I need a motor and I'm not sure what to be looking for. I know the power and torque I need (3 N*m @ 1200 rpm = 1/2 HP). Is finding a motor as simple as matching these two values?

Edit: The motor will be used essentially like a fan motor; It will have to maintain a (fairly) constant speed for long periods of time. It's power will come from the wall (120V) so it could probably be AC but doesn't have to be. It'll be sitting still at room temperature.

I'm mostly trying to understand how to choose a motor based upon torque/power requirements as I don't know what I'm doing when it comes to this.

EDIT: I can incorporate something like a gearbox, I just don't know the process of picking a motor + gearbox that does what I actually want.

  • \$\begingroup\$ Is your application flexible to more standard motor base speeds, say 1725 RPM? \$\endgroup\$
    – HikeOnPast
    Dec 9, 2012 at 0:26
  • \$\begingroup\$ @HikeOnPast No, my design would just fly apart at 1725 RPM. It would have to be from 600 to 1200 RPM. I want to shoot for 1200 RPM and be able to slow it down if I need to. Any information on what I can do with a gearbox directly attached to it would be appreciated. \$\endgroup\$ Dec 9, 2012 at 1:21
  • \$\begingroup\$ If you had to trade between low cost on one side and lots of adjustability/flexibility on the other side, which way would you lean? \$\endgroup\$
    – HikeOnPast
    Dec 9, 2012 at 1:48
  • \$\begingroup\$ Honestly I'd like to understand both. I want to understand how to pick a motor. \$\endgroup\$ Dec 9, 2012 at 2:23

3 Answers 3


Things to ask yourself:

What is my input voltage? AC or DC? Are you running this off of a battery or line voltage or a control or something else?

What am I doing with the motor? Is it for a fan and will be running at one speed for 8 hours a day? Is it for a servo type application? What's the duty cycle?

What is the environment it will be running in? Does it need to be dust proof? Water-proof? Explosion-proof? Will the ambient temperature be 20 C, 65 C or -40 C?

How many hours does it need to run before failure? 5000 hours, 20,000 hours, 50,000 hours?

Your answers to those questions would give me a pretty good idea of the type of motor you need and what its rating should be.

EDIT: In general, sizing a motor is as easy as you suspect. If you know the power you need the motor to output, then you find a motor that is rated for that power. The rated power on the nameplate of a motor is the power that motor can continuously output without overheating. Generally these ratings tests assume that the ambient temperature of the environment is below 40 degrees Celcius. So if you have a motor that is going to be running at a constant torque and speed at a temperature below 40 C, finding a motor is simple. Things get trickier when you have short duty cycles or a duty cycle where the torque changes abruptly or higher ambient temperatures.

  • \$\begingroup\$ I've edited the question \$\endgroup\$ Dec 8, 2012 at 22:45
  • 2
    \$\begingroup\$ You are probably going to want a single phase, 6 pole, AC induction motor. That will have a synchronous speed of 1200 RPM but when loaded will probably have a speed 5-10% lower than that. Motors are rated for continuous speed unless they say otherwise, so for your application if you got a 1/2 hp motor, you should be fine. \$\endgroup\$
    – Eric
    Dec 9, 2012 at 2:27

EricE did a great job summarizing some of the major points of motor selection. This is a really broad topic, so I'll attempt to summarize a few other (non-comprehensive) factors for consideration:

  • What is the peak torque requirement?
  • Is the load torque somewhat proportional to the shaft speed (like a fan), constant regardless of shaft speed (like a winch), or decreasing with shaft speed (like a flywheel). The load characteristics during motor startup (i.e. from stop to running) can be a major design driver.
  • What is the speed requirement? Is some speed variation (slip) acceptable with changes in load, or is tight speed regulation required (synchronous motor or closed loop VFD/servo).
  • Must the motor speed be controllable? Within what range? (If so, a VFD or servo drive may be required. If very low speed, high torque operation is required without a gearbox, an external forced air fan may be required).
  • Are there efficiency considerations?
  • Does form factor matter? Brushless DC (BLDC) motors are available in a wide variety of aspect ratios, while induction machines tend to be of a similar aspect ratio.
  • Does motor mass matter? Will the motor be mounted on a moving assembly?
  • Does the application require a particular acceleration or deceleration profile? If so, the peak torque requirement may increase - it may be driven by acceleration/deceleration torque rather than continuous running torque.
  • Is monitoring of the motor speed or torque required?
  • Are commercially available motors available in the desired speed range (if fixed speed)? If not, a gearbox or other reduction system may be more cost effective than a special purpose motor.

I'm sure I've missed a few key ones, but those are the ones that jump out (and are complimentary to EricE's post.)


I'm sorry to tell you that this equation is wrong.

3 N*m @ 1200 rpm = 1/2 HP

it should be corrected to

3 Nm x 1200 rpm x 2 pi = 1/2HP

Just selecting a 1/2 HP won't give you 3Nm and 1200rpm either.[Note that 1200rpm is wrong anyway].

DC motor could be easily controlled with filed voltage. Well AC motors too, but not inductor motors or synchronous motors. AC commutative motors could be controlled with regulating voltage.

EDIT: I can incorporate something like a gearbox, I just don't know the process of picking a motor + gearbox that does what I actually want.

No need to worry that much of. Motor have it's torque-speed curve. So for a application like a printing press[Hamada 700 that I worked with] it uses a mechanism that adjust the tension of the slip belt to adjust it's operating point and operating speed. In otherwords you could mechanically do it or do it using a feedback system that precisely controlled with a microcontroller with the help of control theory.

  • \$\begingroup\$ Your equation is not right either. HP = torque(lbfft)*2pirpm/33000... and 3 Nm at 1200 rpm *is 1/2 HP \$\endgroup\$ Dec 9, 2012 at 18:03

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