Re: [CAD_CAM_EDM_DRO] How do you compare steppers to servos

Brushed motors also wear out- the brushes need maintenance, and the
rotor contact rings have a finite lifespan. Asking for higher power
will increase the wear.

Brushless has no such limitations, and its life is probably ultimately
limited only by bearing life.

Danny

On 2/25/2011 10:09 PM, Jon Elson wrote:
> Glenn wrote:
>> Suppose a system is advertised using nema23 steppers of some size, say 260 oz-in.
>>
>> You want to build the same system but instead use servos.
>>
>> How do you compare the compare power of the the steppers to the servos
>>
>>
> Ohhhh! This will start another culture war!
>
> Well, steppers are rated by HOLDING torque, ie. the torque they can
> resist at standstill. The running torque, ie. the torque they can
> deliver at some speed, is less. Above 100 RPM or so (varies by motor
> and driver) the torque drops off as speed increases. But, you can try
> to get a torque vs. speed graph for the specific motor and driver
> combination, at some specific DC supply voltage. Then, figure out what
> motor speed will be required at the fastest cutting feedrate, and see
> what torque the motor will deliver at that speed. Then, figure back
> through the drive belts and leadscrew to figure the delivered linear
> force on the table.
>
> Servos have a generally flat speed vs. torque rating, and generally can
> run MUCH faster than steppers, so in many cases a belt reduction is used
> to allow the motors to spin at a more optimum speed. (Power out is
> speed X torque).
> Where it gets a LOT more complicated is that servo motors have a
> continuous and a peak torque rating. They can deliver the continuous
> torque all day, and the peak torque maybe for a few seconds out of every
> minute, but the peak rating may be 4 X higher than the continuous. But,
> most lower-cost systems don't have an RMS integrator to protect the
> motor against using the peak rating too much of the time. If you do,
> the motor just burns up.
>
> Brushless servo motors have a big advantage over brush type, as the
> coils are on the stator and have a good heat conducting path to the
> outside. Brush motors have the coils on the rotor, and removing the
> heat is a problem.
> But, brushless drives are more complicated, hence cost more.
>
> The bottom line is if the continuous rating of the servo motors equal
> the holding torque rating of the stepper, you can't possibly go wrong.
> But, this will result in larger motors than you really need. In most
> cases, if the peak rating of the servo is equal to the stepper's holding
> torque, you should be OK, but this is a VERY rough approximation.
>
> If the stepper is run at a very high speed, such as driving a 20 TPI
> leadscrew on a Taig or Sherline, then it actually develops very little
> torque under those conditions. (60 IPM on a Sherline, for instance,
> requires 1200 RPM. Drivers with a low supply voltage won't have much
> torque left at that speed, if they can even get there.) With a servo,
> you might use 2:1 or even 4:1 belt drive to run a servo at a better
> speed, and get even more torque.)
>
> Jon
>
>
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