The primary job of an inverter is to convert the DC (Direct Current) power from the battery bank or solar panels to AC (Alternating Current) power needed for most appliances. To do that, it must take the constant DC voltage and change it to a sine wave curve that goes above and below 0 volts. When inverters first came out, the most common way to do this was to make the voltage go straight up and down, creating a blocky signal. This is called modified sine wave, seen in orange in the image below. More advanced modified sine waves make multiple steps, trying to come close to a pure sine wave.
In the following images, you can see an output of a modified sine wave on an oscilloscope at the left. A Pure Sine wave is shown on the right. Other than how the signal looks, what’s the difference between the two outputs?
A modified sine wave inverter can be used for simple systems that don’t have any delicate electronics or audio equipment that may pick up the choppy wave and produce a hum. Old tube TVs and motors with brushes are usually ok with modified sine wave. Your digital clock will likely act funky, and battery rechargers quite often just plain won’t work. Some equipment may seem to be working fine, but may run hotter than with a pure sine wave and reduce the life of it.
Take heed if you’re considering buying a modified square wave inverter to shave a few bucks off your system costs. A whole raft of modern appliances won’t run as well and some not at all on this waveform:
- Laser printers, photocopiers, and anything with an electrical component called a thyristor
- Anything with a silicon-controlled rectifier (SCR), like those used in some washing machine controls
- A few laptop computers
- Some fluorescent lights with electronic ballasts
- Some battery chargers for cordless tools
- Some new furnaces and pellet heaters with microprocessor controls
- Digital clocks with radios
- Appliances having speed/microprocessor controls (like some sewing machines)
- X-10 home automation systems
- Medical equipment such as oxygen concentrators
In general, because the total harmonic distortion is higher in modified square wave inverters, motors will run hotter (less efficiently, consuming up to 30% more energy than with pure sine wave inverters), and likely not last as long. Additionally, a modified square wave inverter will often cause a “buzz” to be heard from audio devices and sometimes other appliances like ceiling fans and microwave ovens.
We liken using a modified sine wave inverter to running a car with square wheels versus a pure sine wave inverter like running a car with round wheels. In the first case the ride is going to be awful rough and depending on the sensitivity of the car’s occupants they may not survive the ride.
Pure Sine Wave Inverters are Preferred for Many Electronics
Pure sine wave is always needed for a grid tie system. It is generally needed for newer LED TVs, CFL light bulbs, and inductive loads like brushless motors. Clocks and audio equipment will behave much better on a pure sine wave.
Differences in Cost Between Modified and Pure Sine Wave Inverters
Generally, modified sine wave inverters are less expensive than a pure sine wave inverter, so they are still used in simple systems. But as technology advances, the cost of pure sine wave inverters is coming down, making them much more affordable and the favorite option.
Off-Grid Inverters Working with 12, 24, or 48V Battery Banks
Once the charge controller has charged up the battery bank, the off-grid inverter converts the 12, 24, or 48VDC battery bank to AC voltage. The AC output depends on your requirements. In North America, most residential and business use is 120V single phase. Homes will often use 240V split phase for larger loads such as a clothes dryer, well pump, and electric stove. Commercial buildings that use a lot of power will generally have 208V or 480V 3-phase delivered to the building, and it is then broken out to 120V when sent to the outlets. Depending on how you wire the output of the inverter, and which inverter you get, you could have both 120V and 240V as an output. You need to determine what your loads require, and select and configure the inverter accordingly.
Most off-grid inverters can not sell extra power back to the grid, with the exception of inverters like the Apollo Solar TSW series inverters.
However, an inverter/charger can connect to the grid (if available) to act as a battery charger. For instance, if you have a boat or RV with an inverter/charger, when you connect to shore power, you can use the AC power from the electric grid to charge your battery bank when the solar doesn’t provide enough power. But that AC connection is one-directional, it will only take from the grid, not send back. Likewise, you can often connect a generator to the AC input of an inverter/charger to top off the batteries when needed. This is a common configuration for off-grid homes that need more power in the winter than the sun can provide.
