The properties of motors should be thoroughly understood to avoid some chronic misconceptions. Before deciding to remotor, it is wise to know what you have and what improvements are available for replacement. A thorough ANALYSIS OF MOTOR PERFORMANCE is required to avoid disappointment and wasted time, effort and money. Each motor has its pros and cons, dependent on application. Is it worth performing major surgery on a loco, destroying the original mount, only to find nothing was gained or that things are worse?
A motor is only one integral part of the power train in which all parts must match. A thorough REPOWERING ANALYSIS should be done before deciding to remotor. Plan carefully, avoiding hype. In the field of remotoring, there are many self appointed gurus, who will readily preach the wonders of brand or type X or the evils of Y, without any substantiation. Compare some repowering alternatives to get a better feel for various methods. See EXAMPLES .
Only common types of DC permanent magnet motors will be discussed. These motor types contain two fixed permanent magnetic poles (stator) with a rotating armature (rotor) between them. Rotation is developed from the interaction of the stator field with a rotor field caused by current flowing through the armature windings wrapped around the poles. These are essentially electromagnets, whose fields are rotated, so externally the effective field is quasi stationary. Brushes and segment plates on the commutator act as switches to accomplish this feat during rotation. Practically all permag rotors have an odd number of poles to prevent lockup at startup.
Polarities are defined as N (north seeking) and S (south seeking) from compass use. A basic pole law states: like repel and opposite attract. The upper S rotor pole is repelled from the left-hand S stator and attracted toward the N stator pole and vice versa on the lower N pole, producing a clockwise rotation. The right hand rotor pole has no current and thus no flux or polarity. During rotation this condition exists for a very short time while the respective brush spans two commutator segments. Just prior the polarity was S, while just after it will be N.
Magnetic fields are closed loops and motor types differ mainly in how the stator flux is routed around the loop from the source magnet(s) through the armature and back. Open frame types usually have visible magnets, armatures, brushes and commutators.
Motors are available in a very wide variety of shapes, sizes and design configurations. While many are application specific, others are available for general use. Being acquainted with the general types, will yield a better chance in selection. Many variations of the general types below exist, but analysis can usually narrow typing.
One single shaft type has a BAR MAGNET MOUNTED AT ONE END _ of the shaft with a permeable steel pole piece attached to each bar end. Running parallel to the shaft, they are partially wrapped around the armature to complete the loop.
A second type, often double shafted, has a bar MAGNET MOUNTED ALONG SIDE _the armature with the pole pieces crosswise to the shaft.
Careful selection of material from the ALNICO (ALuminum-NIckel-CObalt steel) alloy series is crucial to obtain the maximum field strength for the bar dimensions. These motors are always magnetized after assembly for maximum magnetic retentivity.
CAUTION: Do not remove
the armature from these motors; as this interrupts the flux loop,
drastically reducing field strength. If removal is imperative, a permeable
keeper such as an iron clamp must be placed across the poles to provide an
alternate flux path. Placing AC across the motor will accomplish a far
better demagnetization. Even quick reversal at higher RPM can deteriorate
field strength. Symptoms of deterioration are increased RPM, decreased
torque and over heating due to increased current draw for reasons explained
below. Some manufacturers will remagnetize for a nominal fee.
DANGER: Do not attempt to
remagnetize a motor with a loop of wire placed across a car battery's
terminals. The wire may fuse (melt) immediately, causing severe damage and
injury at a cost many times that of a new motor.
In CAN MOTORS _ there are two flat curved ALNICO, ceramic or ferrite magnets fastened to the inside of the shell, partially wrapped around the armature. The flux loop is completed around the shell and the metal end, if there is one.
A sort of hybrid SQUARE TYPE OPEN FRAME _ uses the can magnet locations but the flux path is completed through pole pieces and metal sheets at the ends of the armature.
CORELESS motors have been around since about the 60's, but only with the development of newer magnets have sizes and shapes been made, which are suitable for locos. Many are designed for stepper or servo positioning uses. The most useful ones are derived from designs for miniaturized battery operated devices such as camera zoom and focus drives. Externally many resemble can motors, but some are flattened with large diameters like pancake motors. Built on an armature design similar to squirrel cage motors, brushless versions are not very adaptable for loco use.
To eliminate iron core losses due to eddy currents and hysteresis, the metal cores have been eliminated. Skewed windings are supported by plastics of various types. The net results are faster acceleration, more torque and power, less current draw, higher efficiency and smaller size. On the negative side they are very susceptible to heat, since the heat sinking metal cores are absent. Most are continuously rated at about 80 ° C. with a few a 100. With poor thermal paths and a reduction of inductance, they are readily overheated by rapid current changes in any sharp rise or fall times used in pulsed power. Pulse width modulation (PWM), commonly used in DCC, is particularly bad. Rumor has it that some DCC manufacturers have solved the problem by sensing coreless motors and reconfiguring modules. But no references have been found in ads. Very expensive motor versions have internal capacitor rings to reduce the effects. Caution is advised to avoid overheating and softening or melting plastic armature.
