What is the major difference in construction between an AC generator and a DC generator?

Is it in the number of coils? For an AC generator, how does the coil continually rotate up and back down again? Does it have to do with force from a magnetic field? Any diagrams helpful. Thanks.

One Response to “What is the major difference in construction between an AC generator and a DC generator?”

kirchwey Says:
15. June 2010 at 4:05 pm

First, rotation is continuous and in one direction in either type.
The idea is to get a conductor to move through a magnetic field, generating what is called "motional EMF" (1st ref.). For a practical machine you want many of these conductors connected in series and moving through a field so you wind a long wire into a coil of many turns, and you rotate it through a field so the motion can be continuous.
The main difference between AC and DC generators is that the rotation naturally produces an AC voltage, with the AC frequency equal to the rotation rate or a small-integer multiple of it. So getting AC from the generator rotor to an external load is relatively simple. See the 2nd ref.
To generate DC you have to find a means of rectifying the AC or reversing the connections from the rotating coil so the current to the load flows only one way. This is often done by a commutator but diodes are gaining popularity as a means to do the same thing with less dependence on the mechanical health of the moving parts. See the 3rd ref. (which is actually about a DC motor, but a motor has many points in common with a generator).
This description has assumed that there is always a rotating coil, and a rotating coil requires some way of making a current connection from it to the load. These connections are made with metal strips that are attached to the rotation shaft, and "brushes" that are stationary and maintain sliding contact with the strips. In an AC generator the contact strips are full circles called slip rings and ref. 2 shows their function. In a DC generator the strips are segments of a circle and the mechanism is called a commutator, which converts the AC current in the coil to a DC output current by synchronous coil-connection switching.
There are also generators in which a stationary coil generates the output while the rotor provides a moving magnetic field. In fact automotive alternators work this way. However I’ll end this answer here because it’s getting too involved in the many possible variations of motors and generators from the basic types, which deserve several more answers. (E.g., the 4th ref.)

kirchwey Says:
15. June 2010 at 4:05 pm
First, rotation is continuous and in one direction in either type.
The idea is to get a conductor to move through a magnetic field, generating what is called "motional EMF" (1st ref.). For a practical machine you want many of these conductors connected in series and moving through a field so you wind a long wire into a coil of many turns, and you rotate it through a field so the motion can be continuous.
The main difference between AC and DC generators is that the rotation naturally produces an AC voltage, with the AC frequency equal to the rotation rate or a small-integer multiple of it. So getting AC from the generator rotor to an external load is relatively simple. See the 2nd ref.
To generate DC you have to find a means of rectifying the AC or reversing the connections from the rotating coil so the current to the load flows only one way. This is often done by a commutator but diodes are gaining popularity as a means to do the same thing with less dependence on the mechanical health of the moving parts. See the 3rd ref. (which is actually about a DC motor, but a motor has many points in common with a generator).
This description has assumed that there is always a rotating coil, and a rotating coil requires some way of making a current connection from it to the load. These connections are made with metal strips that are attached to the rotation shaft, and "brushes" that are stationary and maintain sliding contact with the strips. In an AC generator the contact strips are full circles called slip rings and ref. 2 shows their function. In a DC generator the strips are segments of a circle and the mechanism is called a commutator, which converts the AC current in the coil to a DC output current by synchronous coil-connection switching.
There are also generators in which a stationary coil generates the output while the rotor provides a moving magnetic field. In fact automotive alternators work this way. However I’ll end this answer here because it’s getting too involved in the many possible variations of motors and generators from the basic types, which deserve several more answers. (E.g., the 4th ref.)