is a block
diagram of the system.
|
(Click
here to enlarge)
|
Photo
1—Take a look at the finished prototype PCB. The
power stage is on the left. The CY8C27443 development
pod is on the bottom right. The linear power supplies
are in the top right corner. |
|
(Click
here to enlarge)
|
Figure
2—The power stage (bottom) is connected to the CY8C27443
(top). The solar panels provide DC power, which
is converted to AC power using the power stage and
transformer that’s suitable for driving the pump
motor. |
The
control section makes extensive use of the Cypress CY8C27443’s
mixed-signal array, reducing it down to a single chip
(see Figure 3). The CY8C27443 is responsible for PWM
generation, signal conditioning, over-current and fault
protection, temperature monitoring, constant voltage-to-frequency
control, and maximizing the power from the solar array.
|
(Click
here to enlarge)
|
Figure
3—The main prototype includes dual THD/SMD footprints
for the CY8C27443 microcontroller. A simple linear
power supply provides control power. The RESET line
keeps the control inactive when turned off. |
The
power stage consists of three H-Bridge driver ICs connected
to an integrated three-phase MOSFET module (see Figure
4, page 66).[3] The power-electronic topology is that
of a hard-switched buck converter (with the solar panels
connected directly to the power stage) and a three-phase
delta-wye transformer responsible for generating the
230-V nominal line-to-neutral voltages for the submersible
pump motor.
|
(Click
here to enlarge)
|
Figure
4—The prototype power stage schematic shows the
three H-Bridge gate drive ICs connected to the integrated
three-phase MOSFET module. A chunky (yet inexpensive)
linear regulator powers the controller. You can
probably improve this with the extended voltage
range simple switcher. |
