Modified sine wave inverter

[ians]

I am not a fan of using modified sine wave inverters ... if It is my choice ...it would be a pure sine wave even though it cost a lot more.

I was asked to connect a modified sine wave inverter in an office which has a few computers ... screens ... printer ... router ... you know the usual stuff.

A couple weeks later a computer fries ... the IT guys collect the unti and replace a pile of stuff and retrun the unit to site ... all seems good ... then I gget a call because the printer is chewing the battery of the inverter ... so I disconnect the printer ( I expected that to happen ... printers and UPS/inverter etc dont work well together ... unless you have a pure sine wave unit with ample power) ... then a screen pops.

When do some research on modified sine wave inverters ... the results indicate that there is no issues with using a modified sine UPS/inverter for computers ... laptops ... TV's etc ... the results indicate that things which have motors become a problem.

Would you connect a modified sine wave unit to your R10k Xbox or LED TV or office computers ?

[Frans400]

Yes i will, but i will monitor the inverter current and voltage for one hour or so under full load to make sure the voltage drop is not exceeding the tolerated range of the equipment in use. Most of the time i found broken dc power supplies (it equipment) the voltage was not in range when the installation was running at full potential.

[Justloadit]

In all electronic equipment today, they use switchmode power supplies.
Basically they take a bridge rectifier, and rectify the mains, and dump it into a capacitor. The switchmode then uses this voltage to supply your equipment.
It varies a switching FET to drive a transformer to produce the required voltage for the equipent. By varying the switching time it controls the output voltage. It actually drives a squarewave. This is the reason that switch mode power supplies can work from 85V- 230V AC. The lower voltage just ensures that the switching has a longer perid of on time to produce the required output. Look there is a little bit more to this than the explanation I have given, but it is close enough to understand.

Now if you take 230V, and send it through a rectifier and smoothing capacitor, you will find that if you measure the voltage, it sits around 315V D.C.
The modified sine wave inverter is a square wave with peaks of 315V, same as your rectified sine wave mains.
As far as the switchmode is concerned no problem, the internal electronics are invariably designed to handle 600V DC anyway so who cares.

However the problem arises when you wish to run an electric motor. The electric motor is designed for a sine wave, which has this nice slow rise and fall times, and the motor design is made for this.
Chuck a square wave into it, and you are going to have issues, as the motor does not see 230V RMS, it actually sees 315V, so the motor overheats, as the windings were made for 230V not 315V.
Not saying a motor can not run with a modified sine wave inverter, it can if the motor is designed for the higher voltage.

So when using a motor to directly connect to mains, then a pure sine wave inverter is the only way to go.
Now if you have a VSD driev, then it will work with a modified sine wave inverter, because what it does, is that the mains is rectified and dumped into a capacitor bank, and the motor voltage is then fed of this DC bus. Fancy electronics and filters then change the DC voltage into sine wave with a varying frequency and amplitude to make your motor turn. To make the motor work there is a formulae used to match the motor voltage to the frequency it can handle. So the 315V will urn out to be 230V AC at 50Hz. By varying the speed pot, the motor voltage changes along with the frequency. There is a relationship in motors that voltage must match the frequency to produce a cool motor at what ever RPM has been selected.