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IndustryArena Forum > Other Machines > General Other Machine Discussion > How to calculate wattage on cordless tools? Along with questions about brushed vs bru
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  1. #1

    How to calculate wattage on cordless tools? Along with questions about brushed vs bru

    Is there a easy way to calculate the wattage of a cordless power tool? I see tools listed with 40v, 20v, 18v and so on. I see batteries listed as 4mah or 5mah etc. To me these numbers only tell part of the story. Technically one could have a high voltage motor that had a low amp draw. Or someone could have a huge mah battery running a tiny motor. I see some that list volts and amps but most of them don't.
    My leaf blower for example uses 40v and has a 4mah battery. It has a brushless motor with variable speed. The manufacturer states it can run "up to 20 minutes on 4mah". In theory could I assume it's using 12mah in 20 minutes and multiply that times 40v giving me approx 480w? If this is correct it still only answers part of my question. When it states "up to 20 minutes" I'm going to assume that's on low setting. On high setting or under a load it may only last 15 minutes which would mean it's closer to 750 watts? I'm not even sure if I'm on the right track here or if that even makes sense lol.

    Next question is regarding brushless vs brushed. It is my understanding brushless motors are going to run more quite, more efficiently and produce more power compared to a brushed motor? Is a 500w brushless going to produce more than a 500w brushed? If so is there a standard multiplier to go by? For example a brushless is going to on average produce 80% more power or get twice the battery life (I completely made those numbers up).

    Next it is my understanding a brushed motor varies speed by increasing/decreasing voltage. I know this works for most DC motors I've played with. Can pwm (pulse width modulation) adjust speed on a DC motor as well? It is my understanding pwm is basically rapidly cycling on/off. Lastly based on what I know a brushless motor adjusts speed by changing the frequency. Voltage would remain the same but frequency would increase/decrease. It is also my understanding that a brushless motor is going to produce comparable power whether it's running at high speed or low speed. Is this assumption correct? If so would a brushless pull the same amperage whether it's running at high or low speed? Would it pull more juice working under a load vs no load?
    Not sure if I'm on the right track here or not. Really just trying to better understand how this stuff works. Hopefully that all makes sense!

    Edit to add: It is my understanding one advantage of a brushless motor is the ability to maintain a consistent speed whether it's under load or not. Obviously if you are completely digging a motor out it's going to bog down or stall but in theory is the speed controller going to use feedback to try to compensate for the working load? If so is this going to be a feature common with all variable speed brushless motors ranging from my cheap brushless leaf blower up to a high end spindle motor. I'm not saying they will do it equally as well but asking if they are both going to try to maintain a constant speed?

  2. #2
    Community Moderator Jim Dawson's Avatar
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    Dec 2013

    Re: How to calculate wattage on cordless tools? Along with questions about brushed vs

    I have found the brushless (BLDC) motor power tools seem to have better torque and maybe a bit more battery life per charge than brushed motor powered tools. Calculating run time/charge is pretty much impossible unless you can keep the load constant, which in most cases is impossible. But maybe possible in the case of a leaf blower since it is pretty much a constant load.

    750 Watts is 750 Watts the RPM and torque are the variables. You could have 1 RPM and very high torque or 10,000 RPM and very low torque and the power output (work done over time) would be the same.

    Most modern brushed power tools do use a PWM speed control. This keeps the torque more or less constant through the operating range. Precise speed control is not really a factor, take the case of a dill, if the motor bogs down, you just add a bit more pressure on the trigger to increase the average voltage (and thus the amps) being supplied to the motor.

    BLDC motors are 3 phase AC motors, and as you said the RPM is dependant on the frequency. They are controlled by a mini-VFD in the tool.. The generated sign wave is actually a modified sign wave and is a stepped voltage PWM. BLDC motors may have a bit better speed control, but normally in a power tool this is not critical. In most cases you are the speed control with your finger on the trigger.

    Getting out of the hand tool realm and into spindles, this is a different world. Now you may have a controller with a speed knob on it, or may be under computer control, and the system will have some type of feedback to maintain the set speed. This may be done electronically in the controller by sensing the rotor position, or could have an encoder actually providing the feedback. In either case, the controller adjusts the voltage and/or PWM signal to add or reduce torque as needed to maintain the set speed.
    Jim Dawson
    Sandy, Oregon, USA

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