K
Kludge
Guest
As promised, here is the beginning of the thread on DC motors. I have no clue what direction this will go since that will depend an awful lot on questions and problems along the way. There is no possible way I can claim to be any form of expert; I just like them and far prefer them over AC motors. Actually, there is one sort of AC motor I like but it's used and controlled like a DC motor which just serves to confuse issues. I'll get to it another time.
In describing a DC motor, I'm ignoring a few trivial cases which would not be found in a shop other than as toys or experiments. I may hit a few of these later just for fun.
A present day DC motor has two major parts, the field magnet and the armature. Smaller ones use fixed magnets for the field while larger ones use electromagnets. Applying voltage to the armature of either type causes the windings to form mini-electromagnets on the armature pole pieces (those things that stick out that look a little like headsman's axe heads viewed end on) which are either attracted to or repelled by the field magnets. This causes the armature to turn.
These motors vary in size from the motors that operate the vibrators in pagers on up to locomotive sized and in complexity from a single permanent field magnet & three armature poles (Two in the case of the St. Louis motor which I'll describe at the end of this message) on up to multiple field electromagnet windings and armature poles. Speed and torque vary directly with field strength (ie: the strength of the permanent magnets or the field current) and armature voltage. I won't get into the math (Sorry, Marv), mostly because it's not really needed in everyday use and I've forgotten a lot of it myself through disuse. Actually, the only times I did use it was when I was modifying motors for my own evil purposes.
Among other places, DC motors are found in cordless tools, electric golf carts, wheel chairs, engine starters, pager vibrators and a great number of toys the packages of which are marked "Batteries not included."
A special case of the DC motor is the universal, AC/DC or series wound motor. They're all just different names for the same thing, a DC motor that has its field and armature wired in series to allow it to run from AC. Sewing machines, most (if not all) corded electric hand tools and virtually all corded kitchen appliances use these motors because they're cheap and easy to manufacture. They are operated from AC normally but they are much happier as DC motors The design dates from when there was still some question as to how the national power grids were going to go, and some areas used AC while others used DC. (Long historical note deleted due to its being boring.) The appliance manufacturers had to find a way to sell to everyone so the universal motor was born.
One of the features, which also is one of the problems, with the universal motor is the interdependence of the field current and armature voltage. Vary one and the other will go right with it. This is great as an Ohm's Law demonstration but lousy for practical use especially at reduced input. On the other hand, the current increases under load as with most motors which helps bump the field up some for increased strength.
Speed control for a DC motor can be handled via a simple rheostat or with what's called a Pulse Width Modulator (PWM). A rheostat controls the actual voltage reaching the motor and wastes a lot of energy in the form of heat. A PWM sends varying width full voltage pulses to the motor with a great reduction in wasted energy and, since the pulses are at full voltage, a greater control over the motor, especially at lower speeds. The pulses come at a fixed rate determined mostly by the design of the controller and range from very narrow for slow speeds on up to always on for top speed.
Even with a PWM, there are several factors that determine the actual motor speed however some of these can be overcome through some rather simple (Okay, maybe not so simple) methods. These will come in a bit.
I mentioned the St. Louis motor above. This type of motor is often used in varying degrees of complexity for experiments in motors and generators due to their almost irreducable minimum count of elements. There are two field poles and two armature poles with a commutator that allows a variety of configurations. The larger versions have field coils instead of field magnets to allow an expansion of the experiments that can be done with them. I ran into and got to play with one of these a great number of years ago which was quite an education.
Now if only I could remember what I learned ... ???
Best regards,
Kludge
In describing a DC motor, I'm ignoring a few trivial cases which would not be found in a shop other than as toys or experiments. I may hit a few of these later just for fun.
A present day DC motor has two major parts, the field magnet and the armature. Smaller ones use fixed magnets for the field while larger ones use electromagnets. Applying voltage to the armature of either type causes the windings to form mini-electromagnets on the armature pole pieces (those things that stick out that look a little like headsman's axe heads viewed end on) which are either attracted to or repelled by the field magnets. This causes the armature to turn.
These motors vary in size from the motors that operate the vibrators in pagers on up to locomotive sized and in complexity from a single permanent field magnet & three armature poles (Two in the case of the St. Louis motor which I'll describe at the end of this message) on up to multiple field electromagnet windings and armature poles. Speed and torque vary directly with field strength (ie: the strength of the permanent magnets or the field current) and armature voltage. I won't get into the math (Sorry, Marv), mostly because it's not really needed in everyday use and I've forgotten a lot of it myself through disuse. Actually, the only times I did use it was when I was modifying motors for my own evil purposes.
Among other places, DC motors are found in cordless tools, electric golf carts, wheel chairs, engine starters, pager vibrators and a great number of toys the packages of which are marked "Batteries not included."
A special case of the DC motor is the universal, AC/DC or series wound motor. They're all just different names for the same thing, a DC motor that has its field and armature wired in series to allow it to run from AC. Sewing machines, most (if not all) corded electric hand tools and virtually all corded kitchen appliances use these motors because they're cheap and easy to manufacture. They are operated from AC normally but they are much happier as DC motors The design dates from when there was still some question as to how the national power grids were going to go, and some areas used AC while others used DC. (Long historical note deleted due to its being boring.) The appliance manufacturers had to find a way to sell to everyone so the universal motor was born.
One of the features, which also is one of the problems, with the universal motor is the interdependence of the field current and armature voltage. Vary one and the other will go right with it. This is great as an Ohm's Law demonstration but lousy for practical use especially at reduced input. On the other hand, the current increases under load as with most motors which helps bump the field up some for increased strength.
Speed control for a DC motor can be handled via a simple rheostat or with what's called a Pulse Width Modulator (PWM). A rheostat controls the actual voltage reaching the motor and wastes a lot of energy in the form of heat. A PWM sends varying width full voltage pulses to the motor with a great reduction in wasted energy and, since the pulses are at full voltage, a greater control over the motor, especially at lower speeds. The pulses come at a fixed rate determined mostly by the design of the controller and range from very narrow for slow speeds on up to always on for top speed.
Even with a PWM, there are several factors that determine the actual motor speed however some of these can be overcome through some rather simple (Okay, maybe not so simple) methods. These will come in a bit.
I mentioned the St. Louis motor above. This type of motor is often used in varying degrees of complexity for experiments in motors and generators due to their almost irreducable minimum count of elements. There are two field poles and two armature poles with a commutator that allows a variety of configurations. The larger versions have field coils instead of field magnets to allow an expansion of the experiments that can be done with them. I ran into and got to play with one of these a great number of years ago which was quite an education.
Now if only I could remember what I learned ... ???
Best regards,
Kludge
