# Why would venting an engine create a negative amperage on a generator?

I'm writing simulation software for a civilian helicopter.
The helicopter has two engines and two generators.

Normally, we calculate the amperage reading of a generator through Ohms.
I = W / V.

However, there is a venting switch on the overhead panel, that "vents" a particular engine. From what I've read, this also enables a starter relay, and I've been told that it is similar to "spinning up a generator without actually starting the generator."

By observing videos from the actual craft, venting an engine immediately drops the corresponding generator amperage from 0 to -270.

I do not know enough about electrical engineering or automotives to explain why this happens.

Why does the generator drop to a negative amperage?
Is the engine somehow providing a negative load to the generator?

Image shows an arrow to the venting engine.
The negative amperage of the generator is circled.

• Is it possible that the generator and the starter are one and the same device? When acting as a generator it draws mechanical power from the engine to produce 'positive' electrical power, but when acting as a starter it draws 'negative' electrical power to turn the engine. – brhans Mar 9 '17 at 17:54
• Sounds like "venting" means to crank the engine without actually letting it start. It's usually used to deflood the engine. In that case, you'll need energy to run the starter motor for cranking the engine. That's why power is being drawn into the generator. – tangrs Mar 9 '17 at 17:54
• Negative amperage is just current going the other way :) I suspect the generator/engine system is consuming (as opposed to generating) 270 Amps while venting. – bitsmack Mar 9 '17 at 17:57
• Normally, Generators have carbon brushes for longevity, and starters have copper brushes for torque current. but for helicopters they combine purposes Starter-Generators perhaps with hybrid brushes – Sunnyskyguy EE75 Mar 9 '17 at 18:15
• A bipolar Ammeter is needed to show starter current when venting and Charger current when running. – Sunnyskyguy EE75 Mar 9 '17 at 18:21