Magnetic flux

[mary20]

Help Please two questions but kinda the same i think

1.Explain why the increasing the magnetic flux in the main field of a d.c motor causes the armature current to reduce

2.Explain how magnetic flux in the main field of a d.c motor may be increased and what effect this will have on the motor armature current.

I know that when the flux is increased the speed is reduced and when the flux is reduced the speed is increased.

And how do i post a sticky thread Thanks..

 

[RogertheBodger]

Good question, is there a bit more information to go on?
The easy bit is that only the Moderators can make a thread sticky

 

[RogertheBodger]

This question has got me interested, feel like I should know the answer but can't find any information, anybody know much about This?
Any ideas where to look?
Cheers ........

 

[Knobhead]

Have a look at http://www.lmphotonics.com/ you will find it very useful, I still do.

 

[leena2k6]

Hi, I still did not find any information on this. Any inputs on this will be highly appreciated.

Thanks!

 

[Marvo]

It would help if you could give a better description of the motor concerned. Are you talking about a universal motor?

There's some very basic motor theory here that covers the relationships between magnetic flux and rotor currents etc.

 

[leena2k6]

The question says - DC motor.
Re-writing the question once again - Explain how the magnetic flux in the main field of a DC motor can be increased and what effect this will have on motor armature current.

 

[Knobhead]

I’m going to use the Google escape pod at the moment. Have a look for Fleming’s Left Hand rule, then look for Back EMF. The LH rule gives the basics of a motor, back EMF is what is controlling the current. Back EMF is governed by the density of flux X the speed the lines of flux are cut. If you increases the flux then as the motor rotates the rotor windings “cut” a higher density of flux lines at a lower speed to maintain the back EMF. Reduce the flux then a higher speed is required to maintain the rate at which the lines are cut. I’m sorry about the link I gave in that it didn’t work. Last time I looked it was fine it’s just the DC stuff that seems off line.

 

[leena2k6]

1. I get the relation between flux and speed. But how do we increase the flux which is the first part of the question.

2. Also back emf controls the armature current. If we increase the flux, speed decreases but back emf is constant, which means the armature current will not change with change in flux. Is that correct?

 

[leena2k6]

Sorry, I have got it right now. If flux increases, back emf increases and hence this decreases the armature current. I only need help with how can we increase the flux?

 

[uncle betty]

Sorry, I have got it right now. If flux increases, back emf increases and hence this decreases the armature current. I only need help with how can we increase the flux?

For a shunt/parallel motor - put variable resistor in series with field winding.

For a series motor - put variable resistor in parallel with field winding.

 

[Knobhead]

Sorry, I have got it right now. If flux increases, back emf increases and hence this decreases the armature current. I only need help with how can we increase the flux?

Quite simply speed is controlled by raising or lowering the current through the field winding. But I’ll be honest its not always that simple. Series motors there’s not a lot you can do about it other than govern the supply voltage. Shunt motors usually a constant voltage is applied to the armature and a separate variable voltage to the field, high current in the field gives low speed with high torque, low current gives high speed with low torque to the point that field failure occurs and the motor stalls. (At this point start looking for the fire extinguisher). Compound motors gives the best of both worlds, you can vary the fields separate supply without the fear of field failure due to the series field winding, but most control systems will shut down it the shunt component of the field fails.

 

[Marvo]

It would help if you could give a better description of the motor concerned. Are you talking about a universal motor?

The question says - DC motor.
Re-writing the question once again - Explain how the magnetic flux in the main field of a DC motor can be increased and what effect this will have on motor armature current.

Thanks for rewriting the question again but I didn't mis-read it the first time. A universal motor is either AC or DC....hence the name universal motor.

 

[uncle betty]

Thanks for rewriting the question again but I didn't mis-read it the first time. A universal motor is either AC or DC....hence the name universal motor.

I have the feeling that these questions are for an upcoming exam rather than a practical problem.

So I would say it's a theoretical DC motor that's being asked about. Thus there are two possibilities - the field is either in series with the armature, as with a universal motor; or else the field is parallel with it.

That's why I mentioned variable resistors to increase flux - put one in parallel with the field winding of a series motor. Put one in series with the field winding of a shunt motor.