When selecting an inverter, you must determine what your maximum wattage draw will be if all of your appliances that may be on at the same time, are on. If you have an 800W well pump, a 100W fridge, five 10W lights, and a 50W laptop, you need to add the wattages together to get at least a 1,000W inverter. You also have to make sure the inverter is able to handle the surge as motors turn on. For example, if your fridge and well pump both turned on at once, the surge could be three or four times the rated wattage. You must be sure the inverter can handle that. Inverters are rated in both continuous wattage and surge.
Battery based inverters have a lot of options to choose from. Not all of the inverters have all of the features, so you need to decide which features are required or desired, and select the inverter based on which has them.
Some of the features include the ability to charge the battery bank from an AC source like the grid or a generator; even automatically starting the generator to power the built-in AC charger in the inverter to charge up the deep cycle batteries when they are low, and turning it off when they are charged. Some can automatically use the generator to assist with high loads, reducing the need to oversize the inverter for occasional heavy use.
Since the inverter is often installed in an out of the way location near the batteries, an inverter remote control or display in the living area is useful to keep an eye on the system. Some inverters even have the ability to monitor the system remotely via the web. This is very useful for part time locations that you are not always there to keep an eye on.
Many inverters can be stacked to increase either the voltage or the current, or both. This allows you to use multiple inverters in a master/slave configuration, automatically turning on only the inverters as needed, conserving battery power, as you are not providing power to the second inverter when it is not needed.
Advantages of a Pure Sine Wave Inverter vs the Modified Sine Wave Inverter.
The Comparison - The output voltage of a sine-wave inverter has a sine wave-form like the sine wave-form of the mains / utility voltage. Please see sine-wave represented in the Fig. 1 and Fig. 2. Fig. 1 shows modified sine-wave and square wave for comparison. In a sine wave inverter, the voltage rises and falls smoothly with a smoothly changing phase angle and also changes its polarity instantly when it crosses 0 Volts. In a modified sine wave, the voltage rises and falls abruptly, the phase angle also changes abruptly and it sits at 0 Volts for some time before changing its polarity. Thus, any device that uses a control circuitry that senses the phase (for voltage / speed control) or instantaneous zero voltage crossing (for timing control) will not work properly from a voltage that has a modified sine wave-form.
As the modified sine wave is a form of square wave, it is comprised of multiple sine waves of odd harmonics (multiples) of the fundamental frequency of the modified sine wave. For example, a 60 Hz. modified sine wave will consist of sine waves with odd harmonic frequencies of 3rd (180 Hz), 5th (300 Hz.), 7th (420 Hz.) and so on. The high frequency harmonic content in a modified sine wave produces enhanced radio interference, higher heating effect in motors / microwaves and produces overloading due to lowering of the impedance of low frequency filter capacitors / power factor improvement capacitors.
Advantages of a Sine Wave Inverter
- The output wave-form is a sine-wave with very low harmonic distortion and clean power like utility supplied electricity.
- Inductive loads like microwaves and motors run faster, quieter and cooler.
- Reduces audible and electrical noise in fans, fluorescent lights, audio amplifiers, TV, fax and answering machines.
- Prevents crashes in computers, weird print outs and glitches in monitors.
Examples of devices that may not work properly with modified sine wave
Note amage may occur in these devices without the correct inverter.
- Laser printers, photocopiers, magneto-optical hard drives.
- Built-in clocks in devices such as clock radios, alarm clocks, coffee makers, bread-makers, VCR, microwave ovens etc may not keep time correctly.
- Output voltage control devices like dimmers, ceiling fan / motor speed control may not work properly (dimming / speed control may not function).
- Sewing machines with speed / microprocessor control.
- Transformer-less capacity input powered devices like Razors, flashlights, night-lights, smoke detectors etc…Re-chargers for battery packs used in hand power tools.
- Devices that use radio frequency signals carried by the AC distribution wiring.
