D.C. Motor theory.

[SteveG83]

Hi People,

Could you help me out? I'm wanting to know couple of things with DC motors:-

1 . I know that when you decrease the field excitation the speed increases and torque reduces but does anyone know how?

2. And when the armature current is increased the torque is increased but again how?

If anyone can give me a basic explanation of how the alteration of those affects the out come it would be awesome!

P.S reason I'm asking is my apprentice has asked and I can't remember half of stuff I learnt at college let alone what I had for tea yesterday!:44:

 

[Pete999]

Hi People,

Could you help me out? I'm wanting to know couple of things with DC motors:-

1 . I know that when you decrease the field excitation the speed increases and torque reduces but does anyone know how?

2. And when the armature current is increased the torque is increased but again how?

If anyone can give me a basic explanation of how the alteration of those affects the out come it would be awesome!

P.S reason I'm asking is my apprentice has asked and I can't remember half of stuff I learnt at college let alone what I had for tea yesterday!:44:

https://www.google.co.uk/search?q=b...nt=firefox-b&gfe_rd=cr&ei=XMBmV7OCO8HBaMT3ozg

 

[Peter Monfort]

The field can be considered to be constant since it is separately excited.
The speed of a dc motor is proportional to back emf / flux i.e Eb / φ so if the field and therefore flux is reduced the speed will rise. Here is a useful link.
http://people.ucalgary.ca/~aknigh/electrical_machines/fundamentals/f_dc_sep.html

 

[marconi]

http://schoolphysics.co.uk/age16-19...e_on_a_current_in_a_magnetic_field/index.html

The force(torque) on a current carrying conductor depends on the current flowing in it, its length in a magnetic field and the strength of the magnetic field. F=B x I x l and B is proportional to the strength of the excitation current in the field winding, so Force(torque) is proportional to Ia x If, where a is armature and f is field.

Here is an interesting project for your apprentice to do:

You can use ordinary solid insulated wire for the rotor. Strip the insulation of the coil 'wings', then lie it flat on a table and using a permanent marker pen deposit some ink along the uppermost surface of the exposed copper and then let it dry. Do this a couple of times to make a thicker layer of insulating ink. It does though wear away with use.

There are other designs to try too. I use a small block of wood rather than the cardboard.

The apprentice can experiment with different currents, number of turns, diameter of coil, strength of magnetic field (eg bring a second magnet closer to the coil) and separation distance between the coil and magnet.

You can obtain very strong small neodymium permanent magnets from a number of suppliers by searching on line. Keep them well away from watches!

 

[Lucien Nunes]

All the constructional parameters (size, number of windings, etc.) of an ideal DC motor can be rolled up into one factor termed 'flux constant' or kφ that relates the mechanical variables (torque, speed) with the electrical (current, EMF). Amongst other things, kφ is proportional to airgap flux which, in a wound-field machine depends on field current. Using kφ simplifies the two main motor equations to:

1. Τ = kφ.Ia (torque is proportional to armature current and flux)
2. ω = Ea/kφ (speed is proportional to EMF but inversely proportional to flux)

These are true almost by inspection, from a basic appreciation of the interaction of a moving wire with a fixed magnetic field; more armature current means more force acting on each turn of the winding, and higher velocity means more voltage generated, for a given flux. To answer your first question, we can see from 1) that decreasing field current and hence kφ will reduce the torque for a given armature current, and from 2) that the speed will have to increase to maintain the same armature voltage (so that the generated EMF equals the applied voltage). The answer to the second is equation 1) itself.

B is proportional to the strength of the excitation current in the field winding, so Force(torque) is proportional to Ia x If,

Torque is proportional to the total flux, and certainly increases with field current, but I would hesitate to say that it is 'proportional' to the current because the B/H curve of the field iron is not a straight line at full excitation on a commercial machine. It is more nearly proportional to Ia at typical loads because the MMF of the armature reaction is lower than that of the main field.

 

[marconi]

Lucien, I agree with your clarification. Thank you.