[CAD_CAM_EDM_DRO] Re: Stepper Motor Power Calculations

Excellent answer, thanks. So the short story is that overkill is not a bad
idea unless you are designing a commercial power supply for volume
production and need to squeeze every penny. I was thinking about the dynamic
effects and it makes sense to use the highest supply voltage possible for
the best high-speed performance, since torque drops off at higher RPM and
inductive reactance increases at higher frequencies. I presume most
solid-state motor-control current regulators dynamically adjust the PWM duty
cycle to compensate for changes in power consumption at different step
frequencies, but have not seen data sheets state that specifically. If not,
I wonder if it makes sense to have an external microcontroller that does
that. I have observed that stepper motors dissipate a lot more heat when
holding than when spinning, if holding current is not reduced. I would think
that the motor current ratings are for holding current, and could be
substantially exceeded when stepping at high speed without damaging the
motor, since heat dissipation is the main reason for current limitation.
That would also make a case for heat sinks and fans on stepper motors.
Wouldn't it be funny if people started juicing up their stepper motors the
way people juice up their Pentiums now. Maybe empirical observation is the
only way to find out, as you suggested. The thing is, I've re-invented so
many wheels in my life time that I'm a little gun-shy now. It's worth
spending some time to find out if someone has already done it.

>
> Power is measured in watts and has a voltage, current and time factor.
>
> The average wattage over time is a function of the voltage times the
> current times to period of the waveform. In a chopper drive the
> voltage is applied to the coil and due to reluctance it takes a finite
> time to move the charge into the coil. The higher the voltage the
> faster it moves into the coil. The drive pumps voltage into the coil
> until the current is reached then the pulse is cut off. The energy is
> only being transferred during the time the pulse is on. SInce the
> current will reach the cutoff point before a 100% waveform the energy
> is whatever the percentage of on time is. The less load the motor has
> the less time it takes to reach the cutoff point.
>
> You are correct about the wattage. If the motor would dissipate 18W
> at it's stated current and voltage it would still dissipate that
> amount with a higher voltage but limited to a waveform that gives the
> required current. What changes is the fact that you have to put more
> energy into a system than you get out so if the motor is working it's
> going to dissipate more energy and needs more watts. No motor is 100%
> efficient so you have to build in enough reserve power in the
> calculation to compensate for loses and loads.
>
> The (very) conservative number is about 60% of the total current
> ratings of the motors BUT in most circumstances that is an extremely
> simplistic approach. As the supply voltage goes up the average
> current is going to diminish since the period of the waveform will be
> less. In a lot of applications not all three motors are running
> heavily loaded at the same time and since we are dealing with power
> it's over time. Linear power supply components have a lot of
> "inertia" and can handle 50 to 100% overloads for a certain duty
> cycle. I find that even 50% of the total motor ratings running at
> least 10X the motor voltage rating is sufficient in all but the most
> demanding aplications.
>
> There is a general belief that if you need a 300W supply then a 500W
> is better and a 1000W is even better than that (arh, arh). In reality
> it's just wasted money and weight siting there doing nothing. Sort of
> like having a 300 hp go cart (:-)
>
> The best way to design a power supply is to actually measure the
> current of the machine under the most heavily loaded condition and
> then add a 20% fudge factor. Of course that assume you have some way
> to power it to begin with with most HSCNC'rs don't so they just build
> a monster power supply and go on. Given the nature of components a
> power supply that is double the size you need won't cost double the
> price. The expensive componets like the transformer and filter cap
> will cost at least 50 to 70 % more and since to the home builder space
> is plentiful and labor is free those are not factored into the actual
> cost.
>
>
>
>
>
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