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How to Size a DC Motor: Guide to Electric Motor Sizing

May 18, 2021

When it comes time to select a DC electric motor, whether for replacement or initial installation, there are a few factors to consider before you settle on a specific model. Here, we look at the physical, mechanical, and electrical requirements that you need to meet to ensure the motor you have chosen is entirely up to the task and can complete the work efficiently.

Selecting a Replacement DC Motor

While searching for a DC electric motor for an initial installation requires quite a bit of investigation, finding a replacement motor can be a much simpler endeavor. First, you must locate the motor nameplate and take note of the different ratings recorded. Second, view and take note of the motor's operating location and any related components (like gear reducers, motor controls, etc.). Finally, if the old motor is being replaced due to a failure, you must find the source of this failure.

Motor Nameplate Ratings

If no changes need to be made to the application, a new motor can be selected easily by matching the old motor's nameplate ratings to the new one. You can find your motor's nameplate affixed to the enclosure or etched onto the body (in the case of stainless steel washdown motors) of your motor. It provides you with valuable information about your motor, manufacturer information, model and catalogue numbers, and ratings like HP, RPM, voltage, enclosure, insulation class, and duty cycle. By matching these ratings to the new motor, you can easily find a replacement that is up to the task.

You can also utilize the cross-reference search-ability on our website. Input your motor's catalogue number into the Smart Search bar to bring up that exact motor and any other motors that can be used as replacements.

Location Requirements

While matching nameplate ratings will give you a motor with the same capabilities, it doesn't always give you a motor that fits in the physical location it's intended to work. Before replacing your DC motor, make sure to take note of the physical location requirements. How large is the space available, and are there any stationary objects that will hinder the motor's installation or operation? Are there any attached components like gear reducers or motor controls that will need to work with the new motor? How is the motor mounted into place? Noting size requirements, any related components, and mounting options will help you select the correct replacement motor the first time around.

This is also a great time to evaluate the application's atmosphere. Is there a lot of dust? Are the surfaces cleaned often? Are you in sub-zero temperatures? Making sure that the enclosure type is appropriate for the motor's operating atmosphere ensures that it has a long and productive life.

Motor Failure

It is essential to note the reason you're replacing your electric motor. If you're replacing your motor due to a failure, replacing it with the same motor could produce the same issue. The motor might not be up to the task in the first place, or if the failure was due to a related component, that component would eventually cause the new motor to fail in the same way. It would be incredibly beneficial to hire a professional electrician to perform tests on the failed motor to pinpoint the failure's root cause. Once you find the failure source, you may need to size up the new motor or purchase gearing or control accessories that have the correct rating for the task.

Selecting an Initial Installation DC Motor

Selecting a DC motor for an initial installation is a much more complex undertaking. You'll need to complete a few calculations to ensure you choose a motor capable of performing optimally under the intended conditions. By learning the electrical and mechanical requirements of the application and matching those ratings to a motor, you'll be able to match the correct motor to the job at hand. These application requirements include power, speed, torque, and physical size.

Power Requirements

DC electric motors run on direct current (DC) power, the power available from batteries or thru a DC Speed control. The supply voltage is most commonly 90V or 180V, but could be as high as 500V, or as low as 12V, depending on the application's requirements.

Once you determine the type of source power available at the application's location, you can select a motor capable of running on that power. Keep those motors in mind as we move on to more complex calculations; we'll need to match each rating to find the correct motor for the job.

DC Motor Sizing Calculations

Here are four basic equations that will give you a great sense of the abilities a motor must have to be an excellent fit for your application. Once you have made these calculations, you can match motor ratings to the requirements you uncovered.

1.Load Inertia (oz-in2) = the resistance of the motor and the driven load to any change in rotational speed. A simple equation to use is I=mr2, where m=mass and r=distance to rotational access. Load Inertia is recorded as oz-in2.

2.Load Torque (T) = the amount of torque constantly required for the application. Use this equation: T=Fr, where F=Force of moving direction calculated in newtons (N) and r=distance to rotational radius calculated in metres (m). The motor you choose must generate more torque than this calculated value, and you must keep in mind that the torque needed at acceleration may be higher than what is necessary at constant speeds.

3.Speed (RPM) = how many rotations of the motor's shaft are completed within one minute. This can easily be calculated as distance divided by time. It is important to note that torque will drop as speed increases and vice versa. Alternatively, if the load requirements go up, the speed will decrease. Also, the rate of acceleration will go up as torque goes up. Plotting the torque-speed curve will show you if the motor can handle the required load across the entire speed spectrum.

4.Motor Constant (Km) = the ability of the motor to convert electrical power into mechanical power. Km=M/ √(P), where M=Torque calculated in newton-metres (mNm) and P=Resistive Power Loss calculated in watts (W).

Motor Physical Parameters

As noted earlier, sometimes you may have a situation where the motor's weight and size are constrained by the location you intend to mount it. In this case, you would find a motor that fits within the size and weight constraints and modify the mechanical abilities by purchasing accessories like gear reducers and motor control units.

You will also need to take note of the atmosphere in the location the motor will be mounted. You will have to select a motor with an enclosure from diverse options like explosion-proof, washdown, or totally enclosed fan cooled, to name a few. Choosing the correct enclosure type for the application will help ensure that your expensive equipment and its internal components will last for years to come.

Brushed vs Brushless DC Motors

Brushed DC motors have commutator brushes that contact the motor's armature to apply power, and these brushes are a wearable component that requires regular maintenance. Brushless DC motors don't have this wearable component, providing more efficiency and less downtime for maintenance. BLDC motors (brushless dc motors), while quieter and capable of higher speeds than their brushed counterpart, require more complex motor controllers. However, brushed dc motors are better for applications where a gearbox is required. The benefits and payoffs need to be examined closely to determine the best option for your application.


As you can see, selecting the correct replacement DC electric motor can turn into quite the undertaking. However, understanding your application requirements and matching a motor with manufacturer ratings capable of meeting those requirements is detrimental to your electric motor's lifespan and the equipment it drives. Not to mention, it will save you money when it comes to efficiency, maintenance, and equipment replacement. While selecting a replacement DC motor is much simpler since you have a legacy motor to compare the new motor to, selecting an initial installation motor requires much more research. You may see the benefits of hiring a mechanical engineer to make recommendations. Here at eMotors Direct, our motor experts are standing by to give you a hand with your electric motor project when needed.

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