Download Inverters deliver power solution

FROM THE WORKSHOP:
Power inverters are a handy source of mobile
electricity great for powering power tools, battery
chargers, lights, computers and a host of other mobile
electronic equipment. But they have their limitations
and if not used correctly can damage or destroy the
appliances drawing from them.
Inverters deliver power solution
Josh
Giumelli
KONDININ GROUP
As a mobile means of providing
electricity, power inverters are difficult to
beat for convenience.
Inverters are quiet, compact and do not
require fuel unlike a generator.
While the price has fallen considerably in
recent years, there are several limitations to
their use to consider to avoid damage when
operating 240-volt equipment.
Feel the power
At a glance
Power inverters come in a number of
different capacities suitable for a
variety of applications.
It is essential to match the drawing
capacity of an appliance with the most
appropriate inverter, failure to do so
can damage or destroy the appliance.
There are two different types of
inverters: modified sine wave — best
suited to 90 per cent of electrical
equipment; and pure sine wave —
best suited to sensitive electrical
equipment such as computers.
Avoid installing inverters in the engine
bay as heat from the running motor
can prevent the inverter from
cooling properly.
Inverters use 12V or 24V direct current,
usually from a vehicle and convert it to 240V
alternating current.
The major limitation is the rated current
output, which determines the size of 240V
load, which can be operated.
Smaller inverters, usually about 150-watt
output, are inexpensive and can be
connected to the cigarette lighter connection
in a vehicle. They are handy for operating a
phone charger but will not power a small
angle grinder.
Avoid installing inverters in
the engine bay, as the heat
from the running engine will
prevent the inverter from
cooling effectively.
Larger inverters of 300W and above need
to be connected directly to the vehicle’s
battery as they will blow the accessory plug
fuse if used to capacity.
Depending on the size of inverter and
the load placed on it, choose the cable
connecting it to the battery carefully or it
will overheat and reduce the available
current to the inverter.
Two varieties
There are two types of inverter: modified
sine wave and pure sine wave. Most inverters
sold are modified sine wave units suited for
general use, while pure sine wave inverters
are more suited to specialist applications.
32 Farming Ahead September 2007 No. 188 www.kondinin.com.au
What’s inside the box?
While stepping up AC voltage is as
simple as passing it through a transformer,
DC voltage will not work through a
transformer as the current direction is
always the same. As transformers need an
alternating magnetic field, they must be
supplied with alternating current, achieved
in an inverter by switching the 12V supply
on and off many times a second.
A transistor device called an ‘H-bridge’ is
used to shape the voltage waveform fed into
the transformer. Grid supply has a sine wave
(see Figure 1) that needs to at least be partly
replicated for electrical devices to function
properly when powered by the inverter.
Modified sine wave — This is sufficient
for about 90 per cent of electrical
equipment and it means the inverter is
cheaper to construct.
Pure sine wave — As more complex
electronics are needed to achieve this output,
these inverters are more expensive but better
suited to expensive equipment. They are less
efficient and draw more current when idle.
Devices such as cordless drill chargers,
computers and printers, lights with dimmer
switches or sensitive electronic equipment
are best run off a pure sine wave inverter.
FIGURE 1 Wave forms
A modified sine wave inverter creates a square
wave form in four steps which approximates the
sine wave. A pure sine wave inverter achieves a
very close approximation of the sine wave by using
about 50 steps to closely replicate the sine wave.
Source: Kondinin Group.
Photos: Josh Giumelli
MOBILE ELECTRICITY
© Kondinin Group
This article has been reproduced with permission from Farming Ahead.
For more information about Kondinin Group phone 1800 677 761.
Further duplication of this article is not permitted.
Matching the load
TABLE 1 Cable sizes
Inverters are rated in watts, which refers
to the maximum continuous power it can
supply to a 240V appliance and often will
have a peak rating (the amount of power that
can be supplied above the continuous rating
for a short period). For example, a 2500W–
5000W peak inverter can cope with a starting
or peak power consumption of 5000W but
will only operate continuously at 2500W.
Many electric motors and other devices
will have a starting load several times their
running load. It is this starting or peak power
requirement that often dictates if an inverter
can operate the device or not. A 1500W
electric motor could draw 4500W on start-up
and will not operate off a 1500W inverter.
Modern inverters have protection systems
to ensure they are not damaged by
overloading but do not rely on these systems
continuously to protect the inverter.
If an electrical appliance is rated in
amperes rather than watts, the power
requirement can be calculated as:
Maximum
watts at 240V
300W
600W
1000W
1500W
2500W
Watts = amps x 240
If in doubt ask
Check with the appliance manufacturer
for starting load as this is not always
included in the specifications.
Pumps, grinders, compressors air
conditioners and microwaves all draw
significantly higher currents on start-up,
with some devices momentarily consuming
up to seven times the running current.
Current draw
from battery
30A
60A
100A
150A
250A
FROM THE WORKSHOP:
MOBILE ELECTRICITY
Cable size
gauge mm
#10
2.6
#6
3.3
#4
5.2
#2
6.5
2–0
9.3
Source: Kondinin Group.
Lights on
Fluorescent lights can cause problems
when running from an inverter. While the
light tube itself has a low wattage rating, it
can consume a large amount of current.
When operating off 240V mains supply, a
capacitor inside the light fixture corrects
the lagging power factor. But connected
to a modified sine wave inverter, the
capacitor tends to drastically over-correct
the power factor, drawing a high current
and potentially overloading the inverter.
Keep inverters away from
moisture or explosive
environments such as
petrol fumes.
If the fluorescent lights never will be
used again on mains supply an electrician
can remove the capacitor, allowing easier
operation on an inverter.
Safety first: Small inverters can be run from a
vehicle’s 12V accessory outlet but it is generally
only safe to draw up to 150W at 240V, which
corresponds to 15amps, without blowing a fuse.
Installation
Inverters, especially larger units, are
often installed in vehicles and hard-wired
to the battery. Ideally, the inverter can be
connected to the second battery in a dualbattery system. Units smaller than 1000W
can be temporarily connected to the battery
using alligator clamps but larger units need
a more permanent connection capable of
handling the higher current draw.
Locate the inverter where it can cool
effectively and the internal fan has a ready
supply of dust-free air. Keep the battery cable
short to reduce current loss (see Table 1) and
ensure the inverter chassis ground terminal
(if fitted) is connected to the vehicle body.
CONTACT Josh Giumelli is the engineering manager
for Kondinin Group and writer for Farming Ahead.
[email protected]
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Farming Ahead September 2007 No. 188 www.kondinin.com.au
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