View single post by mwiz64
 Posted: Fri Jul 26th, 2013 03:04 pm
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mwiz64



Joined: Mon Mar 26th, 2012
Location: Fenton, Michigan USA
Posts: 1329
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Oh and I missed one other important cause for that initial jerk into action... I don't have the technical terms here but I think you'll get the picture.

Ever play with an electric motor by just spinning the shaft? Ever notice the coging effect? The motor turns in a jerky manor. This is called the coging effect. And depending on design, some motors cog harder than others. If you HAVE ever spun an electric motor by hand, you know what I'm talking about. What causes this? Well, it's the magnets in the motor and the teeth on the stator (core) interacting with each other. They try to rest in the position that leaves them in a state of equiliberium. If the motor is of high quality this position is exactly same for both poles on the core or the many teeth of the stator.

To try to under stand that, picture two magnets glued down closely together on a table and you with a thin strip of metal in your hand. Now try to put that piece of metal between the magnets such that it doesn't jump to one magnet or the other. That would be pretty hard, wouldn't it? But if you could place the metal exactly at the point where the attraction was the same in both directions you could prevent it from jumping to either side.
This is essentially what causes coging in an electric motor. The stator/core finds that perfect resting position and it takes an effort to be moved from it.

So with that long winded explanation comes the point. Every electric motor has a start up voltage. That's the power it requires to break that hold and move. Every electric motor also has an RPM/V constant. The minimum voltage needed to move the motor can be high enough to cause the motor to spin quite fast initially. This is that initial lurch into action that all electric motors have. Again, some designs are much better than others but there are tradeoffs to that.

How can we reduce the effect of that initial lurch into action? Deep gearing or a high gear ratio if you prefer that term. So that little 10 tooth pinion on the motor shaft spins that large spur gear at a slower RPM thereby masking that lurch into action. Worm gear transmissions, like many trains have, will need to have a fine pitch to them to smooth out this initial lurch into action. A motor that is precisely made and optimised to not have a high start-up speed will make a big difference too.

See, even us "simple" Civil Engineers remember some of the mechanical and electrical engineering we were taught back in school. The fact that I've been a "gearhead" of one type or another all my life has helped too.

Last edited on Sat Jul 27th, 2013 02:04 am by mwiz64



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