Posted on by & filed under Mechanics.

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.

 

BASIC SPECIFICATIONS

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) = 1N

How Do I Interpret DC Motor Specifications?

 

IDEAL SPECIFICATIONS

Many motor manufacturers are now listing additional information that can be very useful when selecting the right motor. Below is some additional information you might come across when searching for DC motors:

Voltage vs. RPM:

Ideally, the manufacturer would list the graph of a motor’s voltage vs. rpm. For a quick approximate, consider using the no-load rpm and nominal voltage: (nominal voltage, rpm) and the point (0, 0). See “gear down” below for motors with a gear down.

Torque vs. Current:

Current is a value that cannot be easily controlled. DC motors use only as much current as they need. Ideal specifications include this curve, and approximations are not easily reproduced. The stall torque is related to the stall current. A motor that is prevented from turning will consume maximum (“stall”) 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 make the motor, the magnets and other mechanical factors.

Technical specifications or 3D CAD drawing:

Many robot builders like to draw their robot on the computer before purchasing the necessary parts. Although all motor manufacturers have a CAD drawing with the dimensions, they rarely make it available to the public. Ideal motor dimensions include the basics listed above, as well as mounting hole locations and thread type. Ideally the materials used to make the motor, gears and winding as well as separate dimensions for the motor and the gear down would also be given.

Gear down:

DC motor manufacturers that also produce the corresponding gear down for a motor must list the gear down ratio. The gear down acts to increase torque and reduce rpm. The No Load RPM value given is always that of the last output shaft after the gear down. To find the angular velocity of the motor shaft before the gear down, multiply the value by the gear ratio. To obtain the motor’s stall torque before the gear down, divide the stall torque by the gear down. The material used to make the internal gears is usually plastic or metal and are chosen to be able to withstand the maximum torque rating. Calculate the gear down below given the values before and after gear down:

How Do I Interpret DC Motor Specifications?

Accessories:

An optical encoder is the most common accessory for a gear motor. Finding the right size of optical encoder for your motor can be very difficult if it is not made from the same company. An optical encoder allows you to track both the direction of rotation and number of revolutions of the motor. With the right code, an optical encoder can also give you the angle of the shaft.

Hubs and Shaft Couplers:

Secondary items such as hubs (used to connect the output shaft to other items) are slowly becoming available for varying sized output shafts. Only a few manufacturers provide generic shaft couplers. If you cannot find the appropriate coupler, consider using spur gears to offset the shaft to that of a different size. The image below shows three different types of couplers. The hole in the hub is for a threaded screw (“set screw”) which presses tightly against the shaft.

How Do I Interpret DC Motor Specifications?

Tags:

34 Responses to “How Do I Interpret DC Motor Specifications?”

  1. Zaeem Usman

    What happens if we exceed the nominal voltage for example if we have a 3v dc motor and connect it to a 9v battery is there a chance that the motor will burn?

  2. Carlos Asmat

    If you exceed the tolerances of a components, you will release its magic blue smoke and your components will not work anymore (i.e. you will fry it) :)

  3. Jeanine

    I have a small DC hobbyist motor for which I cant find the current spec. It is 1.5v to 3v rated. How do I measure the watts to estimate the amps as you explained? And then can I just add a resistor to decrease the voltage in the circuit if I want to run it off a 9v battery?

  4. shiva

    Stall current : New terminology, will be better if we use a common universal terminology

  5. RAJ

    if 30A is rated value of current give on data plate of DC motor. What does it specify line current or armature current

  6. sathish

    I need to design a robotic vehicle which should carry 20kg of weight and speed of 500 rpm.can you recommend me the geared dc motor to choose.how torque is calculated ?

  7. krushna

    I want to use two dc motors with 12 v battery should i go both for 12 or use 6

    • Coleman Benson

      @krushna The battery pack you choose should ideally correspond to the motor’s nominal voltage (greatest efficiency). If your motors are 12V, then choose an 11.1V LiPo or 12V NiMh / lead-acid.

  8. Dinesh kumar

    I tried 0.4v salt water battery, and I used toy car DC motor , I was measured multimeter 8 to 10 amps but did not run this motor, what can I do ? How to run the motor , which is the problem , please replay me . Thank you.

