Several manufacturers are now marketing devices that promise to triple current output from a PV panel. These linear current boosters (LCBs) help eliminate the need for storage batteries or oversized arrays when running electric motors directly from the panels. Is this magic or simply alot of hype? Actually it's neither. In keeping with Home Power's philosophy of delivering accurate and useful information on new renewable energy products, these devices were tested and their performance documented. The following article explains their operation and gives the facts and figures in how well they perform.
When photovoltaic panels are connected to storage batteries, the match between load and source is pretty close to ideal. The panel is able to deliver close to maximum available power over a wide range of solar intensity, current, and voltage. It is only when PV panels are called on to power electric motors directly that a poor match takes place and we find that the panel is unable to deliver full power and operate the motor.
In order to better understand why this occurs, let's take a look at how a photovoltaic panel delivers volts and amps to a load. A photovoltaic panel is essentially a constant current source. It can deliver a fairly constant amount of current even as the voltage falls. We can see this if we connect an ammeter to a panel and short the output. The current may be as high as three Amps even with the Voltage essentially zero. The power output at this point is also zero since Volts x Amps = Watts. (For those who need to brush up on this see Home Power #1 and #4 for R. L. Measures' fine articles on basic electricity). When the panel is connected across a motor that requires close to the maximum power output of the panel, the motor is essentially a dead short and Voltage drops to zero. With no Voltage, there is no power and without power there is nothing to run the motor. A motor that requires as little as eighty Watts to run at full power and speed may require 150 Watts of panel capacity. This leads to inefficiency and higher costs. Now thanks to modern electronics this problem can be eliminated.
Without getting overly technical and trying to explain the inner workings of the various current boosters or power trackers, here is an explanation of how they do their job.
Power or Watts is the product of Volts times Amps. Whether we have 40 Volts at one Amp or 40 Amps at one Volt the power is still 40 Watts. The boosters we are talking about do basically two things. First, they "fool" the panel into thinking that the load it is supplying, in this case a motor, is really smaller than it is. This allows the output current and voltage from the panel to remain at maximum, thus delivering full available power to the booster.
The second function, and really the "magic" that these devices perform, is their ability to covert volts to amps. Using high speed switching power supply technology, an input of three Amps at 24 Volts may, depending on load, be output at 6 Amps at 12 Volts. Power out then equals power in (minus 8% efficiency loss approx.) only at a lower voltage and higher amperage. When this higher amperage is input to the motor to overcome internal friction, and reactive loading.
Permanent magnet motors are the only types that these devices work on. The reason for this is that wound field motors need a higher voltage applied to the field to create the magnetism for the field flux. The magnetic field in permanent magnet motors is independent of applied voltage so it is only concerned with input amps to create the torque needed to start. The trade off is in the motor RPM. Lower voltage means lower RPM.
Because of the units simplicity and low cost, as well as fine technical support from the factory, the LCB or Linear Current Booster from Bobier Electronics, Parkersburg, West Virginia was selected for this article. The device is a small metal can weighing less than 1/2 pound and measuring less than three cubic inches. It is rated at 3 Amps maximum input, 4 Amps continuous output and 8 Amps surge. Connection is via a plus and minus input from panel and plus and minus input to load. Ten inch leads are provided and connections are clearly marked and color coded.
The model tested had what is called by the factory a "Tweeker" adjustment that allows the device to be adjusted to match any load between 12 and 24 Volts. When the device was first taken from the box, the urge to really give it the acid test came over me. I couldn't wait to hook it up. In my shop I have a 24V 1 HP permanent magnet motor that really is stiff and hard to turn over. It seemed much too large for the test, but then I wanted to put the ultimate load to the device. A 36 Watt Solavolt panel was connected to the L.C.B. I then connected the motor leads and nothing happened.
Following the instructions that came with the device I used a jeweler's screwdriver to adjust the "tweeker" on the back side of the case. After a few turns, I heard a high pitched squeal come from the device, and before I knew it the motor had rolled from the deck and on to the ground. The motor was not held in place so the sudden torque of the starting caused it to roll away. Holding the motor in place, I again connected the leads and was very surprised at the sudden torque and quick rise in RPM. I couldn't help being impressed when I realized that this was a one horsepower motor with lots of friction loss, starting and running on less than 40 Watts of power. The booster was putting out over seven Amps to start this motor.
In order to assure myself that the device really worked I connected the panel directly to the motor. I couldn't even get it to hum. I was convinced that the device really did start motors, but accurate lab testing for speed, efficiency, and operating horsepower now had to be run.
In order to assure a fair and accurate test of these devices, proper laboratory testing procedures had to be followed. All testing was done at high noon, clear sky conditions at 4,200 feet elevation. Meters and test instruments were calibrated before using. The following diagram shows how connections were made. Input and output current and voltage were monitored simultaneously as motors were tested. Power came from 2 SOLAVOLT 36 Watt panels connected in series to give a nominal 24 Volt 2.5 Amp output. A small prony brake was used to record the output torque from the motor and a hand held tach was used to measure RPM. Using the torque and RPM readings the horsepower was determined. Although five motors in all were tested at 12V-24V-36V, only one was sized to give an accurate picture of performance based on the array size. The motor selected was a 24 Volt 15 Amp 2000 RPM continuous duty unit. The chart on page 14 shows performance figures for loading from no load to approaching full stall when connected to the linear current booster.
From the chart we can see clearly that the booster does indeed supply more current than the panel can by itself. Looking at the input current and voltage, it is obvious that the power is remaining quite stable over the entire range of loads. Close to maximum power is being delivered to the booster. Although we incur some losses (8% average) in the conversion, the power out is still close to power in. The most significant changes we see are in the drop in RPM and the dramatic increase in torque. This increase in torque is the boosters greatest contribution to running motor loads. Not only does this torque boost help in starting a motor, it also allows the motor to power a fluctuating load, or keep a pump operating as a cloud passes. Using this same motor and booster setup, a small rotary vane pump was able to continue pumping even when the sun was hidden behind modest cloud cover. The RPM and delivery rate dropped off, but it kept pumping. On array direct operation, the pump stopped as soon as the clouds rolled in.
Before running the motor on the booster, it was tested on panel direct operation in order to develop a baseline for torque and RPM. With 34V and 1 Amp input, the motor spun to over 2,400 RPM. As soon as the prony brake approached a load of 30 ounce-inches, the voltage dropped very quickly and the motor started to stall. With the booster I was able to load the motor to well over 130 oz.-in. and still not stall the shaft.
DC Ammeter measuring LCB output current
2 @ MSVM 4010 Solavolt PV Modules wired in Series 36 Watts
DC Ammeter measuring panel current
DC Voltmeter measuring panel voltage
DC Ammeter measuring LCB output current
DC Voltmeter measuring LCB output voltage
Tachometer measuring motor RPM
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