Motors and Generators
These are DC gear motors I bought for about $8 apiece on the surplus market and are no longer available from my source. I am experimenting with other motors to compare their behavior with these, which seem to be working quite nicely. The gear motors came from All Electronics. The aluminum crank handles came from Reid Supply, they arrived with solid hubs and I drilled the ¼” blind holes in them. I then used two-ton epoxy to fasten the handles on the shafts. A vice or a clamp is helpful to encourage the shafts to mush all the way down into the epoxy. The motors came with four wires, two of which went to either a tachometer or a clutch of some sort. I cut the extra wires and soldered them together with the motor wires to extend the wires’ reach. Shrink tubing helps support the connections. Heavy alligator clips from Radio Shack completed the assembly. If I were to rebuild these, I’d give each motor at least two feet of wire and stagger the clips so they would be less prone to shorting themselves. Clips with plastic caps on the handles are a luxury to be considered. Avoid hooded clips, they will restrict your possibilities for connections.
At the motor station, I typically ask a student to ‘crank me out some electricity’ on a motor that is not attached to anything. Questioned, she will report that it is easy. I then short circuit the motor by touching the clips together. Suddenly the cranking is not so easy any more. I explain that now we have a circuit, and electricity (electrons) is (are) flowing through the wires. When there is no circuit, we are not moving electrons, not doing work, and indeed we all know that it is easier not to do work than it is to do work.
Next, I connect two motors together and let the student explore what this does. Often I find this is too early to try to explain that a motor and a generator are interchangeable depending on where the energy comes from and how it is being changed. A motor takes in electrical energy and changes it into motion; a generator takes mechanical motion and changes it into electricity. I also show them that if a student cranks a generator and I grab the spinning handle on a motor, the person cranking can feel me holding the motor. This is why we pay for electricity, and should give us renewed respect for batteries and power plants. At this point I step back and let them experiment.
Students will connect motors in series circuits and in parallel circuits, they will discover on their own that they can get shocks, and they will find that the motors can ‘break dance.’
I try to show them that no matter how fast they crank one motor, they can’t get another to go that fast. That would defy the Second Law of Thermodynamics; entropy always increases. (Friction being as relentless as death and taxes.) If they have a series circuit, though, they can add their voltages together to increase the speed of a third or fourth motor. This does not work in a parallel circuit with each of the motors connected to each of two common nodes. In the parallel circuit, the voltages don’t add, although I think the currents do, suggesting that the driven motor(s) might have more torque. Voltage governs speed in these motors.
Students typically don’t realize they can make the motors arm wrestle, and that is fun to show them, taking this opportunity to show them that switching wires can change the direction of a motor’s rotation. They will also discover that there are differences of effect depending on which way they crank and whether they have parallel or series circuits.
By now the time is usually used up, but if students are motivated, we explore series-parallel arrangements. Connect ‘generators’ in series to a set of parallel ‘motors’ for best effect. Interested students will discover that they only get shocks if they are touching the metal while they are disconnecting clips and someone is cranking. Show them that if a series circuit is broken anywhere else, they don’t get a shock. Show them that the shock is current and not static electricity. The shock doesn’t happen when we are just holding two clips. Connecting them together diverts any current through the wires, but disconnecting them results in a sudden mild pulse of electricity.
Inside each motor is a coil of copper wire. When current flows through this coil, it sets up a magnetic field. When the current stops as, for instance, the circuit is broken, this magnetic field collapses, sending a sudden but short-lived spike of electricity through the wire.
The hand generators are an experiment, and I see them as being in destructive testing. I am amazed at how well they have lasted, the only repairs being two handles needing reattachment to their shafts. The clip leads are holding up well, the gear boxes seem to be doing well, and after numerous drops to the floor from table height and plenty of time break dancing, the motors seem to be durable. I feel very lucky to have found these particular motors at a great price, the plastic cases are a plus as they bang around on tables.
