Author Topic: Installing a Capacitor between an inverter and a battery.  (Read 13769 times)

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fabieville

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Installing a Capacitor between an inverter and a battery.
« on: August 16, 2011, 04:23:33 AM »
I was wondering if it would be more efficient if you connect a large capacitor between the inverter and the battery instead of just connecting the inverter straight to the battery?

Flux

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Re: Installing a Capacitor between an inverter and a battery.
« Reply #1 on: August 16, 2011, 03:16:28 PM »
Probably a pointless expense. Batteries in many ways behave as very large capacitors indeed. There are issues sometimes with higher frequencies where capacitors lower the impedance but the inverter will have its own input filter capacitance that will deal with all normal conditions.

The only time it might help is if you have longer than desireable leads to the inverter and you try to start motors, a huge capacitor straight on the inverter terminals may keep the volts up a bit longer to help the situation but you are looking at capacitors much larger than the typical input capacitors of an inverter something approaching fractions of a farad.

Sometimes for motor starting you need to resort to all the tricks possible but for normal inverter loads keep the leads short and forget it.

Flux

alijan

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Re: Installing a Capacitor between an inverter and a battery.
« Reply #2 on: September 13, 2011, 11:39:08 AM »
I have heared about it first time. Is capacitor give long life to battary?

artv

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Re: Installing a Capacitor between an inverter and a battery.
« Reply #3 on: September 13, 2011, 06:54:18 PM »
"for starting motors you need to use every trick in the book"....Hi Flux ,.....I would like to see that list...
is it possible it exist's??......sorry for the interuption........artv

joestue

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Re: Installing a Capacitor between an inverter and a battery.
« Reply #4 on: September 13, 2011, 07:07:38 PM »
if its a pure sine wave inverter then it's already got sufficient capacitors to handle the ~5Khz current ripple. the 120 hz current ripple is a sine wave and thus negligible.

if its a modified sine wave then you would only be extending the life of the capacitors inside the inverter... but they are never the first to die anyway.
My wife says I'm not just a different colored rubik's cube, i am a rubik's knot in a cage.

Flux

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Re: Installing a Capacitor between an inverter and a battery.
« Reply #5 on: September 14, 2011, 03:06:24 AM »
 "  "for starting motors you need to use every trick in the book"....Hi Flux ,.....I would like to see that list...
is it possible it exist's??......sorry for the interuption........artv "

Possibly that was a misleading statement as there really is no book.

Induction motors are pigs to start, 3 phase are easiest and they have starting currents from 1.5 to 3 times running current and it is at low power factor during start.

Single phase motors of any size can pull 5 times full load current during starting and at power factors as low as 0.5 . Small alternators in particular have great difficulty starting them as the kVA available is not much more than the full load kW rating before the volts start to collapse.

Inverters do tend to be able to handle the higher kVA a little better but only for a fraction of a second. few motors start in such a short time. Those with an inertia load can often take several seconds to start.

With alternators adding capacitors will power factor correct the lagging current of the motor during the start and often make a big improvement but you need to be careful to have the capacitor connected to the motor, otherwise adding a high capacitive load to the alternator will cause it to produce excess volts and saturate the core, both of which are damaging to the alternator and anything connected to it.

With inverters ,adding a bit of capacitance as pf correction may work with a pure sine inverter, it is very risky with modified sine as these really don't tolerate capacitive loads and if you must try it, again make sure that the capacitor is in parallel with the motor before switching to the inverter. If the input cable is totally inadequate for the inverter you may gain a little with a monster capacitor directly on the input terminals of the inverter as this question was originally about but with proper input leads it will do little good.

I can't go into this in much more detail but any further tricks normally rely on reducing the starting requirements of the motor itself, mechanically or electrically. Resistance in the line ( long extension leads) may reduce the starting current to something the inverter can handle but sadly motors tend to take more current as the volts are reduced to try to maintain the starting torque unless you can reduce the frequency at the same time ( possible with an alternator but not many inverters).

Fridges and freezers are a bad case and unless you can get a start in about 3 turns before the compressor builds up pressure you won't get it away. If you fail then wait several minuted for the pressure to drop before trying again.

High inertia loads such as fans can be eased by removing the fan load during the start ( block the air in or out) once the motor is running let the air flow. There are other tricks such as a pilot motor running light before the main load motor is added but not so effective on single phase, it works far better on 3 phase . You can also play with changing the starting capacitor on single phase motors during the start but it is near impossible to give any real guide on this as each case behaves differently.

Shall have to be more careful about these throw away remarks in future but if it does help someone then all to the good.

Flux