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What are the main parameters for micro DC motor selection?

Date:2023-11-25   Author:XINDA MOTOR

Selecting the correct DC motor or DC geared motor for a specific application can be a daunting task, and many manufacturers may only provide basic motor specifications. These basic specifications are not enough to meet your needs. Below we enumerate the specifications of micro DC motors and provide an approximation if possible.

  Below is a very common specification that a DC motor manufacturer might cite. For most buyers, this basic information is enough to make a purchase decision.

  1. Rated voltage:

  Voltage corresponding to high motor efficiency. Try to choose a battery pack that matches the voltage rating of your drive motor. For example, if the motor's rated voltage is 6V, then use a 5-cell 1.2V battery pack to get 6V. If your motor runs at 3.5V, use 3 AA or 2 AAA battery packs. The motor's efficiency decreases if it is operated above its rated voltage, which typically requires additional current, generates large amounts of heat and reduces the motor's service life. In addition to the rated voltage, DC motors also have an operating voltage range, and manufacturers do not recommend operating the motor beyond this range.

  2. No-load speed:

  Assuming there are no connections, this is the fastest rotation speed of the output shaft (angular speed). If the motor has been decelerated and the motor speed is not displayed separately, the motor rpm is proportional to the voltage input value. "No load" means the motor is not experiencing any resistance (the hub or wheel is not mounted to the end). Normally, the supplied no-load speed is linked to the rated voltage.

  3. Rated power:

  If the motor power is not listed, it can be approximated. Power (P) is related to current (I) and voltage (V). The formula is: P=I*V. Use no-load current and rated voltage to approximate the power output of the motor. Use locked rotor current and rated voltage (rather than maximum voltage) to derive the maximum power of the motor (this can only be used for a short period of time)

  4. Stalled rotor torque:

  This is the maximum torque that the motor can provide when the shaft is not rotating. If the motor is stalled for more than a few seconds, the motor will suffer irreparable damage. When selecting a motor, you should consider that it should not exceed 1/4-1/3 of the stalled torque.

  5. Locked rotor current:

  This is the current consumed by the motor at maximum torque. This can be very high, and without a controller to control this current, in large cases it will suffer damage. If neither locked rotor nor rated voltage is provided, then please try to use the rated power and rated voltage of the motor to estimate the current: Power [watt] = Voltage [volts] * Current [Amps]

      General specifications:

  General specifications for a DC motor usually include weight, shaft length, and shaft diameter as well as motor length and diameter. Other useful specifications include mounting hole location and thread type. If length or diameter is provided, refer to pictures, photographs, or scale drawings to get a feel for other dimensions.


  "Torque" is calculated by multiplying force by distance. A motor rotating at a locked-rotor torque of 10Nm can rotate within 1m

  Maintain 10N. Likewise, it maintains 20N within 0.5m. Note: 1kg*gravity (9.81m/s2)=9.81N (10N is for quick calculation)

  Ideal specifications:

  The additional information listed by many motor manufacturers can be very helpful when choosing the correct motor. When searching for DC motors, you may encounter some of the following information:

  Voltage vs speed

  Ideally, the manufacturer might list a graph of voltage versus speed for the motor. For a quick approximation, consider using no-load speed versus rated voltage: (rated voltage, speed) and the point (0,0).

    Torque vs Current:

  Current is a value that is not easy to control. DC motors use only the required current. Ideal specifications include, curves, and approximations that are not easily replicable. The stall torque is related to the stall current. A motor that is inhibited from spinning will draw maximum ("locked") current and produce the maximum torque possible. The current required to provide a given torque is based on many factors, including the thickness, type and configuration of the wires used to build the motor, as well as the magnets and other mechanical factors.

  Technical specifications or 3D CAD drawings:

  Many robots like to draw pictures of the robot on the computer when purchasing the necessary parts. Although all motor manufacturers have CAD images with dimensions, they rarely make them available to the public. The ideal motor size includes the above information, as well as the location of the mounting holes and the type of threads. Ideally, the materials and dimensions used to make the motor, gears and windings would be provided.

  Reduction ratio:

  The manufacturer of the DC motor must provide the corresponding reduction ratio when producing the corresponding reduction motor for the motor. Deceleration is used to increase torque and reduce speed. The given no-load speed value is always the value of the output shaft after deceleration. In order to obtain the angular velocity value before deceleration, this value (no-load speed value) needs to be multiplied by the reduction ratio. Before decelerating, for the stall torque of the motor, divide the stall torque by the reduction ratio. The material used to make the internal gears is usually plastic or metal, and is chosen to bear the maximum torque rating.

  Accessories: For geared motors, encoders are commonly used accessories. Finding the right encoder for your motor can be difficult if you're not sourcing it from the same company. An optical encoder allows you to find the direction of rotation as well as the motor's RPM. Along with a suitable encoding, an optical encoder can also provide you with the angle of the shaft.

  Hubs and couplings:

  Hubs (which are used to connect the output shaft to other components) are increasingly adapted to different sizes of output shafts. Only few manufacturers offer original couplings. If you cannot find a suitable coupling, consider using spur gears to offset the shaft to another size.