Essentially all types of permanent magnet motors operate in a similar manner with varying differences in characteristics.
Examination of two fundamental motor equations will enable an understanding of these differences. First:
Usually less than one, the k in both equations are different constants, known only to the designer, lumping together all the hidden factors such as flux and copper losses, friction etc. Of little concern at the moment, they affect the final quality and characteristics of the motor. Increasing anything on the right side that is multiplied increases the left side. Increasing anything that is divided decreases the left side. In measurement testing, applied voltage is held constant, but as can be seen while operating a throttle and watching a voltmeter, speed increases with voltage. RPM are reduced if flux through the armature is increased by using a stronger magnet or reducing loop losses.
The voltage and RPM appear to be directly proportional, but this is not true at all points. Since static friction is normally greater than dynamic, it must be overcome before an unloaded motor turns at some minimum voltage. Once past this point friction drops. If plotted there is a hump in the lower part of the voltage line. Thus the slowest running voltage will be lower than the starting voltage. Even beyond this point, the line may curve. This is of no consequence in operation, since loco or train loads will mask it with higher friction. The amplitude of the hump increases with load as does the starting to minimum running ratio. Contrary to published data, to determine the speed to voltage ratio the minimum running speed should be used, since only the section above this is quasi linear. With all the other variables, deadhead loco values are not a true indication of performance under train loads.
Can motors are practical using much weaker ceramic magnets, since losses are lower resulting from a smaller armature-stator gap and a shorter flux path through the shell. Open frames have larger gaps to cut alignment costs, while the pole pieces are relatively poor paths because of their lengths.
Pole here refers to the number of rotor poles or armature segments, always odd to eliminate lock-up positions thus allowing motors to be self starting. Most motors are 5 pole with 3 pole common in N and some low cost HO imports and 7 pole only in the larger scales. A 5 pole motor should rotate at 3/5 or 60% the RPM of a 3 with the other parameters constant. More poles produce lower RPM, reduced cogging or jerking and less possibility of stalling at low motor RPM.
A more recent improvement, skewing of the rotor pole pieces at an angle to the shaft, reduces the effective distance between adjacent poles and thus cogging. Available on most newer Japanese can and NWSL square motors, Bowser recently released a SKEWED DC-71 _ open frame version with new magnet material, for upgrading many older locos. They should be encouraged to convert their other motors.
Note angle between magnet edge and winding slot.
The effects, only evident at very low RPM, may not be noticed when using a power pack with good low end pulses or a fly wheel. Increasing the gear ratio permits higher motor and flywheel RPM at all speeds. Thus at low loco speeds, cogging is reduced, along with the necessity for skewing. However, using all these, some very smooth low speeds can be achieved.
Last, increasing the number of turns in the windings reduces the RPM. Smaller motors, with less volume available, usually run faster.
Presented RPM ratings are often very misleading. Some do not specify the point of measurement, while others state: no load, maximum power or maximum efficiency values. The max power occurs at approximately one half and efficiency from about .6 to .9 no load RPM . The one we really need to know: RPM under layout conditions can be derived from SPEED MEASUREMENTS or indirectly from operating current, using GRAPHICAL METHODS. Measure with a nominal train.
The second equation interacts with the first:
TORQUE is the force exerted by a motor at a unit distance from the shaft center. This eventually translates into tractive effort at the rail. Higher torque yields less slowing from load increase. When the number of turns or the flux increase torque also increases, while in the first equation they decrease RPM. This explains why a demagnetized motor has a very high RPM and very little torque.
As the load increases on a loco by adding cars or ascending a grade so does current draw to increase the torque until it matches the load. A hidden factor explains why current increases under load. All motors act as generators while turning, producing an RPM dependent voltage of opposite polarity to that applied. This is commonly and erroneously referred to as counter or back EMF (electro-motive force). The difference between the applied and the counter voltage is called the effective voltage which produces the current. When RPM drops due to load, counter voltage also drops, increasing the effective voltage, causing more current to flow. This increases torque until it equals the load and RPM no longer decreases. Any analysis of armature rewinding is far beyond the scope of this discussion.
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In normal testing, a well regulated power source is used with applied voltage held constant, independent of the load. GRAPHS are generally drawn to present the parameters in perspective. Usually measured at rated voltage (12 volts), stall torque is a maximum value developed by a maximum stall current when a motor stops under full load. With no rotation there is no counter voltage, resulting in a current produced by the full applied voltage. The total effective resistance of a motor, in ohms, can be found by dividing this amperage into the voltage. Stall torque, at starting voltage, is translated into starting tractive effort at the rails.
