PV Solar & Battery Installation - UK

[Deleted member 56522]

You have to see a graph of the different inverter wave forms.
The square wave and the "modified" sine wave go positive then come down to zero before going negative in each AC cycle.

However, the wave hangs at zero just long enough for a motor to believe that the power has been turned off. So you're back at the "dead short" start with the 2X inrush current to get rolling again but you're on it all the time. Every half AC cycle in fact. Hard on the inverter, too.

The pure sine wave inverter wave form whips straight across zero and goes negative without a pause. As if the power is "ON" all the time. So you have a bump to get the motor rolling and that's all.

I could install the square wave inverter in my GMC Suburban and decorate the pig with Christmas lights. It would all run just fine. At 60cps, you would not be able to detect any flicker at all.

The problem is that with an inductive load like a motor, the motor itself is generating currents and those currents generate their own extra voltage. At some points of the cycle, these currents will be OPPOSITE to the inverter voltage. This is a difficult situation for an inverter to handle and can result in large voltage spikes being developed, so the inductive load an inverter can handle is much less than a resistive load (like a simple old style lightbulb).

The benefit of a sinewave inverter, is firstly that they are more expensive and therefore usually higher spec and therefore usually better able to deal with inductive loads, secondly a pure sinewave reduces the various voltage spikes that an inductive load can produce, and thirdly that some equipment particularly using a transformer, can be very sensitive to the voltage spikes that can be produced using a simple inverter. (it behaves as if you've turned up the mains voltage and that can overload the circuits).

Basically
- if your load is very simple "resistive" ones ... the inverter will work at spec (note they usually have peak ... and (hidden) continuous use specs
- if it is inductive (or capacitive), it will only work at the inverter's rated capacity for inductive loads
- if it is sensitive equipment - like electronic equipment, you could risk damaging it, and so a pure sine wave is recommended. Having said that. I've used cheap inverters for laptops for decades without problems.

 

[TLM]

Better inverters have inductive (much more common) and capacitive (fairly rare) load phase compensation so the other equipment on the same circuit don't see it so badly.

 

[Robson Valley]

I look at the wave forms. Both the square wave and the modified sine wave inverters pause and shut off at zero for several milliseconds. It is measurable. Electric motors with universal winding can see that zero = off. Every half AC cycle is a total restart.

A pure sine wave inverter has a far larger parts count in the circuit design, even if its all pick-and-place. More $$$. I can't recall the brand but the PSI I'm running now is very reliable.

At least it was in yesterday's fekking 75 minute power failure.
It quits and the alarm goes off at about 10.5 VDC and that's deep cycle marine batteries. I have 1.2kW @ 117VAC delivered to my upstairs kitchen. Plenty to putz around with now that I don't need it for the wood pellet stove.