Xantrex Technology SW Series - Manual (Page 119)

TECHNICAL INFORMATION
 2001 Xantrex Technology, Inc.
5916 - 195th Street N. E.
Arlington, WA 98223
Telephone: 360/435-8826
Fax: 360/435-2229
www.traceengineering.com
SW Series Inverter/Charger
Part No. 2031-5
Rev. C: February 2001
Page
111
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.