Car starter as E- motor?

Discussion in 'Electric Bicycles' started by professor, Oct 8, 2009.

  1. professor

    professor Active Member

    Other thoughts are coming to my head for powering my gas/electric Mongoose.
    I will be powering this by a "Genset" (what we sometines call a generator, n my case, a wacker engine tied to a Ford alternator).

    Origional plan:
    250 watt scooter motor.

    But, could a starter motor (IF it fits) from a Toyota (or similar) with it's unitized gear reduction output be used? These are superb units, ball bearings, ultra- reliable and ultra cheap at the U pick it junkyard.

    Seeing I am powering this from a variable- current source, WITHOUT a huge inrush of power (like the normal battery would give). Would the starter motor be viable? I guess they are rated around 2 hp but I would not be using it all.
    The only issue I have right now is the legnth of the whacker- direct driven -into the alternator- more than a foot. This is going on a rack (so far)

  2. spad4me

    spad4me Member

    Just off the top of my head .
    Don't starter motors get really, really HOT after about a minute of running.
    Rewind it ?? For cooler operation.
  3. professor

    professor Active Member

    Probably, but if you were spinning over a big v-8 you would get hot too. I am thinking light load= low effort.

    This electric motor world is a different dog for me.

    What is the difference between DC permanant magnet motors that would negate this motor?
  4. safe

    safe Active Member

    Starter motors are designed for peak torque and not for efficiency or long term use. It's doubtful they will work well.

    If you want "progress" in the ebike the history went:

    :D DC Brushed Permanent Magnet motors single speed (overall Efficiency ~60%)

    :D DC Brushless Permanent Magnet motors single speed (overall Efficiency ~70%)

    :D DC Brushed Permanent Magnet motors multiple speeds (overall Efficiency ~75%)

    :D DC Brushless Permanent Magnet motors multiple speeds (overall Efficiency ~80%)

    :D AC Induction motors single speed (Six Phase or at least Three Phase) (overall Efficiency ~85%)

    ....and the last one is still not proven, but just theory.

    Something like a starter motor is likely to drop you down to below the brushed single speed.
    Last edited: Oct 8, 2009
  5. professor

    professor Active Member

    Here is how I think- I drive a simple car (listed as a "Classic" now) without- computer or fuel injection, or smog testing. Something I can fix.
    With efficiency comes complication.
    Complication brings expense- that expense makes up for quite a bit of my loss of efficiency.
    I do plan on using gears. Not single speed.
    I am thinking the starter motor is very robust for what I intend to do with it. I don't know what the Ford alternator puts out, let's say- 70 amps (like a run of the mill old Delco). Seventy amps x 12v = 840 watts. Right around one hp. That is way less than the amperage coming out of a car battery.

    My buddy electrican at work says- put it on a bench and run it for a half hour- see if it gets hot. Course I have to go get one first.
  6. safe

    safe Active Member

    Starter Motors Are Series Motors

    The advantage of a series-wound motor is that it develops a large torque and can be operated at low speed. It is a motor that is well-suited for starting heavy loads; it is often used for industrial cranes and winches where very heavy loads must be moved slowly and lighter loads moved more rapidly.


    Figure 9 Torque-vs-Speed for a Series-Wound Motor

    My comments:

    The problem (as I see it) is that the central disadvantage with permanent magnet motors is that they have a very narrow usable powerband. The series wound starter motor shares a narrow powerband, but it's shifted downwards so that the best torque is squeezed to the lowest rpms. This motor would be fantastic in a drag race for the first 10 feet, then the torque will start to fall off.

    The AC Induction motor can deliver a flat torque curve from near zero rpm up to it's designed frequency and then tapers off afterwards rather slowly. The total powerband can be twice or three times the width of a permanent magnet motor.

    So from the theoretical standpoint I just don't see it... that low end torque is going to want to stress your chain and other parts when at low rpm, but then it doesn't help much after. :lurk5:
    Last edited: Oct 8, 2009
  7. arceeguy

    arceeguy Active Member

    Years ago, me and my buddies made a motorized bar stool with a Chrysler starter motor (built in gear reduction). It ran off of a six volt battery so it wouldn't overheat from the continuous operation.
  8. professor

    professor Active Member

    OK, what kind of motor is in the typical brush dc jobs that are on the bike kits? Are they series wound? Like a starter motor?
    The rain is supposed to subside by tomorrow and I am feeling better, so it looks like a trip to the U-Pull junkyard.
  9. safe

    safe Active Member

    Series wound motor
    A series wound motor has a low-resistance field winding connected in series with the armature. It responds to increased load by slowing down and this reduces the armature current and minimises the risk of overheating. Series wound motors were widely used as traction motors in rail transport of every kind, but are being phased out in favor of AC induction motors supplied through solid state inverters.

    Permanent magnet motor
    A permanent magnet DC motor is characterized by its locked rotor (stall) torque and its no-load angular velocity (speed).

    As I see it the Series wound motors are great for something that is very "sticky" to start. Once the "sticky" torque is passed the torque drops to a very low value. So the Series wound motor is great for something like a train because it has the low end torque to overcome a stationary train's starting friction. Series motors typically use things like slip rings (brushes) to transfer energy to their field coils.

    A permanent magnet motor usually has four magnets and a rotor that is wound with magnet wire and spins. It's the spinning part that holds the commutator and that's where the brushes make contact.

    Like I wrote before... a Series motor is a good dragster motor because it's really going to be able to jump off the line, but after that the torque is weak. It would be interesting to see you do it though, so don't let us stop you if that's what you want to do. As far as low end torque the thing is going to be a real tractor. :cowboy2:
    Last edited: Oct 9, 2009
  10. professor

    professor Active Member

    Brought back a starter.
    Hooked it up to a 12v battery with some small leads.
    After about 5 seconds they got hot. Quite hot.
    Disconnected the solenoid and it made no difference.

    Might look into using the gear reduction and one way clutch adapted to the scooter motor. But for sure, starter motors are OUT.
    Well, I had to find if it would work.
  11. professor

    professor Active Member

    Took the starter apart to see internals. BIG windings on field magnets, brushes made of copper with lots of drag.
  12. safe

    safe Active Member

    Copper gives something like four times the conductivity compared to carbon brushes. Higher brush pressure translates into higher currents without overheating. (it lowers electrical resistance)

    The starter motor is entirely designed around the idea of massive starting torque and nothing else... it's just not right for an ebike.