Radio Control Car Action - September 2009

S MOTORS

HAVE

WEAKER COGGING

ments of the can’s magnetic field. Brushless motors often have less cogging than brushed motors. ; WHY IT MATTERS: This term is often mistakenly applied to the motor hesitation that can occur when a battery pack can’t output the current load needed in a brushless motor system.

kV

DEFINITION: Motor rating that states how many RPM it will run at per volt of electricity. ; WHY IT MATTERS: The closest thing to a common brushless motor rating standard we have is by kV. For example, a motor rated at 5700kV will run at 42,180rpm at top speed on a 7.4V battery pack (5700 x 7. 4)—no matter how many winds it has or whether it’s sensored or sensorless.

Li-Mn

DEFINITION: Also known as lithium manganese (LiMnO4), these batteries have a cathode made from manganese and oxygen. They have a rating of 3.7V per cell, and do not produce as violent a reaction when shorted as LiPo cells. ; WHY IT MATTERS: Apogee Batteries is best known for using Li-Mn cells. They are durable and safer than other LiPo cells, and also come in large capacities. They can be charged safely at 2C.

VOLTAGE

DEFINITION: The measure of poten-tional difference of electricity between two points.

; WHY IT MATTERS: Nickel-based batteries output a nominal 1.2V per cell, while the magic number for lithium cells are 7. 4, 11.1 and 14. 8, referring to voltage of 2-, 3-, and 4-cell lithium packs, respectively. Nickel packs are usually 6-cell ( 7.2V) or 7-cell ( 8.4V). Higher voltage usually means you car will be faster.

STACK

DEFINITION: The frame, usually made from iron, that the motor windings are wrapped around. ; WHY IT MATTERS: In a brushless motor, it’s inside the walls of the can; in a brushed motor, it’s part of the rotating armature.

mAh

DEFINITION: The standard for rating battery capacity; short for milliamp output per hour. ; WHY IT MATTERS: A 5000mAh battery pack can also be called a 5AH pack, but we always use the milli rating—it sounds bigger. This means this pack can deliver 5 amps or an hour when fully charged.

STATOR

DEFINITION: The fixed, non-rotating portion of a motor. ; WHY IT MATTERS: In a brushless motor, it’s the winding structure inside the can; in a brushed motor, it’s the magnet structure.

Ohms

DEFINITION: The measurement of how much a circuit resists voltage.

; WHY IT MATTERS: Copper wire of a specific diameter and length will have a calculable resistance. Knowing that, motors of all types can be tech’d by measuring the ohms resistance between winds.

MOTOR LINGO

COGGING

DEFINITION: The resistance felt in a motor armature as the stator or armature passes through different seg-

WATTS

DEFINITION: A measure of electric power calculated by multiplying the volts and the amperes used in a circuit. ; WHY IT MATTERS: Some companies rate its motors by their wattage, instead of by winds or kV ratings.

AR.MA.TURE (ar’ma-choor’) n.

DEFINITION: The rotating spindle assembly on a brushed motor that contains the motor shaft, the wire-wrapped stack and

the commuator. It’s supported by bearings or bushings.

WHY IT MATTERS: If the performance on an old brushed motor has dropped off, you can usually just replace the armature, rather than the whole motor.

WRAP-UP It’s always a good idea to do your homework before jumping into something new, which is why we’ve provided this “study guide.” Not only will you be better prepared to shop for and take care of your new electric gear, but you also won’t sound like a dolt when you call that LiPo cutoff a “battery-saving doohickey.”