How Much Power Do I Need?

Owned by Jack Degrano
Last updated: Aug 05, 2024 by Eric Rojas (Unlicensed)
3 min read

How much power do I need for my application?

Here is a very reliable way to determine how much power your application will require so that a properly sized power supply and/or transformer can be selected. Follow these steps:

  1. Determine your continuous and peak requirements for mechanical shaft power in terms of speed and torque (just peak requirement for stepper applications). If you do not know what the exact requirements are for your application, play it safe and assume worst case by choosing a point on the "knee" of the speed/torque curve for your system. Make sure that your speed is in units of revolutions-per-second (rps) and that your torque is in units of ounce-inches (oz-in).

  2. Multiply your speed value and your torque value. Divide the product by 16,800 which will result in a value of shaft power in units of horsepower (hp) for that particular moment.

  3. Convert your mechanical power to electrical power by multiplying your mechanical value by 746 which will result in a value of electrical power in units of watts. This value is the electrical output power required from your drive. Remember that in a servo application you must do this for both continuous and peak considerations.

  4. No drive amplifier is perfect, and there is always some loss between the input power and the output power. So the next step is to determine how much input power is required by your drive in order to deliver the output power that you have just calculated. It is safe to assume an efficiency of approximately ninety percent over the entire range of Parker drive offerings. Therefore multiply the output power that you calculated in Step 3 by 1.11 which will result in a value of electrical power also in units of watts. Again, remember that in a servo application you must do this for both continuous and peak considerations.

  5. Just like the drive, no motor is perfect so you must compensate for this. All rotary motors are approximately 80 percent efficient throughout their speed range. Therefore multiply the output power that you calculated in Step 4 by 1.25 which will result in a value of electrical power also in units of watts. This value is the electrical input power required by your drive. Again, remember that in a servo application you must do this for both continuous and peak considerations.

  6. If you are sizing a DC power supply for your application, your work is nearly done. You have calculated the amount of continuous and peak power that your drive will require. If you plan to use a power supply for more than one axis, you must perform the previous steps for each axis, and you must select a power supply that will satisfy the total continuous and peak wattage for all of your axes. If you are sizing a transformer or a power supply that uses rating units of volt-amps (VA), you must now convert your requirements (in units of watts) into units of volt-amps by dividing the value in watts by a power factor of 0.66. Again, you need to consider both continuous and peak requirements for all of your axes.

IT IS IMPORTANT TO SELECT A TRANSFORMER OR POWER SUPPLY WITH A WATTAGE OR VOLT-AMP RATING THAT IS GREATER THAN THE TOTAL PEAK POWER REQUIREMENTS OF ALL YOUR AXES. IF THE RATING IS NOT GREATER, THEN YOU MUST CONSIDER USING A POWER DEVICE WITH A GREATER RATING OR YOU MUST CONSIDER USING MORE THAN ONE POWER DEVICE.