Motor specifications describe how a motor behaves electrically and mechanically: voltage, current, speed, torque, and power. Reading them correctly tells you how much a motor will draw, how hard it can pull, and, most important, what current rating your motor controller needs to drive it.
When you choose a motor for a robotics or motion project, the motor’s specifications dictate everything downstream: the battery, the wiring, and above all the motor controller. This guide explains the most common motor specifications, what each one actually means, and how to turn them into the numbers you need to size a controller.
What Do the Basic Motor Terms Mean?
Four quantities show up in almost every motor conversation. Everything else on a datasheet builds on these.
Speed is the rate at which the motor shaft rotates, almost always given in revolutions per minute (RPM).
Torque is the rotational force the motor produces at the shaft. Large motors are usually rated in foot-pounds (ft·lb) or newton-meters (N·m); small motors in ounce-inches (oz·in) or kilogram-centimeters (kg·cm). Whatever the unit, torque tells you how hard the motor can pull against a load.
Current is the flow of electricity into the motor, measured in amps (A). Current tracks load: the harder a motor works, the more current it draws.
Power is the rate at which the motor does work, measured in watts (W). Power combines speed and torque: it is the product of torque and angular speed, not simply torque times RPM. In practical units, mechanical power in watts is approximately torque (N·m) × speed (RPM) ÷ 9.55. A motor can make high torque at low speed or low torque at high speed and produce the same power in each case.
What Do the Common Ratings on a Motor Datasheet Mean?
The ratings below are the ones you will actually compare when selecting parts. Pay closest attention to the current figures, because they are what your motor controller has to handle.
Voltage (often called the nominal voltage) is the voltage the motor is designed to run at, and the reference point for every other rating on the sheet. A motor can be run below or above its rated voltage, but its behavior shifts as you do. Running above the rated voltage can extract more performance at the cost of a shorter motor lifespan.
No-load speed is how fast the shaft turns with nothing attached to it. It is the motor’s maximum speed. Add load and the speed drops.
No-load current is the current the motor draws spinning freely with no load, essentially the current needed to overcome its own internal friction. It is the low end of the motor’s current range.
Stall current is the current the motor will draw when the shaft is not rotating with its full rated voltage applied. This is the maximum current the motor can pull. It is one of the more important numbers for controller selection, because the controller must be able to handle the stall current for the motor to reach its maximum torque output. Size the controller so its rating meets or exceeds the motor’s stall current. A sustained stall also generates a great deal of heat, which will eventually damage the motor itself.
The catch is that many motors never list a stall current on their datasheet or placards. Fortunately, there is a relatively easy way to determine the approximate stall current.
The relationship comes straight from Ohm’s law: stall current equals the rated voltage divided by the motor’s terminal resistance, or Stall Current = Voltage ÷ Terminal Resistance. Measure the terminal resistance across the two motor leads with a multimeter (turn the shaft by hand and take a few readings, since the value changes slightly with brush position, and use the lowest). The calculator below applies this equation for you: enter your supply voltage and the measured resistance, and it returns the estimated stall current along with a suggested minimum continuous controller rating that includes a safety margin.
Estimate stall current from the motor’s rated voltage and winding resistance using Ohm’s law: stall current = voltage ÷ terminal resistance. Measure the terminal resistance across the motor leads with a multimeter, or take it from the datasheet.
Stall torque is the torque at the shaft when rotation is zero, the most torque the motor can produce (or, equivalently, the load that stops it). It pairs with stall current: both occur at the same locked-shaft condition.
Rated current is the current the motor can draw continuously without overheating, running at its rated voltage. It sits between the no-load and stall values. Note that it is not the same as stall current: the motor can briefly exceed the rated current, but not run there indefinitely.
Rated power is the maximum power the motor is designed to deliver at its shaft. Whether the datasheet lists it in watts, kilowatts, or horsepower, this figure refers to mechanical output power. The electrical power drawn from the supply is always higher than the rated mechanical output, because some energy is lost to heat and friction inside the motor.
Starting current is the surge of current a motor pulls at the instant it starts from rest. Because a stationary shaft looks electrically like a stall, this inrush briefly approaches the stall current before the motor spins up and the current settles toward its running value. It is one more reason to size the controller around the stall figure.
How Do Torque, Speed, and Power Relate?
For a brushed DC motor held at a fixed voltage, torque and speed trade off against each other in a predictable way, and power follows from the two.
Torque versus speed: the relationship is linear. With no load, and therefore no torque demanded, the motor spins at its no-load (maximum) speed. As load increases, speed falls in proportion, until at the stall torque the shaft stops turning entirely. The two endpoints of that line are no-load speed and stall torque.
Power versus torque and speed: because power is torque times speed, it is zero at both extremes: zero speed at stall, and zero torque at no load. Peak power falls in the middle, at roughly half the stall torque and half the no-load speed. Motors are usually operated well away from stall, on the higher-speed side of that curve.
| Specification | What it tells you | Why it matters for controller selection |
|---|---|---|
| Voltage (nominal) | The voltage the motor is rated to run at | The controller’s voltage rating must cover it, with headroom for regen spikes |
| No-load current | Current spinning freely, no load | Low end of the range; rarely a limiting factor |
| Rated current | Continuous current without overheating | The current the controller carries in normal operation |
| Stall current | Maximum current, shaft locked | The number to size the controller’s current rating against |
| Stall torque | Maximum torque, zero rotation | Sets the peak load the motor can hold |
| Rated power | Maximum mechanical output power | Sanity-check against your load and battery capacity |
Next Steps
Once you can read a motor’s specifications, the next step is turning the stall current into a controller choice, and sizing the battery that feeds it:
- Motor Controller Selection, covering how to match a controller’s current and voltage ratings to your motor.
- Selecting the Proper Battery, covering how to choose a pack that can supply the current your motor and controller demand.
