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