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Forum topic by Willscary posted 09-29-2018 09:43 PM 439 views 0 times favorited 7 replies Add to Favorites Watch
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Willscary

25 posts in 337 days


09-29-2018 09:43 PM

Topic tags/keywords: bandsaw

This topic is easy to find both on this site and also all over the web. What is stated in most cases is that roughly 746 watts equals 1 HP.

From there, you need to look at inefficiencies, and it seems that although there is a spread of AC induction motor efficiency ratings, the general consensus is that these motors in the smaller range of 1/2 to 2 HP are all roughly 65% efficient.

I just purchased a late 90’s Delta DJ-15 with a USA made Marathon motor. The motor plate lists it as a 3/4 HP motor. The rest of the plate states 60 Hz single phase 115/230V and 11.4/5.7A. It is continuous rated with a service factor of 1.0.

Is this a 3/4 HP motor? Doing the math it seems like it is a solid 1-1/8 HP.

115V X 11.4A = 1,311W

1,311/745.7 = 1.758 HP

At 65% efficiency this works out to 1.143 HP.

What am I missing? Is there something else to convert?


7 replies so far

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splintergroup

2728 posts in 1640 days


#1 posted 10-01-2018 10:02 PM

Welcome to the mystery of electric motors 8^)

HP ratings are a lot like the stereo amplifier “Watts” battles of the 70’s. Lots of misinformation.

Many motors list a current that is “RLA” or rated load amps and some list FLA or full load amps. Each is dependent upon how the motor was tested. There are standards, but it still is a somewhat sloppy business.

Your motor will most probably run at much less than the nameplate current while under operation because it is not being fully loaded. Lock the motors rotor and turn it on and you might see the 11.4A pop up on the meter.

Sorry, not exactly the explanation you were looking for but your underlying assumptions are correct. Measure the current while running with the rotor not attached to anything. Load the motor up with the tool/machine, read the current difference and use this to calculate the motors applied HP (with an assumed efficiency)

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runswithscissors

3052 posts in 2443 days


#2 posted 10-02-2018 12:39 AM

Reminds me of when stereo was coming in (so, you can guess how long ago), I went with a friend to check out some hot shot new vendor who was bragging about the output of his speakers. He would play something over the system, and then point to the wiggly line on an oscilloscope to show us how great the frequency range was. Trouble is, he was showing us the oscilloscope’s reading, not the real output of the speakers. As usual, caveat emptor.

-- I admit to being an adrenaline junky; fortunately, I'm very easily frightened

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Willscary

25 posts in 337 days


#3 posted 10-03-2018 01:16 AM

As a big “hi-fi” guy myself, I get the inflated specifications part of it. My thought though is that amps are amps, and if this motor is rated at 11.4 amps at 115 volts, that is more than 1-1/8 HP at the 65% common efficiency of motors of this age, size and type.

My thought was that this motor was probably VERY conservativiely rated.

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TheFridge

10858 posts in 1903 days


#4 posted 10-03-2018 02:41 AM

that’s still a beefy motor with even 65% efficiency figured in.

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

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Nugs

67 posts in 2295 days


#5 posted 10-03-2018 06:33 PM

What am I missing? Is there something else to convert?

- Willscary

You are missing the power factor, usually the nameplate will have both the power factor and efficiency on it.

The power equation for a motor is; power = voltage x current x efficiency x power factor

https://en.wikipedia.org/wiki/Power_factor

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Willscary

25 posts in 337 days


#6 posted 10-03-2018 07:30 PM

I had originally posted the PF as 1.0, but then noticed that the faceplate did not list the power factor, but instead the service factor = 1.0.

Here is the entire plate:

You are missing the power factor, usually the nameplate will have both the power factor and efficiency on it.

The power equation for a motor is; power = voltage x current x efficiency x power factor

https://en.wikipedia.org/wiki/Power_factor

- Nugs


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splintergroup

2728 posts in 1640 days


#7 posted 10-04-2018 01:45 PM

The power factor changes as a motor is loaded and is one of the primary reasons that for maximum efficiency (most work done for your electric bill payment), you want to run the motor near the maximum load.

Typically people only really care about PF for mulit-HP 3-phase motors, but give this calculator a try (nice revalation Nugs!)

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