Course work help

I have forgotten all of my DC theory, but does this help help a little?

https://www.bing.com/videos/search?q=DC+motor+back+emf+armature+current&docid=608049412753264334&mid=35341B7A3D131B25C40735341B7A3D131B25C407&view=detail&FORM=VIRE


Z.

I have just noticed the when we consider Eb=Ia X Ra, Ia X Ra is like Ohms' Law where V=I X R.


So if Ia and Ra are concerned with the armature values they may help us to understand more.


So, we are saying that Eb = Ia X Ra, or that back E.M.F. is supply Voltage minus armature Voltage.


Z.

Hi Daniel,


The two formulae are essentially the same. Going to first principles, the mechanical equation for power is P=ωT where ω is the angular speed. Substituting the electrical equation P=VI we get VI=ωT which can be rearranged to I=ωT/V (formula 1). Since ω=2πN where N is the number of rotations per second it can be restated as I=2πNT/V (essentially formula 2 if you use armature current and armature voltage).


If you get different answers then perhaps you should look at the numbers you are putting in to the equations to see where the problem is. Hope this helps to get you started - if you still have problems come back for further help.

A suitable textbook is Hughes, Electrical Technology. You will find many worked examples and the underlying theory well explained. It is well worth having for all kinds of stuff. ISBN 0-582-22696-1 although there may well be a newer edition.

Torque and Watts.

Torque (Nm) to Watt (W) Conversion Calculator | Electrical4u


Z.

If your mind works that way try  to do this without memorising some magic formula you cannot remember the units for.

Just rebuild the formulae from the basics.

Power is force times distance moved in one second

Power is volts times amps across a resistor (voltage sources have no internal resistance.)

generator voltage is more or less proportional to revs. for small changes. (depends a bit what you assume about the field - series wound versus permanent magnet differ in that way, as  in one case the magnet changes as well as the armature current.)

What do we know

Well we know the torque (that's a force and a radius (- to convert to force times distance you need to realise that there s a a factor of 2pi between radius and the circumference of the circle. )

So we know the force, and as we know the distance that force moves in a second (n revs of the circumference)  we can get to watts.

Do not be tripped there are 60 seconds in a minute, we need revs per second....


If we know supply voltage and watts of work being done, we know current for the whole thing, if it was lossless.


So we know that spinning at that speed the as a genset the motor generates 115V, in opposition to the 120V supply. The extra 5V is what is pushing the current we just calculated, and we can get back to an effective load resistance from that and the current we have just calculated.


Now we want half as much power again, but the load resistance does not change so the voltage across it must rise, and therefore the back EMF must fall, and as this generator voltage is reduces ,  pro-rate do the revs.


Give it a go and come back. I have the rough numbers in my head but I do not want to poison your thinking too early.

M

Hi Dan,


Would you please tell me, for which DC motor configuration (series, shunt, long shunt, short shunt etc), the question has been framed?

This is amazing, thank you all for your help, I shall read and digest.

So greatfull

Hi Mr R Trivedi

That is all the information I have been given.