Hi all,

I'm using an inverter-charger unit as a UPS for a a hydraulic pump (single phase induction motor), one of these:

(https://zlpower.en.made-in-china.co...brid-Charger-Power-Inverter-Lw1000-6000W.html)

It works well enough, when the mains a.c. drops out it switches over to inverter output seamlessly but when the mains comes back on and the unit switches back into charging mode rather than inverter mode, after a second or so the circuit breaker trips. I connected up a current probe and got this:



A current spike of about 320 A. Understandably the voltage wobbles a bit too. It only seems to happen when switching back to a.c. mains from battery. I have a couple of theories on what is happening but wanted to ask here if anyone has seen this before or has any knowledge of what might be going on to cause this?

Many thanks
Dean

 

... not completely clear on your connection scheme. First guess would be that the problem is somehow related to the induction motor start-up current transient. A diagram or sketch of the layout might clarify the situation, if that is not too much trouble.
... Thinking that the UPS charging load, in addition to the motor start-up load, is exceeding the breaker limit. ...

 

... not completely clear on your connection scheme. First guess would be that the problem is somehow related to the induction motor start-up current transient. A diagram or sketch of the layout might clarify the situation, if that is not too much trouble.
... Thinking that the UPS charging load, in addition to the motor start-up load, is exceeding the breaker limit. ...

Hi, no problem here's a rough idea:



It has a transformer in the inverter which must have some inrush, and also the motor has an inrush of around 60 A (I fitted a larger CB to take the load whilst making that measurement above BTW). You are right the charger would also draw come current. Maybe all of those things combined would be the perfect storm to trip it but 320 A seems so high.

Thanks
Dean

 

... On one occasion I measured the start-up current on a 120 volt, 1 Hp induction motor connected to the grid ... It was an air compressor. For a few seconds, the current was 70 amps, before coming down to the rated value ... just a few amps. Your motor is 3 or 4 times larger than that ... making me think that it could be taking 2 or 3 hundred amps for several seconds.
... If that is what is actually happening, then the problem would be how to reduce the motor inrush for just a few seconds when the AC mains re-starts power to the UPS.

 

... On one occasion I measured the start-up current on a 120 volt, 1 Hp induction motor connected to the grid ... It was an air compressor. For a few seconds, the current was 70 amps, before coming down to the rated value ... just a few amps. Your motor is 3 or 4 times larger than that ... making me think that it could be taking 2 or 3 hundred amps for several seconds.
... If that is what is actually happening, then the problem would be how to reduce the motor inrush for just a few seconds when the AC mains re-starts power to the UPS.

 

I have data for the inrush we locked the rotor and measured 58 A r.m.s. I guess this would equal about 70 A peak. It’s on 240 V a.c. If I start the motor on battery or mains there is no problem - only when switching from battery to mains. I will try and get hold of a time delay breaker I think as reducing inrush isn’t possible.

 

Ok ... starting to get some idea what is happening ... maybe.
If the UPS charging mode is somehow involved, then the internal battery might be drawing too much current. Those batteries are usually replaceable if it turns out to be defective.