Date: Sun, 21 Apr 1996 11:44:52 -0700
From: email@example.com (Evan Soule)
Subject: COMMUTATORS FOR NEWMAN MOTOR/GENERATORS - AN OVERVIEW
From: firstname.lastname@example.org (Ralph Hartwell)
Organization: The Energy Machine Information System 504-733-8380
COMMUTATORS FOR NEWMAN MOTORS - AN OVERVIEW
(C)opyrighted 1991 by
Ralph M. Hartwell
715 Jefferson Heights Avenue
Jefferson, Louisiana 70121-1110
This discussion considers only the standard circular rotating drum-type commutator, and not Mr. Newman's reciprocating commutator (nor the recent magnetic rotor design).
OVERVIEW OF THE PROBLEM -
Commutators have been used in electric motors and generators for many years, and their theory and operation had become a matter of textbook theory and well established commercial and industrial practice. The invention of the Newman motor has caused designers and engineers to take another look at the design of the previously well-understood commutator. Experimentors have quickly discovered that attempting to use a standard commutator with a Newman motor was a failure. Something new and obviously different was required.
Perhaps the best way to explain the difference between a standard commutator and what I shall call the "Newman" commutator is that unlike the conventional commutator which is designed to prevent brush arcing if at all possible, the Newman commutator must allow a spark to occur after each conducting segment is passed by the brush.
A rotating commutator has one simple function - to regularly interrupt the flow of electric current passing through it. A secondary function is to prevent its own destruction by overheating usually caused by friction and electrical arcing, and destruction of the commutator surface, caused by mechanical wear and pitting caused by the effects of the arcing between the commutator conducting segments and the brushes.
These destructive effects are aggravated in a Newman commutator due to the fact that the Newman commutator must be designed to allow an arc to occur after the brushes pass each conducting segment in the commutator. This is necessary because in Mr. Newman's theory, the generated power is released when the current flowing through the coils of his motor is interrupted in a very rapid manner. This means the current must be abruptly switched off by a rapid and complete breaking of the circuit. So far, no one has developed a solid-state switching circuit capable of handleing the extremely high voltages generated by his machines - even the smaller machines generate voltages in the multi-kilovolt region.
A standard rotating commutator used with a conventional motor or generator consists of conductive segments spaced regularly around the outer surface of a drum or wheel. Between each of these conducting segments are insulating segments to prevent the conducting segments from shorting out to each other. The spacing of the conducting segments is quite close; usually the ratio of widths of the conducting to insulating segments is about 20:1.
The width of the brushes which ride against the commutator varies from about 50% of the width of one conducting segment to almost twice the width of a conducting segment. The exact width used will depend on the design of the particular motor or generator, and is influenced by such things as the number of turns per coil, the number of coils in the armature, the operating voltage of the motor or generator, and the horsepower the device must handle.
STANDARD COMMUTATORS -
A conventional design commutator is specifically designed to prevent any brush arcing, or to at least reduce it to a minimum. It accomplishes this by insuring that the brush is always in contact with at least one commutator segment, and is never completely disconnected from a segment so as to cause an open circuit to occur. This means that before the brush disconnects from the segment it is leaving, the brush will connect to the next segment, shorting the two segments together momentarily.
Since the adjacent segments of the commutator are at slightly different voltages, this temporarily short circuit will result is a small spark between the commutator segment and the brush at the time fo connection or disconnection by the brush. In addition, the short circuiting of adjacent segments results if the flow of wasted power through the coil that is connected between the two segments which are short circuted. The lower the voltage in the coil, the less the arcing.
The more segments the commutator has (and by definition, the more coils the motor has), the lower the voltage, and the less the arcing. This is the reason that better quality motors will have a greater number of commutator segments than a cheaper motor. Motors designed to operate on high voltage will also require a larger number of commutator segments than low voltage motors.
NEWMAN COMMUTATORS -
By contrast, a Newman commutator typically is connected to the single coil in the Newman motor, but the commutator has the job of interrupting the current flow in this single coil many times per revolution of the armature. In addition, the commutator must reverse the polarity of the applied voltage twice during each revolution of the armature.
The interruption of the current flowing in the Newman motor by the commutator is of vital importance to the proper operatin of the motor, and it is necessary to understand Newman's theory to be able to design a successful commutator.
For purposes of commutator design, a Newman motor may be considered as a huge inductor in series with a pulse generator. When the commutator connects the input power supply to the motor coil, current starts flowing through the coil, and this current gradually increases to the final steady-state value as determined by inductor theory. A portion of the time required for the current to reach the maximum level according to Ohm's law through the inductor is considered by Newman to be the "charging" time of the motor coil. This charging time is rather short when compared to the time required for the curent to reach steady- state value, typically 10-30% of the total time to reach steady- state current levels.
