# Stator Current & RPM - Motorcycles

Short and simple - I want to understand the relationship between RPM (specifically on a motorcycle crankshaft) and AC stator current. In most motorcycles there is a magnetic field generated at the stator (of varying design) which I am hoping to prove generates a current that proportionally increases with the speed of rotation at the magnetic field (ie the crankshaft, as there is often a flywheel with magnets attached).

What is the relation between the stator current and motor rpm for an AC motor?

But am still a little confused.

If you are asking about generated current, you are asking about a generator, not a motor. In this situation, I believe that you are talking about a magneto that is generating AC voltage in a stator coil. The generated frequency and voltage will be proportional to the engine speed. The current will be determined by the characteristics of the load, but would tend to be proportional to the voltage. If the power is used for loads other than the ignition system, such as charging a battery, there will be a rectifier to change the AC to DC and a voltage regulator to make the voltage constant.

Edit: The current will tend to be proportional to the voltage only to an extent that increasing voltage with a given load will tend to increase the current. However some loads have characteristics that tend to maintain constant power. In that case, increasing the voltage may cause the current to decrease.

• Yes you are right, thanks. I want to measure the AC stator current at varying RPM. If it is proportional, I imagine I would need to measure this across one of the phases to ground (measuring current ie amps and not voltage)? – Shidoran Nov 29 '15 at 21:38
• Current is measured by inserting the ammeter between the source and load connected in series with the load. – Charles Cowie Nov 30 '15 at 2:03
• Thanks, can you please define "load" so I am not confused. My understanding is that the load in this particular case could either be (1) the reg/rec as it receives the current or (2) the battery as it "consumes" the current? Noting that after the reg/rec the voltage would be changed to DC as you stated. I have noticed the "charging" DC current to the battery does vary with RPM so I am wondering if that is what I should be measuring. The purpose here is to find a proportional "max current" at a specified RPM. I am not really concerned if it is DC or AC - it just needs to be proportional. – Shidoran Dec 2 '15 at 1:50
• The load on stator would be (1) and the load on the stator and the reg/rec working together would be (2). – Charles Cowie Dec 2 '15 at 3:32

The Stator AC output does rise and lower with engine RPM. ..but this work has all been done! A stator is made by component manufacturers like Denso, and they are just like electronics components. It's an off the shelf part, manufactures/designers (Honda,Yamaha, Kawasaki,Suzuki) pick which ever they think is best for the specific model.