Inverters November 2007, Alex Righolt

Overview 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: 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 – Traction – so cars; may be discharged 50% – Deep cycle – or “solar battery”; may be discharged 80% Gel or wet Lithium, etc.

Batteries Battery capacity: Battery capacity is expressed in Ampere Hours, Ah In 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

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 split- second.

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 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