Understanding Motor Specifications
Basic motor specifications you need to know
Motors are rated by their voltage, current draw, output speed, and torque.
Operating voltage specifies the nominal (normal) voltage the manufacturer recommends for the motor. Most small DC motors are designed for 1.5 to 12 volt operation, with the majority in the 3-6 volt range. Larger DC motors designed for heavy-duty applications usually require 12 to 24 volts, with some needing upwards of 90 volts. Often, but not always, the higher the voltage, the more powerful the motor (this does not apply to stepper motors, where very low voltages — on the order of just a few volts — are common for heavy-duty motors). Most motors can be run at operating voltages higher or lower than the specified rating.
- Lower voltages reduce torque and speed.
- Higher voltages increase torque and speed.
(Note: For stepper motors, speed is not greatly altered by changing the voltage.)
Avoid applying excessively high voltages to a motor, or else it could overheat and burn out. R/C servo motors contain their own electronics which are not designed for use over about 7.2 volts.
Current draw is the amount of current, specified in milliamps or amps, that the motor requires to produce a certain amount of torque. Motors consume different amounts of current depending on how they are operated:
- No-load. A motor that doesn’t have anything attached to its shaft isn’t doing any work, and is said to be free-running. No-load current tends to be very low.
- Load. As the motor does work, its load and current draw increases. Manufacturers rate the current draw under load using different standards, making it hard to judge a motor using this specification alone.
- Stalled. When the motor shaft stops rotating, it “stalls” and draws as much current as will flow through the windings. This specification is useful for “worst case scenario” engineering planning.
- Shorted. Maximum current flows into the motor when the coils are shorted out. The motor will not run, and like any short circuit, if operated in this way for any length of time, serious damage can result to other systems on the robot.
Torque is the way the strength of the motor is measured. It is typically calculated by attaching a lever to the end of the motor shaft, and a weight or gauge on the end of that lever. The length of the lever usually depends on the unit of measurement given for the weight. Examples:
- Metric measurements use Newton meters (Nm), kilograms-force meters (kgf-m), or sometimes gram-centimeters (gm-cm).
- Standard measurements use ounce-inches (oz-in), or pound-feet (lb-ft), or pound-inches (lb-in). It’s common to reverse the nomenclature and call it foot-pounds and inch-pounds.
The speed of the motor indicates how fast its shaft is turning. DC motors without a gearbox spin at 3,000 to over 12,000 RPM (revolutions per minute). With a gear box, the speed can vary from under 1 RPM, on up.
Stepping motors are not rated in RPM, but pulses (or steps) per second. The speed of a stepper motor is a function of the number of steps required to make one full revolution, times the number of steps applied to the motor each second. Typical values are 200 or 300 PPS.