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You have two motors, one being 1.5HP and the other being 3HP. All other ratings are the same (e.g. motor efficiency ratings).

You attach a load to each motor. The load on each motor is identical at any given point in time. This load will range from no load to some stall load (obviously the stall load would be greater with the 3HP motor).

So for example, no load on each, take reading (how much power draw), add load, take reading of power draw, add more load, take reading of power draw...all the way to stall load...take power reading.

My question is, up until the smaller motor stalls, will the larger motor ever consume less power? Same goes for say a 2HP vs a 1.5HP. Can someone point me to the formula for doing this type of calculation?

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  • \$\begingroup\$ Just depends on the efficiency of each. In an ideal world, both would consume the same power until the load reached 1.5HP, when the smaller motor fails. \$\endgroup\$ – DSWG Jul 19 '18 at 4:14
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    \$\begingroup\$ @DSWG What about in practice? Say picking a middle of the road efficiency design. What this comes down to is the end result of sizing an electric motor for a job. Lot of people scream to always put in the smallest motor you can use as it’s more efficient. Ive also heard the logic of having a slightly larger motor wouldn’t have to “work” as hard to deal with the same load resulting in less electricity being used. \$\endgroup\$ – Kevin Vasko Jul 19 '18 at 12:14
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For most motors sold for general purpose use or purchased by manufacturers for use products, the motor manufacturer will provide reasonably complete performance data. That would generally include input power or efficiency at 1/4, 1/2. 3/4 and full rated load torque. For many motors, manufacturers will provide performance curves that include a continuous plot of efficiency or power vs. load torque. The efficiency curve shows zero efficiency at no load, rising rapidly as load increases and then leveling of somewhat above 1/2 load. Peak efficiency often occurs at or slightly above rated load, but sometimes occurs closer to 1/2 load.

If you have a 1.5 Hp load, you really should use a 2 Hp motor if there is any chance at all that the load might actually be more than 1.5 Hp or might increase as the machine ages or the process changes. There will probably not be very much wasted power. You should not use a 3 Hp motor unless you absolutely can not be confident that the load is less than 2 Hp even then, you should consider replacing the motor if you later find it is larger than necessary.

Since higher power motors are generally have a higher peak efficiency than lower power motors, there is a possibility that a higher power motor will use a little less power than a lower power motor at some load, but that would rarely be the case and the difference would be quite small.

Here is an example of a motor data table that provides the information needed for this type of comparison:

enter image description here

Note that the table and the above discussion assumes a 3-phase induction motor, the type of motor usually used for fixed-speed motors 1-1/2 Hp and larger.

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  • \$\begingroup\$ The two motors you highlighted do not have the same ratings. \$\endgroup\$ – Britt Kelly Sep 16 '18 at 19:22
  • \$\begingroup\$ @Brit Kelly: Read the question and answer. The example shows two motors of different ratings running at the same load, 1.5 Hp. They have the same efficiency at that load and thus use the same power. That demonstrates the statement in the last paragraph of my answer. \$\endgroup\$ – Charles Cowie Sep 16 '18 at 21:20
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The answer is no if the following conditions are what you are asking about:

  1. All motor ratings are equal except hp - meaning you are using both motors at those ratings.
  2. Both loads are identical
  3. Motor is assumed to be running continuously at a steady state.
  4. Identical system inputs (you did not specify this)

It’s important to note that when you say, will it ever use less power, you would typically mean real power because that is how we determine efficiency. This ratio helps us compare different sized motors since it will always be less than one. Also real power does the mechanical work.

Efficiency = PowerOut/PowerIn

Since you have already stated that the motors have the same efficiency, and same load, this number will be the same for both under the assumptions listed above.

It’s possible that you could be asking about using less reactive power. Reactive power does not do work. Once again though, since all the ratings (except hp) are the same and the load is the same, you will have the same consumption.

A hp rating is essentially, what is the maximum power that can be consumed by a load connected to that motor. That way you can determine the maximum amount of work that your motor can do.

Now if you have both your motors on different supply frequencies that is one way to change power consumption depending on the load. However you have not mentioned supply conditions so I do not know the situation you are inquiring about.

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  • \$\begingroup\$ I will respectfully disagree with you, Britt. The IR losses should be lower in the larger motor because of the thicker wire in the windings. On the other hand: I would love to be educated more about this if I am wrong. \$\endgroup\$ – Dwayne Reid Jul 21 '18 at 0:19
  • \$\begingroup\$ He said the motors have the same efficiency. I^2R loss is included in that. \$\endgroup\$ – Britt Kelly Jul 21 '18 at 0:21
  • \$\begingroup\$ If the motors have the same load which draws the same power out from the motor, and the motors have the same efficiency, then they must have the same power in. \$\endgroup\$ – Britt Kelly Jul 21 '18 at 1:00
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In order for the load to be the same on both motors, the load cannot be more than 1-12HP. "Same" efficiency would imply a percentage of total power, not a hard value. So a 3HP motor that is 90% efficient at a 3HP load will have 10% losses of it's max rating, but may be 80% efficiency at 50% load, i.e. 1-1/2HP, so the losses are 20% of the 3HP capacity. Whereas the 1-1/2HP motor will be at 90% eff at 1-1/2HP load. So I highly doubt you will EVER see a 3HP motor using less energy on the same load as a 1-1/2HP motor.

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