A direct current electric motor...

[Externet]

A DC motor has two windings, one for the rotor via brushes;

and one for the stator to create the 'bias' magnetic field.

 

Fed with separate supplies, the stator magnetic field consumes 1 Ampere at 10 Volts; and the rotor consumes 2 Amperes at 10 Volts.

That is 10 + 20 = 30 Watt, and the water pump it drives rises a column of water 25 metres high.

 

That same motor gets a transplant of its stator electromagnets, replaced with permanent magnets that produce the same exact magnetic field.

 

Now the motor driven water pump pushes water to the same exact height of 25 metres high as before in the same time, consuming 20 Watts by the rotor.

 

What is going on? -The same work in both cases with now less power demand-

 

How stupid is to say the magnets are contributing with 10 Watts worth of power ? ... Or... saving 10 Watts worth of power ?

What is the mathematical analysis ?

 

 

[John Cuthber]

The problem is that, in the first case, you forgot to use a superconducting magnet.

Using an electromagnet to produce a magnetic field is absurdly inefficient.

 

Maintaining a magnetic field doesn't, in principle, require any power.

If you choose to expend power doing so then that's your choice.

[Externet]

Thanks, John.

It is not about superconducting magnets. For the first case, any plain DC motor has field electromagnets, (unless uses permanent magnets) Like an engine starter motor..

 

----> http://k7nv.com/notebook/ppinfo/13ef4770.jpg

 

Been "absurdly inefficient" about a century, but that is the way they are. Electromagnets for the field. Modern motors are using permanent magnets more and more since better low cost magnets became available.

[swansont]

I'm not sure what the problem is. You can build systems that are more efficient or less efficient. Using permanent magnets is more efficient. An electromagnet dissipates energy because of resistive losses, not because of the field it creates (though there is energy in the field), which AFAICT is what John Cuthber was pointing out. You've eliminated those losses.

[John Cuthber]

Externet, do you not understand that permanent magnets are heavy? Using a lighter, but less efficient motor to start a car engine makes sense because it's not running most of the time , but it is always carried about.

Using permanent magnets would make the car less efficient, because it would be heavier.

 

There are other, similar trade offs in other situations.

Also, a lot of small motors are permanent magnet types.

Edited August 10, 2013 by John Cuthber

[bradpitz]

I thnk electromagnets are more efficient to use. using permanent magnets could have a bigger rotor to match the power of the motor using electromagnets but the power consume will be almost the same cause you will have a heavier rotor by using permanent magnets.