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General Information about Inverters |
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1. What is a
DC-AC inverter?
2.
What are the criteria for selecting the
right inverter for a particular application?
3.
What are the electrical standards pertaining
to DC-AC inverters?
4.
What are “continuous” and “surge” power
capacity ratings for DC-AC inverters?
5.
What types of electrical loads require
“surge power”?
6.
How do “surge power” electrical loads affect
DC-AC inverter operation?
7.
How should I size my DC-AC inverter for
loads that require starting surge?
8.
How should I size my DC-AC inverter to
operate a microwave?
9.
How should I size my DC-AC inverter to power
a water supply pump?
10. What is “idle power” in a DC-AC
inverter?
11. What are Phantom and idling loads?
12. Can a standard DC-AC inverter be
connected in parallel with another AC
source/ DC-AC inverter / electrical utility?
13. How can DC-AC inverters be connected to
multi-wire branch circuits?
14. How should I size my DC-AC inverter to
supply computer or printer loads?
15. What is the difference between a 12V,
24V and 48V inverter?
1. What is a DC-AC inverter?
The utility grid supplies you with
alternating current (AC) electricity. AC is
the standard form of electricity for
anything that “plugs in” to the utility
power. Direct current (DC) electricity flows
in a single direction. Batteries provide DC
electricity. AC alternates its direction
many times per second. AC is used for grid
service because it is more practical for
long distance transmission.
An inverter converts DC to AC, and also
changes the voltage. In other words, it is a
power adapter. It allows a battery-based
system to run conventional AC appliances
directly or through conventional home
wiring. There are ways to use DC directly,
but for a modern lifestyle, you will need an
inverter for the vast majority, if not all
of your loads (in electrical terms, “loads”
are devices that use electrical energy).
Incidentally, there is another type of
inverter called grid-interactive. It is used
to feed solar (or other renewable) energy
into a grid-connected home and to feed
excess energy back into the utility grid.
This inverter is NOT grid interactive
2. What are the criteria for selecting the right inverter
for a particular application?
To choose an inverter; you should first
define your needs. Where is the inverter to
be used? Inverters are available for use in
buildings (including homes), for
recreational vehicles, boats, and portable
applications. Will it be connected to the
utility grid in some way? Electrical
conventions and safety standards differ for
various applications, so don’t improvise.
3. What are the electrical standards pertaining to DC-AC
inverters?
The DC input voltage must conform to that of
the electrical system and battery bank. 12
volts is recommended for small, simple
systems. 24 and 48 volts are the common
standards for higher capacities. A higher
voltage system carries less current, which
makes the system wiring cheaper and easier.
The inverter’s AC output must conform to the
conventional power in the region in order to
run locally available appliances. The
standard for AC utility service in North
America is 120 and 240 Volts at a frequency
of 60 Hertz (cycles per second). In Europe,
South America, and most other places, it is
230 volts at 50 Hertz.
4. What are “continuous” and “surge” power capacity ratings
for DC-AC inverters?
How much load can an inverter handle? Its
power output is rated in Watts. Read details
under “Characteristics of Sinusoidal AC
Power” on page 7. There are two levels of
power rating -a continuous rating and a
surge rating. Continuous means the amount of
power the inverter can handle for an
indefinite period of hours. When an inverter
is rated at a certain number of Watts, that
number generally refers to its continuous
rating. The “surge power” indicates the
power to handle instantaneous overload of a
few seconds to provide the higher power
required to start certain type of devices
and appliances.
5. What types of electrical loads require “surge power”?
Resistive types of loads (like incandescent
lamps, toaster, coffee maker, electric
range, iron etc) do not require extra power
to start. Their starting power is the same
as their running power.
Some loads like induction motors and high
inertia motor driven devices will initially
require a very large starting or “surge”
power to start from rest. Once they have
started moving and have attained their rated
speed, their power requirement reduces to
their normal running power. The surge may
last up to 5 seconds. TVs and microwave
ovens also require surge power for starting.
The manufacturers’ specification of the
appliances and devices indicates only the
running power required. The surge power
required has to be guessed at best.
See below under “Sizing of
inverter for loads that require starting
surge”
6. How do “surge power” electrical loads affect DC-AC
inverter operation?
If an inverter cannot efficiently feed the
surge power, it may simply shut down instead
of starting the device. If the inverter’s
surge capacity is marginal, its output
voltage will dip during the surge. This can
cause a dimming of the lights in the house,
and will sometimes crash a computer.
Any weakness in the battery and cabling to
the inverter will further limit its ability
to start a motor. A battery bank that is
undersized, in poor condition, or has
corroded connections, can be a weak link in
the power chain. The inverter cables and the
battery interconnect cables must be sized
properly. The spike of DC current through
these cables is many hundreds of amps at the
instant of motor starting.
7. How should I size my DC-AC inverter for loads that
require starting surge?
Observe the following guideline to determine
the continuous wattage of the inverter for
powering loads that require starting surge.
(Multiply the running watts of the
device/appliance by the Surge Factor)
NOTE: The surge power rating specified for this
inverter is valid for duration of less than
1 second. This very short duration may not
be sufficient to start motor based loads
which may require up to 5 seconds to
complete starting process. Hence, for
purposes of sizing the inverter, use only
the continuous power rating of this
inverter.
|
Type of Device or Appliance
|
Surge Factor
(No. of times the running power rating of the
device/appliance)
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Refrigerator / Freezer |
5 |
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Air Compressors |
4 |
|
Dishwasher |
3 |
|
Sump pump |
3 |
|
Furnace fans |
3 |
|
Industrial motors |
3 |
|
Circular saw |
3 |
|
Bench Grinder |
3 |
|
Portable diesel / kerosene fuel heater |
2 |
8. How should I size my DC-AC inverter to operate a
microwave?
