Power Inverters – the real thing

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Inverters and UPS

Buck, boost, invert, batteries and PWM – where does all this fit in?

[Editor’s note – this article covers the emphasis of the use of inverter circuitry inside UPS systems. There are many different types of UPS systems available in the market, some only powered by a battery and relay, others by electronic switching, regulators, inverters and power factor correction. We find the APC website (we are not affiliated to them neither do they endorse this website 😀 ) full of invaluable information for anyone wishing to invest in a UPS, whether large or small].

There has been a lot of galloping to the stores of late as Chinese inverters are making their mark for back up power to the home – they are priced well, reliable and just about anybody can install them. There are some pitfalls though. We won’t go into the heady details of R.M.S. power or how to design PWM/chopper circuits.

1) A UPS has an inverter stage.

Line Interactive UPS Explanation: Wiki Commons – DMahalko, Dale Mahalko, Gilman, WI, USA

Inverters and UPS both use similar technology, transistors which are driven by a square wave to switch (chop) a DC supply through a transformer to get a square wave output but average out to sine at country supply voltage (or designed equipment supply) or a pure sine wave inverter which may be driven by a 50Hz oscillator, sine wave. A UPS will detect low mains incoming voltage and boost it or on high input voltage will reduce (buck) the output supply. Both the inverter and UPS will always try to give as close as possible to an output sine wave as can be designed. Whilst a pure sine wave is desirable, they are very expensive, generate a lot of heat and are very heavy. Modified square wave (standard UPS) is great for computer switching supplies but not for inductive loads e.g. your amplifier mains transformer. Inverters however are often more designed around trying to drive inductive loads like fridge compressor motors, swimming pool pump motors and drilling machines. They were not before because of the high cost – Insulated Gate Bipolar Transistors (IGBT) have changed this in many ways.

2) An inverter may not have a buck/boost circuit.

3) A UPS which boosts and bucks is called a line interactive UPS and is the most popular amongst computer users. They are also cheap and reliable.

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Inverters driving UPS, PWM, bucking and boosting

Both the UPS and the inverter use PWM or pulse width modulation, sometimes. Cheaper inverter units are free running (meaning oscillate freely at a designed fundamental frequency).  They may not have a feedback circuit or current sensing. A UPS used for backup supply for a computer or server is rarely free running and do have a lot of fail safe circuits built in.

4) Many cheap low power inverters are free running or set to a fixed frequency without PWM

A UPS can be driven from an inverter, meaning the inverter feeds the UPS. The UPS will then be used to clean up the inverter output voltage. A standard inverter needs a battery supply to run. But then again so does a UPS if there is no mains power. Confused?  The only thing we now see (in some circuits) is that an inverter does not have a buck/boost control. Sometimes we read about the switchover period in milliseconds as the inverter switches to battery. Well, the same applies to a UPS, both hybrid online, double conversion and offline. The online (double conversion) UPS has a very beefy DC charging circuit which when detecting a low or high input will automatically regulate the output without switching to battery, whilst with an off line UPS one can hear the switching and beeping taking place to warn the user that it is switching between mains supply and battery. On line UPS usually transfer to bypass when the load varies significantly. Confused now?

5) An online UPS will have a transfer switch, often utilising silicon controlled rectifiers (SCRs) to bypass the DC and battery charger circuit under heavy loading.

With the online UPS the battery does not get used when bucking or boosting which places less drain on the battery. The off-line UPS can be quite dangerous here because the user does not know the battery has been drained until the system shuts down. An inverter of course runs all the time off batteries. Ummm…

Or so we thought.

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In fact all three topologies are made to run directly off batteries when there is no mains supply – much of what we read is not entirely true, possibly hype and of course economy of scale play a role. A UPS is an inverter circuit. It does run off a battery. There is in most cases a changeover relay. If one had to design an inverter circuit which ran off the batteries all the time but at the same time the batteries were being charged we would in theory have an online double conversion UPS. UPS means Uninterruptible Power Supply. A battery which is being charged while mains power is available and it feeds your household lights is therefore a UPS. There should be no confusion here. Many systems engineers believe the best UPS to have is in fact the line interactive variety because there is less stress, no bulky power supply, no excess loading of small generators, less heat, less cost.  And they also the cheapest to manufacture, even cheaper than an inverter. Strange!

