Screamin' 7.2 Volt Powerplants

These little metal cans, no larger than a pill bottle, can spin around 17,000 - 38,000 RPM and are designed to move a 3- 5 pound R/C vehicle. With ample batteries backing them up, the can send it streaking along as fast as 70 miles an hour. The motor is the second most important part of an electric R/C vehicle and is likely to draw most of your attention in tuning and tweaking your electric-powered R/C.

Motor

The electric motors used in R/C are usually Yokomo 540 motors, designed to run with 7.2 volt 6-cell packs, but like everything else in this great hobby, can be customized and modified to the moon!

Like any DC electrical motors, R/C motors are made from two strong magnets around the inside of the "can," a composite three-segment iron armature core with copper brush windings, and graphite/metallic composite brushes held to the commutator with springs.

In order to make an informed decision on buying an electric motor, or an electric RC in which you'll run one, you'll want to gain a little understanding of how they work. Below is an extended description of the various aspects of motors.

Turns
The motor "turn" reflects the number of times the copper wire is wrapped around each armature. The more turns a motor has, the higher the torque (strength) but the less RPMs it can achieve. Inversely, the less turns a motor has, the higher the RPM's it can achieve but the less torque it will have. This is also described as a longer or shorter wire on the armature, which equates to more or less resistance in the wire.

The "stock" motor used in most sanctioned races is a 27 turn motor and is the optimum balance between RPM and torque for most R/C applications.

To make an R/C faster, many owners select a lower turn motor because of the blistering high RPM's they can achieve. Stock motors average about 17,000 RPM; a 7 turn mod single can reach 38,000 RPM! Care should be exercised in selecting a lower turn motor due to the decreasing torque. You have to remember that the torque is decreasing but the load is it moving remains the same. The motor must be geared down appropriately or the engine will overheat and excess strain will be put on the speed control, much in the way a full size car would act trying to move from a dead stop in third or fourth gear. To take full advantage of the high speed of the RPM, you need to gear down, effectively reducing the load placed on the motor.

Winds
The wind of a motor reflects the number of wires that are actually wrapped around the armature. A 19T double, for example, is two strands of a smaller gauge of wire wrapped around each segment of the armature 19 times. Double- or triple-winds are usually found in motors less than stock to regain some of the torque lost in using less winds in the motor.

Commutator
The commutator is the three-segment copper section at one end of the armature that the two brushes rest against, and are connected to the armature windings. Only two of these segments are in contact with the brushes at any given time. When the brushes are charged with electricity, this turns the armature into an electromagnet that repels against the magnetic poles of the magnets on the inside of the can, turning the armature. As the poles of the armature begin to line up with the poles of the magnet, the brushes now come into contact with the next two segments of the commutator, charging the next two segments of the armature, repeating the process. Exactly WHEN the brushes switch to the next set of commutator segments is a function of motor timing, below.

Although manufactured to precise specifications, the commutator section of the armature, which is pressed into place and cemented, is not always perfectly "round." This causes the brushes to bounce on the commutator as the armature turns which produces electrical arcing (sparking), resulting in loss of available power from the motor. To gain every bit of available power, racers have the commutator "turned" on a on a commutator lathe. A comm lathe is very much like a machine shop lathe; a fixed diamond tip bit is set against the comm as it spins, shaving off excess material, making the commutator a near-perfect circle. This procedure is often used when the comm shows signs of wear.* After the comm has been turned a few times, the diameter of the comm area begins to decrease, changing the timing of the motor beyond usability, and it's time to throw it away.

Timing
The end of the motor holding the brushes, called the endbell, can be loosened and rotated slightly to change the point at which the brushes pass from one commutator segment to the next. This effectively alters the motor's timing, similar to changing the timing in full scale cars. You can advance the timing to gain RPM, or retard it**** to gain torque. But like changing to a motor with a different turn, if you advance timing to gain RPM, you must make appropriate changes in gearing to avoid overloading the motor.

Brushes and Springs
A wide variety of brushes and springs can be used on electric motors to gain performance under varying conditions. The conductivity of the brush is determined by the metallic content; adding more or less copper or silver in the graphite composite material changes it's conductivity. Heavier weight springs and less conductive brushes are often used for high amperage draw (modified low turn motors, off-road) and more conductive brushes and lighter springs are used for lower amperage draw conditions (on-road, small scales.)

If you're going to be playing with motor mods, it's a good idea to have several types of brushes and springs available and expirament with what works best for you.

Cleaning and Maintenance
The arcing of the high amperage passing through the motor causes the comm to blacken and reduces conductivity, which decreases the overall power available to the motor. A regular comm cleaning, every couple of runs, is a low-cost way to keep your motors running well between comm turnings. See the article on electric motor maintenance for more information.

Motor Gearing
While it is beyond the scope of this article to discuss the intricacies of motor gearing, some mention is required of this important aspect of electric vehicles. With any motor, battery, and track condition, correct gearing is essential to getting the most out of your R/C.

The Pinion gear is the small gear connected directly to the motor and drives the spur gear, connected to the transmission or axle, in the case of pan cars. The ratio of the pinion gear to the spur determines not only how fast the vehicle gets up to speed and out of corners, but also the top speed possible for the vehicle.

In a nutshell, a smaller pinion will decrease top speed but increase accelleration, and a larger pinion will increase top speed but decrease accelleration. Which you use depends on the application. For example, a concourse track with lots of turns and slow speed sections will benefit from smaller pinion gears because you need to get in and out of corners quickly. Larger pinions on such a track will not only make you slower, the excess load on the motor will cause overheating and can cause the motor to wear prematurely. On a high-speed track, such as an oval, a taller pinion is best because you won't be slowing down and accellerating frequently and need to get the highest speed possible out of the motor.

1/10 scale pinions come in sizes from 12 tooth to 26 tooth or higher, and the spur can differ depending on the vehicle. Most racers maintain a range of pinions from 14 to 20 tooth, and keep a few different spurs on hand in case one gets worn. Generally the gearing change is done only on the pinion unless a particular size of pinion won't reach the spur (so a larger spur is required,) or there's not enough room to fit the selected pinion (a smaller spur is required.)

Brushless Motors
The last few years have brought a whole new beast to electric RC's, brushless motors. Unlike the standard brushed motors described above, brushless motors require none of the rigorous maintenance and tweaking described above and can hit higher RPM's than their brushed predecessors. Brushless motors require a different speed control than brushed motors and are usually bought as a system with the ESC and motor together.