# Generator Input and Output Power

Hello I try to build a brushless Generator.

Let me try to explain it.

I have a brushless engine. max 6000 Watt 250KV.
And I have combustion engine. 3500 Watt by 16k RPM

If I connect the combustion engine with the brushless motor and the combustin engine turns with 16k
the brushless motor should provide about 1600RPM / 250KV = 64Volt

The combustion engine should have by 16k RPM 3500 Watts,according the performance datasheet.

If the brushless engine would a efficiency of 80% I should get out about 64 Volts with 44 Ampere.

3500Watt * 0.80 = 2800 Watt

2800Watts / 64Volt = 43,75 Ampere

I´m right or not ;)

THX

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anorlunda
Staff Emeritus
6000 Watt 250KV.
I think you mean 250V, not 250KV, correct?

1600RPM / 250KV = 64Volt
No, with most motors voltage is not proportional to RPM.

Can you post a photo of the name plate of the brushless motor? Use the UPLOAD button to post pictures.

What load do you want to supply with this generator? It is the load's needs that determine the voltage/current requirements.

CWatters
Homework Helper
Gold Member
I think the OP means 250 rpm/V. Eg its the motor constant. I'm wondering if this is a brushless DC motor?

Sounds like you are playing with RC brushless motors, at least that is where they typically rate motor as xxxxKV (rpm/V), I'm more used to seeing the EMF constant as xxV/krpm. Contrary to a previous statement, pretty much all (magnetic) electric machines produce voltage exactly proportional to rotational speed, double the speed doubles the induced voltage.

So if you are dealing with a 16krpm, and not 1600rpm, then you will get about 16000/250=64V.

Keep in mind this will be an AC voltage with a frequency determined by the pole pair count of the machine and its speed.

As soon as you put a load on it however things change, you have a frequency dependent impedance from the stator inductance as well as the winding resistance, so while the higher speed generates more voltage, its doing so at a higher frequency which means you loose more voltage with load from the increased impedance.

Plus this thing screaming at 16krpm will be horrible for any period of time!

russ_watters