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Technical How do smaller electric motors..........

Discussion in 'The Hokey Ass Message Board' started by ekimneirbo, Jul 31, 2020.

  1. ekimneirbo
    Joined: Apr 29, 2017
    Posts: 1,848

    from Brooks Ky

    I'm wondering how small electric motors like starter motors generate so much power for their size? A normal size starter motor generates a lot of power for its size. The even smaller "high torque" gear motors will start even bigger motors. I know the gear reduction helps, but the actual motor is extremely small. What is done to these motors that lets them produce so much power ?
    Last edited: Jul 31, 2020
  2. jimmy six
    Joined: Mar 21, 2006
    Posts: 8,752

    jimmy six

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  3. Belle53
    Joined: Aug 13, 2019
    Posts: 27


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  4. Dangerous Dan
    Joined: Jul 10, 2011
    Posts: 366

    Dangerous Dan
    from Graham Wa.

    Series wound DC motors produce a lot of torque and speed when unloaded and with the gearing ratio between the starter gear and the flywheel which is quite high is what makes them work. A unloaded
    dc motor will keep increasing speed until failure.
    ekimneirbo likes this.

  5. Dan Timberlake
    Joined: Apr 28, 2010
    Posts: 1,307

    Dan Timberlake

    And starter motors are designed knowing they don't run for very long at all. The heat build up when they run too long at full load is the limit, like most electric motors. Not much cooling provided on a starter either.
  6. ekimneirbo
    Joined: Apr 29, 2017
    Posts: 1,848

    from Brooks Ky

    But it has to be more than just gearing. Look at the motor for the blower on your heater. Its about the same size as a high torque starter motor, yet adding a gear to it would not make them the same. Even when a gear is added, the starter motor has to still crank the engine fast enough to start it.
  7. Jack E/NJ
    Joined: Mar 5, 2011
    Posts: 645

    Jack E/NJ
    from NJ

    cranking amps x volts = cranking power

    So typical 500 cranking amps at 7 cranking volts for a 12 volt battery = 3500 watts or almost 5 horsepower. Jack E/NJ
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  8. I take it you're wondering about the late-model or aftermarket 'mini-starters'. As said, gearing enters into this but another major improvement is the advent of high-strength permanent magnets used for the fields rather than electromagnets. Electromagnets are limited by 'saturation' i.e. how much magnetic flux that can be generated within the iron core. To get more flux, a bigger core is needed and you get into size considerations.

    How strong can these magnets be? Well, I bought a couple for a home project and they came with a 'separator' between them. When I went to remove it, they snapped together so hard that they took a chunk out of my finger! I had to put the two in my vice and use a big pair of pliers to get them apart...
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  9. A conventional starter motor makes the torque because it is operating at a huge overload. That’s why they are overheating after only 30 seconds.
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  10. No, they're designed to operate like they do. They just have a rather low 'duty cycle' just like most welders.
  11. ekimneirbo
    Joined: Apr 29, 2017
    Posts: 1,848

    from Brooks Ky

    That's more what I'm looking to find out. I understand that the starter has to draw more power from the battery, but I was wondering how it accomplishes it without increasing it's size. Thanks!
  12. squirrel
    Joined: Sep 23, 2004
    Posts: 48,294


    The mini starter is doing the same amount of work as the big starter, so the power draw is similar.

    torque, rpm, and power work the same for electric motors, as they do for gas engines. Smaller motor spinning three times as fast, can make as much power as a larger higher torque motor, not turning as fast.
    ekimneirbo likes this.
  13. Chrysler had the right idea. They created that gear reduction starter that would easily spin a 440 with 12:1 compression and used it on everything down to slant sixes in Plymouth Valiants and Dodge Darts.
  14. Of course they are designed like that. You basically are saying the same thing as I am .
  15. Beanscoot
    Joined: May 14, 2008
    Posts: 1,945


    They were originally inspired by small motors used in cash registers. By the conventional electric motor standards of the time they were invented, they were certainly severely overloaded.

    The brilliant innovation was the realization that they only ran for seconds, so could be many times smaller than a "conventional" motor of the same power rating.

    I think the "PMGR" (permanent magnet gear reduction) starters may actually draw less power for the same work since no power is consumed to energize the field coils.
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  16. That may or may not be true. While there is no doubt a power savings from the magnets, the stronger field also allows the armature to draw more power. The one consideration with these is they don't like rough handling as sharp blows can reduce the magnetic strength... so don't drop them. That's why 'magnetizers' were popular for slot cars back in the day.
    Hnstray likes this.
  17. Beanscoot is right about the idea coming from a cash register drawer. First prize if anyone knows who came up with the idea of the cash register drawer motor, and the successful electric starter motor. It was the same chap. Hint: he was working for NCR at the time and went on to found a very famous auto parts supply company. Second prize for the name of his company.