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No load RPM is the next necessary parameter often seen. However actual measurements can vary drastically depending on lubrication, brush condition and contact force; and bearing alignment. This also holds true for no load current.
In an ideal motor, power and operation at any point can be derived graphically or mathematically from these four parameters.
See PHYSICS for a discussion of the terms and relationships used.
POWER is another parameter bandied about, but what is it? Simply it is the amount of work performed per unit of time and work is a force exerted through a distance. More power means a loco can pull more cars at a given speed. The motor torque exerts a force at the pitch radius of the drive gear, which effectively moves around the the circle once per revolution doing work.
Since torque = r * f and distance traveled per minute = 2¶ * r * RPM
This yields:
where Po is power output and k depends on the units used to measure torque and power.
In the USA motors are rated in horse-power and those considered here are in the area of one thousandth (0.001) horse-power, which I prefer to call 1 mouse-power (MP) eliminating those confusing zeros.
The typical range is from about .5 mp for N scale to about 94 mp for large scale Pittmans. More commonly in the metric system, power is in watts. Ratings can be maximum or continuous without over heating. Since maximum power may occur beyond the maximum continuous current rating, they are not necessarily the same.
The measurements required for determination are very tedious, since a wide range of current, torque and RPM must be plotted to find the max. A good estimate can be made by using half the no load RPM or half the stall torque since in theory everything coincides. In practice they are usually fairly close. At best, advertised values are only a rough guide to selection for performance, since the comparison may be of apples to oranges.
Mouse power can be converted to watts:
1 MP = 0.7457 watt or 1 watt = 1.341 MP.
1 MP ~= 3/4 watt or 1 watt ~= 4/3 MP within 1%
This introduces a discussion of efficiency, the area where most of the misconceptions about motors arise. It is simply the power out divided by the power in, expressed as a percent and varies throughout the range of operation. The input power can be found easily by reading a voltmeter and an ammeter.
Since almost all meters used are DC, measuring effective values, comparisons are valid within the accuracies of the meters.
Advertised efficiencies are usually the maximum value found and are not usually at the same speed or torque as maximum power. Values range from as low as 10% to as high as 80% for some "precision" motors. Efficiency is lowered by bearing and brush friction, copper losses causing heat and flux path losses. Can motors are in general more efficient than open frame because they use smaller commutators with less brush pressure, have less flux losses and less copper losses (I^2*R) by drawing less current. The efficiency of any motor can be improved simply by proper lubrication of the bearings and a small drop of Labelle 101 on the commutator. A tuned ear can soon detect a definite rise in pitch indicating an increase in RPM, while an ammeter will show the accompanying drop in current.
Caution: Labelle 101 is not
compatible with all plastics and may soften or even dissolve some.
How important is higher efficiency with its resultant lower current? Not very, unless a loco is entered in one of those efficiency contests, which unfortunately do not even approach practical layout conditions. In tests an HO steam loco using an open frame motor drew about 0.6 amp pulling 30 cars while the same make loco with a roughly equivalent can motor drew about 0.35 amp. under the same conditions. Double heading with a heavier train would yield 1.2 amp and 0.7 amp respectively. Now enters the controversy. In several lines of power packs the difference in price between approximately 1.25 amp packs and approximately 2.25 amp packs is from ten to fifteen dollars while the cost of replacing each motor exceeds twenty dollars. Choosing larger power packs is far more economical and easier than replacing motors solely for efficiency.
Of far greater importance is the efficiency of the gear train. The losses here can far outweigh any gains in motor efficiency. Not only does a poor gear train require more current, it reduces both speed and torque delivered to drivers, resulting in less total output power and efficiency .
Working with hundreds of locos, only one case arose where motor replacement for efficiency paid off. A two unit brass PRR BP-60 used four old, Japanese open frame motors. Each unit drew 2 amp running light, making it impossible to run the pair on any normal pack. After replacement with can motors, the total current for the pair was about 0.9 amp with a bonus of a lower speed.
The design of motors is a combination of art and science. Many factors are not readily obvious. Spacing and permeability of parts in the magnetic loop are critical to decrease losses. The relationship of wire size and number turns is a compromise. Field strength is not only determined by magnetic material, but also by the physical shape required. There are heat losses due to eddy currents produced by the changing magnetic fields, requiring lamination. The magnetic circuits must be evaluated thoroughly including potential hysteresis loop effects. In addition to these, many other factors influence not only the physical parameters, but those of the elctro-magnetic circuits and performance as well. Due to these properties, all applicable motors are very heat susceptible to rapid current changes from pulses. Increased applied voltage amplitude in conjunction with higher pulse rates, increase residual heat rapidly.
