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I am currently eyeing installing an electric starter to my pocket bike 49cc engine. This question is purely about the electrical part of it, but I am going to give a little context.

The starter engine is very simple, it has 4 anchors that drive a rotor when powered. There is no one way bearing or disengagement mechanism except cutting power. They are mounted directly onto the crankshaft between the engine and the clutch shoe. For completeness at the bottom is a picture of such a starter.

I have watched several videos where people installed and wired them and (successfully) demonstrated starting the engine. It boiled down to this:

  • Starter case connected to negative of battery
  • Positive of starter connected via starter relay/solenoid to positive of battery
  • Button to close the relay

From an electrical point of view the starting makes sense:

  • button is pressed
  • relay is closed
  • power flows to the starter turning the rotor
  • engine starts

Now this is where I get confused. Unlike in a car the starter mechanism has no way to disengage when the engine starts (like one way bearing, ratchet, centrifugal, ...) except letting go of the button for the relay. When the engine starts it will also start to drive the starter shaft effectively turning the starter into an inefficient generator. From my understanding that means that until the relay is disconnected current will flow to the starter battery instead of from it. I checked the starter relays used and they don't have any protection for reverse voltage or current.

So my question boils down to: Do these people risk their starter batteries or am I misunderstanding something?

Image of starter

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  • \$\begingroup\$ Why would you add an electric start to a 49 cc engine? Is there a good reason? \$\endgroup\$
    – Andy aka
    Jan 22, 2021 at 10:16
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    \$\begingroup\$ Mostly just for fun, but also a little bit for convenience, it does not require me to install the engine in a way to make the pull start accessible. \$\endgroup\$ Jan 22, 2021 at 10:18
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    \$\begingroup\$ Once started, remove your finger from the start button. For the small amount of time the motor is generating, i don’t think it will affect your battery negatively. \$\endgroup\$
    – Kartman
    Jan 22, 2021 at 10:31
  • \$\begingroup\$ What kind of battery? \$\endgroup\$
    – K H
    Jan 22, 2021 at 10:36
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    \$\begingroup\$ Then you're probably fine. Measuring the current as kartman suggests would be wise. \$\endgroup\$
    – K H
    Jan 22, 2021 at 10:45

1 Answer 1

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If it's a wound-field series motor (most starters are) it cannot function as a generator as the field automatically reduces as the shaft-speed increases.

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    \$\begingroup\$ This sounds very reasonable, but I feel like there is a typo or two in that answer. \$\endgroup\$ Jan 22, 2021 at 21:52
  • \$\begingroup\$ I'd like to accept this as the answer, but could you maybe add some details to it? What exactly is a wound-field series motor and why does its field reduce when speed increases? \$\endgroup\$ Jan 24, 2021 at 11:09
  • \$\begingroup\$ It's where a winding is used instead of permanent magets for the stator field, because motor current decreases with speed the field also decreases with prevent the back emf from meeting the supply voltage. see also "universal motor" \$\endgroup\$
    – Jasen
    Jan 24, 2021 at 11:15
  • \$\begingroup\$ would the fact, that the whole motor is very magnetic even when not energized be indicating the opposite? \$\endgroup\$ Jan 24, 2021 at 11:16
  • \$\begingroup\$ could be remenant magnetism it's hard to say. \$\endgroup\$
    – Jasen
    Jan 24, 2021 at 11:19

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