REMOTORING
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. S
GENERAL FUNCTION
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.
This simplified drawing shows a rotor in a potential lockup position.
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.
MOTOR TYPES
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 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.