The power rating of the microwave generally
refers to the cooking power. The electrical
power consumed by the microwave will be
approximately 2 times the cooking power. The
“surge power” of the inverter should be 2
times the electrical power (i.e., 4 times
the cooking power). Please note that the
surge power of the microwave is not as long
as the motor load and hence, the surge power
of the inverter can be considered to
determine adequacy of meeting the starting
surge power
9. How should I size my DC-AC inverter to power a water
supply pump?
A water well or pressure pump often places
the greatest demand on the inverter. It
warrants special consideration. Most pumps
draw a very high surge of current during
start up. The inverter must have sufficient
surge capacity to handle it while running
any other loads that may be on. It is
important to size an inverter sufficiently,
especially to handle the starting surge (If
the exact starting rating is not available,
the starting surge can be taken as 3 times
the normal running rating of the pump).
Oversize it still further if you want it to
start the pump without causing lights to dim
or blink.
In North America, most pumps (especially
submersibles) run on 240 VAC, while smaller
appliances and lights use 120 VAC. To obtain
240 VAC from a 120 VAC inverter, use a 120
VAC to 240 VAC transformer. If you do not
already have a pump installed, you can get a
120 volt pump if you don’t need more than
1/2 HP.
10. What is “idle power” in a DC-AC inverter?
Idle power is the consumption of the
inverter when it is on, but no loads are
running. It is “wasted” power, so if you
expect the inverter to be on for many hours
during which there is very little load (as
in most residential situations), you want
this to be as low as possible.
11. What are Phantom and idling loads?
Most of the modern gadgets draw some power
whenever they are plugged in. Some of them
use power to do nothing at all. An example
is a TV with a remote control. Its electric
eye system is on day and night, watching for
your signal to turn the screen on. Every
appliance with an external wall-plug
transformer uses power even when the
appliance is turned off. These little loads
are called “phantom loads” because their
power draw is unexpected, unseen, and easily
forgotten.
A similar concern is “idling loads.” These
are devices that must be on all the time in
order to function when needed. These include
smoke detectors, alarm systems, motion
detector lights, fax machines, and answering
machines. Central heating systems have a
transformer in their thermostat circuit that
stays on all the time. Cordless
(rechargeable) appliances draw power even
after their batteries reach a full charge.
If in doubt, feel the device. If it’s warm,
that indicates wasted energy.
12. Can a standard DC-AC inverter be connected in parallel
with another AC source/ DC-AC inverter /
electrical utility?
The AC output of standard inverters cannot
be synchronised with another AC source and
hence, it is not suitable for paralleling.
The AC output of the inverter should never
be connected directly to an electrical
breaker panel / load center which is also
fed from the utility power /generator. Such
a connection may result in parallel
operation of the different power sources and
AC power from the utility / generator will
be fed back into the inverter which will
instantly damage the output section of the
inverter and may also pose a fire and safety
hazard. If an electrical breaker panel /
load center is fed from an inverter and this
panel is also required to be powered from
additional alternate AC sources, the AC
power from all the AC sources like the
utility / generator / inverter should first
be fed to a manual selector switch and the
output of the selector switch should be
connected to the electrical breaker panel /
load center. To prevent possibility of
paralleling and severe damage to the
inverter, never use a simple jumper cable
with a male plug on both ends to connect the
AC output of the inverter to a handy wall
receptacle in the home / RV.
13. How can DC-AC inverters be connected to multi-wire
branch circuits?
Do not directly connect the hot side of the
inverter to the two hot legs of the 120 /
240 VAC electrical breaker panel / load
centre where multi-wire ( common neutral )
branch circuit wiring method is used for
distribution of AC power. This may lead to
overloading / overheating of the neutral
conductor and is a risk of fire. A split
phase transformer (isolated or
auto-transformer ) of suitable wattage
rating ( 25 % more than the wattage rating
of the inverter ) with primary of 120 VAC
and secondary of 120 / 240 VAC ( Two 120 VAC
split phases 180 degrees apart) should be
used. The hot and neutral of the 120 VAC
output of the inverter should be fed to the
primary of this transformer and the 2 hot
outputs ( 120 VAC split phases ) and the
neutral from the secondary of this
transformer should be connected to the
electrical breaker panel / load centre.
14. How should I size my DC-AC inverter to supply computer
or printer loads?
Computers and Printers generally come with
in-built switch-mode power supplies (see
application notes and
FAQs on switch mode
power supplies (SMPS)) that have extremely
low power factor on account of the extremely
peaky and non-linear nature of the current
drawn by them from the AC source. Hence for
such loads we recommend using inverters with
three times the advertised power consumption
of the computer or printer loads.
15. What is the difference between a 12V, 24V and 48V
inverter?
A 12-V can be fed from a 12V battery or DC
supply. Similarly 24V and 48V inverters can
be fed from 24V and 48V batteries/DC power
supplies respectively. Cars usually have 12V
batteries. Trucks have 24V batteries.
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Copyright 2001-2009 © BOOST Inverters and Controllers USA,
All rights reserved |
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