GENERATORS AND ALTERNATORS
Route 1, Box 163, Lenaby, Minnesota 56651
I read the article, 'Building a Backyard Powerplant' by Carl Lathrop and I thought it was very interesting.
I have a bit of information pertaining to generators and alternators that those who are interested in powerplants might find interesting.
Have you ever wondered what causes the mechanical load on a generator? Even a big generator turns over really easy by hand, but bring it up to speed and start using electricity and you'll notice that the more current you use the harder the engine pulls.
This had me curious, so I set out to find why. In one experiment, I took a small heater blower motor from a junked car. First I connected it to a battery to see if it would run, which it did and by running, it would leave a residual magnetism in the field core. I next hooked it to an electric motor, and turned it in the opposite direction that it turned as a motor. The electric motor turned it like nothing, until I touched both of the heater motors wires together and it created such a load, the HP electric motor had a job running it.
That little experiment proved that it's the currents flow that produces the load. No current flowing in the generator, and no load on the motor.
I took the heater motor apart to see if I could find what causes that big load when it worked as a generator. I used a couple of wires, which I hooked to a battery, and used it as a prod. I touched it on the field windings and found that current flowing through it caused the core to turn into a strong magnet. Next I touched the prods on the commutator in the same position that the brushes contacted it. This turned the armature into a powerful electromagnet.
Now I was on the road to understanding what causes that load. It was magnetism. In order for a generator to produce current, the field windings have to be energized, which creates a magnetic field having a north and a south pole.
When the armature revolves, the field windings induce magnetism into the cores of the armature. Being wire is wound around the armatures cores, this moving magnetism through the core causes current to flow in the windings. This current being generated in the windings and flowing does what an electromagnet would do: its core would become magnetized.
Really it is simple what causes that load. With the field cores producing a magnetic field, and the armature producing a magnetic field of its own, that load is caused by pulling the armature away from the attracting field pole, then turning it towards the other pole, which would repel it. The generator producing current is like a magnetic coupling, as the outside housing would revolve too, if it is not tightly secured.
I read in a manual later on that this load on a generator is called 'Armature Reaction.' And due to this armature reaction, the brushes on a generator are turned in the same direction of rotation, to reduce sparking at the commutator. The more current that flows the more the brushes should be shifted in the direction of rotation, due to armature reaction.
An electric motor has to have the brushes shifted in the opposite direction of rotation to reduce sparking at the commutator.
To reduce the sparking at the brushes, some generators and motors have interpoles and compensating winding. This doesn't reduce the load any, it just reduces sparking.
With the aid of a manual on electricity, armature reaction and the mechanical load placed upon a generator, can be explained more thoroughly.
It will be noted with the armature rotating in a clockwise direction (Fig. 1) that the magnetic lines of force rotate around the wires in a clockwise direction on the left side, and a counter-clockwise direction on the right side.
On the left side, with the magnetic lines of force rotating in a clockwise direction around the wires, it will be noted that it produces a positive charge of electricity. This produces a south pole polarity on the left side of the armature.
Whereas on the right side of the armature, the magnetic lines of force rotate around the wires in a counterclockwise direction, thus producing a negative charge of electricity and a north pole polarity.
Looking at Fig. 2, it will be noted that from line A to B, there is going to be a repulsion felt in the movement of the armature, as alike poles repel, and opposite poles attract. The field magnets have to demagnetize the armatures core, and remagnetize it into an attracting polarity. In doing so, this moving magnetic field, induces or generates electricity to flow in the windings.
When this electricity flows in the armatures windings, this produces an electro-magnetic effect of its own, which so happens to be in the same polarity as to which the field magnets induced into the core, thus producing a strong magnet out of the armatures core.
The more current generated, the stronger these magnetic fields are, and the harder the load is placed upon whatever is driving it.
Also, the more current generated, the slower it is to leave the armature, due to the magnetic field in the cores. Thus the brushes are shifted in the same direction of rotation to compensate for this as a repelling field will speed things up at this point.
Being magnetism and electro-magnetism is very much used in generating electricity, it is important to look at an electromagnet and see what it does.
Any time current flows in a wire, it sets up a magnetic field around the wire. Should the wire be coiled up, this magnetic field adds up thus producing more lines of force.
Looking at the illustration, it will be noted an iron core inserted into a coil increases the inductance, as it provides a better path for magnetic lines of force than air. An iron core will collect and concentrate magnetic lines of force, thus it will produce a magnetic field about 280 times stronger than the air core.
The copper core has the opposite effect, as copper opposes lines of force more than air. The copper core will diminish the magnetic lines of force.
Magnetism is like electricity; both like to flow more on the outside of a conductor, than on the inside. Therefore, a laminated iron core will collect and concentrate magnetic lines of force more so than the solid iron one. Thus producing a stronger magnetic field.
Looking at the armature of an electric motor, or generator, you'll notice that the core is made out of laminated iron. This laminated core is used to reduce eddy currents, which exist in an alternating magnetic field. Eddy currents is current induced into the core from the alternating magnetic field created by the current flowing through the winding. Even though eddy currents have a small current value, it is wasted current, which turns into heat in the core. By using laminations which are thin plates insulated from one another, these eddy currents are reduced.
By having this knowledge of how the core material affects the inductance of a coil, it will be noted that this laminated iron core in the generator's armature will produce a very strong magnetic field, which adds to the load placed upon whatever is driving it.
Next I want to point out how this laminated iron core will place a resistance on the current flowing in the windings. A laminated iron core increases inductance, and an increase of inductance will increase resistance on the current flowing in the coil or winding.
I also want to point out that inductance holds back AC current more than DC current, as AC current drops to zero value and builds up and DC builds up and stays constant.
A DC generator produces AC current in the armature, and is converted to DC current at the commutator and brushes.
A coil of wire is used which is connected in series with a light bulb. When the circuit is energized from a 115-volt AC line, the lamp's brilliance is noted.
With the circuit still energized the iron core is carefully inserted. Note the decrease in the lamp's brilliance resulting from the increased inductance of the coil. A good part of the current is now dropped across the coil.
Next the copper core is carefully removed and the laminated core is inserted. The lamp's brilliance has dropped greatly, and is barely lit, as the laminated core has increased the inductance of the coil more than the solid iron one.
Pretty much the same results would have occurred if DC current would have been used instead of the AC. The resistance would not have been as great though, as DC current is of the same value, or in other words, more constant. With AC current dropping to zero, and building up, this building up current in the coil produces a moving magnetic field which generates an opposing current in the coil. This is called counter electromotive force, or sometimes called self-induction.
This self-induced current in a building up electromagnetic circuit, opposes or bucks the original current. DC current only experiences this upon building up, and the inductive circuit then only acts as a resistor. AC current will have this resistor effect the coil, plus the resistance from self-induction, or counter emf.
I hope this will be of help to those planning on building a power plant, or those who have antique power-plants. The next time you see a steam engine driving a generator, and a switch is closed and current flows, you'll know why the steam engine's governor opened up, and let out a few big chugs out the exhaust