Xantrex Technology 120 VAC/60 Applications, Resistive Loads, Inductive Loads, Page 111

TECHNICAL INFORMATION

APPLICATIONS

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. Even if the inverter could 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 electronic devices have transformers (TV’s, 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 you manage 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 changes AC voltage polarity.

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 grid. Induction motors (motors without brushes) require 2 to 6 times their running current on start-up. The most demanding are those that start under load e.g. compressors and some pumps. The largest motor of this type that the inverter will run varies from 1/2 to 3/4 hp. Of the capacitor-start motors, typical in drill presses, band saws, etc., the largest you may expect to run is 1 to 1.5 hp. Universal motors are generally easier to start. The inverter may start up to 2.5 hp universal motors. Since motor characteristics vary, only testing will determine if a specific load can be started and how long it can be run.

SUBMERSIBLE WELL PUMPS

The well pump is often the hardest load to start for inverter system. Submersible pump motors are particularly hard because the motor is very narrow (in order to fit down the well) and draws extremely high starting current.

When selecting a pump, check the LOCKED ROTOR AMPS in the motor specifications. This is usually the best guide to the maximum load the pump will place on the inverter. It must be less than the AC amps noted under the Maximum Output (RMS) given in the Specifications And Features section starting on page 118. Pump suppliers and well drillers often oversize the pump considerably in order to reduce complaints of poor pressure, low flow, etc. Get several quotations and explain that you are very concerned about the inverter being able to power the pump. It makes sense to operate a smaller pump longer instead of a larger pump for a shorter period of time when you are powering it from batteries or a solar electric system.

When buying a pump, select a three-wire type. This refers to the electrical configuration of the power and starting windings in the pump motor. A three-wire pump requires a separate box at the top of the well for the starting circuit - a two-wire pump has the start electronics built inside. The separate starting box is preferred. If possible, select a relay type starting box instead of an all-electronic type - the relay types have been found to work better with inverters and generators. The relay types also allow use of a larger or additional starting capacitor - it may help if the inverter has a hard time starting the pump. Consult your pump supplier for more information.

If a 220/240-Vac motor is required, you must either use two 120 Vac inverters in a stacked configuration or include a step-up transformer in the system. When used with a well pump, connect the step-up transformer after the pressure switch in order to reduce the load the transformer itself puts on the inverter. This requires that 120 Vac instead of 240 Vac be connected to the pressure switch.

As a last resort, consider powering the well pump directly from the generator instead of the inverter. Many systems do not have enough battery capacity or excess power to handle a load as heavy as a well pump. Using the generator with a storage tank with several days capacity may be more economical than oversizing the inverter, battery and solar array just to power the well pump.

 2001 Xantrex Technology, Inc.	Telephone: 360/435-8826	SW Series Inverter/Charger
5916 - 195th Street N. E.	Fax: 360/435-2229	Part No. 2031-5
Arlington, WA 98223	www.traceengineering.com	Rev. C: February 2001

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