Does your UPS kill spikes and surges?

Many UPS have very primitive voltage spike suppression circuits – in fact all it needs is a Movistor or Metal Oxide Varistor. This component prevents mains surges and/or transient spikes getting through to other electronic circuits causing damage. They are usually placed at the mains input. They come in a variety of sizes, voltage clamping and energy ratings. They are dirt cheap. Then we have the line regulator or stabiliser circuit like the APC -L-R1200. This unit is not an interruptible power supply (no batteries, no inverter) but it does protect your sensitive equipment during brownouts, surges and spikes using a transformer with multiple taps.

6) A UPS may be damaged by a voltage spike.

APC inverters or online or offline UPS

No, not all UPS are built the same. APC (now under Schneider) have long made UPS which are deemed very reliable. Although costing more than many imports they offer better warranty terms than most other UPS manufacturers. This does not mean the others are bad in case you are wondering. Always buy a reputed make or through a reputed supplier.

The expense of batteries

Whilst most UPS are very reliable the batteries may not be. Most users turf their UPS when the batteries fail because either they are not aware that the battery has lost it’s mojo or the replacement price is exorbitant. Always replace with a known good battery and yes, again, the manufacturer or brand name is important.

7) Your UPS is only as good as the battery.

Inverters are for big power, UPS for small power

Another myth is that an inverter can supply many kW of power whilst a UPS is limited to a few kVA. This is not true – APC even have a UPS which delivers 40kVA 3 phase. (and more if you want). When running very high powers the battery supply is usually series connected, sometimes 240V (20 12V batteries in series).  The problem is when a high powered UPS is designed to run off one 12V battery or possibly even two the currents are huge and just not practical. For instance a 2kVA UPS could be made to run off one 12V battery but the current would be in excess of 200A. At 24V this would amount to 100A and of course, 48V, 50A, which is more design friendly. If you are attempting to fire up your own design, never start big. Big current means big fire. UPS design engineers are very clever people – safety is always their first concern.

8) A UPS can be designed to deliver as many kVA as an inverter. It is an inverter.

Bigger is not always better!

There are a number of 5kVA designs on the internet which may or may not appeal to the novice. My advice is to stick to what works – a UPS for the PC, A UPS for the television receiver and satellite decoder/receiver and a dedicated light circuit coupled to an inverter. If one goes, the whole lot does not go. Any fixed wiring from inverters to household circuits must be done by en electrician. The inverter has to be certified by your local power authority or through a local bureau of standards, not the one in outer Mongolia. Your home brewed inverter will not be covered by insurance, neither the damage to your house or apartment in the event of a fire. Poorly installed down lights are often the cause of fire, likewise your vampire sucking mini-supplies used for modem/routers and cell phones.  (Ever seen a small PSU, usually cell phone charger, plugged in at the mains with a curtain hanging over it?).

9) It’s cheaper and safer to buy unless you are a design engineer.

A 5kVA inverter should be run of a minimum of 48V with a current capacity of 100A/Hr. Better still, 48V in parallel – at full load you are only looking at a maximum of 2 hours usage. Use solar panels for charging by all means but never forget the tried and trusted genset. It’s no use having the world’s best inverter when the batteries are flat and there’s no sun.

Inverters are connected to big battery supplies, UPS to small, internally mounted.

Not so, many manufactures allow extended run time batteries to be connected.

10) Many UPS can take extra batteries to extend run time. You can also modify – take heed of warranty and charging circuit.

To conclude…

Whilst many engineers may cast suspicion over what has been written here I can promise the layman that I have worked on many inverters and UPS circuits. I do find switchover relays in online UPS and APC do commission their high powered UPS to ensure installation is 100%. To prevent confusion do think of an inverter circuit forming part of a UPS and not a separate entity entirely. I often do advise users to rather use more than one UPS than many unless they are installing a dedicated circuit to their house which is to be powered by an inverter and many deep cycle batteries. Or a bunch of programmers needing a dedicated UPS on a generator for backup.

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