Description On Inverter Full

DESCRIPTION OF THE INVERTER COMPONENT BLOCK TO BLOCK

Fig. 1shows the block diagram of the Inverter System. The unit has a modular structure with each item in a block representing circuit bread and the arrow head in the block diagram shows signal or power flow. A description of each block now follows: (1).

Voltage Regulator block: the Inverter DC bus is either 24V or 48V

while the main control cirvuit requires a 12V supply. This block is therefore implim\ented using a 1812 linear regulator IC or a series pass regulator built using a Zener diode and bipolar transistor. (2).

Main control/Oscillation circuit: this unit is based on the SG 3524

ppulsemith modulated IC in which an RC network establishes the power frequency of 50HZ while a reference voltage of 5.0V and a feed back input enables a good control over the output voltage. This unit alternately switches the two legs of the inverter at interval of 10msec. (3). Solar PV array: the solar PV array is used as an alternate source for charging the batteries. A simple relay contact separates the DC bus from the PC output; but sometime a charge controller in the form or a DC-DC converter with a maximum power point tracking may be used. The solar RV array is becoming more popular in residential inverter units and street lighting application as a result of the regular power failure seen in Nigeria presently. The sizing for the PV array is usually limited by the clients purse, but for very rich cooperate bodies a 100W module can cater for a 24VX 100AH battery bank

4.

The dc bus 24v/48v: The dc bus is made up of heavy duty storage batteries. Different types of batteries may be used in the battery pool, but each bank of a string corrected batteries (those connected in series) must be of the same type and ampere hour rating. The sizing of the battery pool depends on the average system load, permissible depth of battery discharge and the required backup period in hours. For example suppose an average load of 300w is to be sustained for 10 hrs using a flooded battery which permits to be sustained of discharge using rated 12v x 100AH. How many batteries are required.

No.batteries = = 5.

powerXNo.Hrs battryVoltXAmperHrsXPerUnitDeptofDisch arg e

300 x10 = 5batteries 12Vx100 x0.5

MOSFET Switch Circuitry: Mosfets are used for switching the pulses to the isolating two winding transformer. Since these are voltage switching devices they are easier to switch to saturation then bipolar transistor and the design is as follows: for battery voltage up to 24v dc I use IRF150 Mosfets which have the following important switching parameters. Max reverse voltage 100v, max drain current 40A. Rdon 0.04hm for example suppose such devices are required to construct 600w inverter how many mosfets should be connected in parallel per leg? Since the inverter power rating is 600w. pwer

600

current draw on a 24v battery bank is = voltage = 24 = 25 A .

Max current for the IRFP150 = 40A but for reliable operation this is clade down to 25% which is 40x0.25 =10A. Therefore the required no. mosfets is =

currentdown 25 = = 2.5 ≈ 3 currentdevice 10

Thus to design such an inverter we require 3 MOSFETS per leg and a total of 2x3 = 6MOSFETS in the inverter circuit. 6) The two windings center tap transformer This transformer receives at its input the pulses from the MOSFETS at 24V and deliver at its output 220V alternating voltage at 50Hz frequency. The two separate windings ensure galvanic isolation of the high voltage circuits from the battery and control circuits and this is a plus for safety consideration. It is also possible for a switched 230V mains voltage at the 220V windings to reverse charge the battery bank using the centre tap at the 10w voltage winding and the internal drain – source free wheeling diodes. 7) AC output CT and overloading detection circuit This unit consists of a two winding current transformer (CT) which measures by induction the inverter output AC current. The CT output which is in the form of an AC voltage is rectified filtered and compared with a reference voltage, and when it is above a preset value the signal from this unit disables the supply of DC current to the main control/oscillation circuit. Such an event will leave behind an LED message.

8) The battery discharge control circuit This circuit, which is based on a comparator monitors the DC bus voltage, when the voltage drops below a minimum level, based on the maximum depth of discharge for the particular type or battery, the unit disconnects the supply of DC current to the main control/oscillators circuits and leaves behind and LED message that battery is low on energy. 9) AC charge control unit As overcharging a storage battery could considerably shorten the life span of the battery, this module based on the popular VA 741 IC, bipolar transistor, relay and other passive components, this unit will disconnect mains power to the reverse charging transformers when the voltage across the 24V battery bank reaches 28.6V. In this way possible damage to the battery as a result of overcharging could be avoided. 10)Power switching unit: A change over relay is used For the purpose of connecting the critical load either to the mains or to the output of the inverter.

LIMITATIONS OF THE PRESENT EQUIPMENT i.

A major draw back of the equipment design is the square wave voltage output which has a high harmonic content and this is objectionable to some sensitive loads although most residential loads seems to work perfectly when powered by the equipment.

ii.

Because electromagnetic relays are used for the power switching, some computers powered by the equipment could reboot as a result of the switching speed which is not fast enough.

iii.

Overload conditions could only be detected on the AC circuit but short circuits and faults which could be more serve on the DC circuits could be detected electronically; and protection by fuses seem to be unreliable for high power inverters.

iv.

Although not yet started in Nigeria, it will desirable that the equipment design should have the capability of grid connection; so that in time many of such inverters could start injecting power to the grid to help the ailing PHCN