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working out hp of electric motor

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working out hp of electric motor

Postby mickthetree » 03 Feb 2015, 12:43

Hi Bob
I have a motor query if you could assit!! Its for a drum sander build I am doing.

My motor doesnt give a hp or watt output on the rating plate.

As I understand it hp = volates (240) x Amps (3) x efficiency (???????)

How on earth do I find out the efficiency?

Currently that works out to be a 1hp motor ish (720 watts) but presumes 100% efficiency.

BTW I couldnt load your motor document or I would have read that first too.
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Re: working out hp of electric motor

Postby Robert » 03 Feb 2015, 13:28

You could take the technical approach and measure the current it draws but that will vary with load. There will be a point where the load is too great and it starts to slow too much. When loaded like that it will pull the most current and you might get a guess as to the motors horsepower allowing for power factor.

I'd suggest a better approach would be to lash up the intended use and see it if is up to the job before a proper build.
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Re: working out hp of electric motor

Postby mickthetree » 03 Feb 2015, 14:11

well, the build suggests anything over 1/2hp so as long as it is more than 50% efficient it should work.
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Re: working out hp of electric motor

Postby 9fingers » 03 Feb 2015, 14:20

Power =Volts * amps is an incorrect simplification but it does get you in the right ballpark for the full output.

Power out = volts * amps * power factor * efficiency will get you a bit nearer but has two unknowns that are not trivial to measure. Power factor will vary according to load.

Even with that estimate, how do you know how much power is needed to drive your sander?
there is required torque, belt drive efficiencies etc to bear in mind too.
The power of the motor is one factor but the torque curve is needed and this depends on the type of motor.

I have deliberately not gone into the subject in my paper as it is non trivial.

Robert's pragmatic approach is far better to be honest and will lead to a good feel for the adequacy of your motor for the application.

I'm concerned that you can't download my motor paper. It works here but I'm using the same ISP to access it and store it.
What happens when you click the link?
Has your browser got a pdf reader integrated? Otherwise can you right click the link and use save as to save a copy and then open with your pdf reading application.

Cheers

Bob
Information on induction motors here
https://drive.google.com/file/d/1dBTVXx ... sp=sharing
Email:motors@minchin.org.uk
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Re: working out hp of electric motor

Postby mickthetree » 03 Feb 2015, 14:30

Many thanks Bob

Sounds like both of your suggestions are logical and I shall tryit out first. If its not powerful enough I will need to get another anyway and it wont dictate the build.

It appears the link is not formatted quite right:

http://http//homepage.ntlworld.com/bob. ... Issue3.pdf

Should be:

http://homepage.ntlworld.com/bob.minchi ... Issue3.pdf

Which does resolve correctly in my browser (Google Chrome Windows 7).

HTH
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Re: working out hp of electric motor

Postby 9fingers » 03 Feb 2015, 15:15

Thanks Mick,

I think I've sorted that out now.

Good luck with the project and I hope we will see some WIP postings from you here.

Cheers

Bob
Information on induction motors here
https://drive.google.com/file/d/1dBTVXx ... sp=sharing
Email:motors@minchin.org.uk
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Re: working out hp of electric motor

Postby kirkpoore1 » 03 Feb 2015, 18:49

9fingers wrote:...

Robert's pragmatic approach is far better to be honest and will lead to a good feel for the adequacy of your motor for the application.
...

Cheers

Bob


Since you've already got the motor, this is the way to go. Another approach is to look at what's available on an existing machine (or even an ad for a new machine of similar capacity) and go for that much power. Many older machines were underpowered by today's standards. (Of course, many current machines are under-built by yesterday's standards, too.:))

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