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### Author Topic: How to reduce power of a DC motor?  (Read 3257 times)

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#### phpenguin

• Newbie
• Posts: 23
##### How to reduce power of a DC motor?
« on: October 04, 2014, 05:08:23 AM »

Hi,

I have a circular saw that consumes 1,400W of power when it runs.
The way I look at based on most of my wood cutting works, it has too much power which is wasted.
I want to install an external device that reduces the power down to 500W.

Is there an easy way to control DC motor power?

Regards,
Hughe

#### cheerio

• Sr. Member
• Posts: 306
##### Re: How to reduce power of a DC motor?
« Reply #1 on: October 05, 2014, 00:20:47 AM »
You can use a dimmer i guess. But you have to make sure that the power rating is high enough. You can then measure the power drawn until you got where you wanna get.
The more expensive solution would be a frequency converter(lower frequency -> lower rpm -> less power, higher frequency -> higher rpm -> more power. This would lower the RPM of the motor instead of the delivered power (better approach but way more expensive.)
« Last Edit: October 05, 2014, 00:24:29 AM by cheerio »

#### phpenguin

• Newbie
• Posts: 23
##### Re: How to reduce power of a DC motor?
« Reply #2 on: October 05, 2014, 13:25:45 PM »
Thanks

Is Variable Frequency Drive a sort of frequency converter?

#### kam

• Hero Member
• Posts: 1849
##### Re: How to reduce power of a DC motor?
« Reply #3 on: October 05, 2014, 23:02:57 PM »
There are typically two ways to control the DC motor speed. I suppose you talk about a brushed DC motor, right?

First is by reducing the voltage. And this is the simplest one. I wonder, what powers this 1400 W DC motor??? Are you sure it is a DC motor? I mean, at 1400W, even at 48 Volts it would require  30 Amperes!

As a by-product of the previous technique (but not applicable to your case) a linear regulator can be used to reduce the voltage. But if you want to drop 1000W with a linear regulator, you need a way to dissipate it.

Speaking of which, you can find some sort of load to connect in series and drop this 1000 Watts, like a light bulb or a heater.

Another interesting method is with a switching buck regulator. No need to dissipate (and drop to the bucket) the excess of power. But at that high amperage, you have to use huge wires for the coils.

The other method is with PWM. Here is a typical circuit which does exactly that. But at that amperage, you definitely need to put this circuit on steroids. Use a big MOSFET to withstand the current. And absolutely use fast switching mosfet driver for the gate otherwise the slow turn-on/off times will fry the fet.

But first post some images of the motor. Are you sure we're talking about a DC motor?

#### phpenguin

• Newbie
• Posts: 23
##### Re: How to reduce power of a DC motor?
« Reply #4 on: October 06, 2014, 15:48:17 PM »
@kam

Here is the image of power saw made by Bosch.  Input voltage is 220V AC.

#### cheerio

• Sr. Member
• Posts: 306
##### Re: How to reduce power of a DC motor?
« Reply #5 on: October 06, 2014, 16:39:29 PM »
i totally missed the sentence where you pointed out that it is a dc motor. good thing it is not it is an AC motor, so my post is still valid.

#### kam

• Hero Member
• Posts: 1849
##### Re: How to reduce power of a DC motor?
« Reply #6 on: October 06, 2014, 21:10:08 PM »
i thought so, its an AC. well, i do not think it will be that simple. AC motors hate power interruptions. 2 ways - either with a frequency converter or with a transformer. Since you only need to reduce down to a fixed speed, you go with the transformer solution.

You can either change the speed by reducing the voltage (transformer), or by reducing the frequency (converter). In rare cases you can change the slip with a series resistor to the excitation coil, but i doubt that this motor is such. Chances are this is a squirrel cage motor. So, its either voltage or frequency.

#### cheerio

• Sr. Member
• Posts: 306
##### Re: How to reduce power of a DC motor?
« Reply #7 on: October 09, 2014, 12:54:13 PM »
i use a dimmer to reduce the power consumption of a pump by about 40%
you have to find the sweet spot though