Design and Hardware Implementation of 5kVA Power Inverter
Background Introduction of Power Inverter
Solar energy and battery storage are alternate sources of electricity which leads to the purpose of this project design and construction of 5kVA Inverter with low battery cutoff, Feedback unit for output voltage control, and automatic input changeover and other protective device.
In my country, there is unstable supply of electricity by the power supplying company to the consumers. Hence the use of additional electric power source such as rechargeable DC battery storage with inverters are common. With advancements in power electronic switching devices, inverter based power source provides a better additional power supply. It is less noisy, provides complete automatic switchover function, possess no environmental threats. Furthermore, this system is less bulky and less expensive to maintain.
Most recently the use of semiconductor power devices such as bipolar transistors, thyristors for voltage amplification, particularly the MOSFET as the power switches, make way to the use of Inverter based power.
Working Principle Of the Inverter System:
When the AC mains is not available, an oscillator section generates 50 Hz frequency* for metal oxide semiconductor drive signal (MOS drive signal) and converts the DC battery voltage to AC. This MOS drive signal is amplified by the driver section which is configure in push pull and sent it to the power amplification section. The power amplifier section uses metal oxide semiconductor field effect transistor (MOSFET) as a device for its switching operation. These MOSFETs are connected to the primary winding of the inverter transformer , when these MOSFETs receive the MOS drive signal from the driver section, they starts switching ON/OFF at the speed of 50Hz. This switching of MOSFETs starts an alternating current with frequency of 50Hz at primary winding of the inverter transformer and the inverter transformer step it up to 220V AC which is being sent to the output socket through a changeover relay.
*(note the frequency can be more than 50 Hz but with the help of the variable resistor you can adjust it)
Construction of Power Inverter:
- Resistors (values indicated on the diagram)
- Capacitors (values indicated on the diagram) c7 after the battery should be a polarized capacitor ranging from 10,000 uf and above for filtering once the inverter enters into charging mode
- Diodes (values indicated on the diagram)
- Inductor (values indicated on the diagram)
- Transistors (values indicated on the diagram)
- MOSFET (values indicated on the diagram)
- NE555 timer
- Relays (220 volts AC coil 60amps)
Note: the value of all the components is on the diagram and the circuit has different sections which is also indicated on the diagram
VR1 is used to set the output frequency of the inverter
VR2 is used to set the output voltage
VR3 is used to set the battery low voltage shutdown
Circuit Diagram of Power Interver:
Tools and Instruments used in Power Inverter Construction:
The tools and instruments used include:
- Lead and Soldering Iron
- Lead sucker
- Razor blade
- Digital Multimeter
- Vero and bread board
Calculations Analysis for Power Inverter System:
Determination Of The Oscillating Frequency in Power inverter:
By supplying a 12Volt DC to the IC SG 3525 PWM, the frequency of the oscillating signal was determined using a 10KΩ variable resistor connected in series with another 56KΩ resistor and both connected in parallel with 0.22µF to form the RC time constant network.
Frequency, F = 1/1.1*Ct*Rf where
Time Capacitor (CT) = 0.22µF
Fixed Resistor (RF) = 56KΩ
Variable Resistor (VR) = 10KΩ
Time Resistor (RT) =56KΩ+10KΩ = 66KΩ
It should be noted that the potentiometer was varied till the frequency of the signal was 50Hz.
Determination Of The Transistor (Mosfet) Switching Current:
The MOSFET used is the IRF260 in the power switching circuit due to high switching speed. By using 3.67volts supplied by the two NPN and the two PNP transistors, the switching time (T) is determined from the oscillating frequency as well as the gate switching current IG.
T= 1/f =1/50Hz
T = 0.02sec
IG = Cdv/dt =5000*10¯6*3.67/0.02 = 183.5
IG = 917.5ηA
Power Switch Circuit for Construction of Power Inverter
Inverter power output (P) = 5000Watts
Output voltage, V = 220V
Inverter Input = battery output voltage =12V
Frequency = 50Hz
Power factor = 0.8
Apparent power= Real Power/P.F = 5000/0.8 = 6,250VA
Therefore, the full load current flowing at the transformer primary;
Real power (P) = current (I) * voltage
5000 = I * 12
Hence, I = 5000/12 = 416.66A
Calculation Of Transformer Parameters
The power Rating for the Inverter transformer (KVA) =5.0KA , E2=12V Assuming the efficiency of transformer =85% Then Input rating =output /Efficiency=5000VA/0.85= 5,882.35VA Ip = P/VP = 220V Ip = 5,882.35/ 220 = 26.7A Ip = Po / Vs ; Vs = 12V Ip = 5000 / 12 = 416.66A
Number of turns per volt for both primary and secondary winding is given by;
NT per V= 7/A
Where A is the area of transformer former in sq. inch
Former area A is 2.3inch by 1.5inch = 7.45sq.inch
NT per V= 7/3.45=2
NT per V= 2 (approximate value).
(Read Complete series of articles on transformer design and hardware implementation here.)
Charger tapping winding turns
Np1= NT per V * E1=220V
Np1 = 2 * 220 = 440turns
Inverter (out) tapping winding turns
NS2= NT per V * E3=260V
NS2 = 2 * 260= 520turns
Difference of Inverting and Charging turns = 520 – 440 = 80turns.
For the primary windings, charging tapping is brought out after 440 turns and an addition 80 turns is
made for the inverter out tapping.
Secondary turns Ns= NT per V * E2 = 12V
Ns= 2 *12= 24turns. (Bifilar winding)
SWG Size and Gauge Estimation
(Learn about Wire Gauge Standards and selection )
Standard Wire Gauge Weight, SGW, can be estimated as follow;
Considering conduction current density J (with fixed value of 2.5A/mm2) and windings coil current.
For Ip = 26.7A, the corresponding gauge from tables is 16SWG and
For Is = 416A, the corresponding gauge from tables is 8 SWG.
Precautions for Longer Life of Power Inverter:
The following maintenance practices and safety precautions are suggested to improve the life span of the system and prevent hazards to the users.
- Dead batteries should not be used with the inverter
- The battery terminals should not be removed too often. When it is removed, placement of correct polarity must be ensured.
- The inverter must be kept in a moderate temperature environment.
- The inverter should be shut down when not in use.
- The inverter should always be partially loaded (not more than 75% of its maximum capacity).