# AC voltage drop



## Jan (Oct 5, 2009)

major said:


> Hi Jan,
> 
> This is a bit off topic. I'd be glad to try to help you, or give you my opinions, but perhaps a new thread in the motor section would be appropriate.
> 
> ...


Major, 

How much is this voltage drop? Is it caused by the inverter/motor? How should I take this unknown factor into account?


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## major (Apr 4, 2008)

Jan said:


> How much is this voltage drop?


Hi Jan,

I was speaking of droop, which is the voltage drop due to the internal resistance of the battery. Take a common lead-acid battery. It has a nominal voltage = 12V. If it has been charged and allowed to rest, you can measure the voltage at the terminals with a voltmeter. It will probably read 12.6V. This voltage at the terminals of a disconnected battery (meaning unloaded) is called the open circuit voltage.

All batteries have resistance inside. Called internal resistance. When there is current flowing through the battery, this resistance causes a voltage which is equal to the resistance times the current. The loaded voltage at the battery terminals will be = the open circuit voltage minus the droop (meaning the voltage drop due to the internal resistance).

Example: The 12 volt battery may have an internal resistance of 8 milliohms (0.008 ohms). So at 100 amps load, the terminal voltage would be = 12.6 - 0.008 * 100 = 11.8 volts. At 500 amps, V = 8.6 volts.

The internal resistance varies with the type of battery, the temperature, the SOC, the age, and probably other factors. It is normally listed on a manufacturer's spec sheet. But usually at best case conditions. When testing batteries, I usually find the internal resistance higher than spec.

So how much is this droop? Depends on your system and the load.



> Is it caused by the inverter/motor?


No, the droop is in the battery itself. However you have to consider the whole system which includes cables, connectors, contactors, fuses and the like. Then there is a voltage drop in the controller. This can range from a fraction of a volt to several volts depending on the particular controller.

There are of course voltage drops inside the motor, but these will be accounted for on the motor performance plot.



> How should I take this unknown factor into account?


With research and/or testing, these factors will become known. Or use SWAG.

But to determine the motor performance you have to know the voltage at the terminals of the motor. So you need to account for all the voltage drops in the circuit.

Regards,

major


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## Jan (Oct 5, 2009)

Thanks Major,

Clear and nive to read explanation. But I knew already of the battery voltage dro(o)p. Not that's called droop. 

But I understood it correctly that in a DC system the voltage drops by increasing load due to internal 'phenomena' in a DC motor?

The droop is just one of the plenty reasons to choose the highest voltage possible. Less current, less resistance in every part of the system. 



major said:


> Or use SWAG.


SWAG? Got something to doe with Torque?


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## major (Apr 4, 2008)

Jan said:


> Not that's called droop.
> 
> SWAG? Got something to doe with Torque?


Hi Jan,

A lot of EVers call it droop when referring to the declining battery voltage under load. Just shop talk.

SWAG = Scientific Wild A$$ Guess.

More Torque jokes, just what the world needs  My torque gets no respect, people think it is just power over speed.

major


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## Jan (Oct 5, 2009)

major said:


> SWAG = Scientific Wild A$$ Guess.


Thanks! That's usefull. I can do that.

I really respect your torque. Really.


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## major (Apr 4, 2008)

Jan said:


> I really respect your torque. Really.


Torque and Power are walking down the street. They stop. Power disappears 

Hey, it's hard to find good torque jokes these days.


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## Jan (Oct 5, 2009)

major said:


> Torque and Power are walking down the street. They stop. Power disappears
> 
> Hey, it's hard to find good torque jokes these days.


Major, Torque never walks. Power does sometimes. Torque always takes the car.


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## Sunking (Aug 10, 2009)

Ok this is from the FWIW category running AC motors via battery and inverter. AC motors or any AC circuit has a reactive component in addition to DC resistance. It is known as the Power Factor which I am sure most are aware of. However it has to be accounted for in the conversion process when using batteries and inverters.

For example a 1000 watt motor load will have a pf of about .8 to .9. That means the battery load will be 1000 w /.8 = 1250 watts plus any inverter inefficiency, and wiring losses. The extra 250 watts is reflected back at the inverter and burned off as heat in the distribution wiring.


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