  9. Diego

    Hi, i’m searching on the internet for the differences between stall current and rated current, i mean, the relation between them. I’m thinking about buying this one http://www.aliexpress.com/item/1PCS-24V-DC-200-RPM-Replacement-High-Torque-Gear-Box-Speed-Control-Motor/32294661284.html , but I can’t figure out it’s maximum current so I can know better the driver I’ll be needing , cause , I thought about using the L298, and It only allow 2A per motor. By the way, Excellent post! I hope I was clear with my doubt.

    • Coleman Benson

      @Diego Assuming they use the term “rated” the same way between values, the “rated current” would be the current consumed at the “rated load”. As such, a motor controller which could provide 24V and 2A might be fine.

  10. Jack

    Hi, I am confused about following terms: “Max. continuous current”, “rated current”. Are they the same? if Max. continuous current is the rated current, why it is much more larger than the no-load current.

    Many thanks.

    • Coleman Benson

      @jack “Max. continuous current” is the current required at the maximum rated load (which should be less than the stall current). “Rated current” may mean the current at maximum efficiency (which would not be the same as the current at maximum load and would likely be lower). Normally you would not find both on the same spec sheet. “No load current” is when there is nothing connected to the output shaft of the motor, while “stall current” is when the motor is prevented from moving.

  11. Raffael

    Hi,
    When I read a value from a motor datasheet e.g. “Nominal current (max. continuous current)” = 2A does that mean I have a range of 2A ([0A,2A]) or is the range [-2A,2A] ?
    Another question: I have a brushless EC motor, with a driver that sais input voltage = ±10 VDC (differential). Am I correct that the EC motor uses DC as an input so I can use the whole 20VDC range?
    Many Thanks

    • Coleman Benson

      @Raffael The current would be up to 2A under normal load. It is rare you will ever see a negative current in robotics. “EC” normally means “brushless” and in mobile robot applications, this is always DC voltage. The input would be 10V (not 20V); ± normally means you can connect the wires either way.

  12. Raffael

    Thank you Coleman for your answer.
    So the thing is, I would somehow need to tell the motor in which direction to turn. I wanted to do this with negative current for one and positive current for the other direction. I have the motor in current mode. It works like that with the custom software for the motor driver. However, I want to control the motor through Matlab Simulink Realtime and thus need the conversion from the input voltage (maximum 10V ) to the current for the motor (max 2A).
    So can I input in the range [-5V,5V] which get converted to [-2A,2A] or would it then be [-1A,1A]? Or did I completely misunderstand your answer?
    Thank you.

  13. paa

    please please I am finding it difficult to determine the specification of a DC motor that has been used for a solar water pump project , please can you help out? “” heinzmann D-79677 schonau Gmbh and co.KG tel 490767382080 fax490767382008199 typl 120-2NFB-65 u 30 VDC HZM-Nh712-00-397-00 p 25,9 w m215 Ncm S-nr 0810 276655 I 1.0 A Ausf. 22-50 n990 1/min””

    • Coleman Benson

      @paa It seeems the motor has all of the specifications printed on the label: 30VDC, 25.9W, 1.0A current and can provide 215N-cm or torque. Not sure about the RPM though.

  14. paa

    thank you very much I really appreciate your help , please I will like to know if there can be a voltage operating range for this motor , that is , I know the max voltage is 30v and I want to know is there any minimum voltage at which it can not work ?

  15. tozz88

    Great article. I have looked at several motors with optical encoders built in. They often describe the encoder resolution as “pulses per minute”. I assume they mean the measure of angle not time. However that means a motor with “2 pulses per minute” would have 2*60*360=43200 pulses! That sounds way too high.

    • Coleman Benson

      @tozz88 If the units are pulses per minute, they likely mean just that (not angle). Most are pulses per rotation (PPR).

  16. ashutosh

    Can I operate air blower fan with 12 volt,17000 rpm robotic motor

  17. raji

    im using dc brushless motor(model no.ffb1212ehe) with an arduino uno for speed control.the motor is running well above 100 speed,but its not running when i give the speed level below 100.can anyone send me the coding??
    Thanks……

    • Coleman Benson

      @raji Without knowing which motor controller you are using, we cannot really help in this regard. We suggest creating a new topic on the RobotShop forum and provide as much information about the products you are using as possible.

Leave a Reply

  • (will not be published)