During the 1930's, universal AC/DC openframe motors were common. Compact openframe, DC permag motors, specifically designed for modelrailroad use, were introduced for the emerging HO. Voltages varied until the NMRA set the standard 12V DC. By the late 40's, Mantua, Varney, Bowser, Lindsey and Pittman offered a variety. The Pittman line soon became the standard for comparison. With the introduction of Japanese brass locos, came inferior copies and variations of some of these.
A new process permitted Alnico magnets to be formed into short curved plates, which led to the development of small can motors. Then less expensive ceramic magnets replaced them at the expense of less power in the same physical size. None of these were specifically designed for MR use, but were selectively chosen. The standard of comprison remained the Pittman line. Although these are still the same basic design, a wider market reduced costs to permit tighter tolerances in magnetic gaps, resulting in higher efficiency and lower currents.
The first major change in armatures, came with the use of skewed poles as used on larger motor. These reduce cogging at low motor RPM, which may only be noticed with low gear ratios.
A more recent development is the use of neodymium (NdFeB) magnets, which increases the magnetic field strength and thus all the parameters.
Since a current trend is to use the less expensive automotive accessory can motors, there is no assurance that the poles are skewed or NdFeB is used.
In an existing motor only two of the variables can be changed relatively easily: the number of turns and the flux. With the introduction of neodymium (NdFeB) magnets, some open frame motor replacements may be made, improving them noticeably. These might be used to create a SUPER MOTOR .
As a humorous aside, presented is a risque memory gem used while teaching hysteresis effects in saturable reactors used in magnetic amplifiers. A virgin core material is one that has never been fluxed. Eventually when it has had enough fluxing, it becomes saturated, making more fluxing extremely difficult. But due to retentivity, it is reluctant to give up the residuals of fluxing and must be forced to. It will also accept as much fluxing with positions (polarities) reversed. And finally, once it has been fluxed, it will never be the same.
Before embarking on any repowering project and to avoid wasting your time and money, it would be wise to know what you have, what needs to be changed, why and how. A thorough ANALYSIS should be done. Often the problem lies outside the motor and no new motor will cure it. The optimum goal should be the proper integration of motor, gear train, weight and power supply.
There are many criteria that eliminate potential replacements, some of which are physical. First, can it be fit into the volume without major hacking? Very often a slower motor is larger. Comparable can motors will not fit between steam loco drivers as openframes do. Next, can it be mounted easily to provide connection to gear train? Replacing a slant mount (DC-71 type) with a flat mount will require idler or step gears to adjust spacing. At least new mounting bracket(s) may be needed. Finally, can a desired flywheel be mounted? These will narrow the field considerably.
The all important motor parameters are considered next. I repeat again, efficiency and current are the least important, except in extremely rare cases. Save your money. Skewed poles benefit mostly in starting, or at very slow motor speeds, where they reduce cogging (jumping jerkily from pole to pole). At most higher speeds their effects are imperceptible.
Interdependent power, RPM and torque are the most important factors! Unfortunately, this info is not always readily available. As shown in the graphical analysis section, those quoted are probably not located at the point of operation. However known values can serve as rough guides in selection. Motor curves can vary drastically, making guestimation more difficult . Close comparisons may be misleading, but larger differences greater than 10% can be very helpful. Lower RPM and higher power indicate more torque, which results in less RPM drop from noload to deadheading and full train loading. Power should be maximized for any case. RPM should be about 10-15 % higher than that required for desired full load train speed. The derived values will be dependent on the gear ratio. GRAPHS . can greatly improve the guess factor.
Most cases will not be ideally resolved, but a large improvement can be attained. For a 40 smile/hr freight, 60 is better than 140. Although 12 v may be the ideal for maximum train speed any value above 10 v should provide a reasonable range of control on most better powerpacks. If the results are not close enough, regearing or a combination is suggested.
Pittman in the past, some importers and most industrial manufacturers provide graphs with their motors, containing most or all of the parameters, permitting a very good analysis and estimate of performance. However account must be taken that these represent either a sample motor or an average of motors. Individual motors may vary by as much as 10% in some instances. However only on rare occasions, is close matching necessary.
All who are interested in remotoring should encourage all suppliers to provide these performance graphs, even at a nominal fee, so a fair comparison could be made. NorthWest Short Line published a booklet with graphs for most of the Sagami motors they supplied.
To help provide a method to measure the required characteristics, a DYNAMOMETER is in the development stages. Preliminary tests reveal some very surprising results. See MOTOR EVALUATION
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