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 Post subject: DC motor question
PostPosted: Sat Apr 01, 2017 2:27 pm 
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Elite

Joined: Mon Nov 16, 2009 9:06 am
Posts: 1093
Location: Trenton, Michigan
After too much time trying to get a clear answer from Google, I am seeing if the electrical guys on this site can answer my question.

I have an air cooled DC Spindle, 400 watt, 48 volts and 12,000 rpm. The 400 watts is roughly 0.50 horsepower. It has a remote PWM speed control.

I believe it is brushless since there is no audible or tactile feedback when turning it by hand. Also it returns to certain positions when it stops, which I assume means permanent magnets.

My question is, does the horsepower change at lower speed due to the PWM?

Reason I am asking is that a significant part of speed / feed calculation for my CNC engraver is hp.

Steve.


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 Post subject: Re: DC motor question
PostPosted: Sat Apr 01, 2017 3:22 pm 
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Obsessed
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Joined: Tue Aug 22, 2006 1:39 pm
Posts: 5904
Location: Livonia
SteveStram wrote:
My question is, does the horsepower change at lower speed due to the PWM?


Interested to see the responses you get from "professionals", as my understanding is that the whole point of the PWM controllers is you DON'T sacrifice HP for slower speeds, as you would by simply reducing the voltage.

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>>--Steve Sawyer->>
A bad day working with tools is better than a good day doing most of the other things I have to do.
blog: http://www.stephensawyer.com


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 Post subject: Re: DC motor question
PostPosted: Sat Apr 01, 2017 7:31 pm 
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Master

Joined: Sun Feb 05, 2012 10:04 am
Posts: 299
Location: Farmington Hills, MI
Not a DC motor expert but I think I can shed a bit of light. PWM is a means of speed control where the motor is given pulses at full operating voltage but reduced percentage time duration to reduce speed. The motor in this case would be producing its full torque output when the PWM was high, but since:

HP=TxN/5252

where T is torque and N is rotational speed in RPM the horsepower produced will be lower since the RPM is lower.

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Kevin (Specialist in reconstituting the wood after a wayward toolpath)


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 Post subject: Re: DC motor question
PostPosted: Sat Apr 01, 2017 9:47 pm 
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Elite
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Joined: Fri Aug 05, 2011 4:17 pm
Posts: 966
Location: Farmington Hills
I have a DC motor on my drill press and it has a lot of torque at low speeds, I use it to drill metal with no problem. I was so impressed I put a DC motor on both my wood and metal lathe.


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 Post subject: Re: DC motor question
PostPosted: Sun Apr 02, 2017 6:30 am 
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Master

Joined: Tue Aug 05, 2014 9:09 am
Posts: 164
Location: Ypsilanti
Not an expert either, but...
Torque is more what you should be concerned about. If your system has some sort of feedback where the motor is trying to maintain constant speed then current draw will increase to compensate for load increases.
I would run the fee rate based on the given RPM and not worry about HP until something happened, like you can hear it bogging down.

-Mitch


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