DESIGN OF A NEWMAN MOTOR COMMUTATOR -
There are two slip rings mounted side by side with the segmented commutator. Each slip ring is connected to half of the segments on the commutator assembly, but rather than the usual arrangement of being connected to alternate segments, in the Newman design, all the segments in a 180 degree sector are connected to one slip ring, and the other slip ring is connected to the segments of the commutator.
The connections to the slip rings go to the battery or other power supply, and the brushes running on the segmented commutator go to the coil in the motor. This allows the commutator / slip ring assembly to reverse the polarity of the voltage applied to the coil every 180 degrees as the rotating magnet on the armature reverses its direction. If this polarity reversal is not accomplished properly, the motor will not run correctly, or will refuse to run at all.
The normal polarity reversal position is about 10-15 degrees past "top-dead-center", or the position where the magnet is parallel to the axis of the coil. Note that the position of the reversal is adjustable by changing the angle between the magnet and the commutator polarity reversal position. The most effecient operation is not in the position where the motor runs the fastest, but rather slightly before that position, or "advanced" in timing if this were an automoblie engine. This requires a bit of "playing" with to get it just right - or even close to right!
Commutator design for a Newman motor is basically as follows:
1) Determine the time required for the current to rise to the maximum value through the coil in the motor. This may be determined by commecting the motor coil to a battery with a small resistor in series with the motor. Using an oscilloscope, measure the time for the current to rise to about 90% of the maximum value.
2) Decide the speed in RPM at which you are going to have the motor operate. Remember it is unwise to have the machine run too fast, especially if the armature is large or poorly constructed. The centrifugal force generated during operation can easily destroy the machine - and you, too, if you happen to be in the way when it flys apart!
3) Calculate the time required for the motor armature to rotate one time at the RPM you have chosen.
4) Calculate the time for the current to rise to about 20% of the maximum value as determined in step 1.
5) Divide the time per revolution by the time required for the coil current to rise to 20% as determined in step 4. This should be at least 10, and less than 20. Discard the fractional part of this number, and call the whole number part N.
6) Draw a circle the size your commutator is going to be when you build it. How large should it be? I suggest as a first approximation, make the segmented commutator wheel at least 4" in diameter; smaller sizes are hard to work with unless you have good construction facilities, and smaller sizes are more prone to arc overwhen in operation. Note, too, that high voltage machines need larger diameter commutators so the insulating segments may be bigger to prevent flashover when in operation. The slip rings may be any convenient size.
7) Divide the circumfrence of the circle into as many segments as your calculated value N. The length of one of these segments is the length that ach conducting segment on the commutator must be to allow the current to rise to the value you calculated in step 1 when the motor is running at the speed you selected for its' operation.
8) Obviously, you cannot have the whole commutator periphery filled with conducting segments, which is what would happen is you placed as many segments on the commutator as you calculated would fit there in step 4.
To solve this problem, remove one conducting segment and divide the space thus gained between the remaining segments. You must have an even number of conducting segments remaining, because half of them go on one 180 degree segment, and the rest of them go on the other half of the commutator. If you do not have an even number of segments, remove another one and adjust the spacing again.
9) When you are finished, you should have an insulating (non- conductive) gap between each conductive segment which is between 1/8 to 1/2 the lenght of one of the conducting segments. The exact length will vary widely depending on such variables as the diameter of the commutator and the speed of rotation. The general rule is; the faster the rotation or the higher the operating voltage, the wider the insulating gap must be to prevent flashover when in operation.
OPERATIONAL CONSIDERATIONS -
The insulating segments should be made of a heat resistant substance, such as glass or ceramic. The material should not break down or burn when exposed to the heat of the arc which will occur during operation, nor should it be eroded by the action of the arc.
The insulating segments should be wide enough to prevent "flashover", which is the tendency for the arc to follow the brush from one conducting segment to he next segment, resulting in a complete path for the battery current through the ionized gasses in the arc. If flashover occurs consistantly when the machine is in operation, this is an indication that the insulating segments are too small, and that the commutator* is probably too small in diameter. Try again!! Back to the old drawing board...
Copyright 1991-1996, Ralph M. Hartwell, II
*The latest commutator design enables higher voltages to be utilized. Note: The above article was written several years ago. The principles described above are generally applicable "across the breadth of the technology." However, considerable improvements to the commutator design have been made in the recent past. These improvements are intended to actually reduce the intensity of the sparking by distributing the physical connections over a wider area. The reader should bear in mind that there are TWO totally different design systems (but many sub-configurations within each basic design): there is one commutator design when the energy machine is intended to function as a GENERATOR and a totally different commutator design when the energy machine is intended to function as a MOTOR. The latest design improvements to the commutator system apply to the machine operating as a MOTOR. Subsequent torque can be utilized for mechanical systems or can be used in conjunction with a conventional generator.