# How does a car alternator work?

I am a beginner and my question is the following:

The alternator has an output voltage 13.7V. I know that each load is not a linear resistor but each load represents a certain resistance to the flow of electricity then can we say that the current depends on the resistance of the load.

Example is car alternator has a capacity of 900W of power to give that power to load at 13.7V. Maximum output current is 65A if the load resistance is 0.21077 ohms. This all calculation are based on ohm low.

Can we calculate like this for every equipment that must support rated current?

Summing up: Can we use Ohm's law to calculate the output current of the alternator?

• What is the question? – Marko Buršič Sep 16 '17 at 17:58
• Are you asking, "How does an alternator regulate the voltage for varying load?" or "Is this how to calculate the minimum load resistance for an alternator of power X watts?" – Transistor Sep 16 '17 at 18:28
• to sum up whether we can use ohm low to calculate the output current of the alternator – tor2006 Sep 16 '17 at 18:44
• yes Essentially the Battery is a huge capacitor and voltage is regulated by feedback to the alternator which is used to control magnetic field current to increase current output and keep voltage as set internally. So Ohm's Law works on the battery and alternator the same except at low idle. then the max alternator current is near 0. just enough to create 13V or so. – Sunnyskyguy EE75 Sep 16 '17 at 18:56

## 2 Answers

The alternator has an output voltage 13.7V. I know that each load is not a linear resistor but each load represents a certain resistance to the flow of electricity then can we say that the current depends on the resistance of the load.

Yes, for a given voltage the current will depend on the load. Note that it is a little more complex than that because increasing the load will cause the voltage to drop a little - particularly at low engine speeds. You can verify this by idling your car engine with the headlights on then then load up the system by switching on wipers, fans, rear window demister, etc. The lights usually dim a little.

Example is car alternator has a capacity of 900W of power to give that power to load at 13.7V. Maximum output current is 65A if the load resistance is 0.21077 ohms. This all calculation are based on ohm low [sic].

Yes, if you meant to type "Ohm's Law".

Can we calculate like this for every equipment that must support rated current?

Unclear what you are asking.

Can we use Ohm's law to calculate the output current of the alternator?

Yes. For each load you can calculate the current and then add them up to get the total current.

The last part of your question: can we use ohm law for any power source? Yes, ohm law apply to all kind of similar problem. For example, if a battery gives 12.0 volt no load and 11.9 volt when a load consume 1.0 amp, then you can compute the internal resistance as follow: R=v/i = 0.1 volt / 1 amp = 0.1 ohm. This resistance tells you how much energy is wasted as heat by the batteries: P=v*i=0.1 volt * 1 amp = 0.1 watt.

• the last part of the OP's question asks for the alternator output current... – Solar Mike Dec 3 '17 at 9:04