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How Do I Interpret DC Motor Specifications?

Jun 22, 2017

Choosing the right DC Motor (or DC gear motor) for a specific application can be a daunting task and many manufacturers only provide basic motor specifications. These basic specifications might not be sufficient for your needs. Listed below are ideal motor specifications and whenever possible, ways to approximate values.

Below is a list of the most common specifications a DC motor manufacturer might list. For most hobbyists the basic information is enough to make an informed decision about which motor to purchase.

Nominal Voltage:

The voltage that corresponds to the highest motor efficiency. Try to choose a main battery pack which most closely matches the nominal voltage of your drive motors. For example, if the motor’s nominal voltage is 6V, use a 5x 1.2V NiMh pack to get 6V. If your motor operates at 3.5V nominal, you can use either a 3xAA or 3xAAA NiMh pack or a 3.7V LiPo or LiIon pack.

If you operate a motor outside of its nominal voltage, the efficiency of the motor goes down, often requiring additional current, generating more heat and decreasing the lifespan of the motor. Aside from a “nominal voltage” DC motors also have an operating voltage range outside of which the manufacturer does not suggest operating the motor. For example a 6V DC Gear motor may have an operating range of 3-9V; it will not operate as efficiently as compared to 6V, but it will still run well.

No Load RPM:

This is how fast (angular velocity) the final output shaft will rotate assuming nothing is connected to it. If the motor has a gear down and the motor’s speed is not indicated separately, the no load rpm value is the shaft speed after the gear down. The motor’s RPM is proportional to the voltage input. “No Load” means the motor encounters no resistance whatsoever (no hub or wheel mounted to the end). Usually the No Load RPM provided is associated with the nominal voltage.

Power rating:

If a motor’s power is not listed, it can be approximated. Power is related to current (I) and voltage (V) by the equation P = I*V. Use the no load current and nominal voltage to approximate the motor’s power output. The motor’s maximum power (which should only be used for a short time) can be approximated using the stall current and nominal voltage (rather than maximum voltage).

Stall Torque:

This is the maximum torque* a motor can provide with the shaft no longer rotating. It is important to note that most motors will sustain irreparable damage if subjected to stall conditions for more than a few seconds. When choosing a motor, you should consider subjecting it to no more than ~1/4 to 1/3 the stall torque.

Stall Current:

This is the current the motor will draw under maximum torque* conditions. This value can be very high and should you not have a motor controller capable of providing this current, there is a good chance your electronics will fry as well. If neither the stall nor the nominal current are provided, try to use the motor’s power rating (in Watts) and the nominal voltage to estimate the current: Power [Watts] = Voltage [Volts] x Current [Amps]

General Specifications:

A DC motor’s general specifications usually include weight, shaft length and shaft diameter as well as motor length and diameter. Other useful dimensions include the location of mounting holes and thread type. If only the length or diameter are provided, refer to an image, photo or scale drawing to get a sense of the other dimensions based on the one known value.

Torque

*”Torque” is calculated by multiplying a force (acting at a distance away from a pivot) by the distance. A motor rated at a stall torque of 10Nm can hold 10N at the end of 1m. Similarly, it could also hold 20N at the end of 0.5m (20 x 0.50 = 10) and so on.

Note: 1 Kg * force of gravity (9.81m/s2) = 9.81N (~10N for quick calculations)