Transcript Inverters - A&A Computers Ltd

Inverters
November 2007,
Alex Righolt
Overview
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A&A Computers sell and install inverter
systems as an alternative to UPS-es.
Inverters are easily scaleable; by adding
batteries, many hours of power back are
available.
This presentation explains the technology.
Overview
We’ll look at:
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Inverter connections
Battery types, charging, maintenance and
connections
Cables
Trouble shooting
Calculations
Connections
An inverter is an
electronic device that
produces alternating
current (AC) from direct
current (DC).
The DC voltage comes
from a battery that may be
as low as 12 Volt, or as
high as 200 Volt.
The ‘load’ is PCs, lights,
printers, etc.
Connections
We
mostly sell charger
inverters, where the unit
also has a battery charger.
Connections
When
there’s good
supply power, it passes it
through to the load, and
also charges the battery.
Connections
When the supply is bad,
battery DC voltage is
inverted to 220 Volt AC.
Bad supply can be:
No voltage
Low voltage
Wrong frequency
Bad wave shape
Connections
The
cut-off point for low
voltages can be adjusted,
for example to 192 volts.
The acceptable
frequency range is usually
47 – 53 Hz.
The supply AC voltage
must be sine wave, and
not trapezoid
Connections
The
output of most
inverters is Trapezoid, or
“approximated sine wave”
If you connect a UPS to
the output of an inverter, it
will not work.
Connections
Inverters
(and
also UPS-es) are
connected as
stand-by, when
AC is passed
through.
In an on-line
circuit, power is
always inverted.
Connections
Instead
of a
Tanesco supply,
a system my
also be supplied
by solar panels
(or “photo-voltaic
units”) of wind
turbines.
Connections
The charge
controller avoids:
overcharging of the
batteries; when
they’re fully charged
the supply is cut-off
over discharging,
when the batteries
are empty the load
is switched off.
Batteries
Battery types:
Lead
Acid
– so cars; may be discharged 50%
–Deep cycle – or “solar battery”; may be discharged
80%
–Traction
Gel
or wet
Lithium, etc.
Batteries
Battery capacity:
Battery
In
capacity is expressed in Ampere Hours, Ah
UPS-es we use 12V/7Ah, 12V/12Ah, 12/17Ah.
For
our battery backup systems we use 100 and 165Ah
batteries
Batteries
Charging
The three steps in automatic battery charging are:
Bulk
Absorption
Float (or “Trickle”)
Batteries
Bulk:
charge with
maximum allowable
current. The voltage
increases.
Absorption: During four
hours, charge with a slight
over voltage – the batteries
bubble, or “gas”.
Float: Maintain charge
with a small current.
Batteries
Battery Maintenance
Batteries
should be topped up with distilled water.
Terminals should be kept clean.
A ring of grease drawn around the battery terminals to
avoid current leakage.
DC Cables
We
use 50 square mm2 welding cable.
You must tighten the cables securely, because
they pass a high current:
We add a DC fuse of 250 Amps
DC Cables
In
a 2000 Watt, 12 VDC inverter:
2000 Watt / 12 volt = 167 Amperes
In
a 3600 Watt, 36 VDC inverter:
3600 Watt / 36 volt = 100 Amperes
DC Cables
DC Cables
Battery connections
By adding batteries in
series you increase the
voltage.
By connecting batteries in
parallel you increase the
Ah capacity (and thereby,
the running time)
Troubleshooting
Frequency
The acceptable frequency range for inverters is usually 47 –
53 Hz . Generator frequencies are often set wrong, or they
fluctuate. Set the generator frequency of the unloaded
generator to 52Hz, so it may drop 5 Hz without upsetting the
inverter.
If it drops more, then the generator is weak – often its engine.
The supply AC voltage must be sine wave, and not trapezoid.
Troubleshooting
Polarity
Double check before connecting the battery
cables to the inverter. The inverter will be severely
damaged even if only wrongly connected for a splitsecond.
Troubleshooting
Battery is discharged
If the batteries have been discharged below 10
Volts, the inverter does not start. Charge them
from an alternative source.
Inverter overheats.
Regularly blow dust.
Provide good ventilation.
Troubleshooting
Bad battery contact
Torque all bolts very hard, but take care to not to break
off the battery terminals.
If corroded, clean terminals and lugs with fine sand
paper.
Connected PCs reboot
The cut-off point for low voltages can be adjusted, for
example to 192 volts. If you set it too low, computers
may reboot.
Inverter
Inverter mounting
The
unit is mounted on a wall, horizontally.
Make sure that the indicator lights can be read
There should be ventilation – the batteries
produce hydrogen gas
Battery cables maximum length is 60 cm.
Calculations
Formulas to know:
Power
= Voltage x Current
P = V x I
Example:
Can a 30 Amp AVS support a 220Volt, 5 kWatt
machine?
I = P / V = 5,000 / 220 = 23 Amperes.
Answer: Yes.
Calculations
Energy
= Power x time
E = P x t
Watt-Hour = Watt x Hours
Example:
How many 12 Volt / 100 Ah batteries do you need to supply 1000 W for 3
hours?
Energy = 3 x 1,000 = 3,000 Wh
One battery can deliver 12 x 100 = 1200 Wh
Assume conversion loss of 50%
Effective energy of one battery is 1200 / 1.5 = 800Wh
Number of batteries: 3,000 / 800 = 3.75, so 4 batteries are needed.
Calculations
Conclusion
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You have seen how inverters can be
connected.
You’ve learnt about battery types, charging,
maintenance and connections.
You know what kind of DC cables to use.
You saw some trouble shooting issues
You were introduced to inverter calculations.
Inverters.
The end