Tuesday, July 15, 2014
Magnets, Motors and Machines
Most of us growing up as children played with magnets- and some of us as adults still do. Magnets carry a certain mystery, a certain romanticism and air of mysticism to them. We see how they interact in the world, with each other and with iron bearing metals and we wonder how. For hundreds of years we humans have played with magnets, yet science still has a fairly minimal grasp on the why of what makes magnets behave as they do. Today we have stronger magnets than ever before and the amount of flux, or magnetic force in these magnets make them many times more powerful and sometimes dangerous than we have ever known magnets could be. The magnetic flux intensity in a tiny commercial or industrial magnet today can be thousands of times stronger than in magnets from just 20 years ago.
A magnet is a metal or ceramic object that has a natural and continuous magnetic field around it. Any ordinary piece of iron containing metal can become a magnet though simply by winding that chunk of metal with insulated wire and passing electric current through the wire. This concept of the 'electromagnet' gives us wonderful machines and devices such as motors, generators, alternators, transformers, ignition coils and speakers that turn the electrical music signals from our MP3 player into vibrating air that we hear as sound. Its motors that I want to focus on for the next few paragraphs here...
It probably doesn't get much thought when you turn the key on your car, motorcycle UTV or garden tractor, but the first sound you hear when you turn that key is the whirring of the starter motor converting battery power into mechanical work spinning the engine to get it to fire up. A motor is an electrical device that uses electromagnets to turn electricity into rotational energy in the form of a spinning shaft. That spinning shaft can be used for all manner of work.
Inside a motor is a rotor (as in rotates) attached to the spinning shaft that looks like a starfish or asterisk when looked at from the end, and a stator (as in stationary) on the outside of that. Each of the arms of the rotor is wound with wire to make it an electromagnet when current flows through those windings. By turning the electric current on and off to the correct coils in a motor with exact timing, we can create rotational motion and alot of torque, or twisting force. As a rotor coil approaches a stator coil inside the motor, the current is turned on to it and it becomes a magnet attracted to the other coil causing the rotor to spin toward the pole or coil of the stator that is energized. As the rotor windings go past the stator coil, the electric current is turned off to the rotor and current is turned on to the next coil. This process of on and off happens many, often hundreds, of times per second.
A starter motor is a special type of electric motor called 'series wound' designed to use a massive amount of power to create a large driving torque to spin the engine. A small motor of this type can make as much torque as an engine that weighs ten times more. In a series wound motor, the current must flow through the rotor and then the stator and is switched in the middle which activates the fields simultaneously causing a very strong magnetic interaction. Series wound motors are inefficient but can turn huge loads. They require more electrical energy to operate than other designs of motor and usually cannot run for much time before overheating.
The stator coils of a series wound motor can be replaced with permanent magnets of a ceramic nature to produce a 'per-mag' motor that uses much less electrical power and make much less torque as well. These motors are better suited for small continuous duty applications such as fans, blowers and window crank or windshield wipers. There are many non-automotive uses for per-mag motors in home and industry.
There are other types of DC motor out there as well, series wound and per-mag are just some of the most common. Some characteristics all these motors share, regardless of design, are relative operating silence, smoothness, wide range of operating environments, and- most important to big loads- instant torque at 0 RPM. Almost any motor is much quieter than an engine of any type. A motor makes smooth power typically because there are odd-even interactions in a motor that cause vibration from magnetic pulsation to be broken up and not cause resonance in the motor itself. A motor can operate in extreme cold, or heat, and can be sealed to make it water-proof, dust-proof, or whatever else-proof it needs to be to tolerate most locations or applications. Most DC motors can make full torque at zero speed and will use a huge surge of current to do it- which can cook a motor if it is not allowed to spin up.
That massive torque surge at low or no RPM is what makes electric R/C cars and electric cars like the Tesla Model S or Nissan Leaf such fun to drive. Not having to wait for RPM to build up into the powerband to attain full power is usually a strange feeling for most noobs to electric vehicles. Having raced electric R/C cars and trucks as a teen as well as built a couple of electric motorcycles as proofs of concept, I know how electric motors can be quite the rush if setup properly. The power of electric motors is not new in vehicles. Over 100 years ago many cars were silent, reliable electric designs. Due to poor battery tech though, a meager range, usually less than ten miles, alot of electric cars in those days were dragged back by horse to recharge the batteries.
Motors play significant and crucial roles in our reality. The simplicity of their operation is based on the principle that magnets can attract or repel at a distance and that a magnet can be created by passing electric current through a wire. The electromagnet and permanent magnet can work together as a hybrid system but so long as one magnet can be turned off or even reversed many times per second and with exact timing, continuously rotating, mechanical energy can be extracted and used for all sorts of applications. Tune back in next time for more helpful info, until then, take care of them so they can take care of you...
To healthy, reliable machines...