- Some new furnaces with microprocessor control / Oil burner primary controls.
- High intensity discharge (HID) lamps like Metal Halide lamps.
- Some fluorescent lamps / light fixtures that have power factor correction capacitors. Inverter shut down may accur indicating overload.
CHARACTERISTICS OF SINE WAVE AC POWER:
Polar Coordinate System - is a two-dimensional coordinate system for graphical representation in which each point on a plane is determined by the radial coordinate and the angular coordinate. The radial coordinate denotes the point’s distance from a central point known as “The pole.”
The angular coordinate (usually denoted by Ø or O t) denotes the positive or anti-clockwise (counter-clockwise) angle required to reach the point from the polar axis.
Vector- is a varying mathematical quantity that has magnitude and direction. The voltage and current in a sinusoidal AC voltage can be represented by the voltage and current vectors in a Polar Coordinate System of graphical representation.
Phase, Ø- is designated “Ø” and is equal to the angular magnitude in a Polar Coordinate System of graphical representation of vectorial quantities. It is used to denote the angular distance between the voltage and the current vectors in a sinusoidal voltage.
Power Factor- is designated by “PF”. It is equal to the Cosine function of the Phase “Ø” (denoted CosØ) between the voltage and current vectors in a sinusoidal voltage. It is also equal to the ratio of the Active Power (P) in Watts to the Apparent Power (S) in VA. The maximum value is 1, normally it ranges from 0.6 to 0.8.
Voltage- is designated “V” for volts. It is the electrical force that drives electrical current when connected to a load. It can be DC (Direct Current – flowing in one direction only) or AC (Alternating Current – flow direction changes cyclically).
Amps- is designated by “A” and the unit is Amperes – denoted as “A”. It is the flow of electrons through a conductor when a voltage (V) is applied across it. (Also called current.)
Frequency- is designated by Hz. It is a measure of the number of occurrences of a repeating event per unit time. For example, cycles per second (or Hertz) in a sinusoidal voltage.
Resistance- is the property of a conductor that opposes the flow of current when a voltage is applied across it. In a resistance, the current is in phase with the voltage. It is designated by “R” and its unit is “Ohm” – also designated by “Ω”.
Reactance- is designated by “X”. It is the property of capacitors and inductors in a circuit that opposes the flow of current due to AC voltage applied across the circuit. The phase of the current will either lead or lag the voltage in time. It will lead if the net reactance is capacitive and will lag if the net reactance is inductive.
Impedance- is designated by “Z”. It is the vectorial sum of Resistance and Reactance in a circuit.
Peak Value- is the maximum value. For a sine wave, it is equal to 1.414 times the RMS value. For example, in a 120 VAC sine wave voltage, the RMS value is 120 V and the peak value is 120 X1.414 = 169.68 or approximately 170 V.
RMS Value- Root Mean Square – a statistical average value of a varying quantity that changes between positive and negative values with respect to time. For example, in a 120 VAC system, the RMS value is 120 V.
Active Power (P) or Watts- is designated by “P” and the unit is “Watt”. It is the power which is dissipated in the load due to the resistance. The Energy Meter (Kilo Watt Hour Meter) measures the energy consumed which is = Active Power consumed in Watts multiplied by the time in Hours and the utility companies bill the users based on this power consumption. This Active Power “P” in Watts = RMS Voltage X RMS current X Power Factor (Cos Ø).
Reactive Power (Q), VAR- is designated by “Q” and the unit is VAR. Mathematically, this power Q = RMS Voltage “V” X RMS current “A” X Sin Ø (Sine Function Value of the Phase Ø between the voltage and the current vectors). The magnitude of this power will be 0 if the Phase Ø between the voltage and the current vectors is 0 degrees or the Power Factor is unity.
