Xantrex Technology PS2.5 Appendix B Inverter Applications, Resistive Loads, Inductive Loads

Appendix B: Inverter Applications

AC loads on the inverter differ in the way they perform. There are different types of loads: resistive loads, inductive loads, and problem loads.

Resistive Loads

These are the loads that the inverter finds the simplest and most efficient to drive. Voltage and current are in phase, or, in this case, in step with one another. Resistive loads usually generate heat in order to accomplish their tasks. Toasters, coffee pots and incandescent lights are typical resistive loads. Larger resistive loads—such as electric stoves and water heaters—are usually impractical to run off an inverter due to their high current requirements. Even though the inverter can most likely accommodate the load, the size of battery bank required would be impractical.

Inductive Loads

Any device that has a coil of wire in it probably has an inductive load characteristic. Most electronics have transformers (TVs, stereos, etc.) and are therefore inductive. Typically, the most inductive loads are motors. The most difficult load for the inverter to drive will be the largest motor it manages to start. With inductive loads, the rise in voltage applied to the load is not accompanied by a simultaneous rise in current. The current is delayed. The length of the delay is a measure of inductance. The current makes up for its slow start by continuing to flow after the inverter stops delivering a voltage signal. How the inverter handles current that is delivered to it while it is essentially “turned off”, affects its efficiency and “friendliness” with inductive loads. The best place for this out-of-phase current is in the load. Inductive loads, by their nature, require more current to operate than a resistive load of the same wattage rating, regardless of whether power is being supplied by an inverter, a generator, or utility power (the grid).

Induction motors (motors without brushes) require two to six times their running current on start-up. The most demanding are those that start under load, e.g., compressors and pumps. Of the capacitor start motors (typical in drill presses, band saws, etc.), the largest you may expect to run is ½ to 1 hp (depending on inverter size and surge power capability). Universal motors are generally easier to start. Since motor characteristics vary, only testing will determine if a specific load can be started and how long it can be run.

If a motor fails to start within a few seconds, or it begins to lose power after running for a time, it should be turned off. When the inverter attempts to start a load that is greater than it can handle, it will turn itself off after about 10 seconds.

Problem Loads in Load Sense

Very small loads - If the power consumed by a device is less than the threshold of the load sense circuitry, it will not run. See “Load Sense” information in “Section 3: Configuration” (page 25 and following) for ways to solve this problem. Most likely the solution will be to defeat the load sense feature.

Fluorescent lights & power supplies - Some devices when scanned by load sense circuitry cannot be detected. Small fluorescent lights are the most common example. (Try altering the plug polarity by turning the plug over). Some computers and sophisticated electronics have power supplies that do not

Prosine 2.5/3.0 Installation & Operation Guide 85