    Sent from my iPhone using The H.A.M.B. mobile app
  18. greybeard360
    Joined: Feb 28, 2008
    Posts: 1,660


    Look at motors for slot cars and r/c cars. They use different size wire when winding the armature for different uses. Small wire with more turns gets you high rpm low torque. Larger wire with fewer turns is lower rpm with higher torque. Turns refers to the number of times the wire is wrapped around each pole of the armature. Same type of system is used when building starter motors.... Different size windings yield different levels of torque.
  19. pitman
    Joined: May 14, 2006
    Posts: 5,041


    Delco or Remy?
    And believe DC's offer max torque at '0' rpm.
  20. Lone Star Mopar
    Joined: Nov 2, 2005
    Posts: 3,070

    Lone Star Mopar

  21. ekimneirbo
    Joined: Apr 29, 2017
    Posts: 1,848

    from Brooks Ky

    OK, I asked this question because I had something other than a starter motor in mind but figured that a starter might be close enough to figure out what I wanted to know. Actually I'm building a deep throat bead roller and have some DC motors. I know one would work and it already has an industrial speed control, but I really want to save it for another project I have in mind....... I also have an AC gear motor that will work, but it will not have variable speed. It does however turn very slowly at 13 rpms. Last I have the two DC motors pictured below. Both are 130 volt. The smaller one has 1/17 hp. The larger one is 1/3 hp. Small is .48 amps large is 2.3 amps.
    The speed control came with the small motor but I hooked it up to the large motor and it worked fine with no load.
    The speed control says it has a 2.5 amp capacity. Would it be sufficient to operate the larger motor under load on a bead roller? I don't want to screw it up since it works great with the smaller motor. These DC motors seem to be rated for small HP and good Torq. Like the high torque starter motors they have some serious reduction gears. Notice that the small motor has 65 in lbs and the larger has only 72 in lbs.......hence my original question. Errr, make that lb ins :D

    2 DC Motors.JPG
    Small Motor.JPG
    Large Motor.JPG
    Speed Control 1.JPG
    Speed Control 2.jpg
    Last edited: Aug 3, 2020
  22. Atwater Mike
    Joined: May 31, 2002
    Posts: 10,517

    Atwater Mike

    My uncle Bill fabricated a starter motor for his 1911 (think Cadillac? Not sure) in San Francisco, around '12 or '13.
    Sunday mornings, he would drive it up Market St. to the news stand, always some 'old duffers' standing/sitting around.
    Uncle would step back into the drivers seat, the sitting-arounds would give him a 'disparaging' glance, (thinking he was waiting for one of them to 'crank him up'...)
    He'd hit the starter, engine would roar to life. Sitters would jump back, "What the...?"
    Unk would drive off, chuckling...
  23. Beanscoot
    Joined: May 14, 2008
    Posts: 1,945


    Did your uncle do this in 1912 or 2012?
    If the former, you must be pretty old.
    Cosmo49 likes this.
  24. Nope... Motor current spikes when starting any electric motor (unless you're using a electronic 'soft start' controller, and even those will only reduce the spike) anywhere from 300% to 1200%. So that controller will either let out its smoke or if it has a current-limiting circuit (to protect the controller) the motor won't start.
  25. MantulaMan
    Joined: Jun 19, 2018
    Posts: 40


    Your question is waaaay more complicated than you realise.

    AC motors are designed to run at a set RPM which is set by the frequency pf the supply voltage. Brushed DC motor is probably your best choice as you will be able to control the speed (and more importantly torque) more easilly.

    Brushless DC motors have very little torque at low RPM but have much better control.

    Sent from my SM-A105G using Tapatalk
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  27. winduptoy
    Joined: Feb 19, 2013
    Posts: 2,373


    Oh, now I know I've got a 5 HP arm for cranking the Model T..., no gear reduction there.
    Maybe @squirrel does too.. naw, bet his has an electric starter.
    squirrel likes this.
  28. Mike51Merc
    Joined: Dec 5, 2008
    Posts: 3,856


    Dare I say "size doesn't matter, it's the motion in the ocean" :D
  29. anthony myrick
    Joined: Sep 4, 2009
    Posts: 7,387

    anthony myrick

    The magical flow of electrons
  30. ekimneirbo
    Joined: Apr 29, 2017
    Posts: 1,848

    from Brooks Ky

    The larger motor does run and speed is adjustable with the controller hooked up. Given the small difference between the capacity of the controller (2.5 amp) and the stated draw of the larger motor 2.3 amps, I kinda figured there wasn't enough safety factor to make it work.....but figured I would ask the experts.

    How can you tell if its a brushed motor? The control does work really good at varying the speed. Guess I'll have to look for a similar control with a larger capacity. Any suggestion on what amp capacity I need to work properly with the larger motor?

    Unless you have been on the receiving end, I don't think you can acurately judge the effects. :D:D:D
    VANDENPLAS likes this.

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