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Sep-EX theory and practice

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2K views 3 replies 3 participants last post by  engineer_Bill  
#1 ·
I would like some theory of how sep-ex motors work, formulas if possible. And what effect the field voltage/curent has on RPM and Torque. Why are sep-ex motors wound with a different winding ratio between the armature and the field? What would the effect of changing that ratio cause? Why does field weakening cause RPM increae?
 
#2 · (Edited)
The sepex motor idles at a speed Fi. Load it down below that speed and it draws current and produces positive torque. Push it enough faster than this speed (like going down a hill), and it produces a retarding torque and pumps energy into the batteries.

Cut the voltage in 1/2 on the field, and the motor spins approximately twice as fast. Double the voltage on the field, and it spins about 1/2 as fast.

There are lots of web pages with motor equations. You can derive them yourself by assuming the back EMF is proportional to RPM and field current.
 
#3 · (Edited)
Hi Bill,

You should be able to find a text book which teaches you all this. But in the mean time:

Eg = Kt * w * Flux

Tem = Kt * Flux * Ia

Where:

Eg is the generated voltage in the armature

Kt is the torque constant of the motor, depending on the turns, poles, etc.

w is rotational velocity

Flux is the flux per pole (I can't make those Greek symbols, Phi, I think)

Ia is the armature current

Tem is electromagnetic torque (shaft torque before rotational losses)

Pretty simple. The rest is just Ohm's law and Kirchhoff's equations for circuits.

The flux is determined mainly by the field excitation. Or MMF, magnetomotive force.

Flux = function (N * If)

Where N is the numbers of turns in the field coil

If is the field current.

The function is linear up to a point called saturation. This saturation is dependent on the magnetic circuit design for that particular motor. Once sufficient MMF (or ampere turns) is provided by the field coil, little or essentially no further increase in flux will be realized.

So, if you manipulate the first equation, you will see that for a fixed voltage (Eg taking in consideration voltage drops due to resistance and brushes), the speed (w) is inversely proportional to flux. So, field weakening is simply the reduction of flux to increase speed.

The opposite works only so far. An increase in flux will slow the armature. But one soon reaches a point called full field, where no further increase in flux is possible. The speed at that point is called base speed for that voltage.

All the while, (while you're messing about with flux to change the speed), realize that you are also changing the torque because the same flux is used in that equation. So you decrease flux to increase speed, it decreases torque. So power is about the same. You don't get something for nothing.

This is a pretty basic explanation. For rough approximations, those simple equations will work pretty well. But this is hardly enough for motor, or control, or system design. There are some things to bite you in the a$$, like commutation.

Hope that showed you something.

major