Alternators and electrical motors

[d.smith292]

After reading about a guy who used a permanent magnet dc motor for his wind turbine, I got confused. Can motors be used as alternators?

 

[Drakkith]

You can easily convert most types of motors into generators and vice versa I believe. How hard it is depends on the exact type of motor or generator that you have.

When you look at a generator, the principles are exactly like the principles in a motor, only reversed. In a generator you have something spin a magnetic source which creates electrical energy. In a motor you have electrical energy turning a magnetic source which creates motion.

 

[Averagesupernova]

Check out www.otherpower.com.

 

[d.smith292]

Thank you for your help

 

[Jiggy-Ninja]

How efficient would it motor-turned-alternator be, compared to a purpose-designed alternator? Are there are different engineering and design concerns between the two?

 

[vk6kro]

Purpose built DC generators can have a correction for armature reaction.

This is a method of moving the brushes so that they can collect maximum power from the armature. This is not the same position as a motor would use.

You can read about this effect here:

http://en.wikipedia.org/wiki/Armature_reaction
I'll quote it here since it is important:

Since an armature is wound with coils of wire, a magnetic field is set up in the armature whenever a current flows in the coils. This field is at right angles to the generator field, and is called cross magnetization of the armature. The effect of the armature field is to distort the generator field and shift the neutral plane. The neutral plane is the position where the armature windings are moving parallel to the magnetic flux lines. This effect is known as armature reaction and is proportional to the current flowing in the armature coils.

The brushes of a generator must be set in the neutral plane; that is, they must contact segments of the commutator that are connected to armature coils having no induced emf. If the brushes were contacting commutator segments outside the neutral plane, they would short-circuit "live" coils and cause arcing and loss of power.

Armature reaction causes the neutral plane to shift in the direction of rotation, and if the brushes are in the neutral plane at no load, that is, when no armature current is flowing, they will not be in the neutral plane when armature current is flowing. For this reason it is desirable to incorporate a corrective system into the generator design.

These are two principal methods by which the effect of armature reaction is overcome. The first method is to shift the position of the brushes so that they are in the neutral plane when the generator is producing its normal load current. in the other method, special field poles, called interpoles, are installed in the generator to counteract the effect of armature reaction.

The brush-setting method is satisfactory in installations in which the generator operates under a fairly constant load. If the load varies to a marked degree, the neutral plane will shift proportionately, and the brushes will not be in the correct position at all times. The brush-setting method is the most common means of correcting for armature reaction in small generators (those producing approximately 1000 W or less). Larger generators require the use of interpoles.