(1). This power will increase as the Power Factor decreases below unity (2). This power is not consumed by the load but travels to the load in the (+) half cycle of the sinusoidal voltage and is returned back to the load in the (-) half cycle of the sinusoidal voltage. This back and forth flow of energy is due to the capacitive and inductive reactances in the load. Hence, when averaged over a span of one cycle, there is no consumption of power. However, on an instantaneous basis, this power has to be provided by the AC source and the AC source, the transmission lines and the gear have to be sized accordingly. The Energy Meter (Kilo Watt Hour Meter) does not measure this power but the Utility Companies have to provide this additional power. The Utility Companies require that the Power Factor of the load should be very close to unity (1) so that they do not have to transmit this additional reactive power that is not being paid for. To bring the low Power Factor of the load to near unity (1), the Utility Companies require use of Power Factor correction devices at the load location.
Apparent Power (S), VA- is designated by “S” and the unit is VA. This power is the vectorial sum of the Active Power in Watts and the Reactive Power in “VAR”. In magnitude, it is equal to the RMS value of voltage “V” X the RMS value of current “A”. The AC power source is required to provide this power. Please note that this power is more than the Active Power in Watts.
Load- Electrical device to which an electrical voltage is fed.
Linear Load- is a load which draws sinusoidal current when a sinusoidal voltage is fed to it. (Ex: incandescent lamp, heater, electric motor, etc.)
Non Linear Load- is a load which does not draw a sinusoidal current when a sinusoidal voltage is fed to it. (Ex: a non power factor corrected Switched Mode Power Supply used in computers, audio video equipment, battery chargers, etc.)
Resistive Load- is a load that consists of pure resistance (like incandescent lamps, heaters, etc.)
Reactive Load- is a load that consists of resistance and reactance like electric motor driven loads, fluorescent lights, computers, audio / video equipment, etc.
Sine Wave- In a voltage that has a sine (sinusoidal) waveform (see Fig. 1), is the instantaneous value and polarity of the voltage as it varies cyclically with respect to time. (Ex: in one cycle in a 120 VAC, 60 Hz system, it slowly rises in the positive direction from 0 V to a peak positive value “Vpeak” = +170 V, slowly drops to 0 V, changes the polarity to negative direction and slowly increases in the negative direction to a peak negative value “Vpeak” = -170 V and then slowly drops back to 0 V. There are 60 such cycles in 1 sec. Cycles per second is called the “Frequency” and is also termed “Hertz (Hz)”.)
Cycle- for a sine wave (see Fig.2.1), it is the complete event starting with a rise from zero to a maximum amplitude, its return to zero, the rise to a maximum in the opposite direction, and then its return to zero. 120 / 240 VAC Sine Wave AC Power Distribution for Residential Application: The waveform of the electrical voltage distributed by the grid / the utility companies is like a sine wave. For example, in North America, the grid / utility voltage for residential use is single phase, 120 / 240 VAC, 60 Hz. and consists of two 120 VAC, 60 Hz Line Voltages (also called “Lines” or “Legs”) and a common “Neutral”. The two 120 VAC, 60 Hz.
Lines (Legs) are 180 degrees apart in phase. The voltage between each Line (Leg) and the Neutral is 120 VAC and between the two Lines (Legs) is 240 VAC.
RMS and Peak Values in Sine Wave AC Power- as mentioned above, in a sine wave, the values of AC voltage (Volt, V) and current (Ampere, A) vary with time. Two values are commonly used – Root Mean Square (RMS) value and peak value. The values of the rated output voltage and current of an AC power source are specified in RMS values.
Power Factor in Sine Wave AC Power- is when a voltage is applied to a load, current flows. If a Linear Load is connected to this type of voltage, the load will draw current which will also have the same sine wave-form. However, the peak value of the current will depend upon the impedance of the load. Also, the Phase Ø of the Sine Wave-form of the current drawn by the Linear Load may be the same or lead / lag the sine wave-form of the voltage. This phase difference determines the Power Factor of the load. In a resistive type of load, the sine wave-form of the current drawn by the load has 0 degrees phase difference Ø with the sine wave-form of the voltage of the AC power source. The Power Factor of a resistive load is unity (1).