Re: [CAD_CAM_EDM_DRO] Gecko burned - now motor types

Hi Ron,

Electrically there is very little difference between a stepper motor and
a DC motor. Internally they have a number of coils and magnets. If the
magnetic fields from the coils don't line up with the magnets then they
generate a torque in the rotor, trying to line up. In a stepper this is
the detent torque.

With a stepper motor the coil switching (called commutation) is done by
the drive. With a DC motor the switching is done by the brushes and
commutator. You could use a DC motor as a crude stepper by directly
connecting power to the coils and manually switching them, bypassing the
brushes. Of course in practise you would have quite large steps and the
wires would get twisted off as the motor turns.

With a constant supply voltage the current through the windings
increases as the speed falls. So why don't steppers catch fire when they
are stationary? Simple - At low speeds the drive reduces the supply
voltage to compensate. This can be as simple as some series resistors or
it can be a clever chopper circuit.

The real reason why servos are so much faster than steppers lies in the
commutator. As it is directly linked to the motor shaft the coils are
always switched at the correct time. They cannot get out of sync. If the
motor slows down the switching slows down to compensate. Stepper drives
have no connection to the motor shaft so they have to assume the motor
can keep up. This severely limits the motor's operating envelope.

Brushless motors use a position sensor to tell the electronics exactly
where the motor shaft is so they can perform the commutation. As there
are no brushes the motos have lower inertia and are more reliable.

Mariss is working on a system that monitors the stepper's shaft position
and feeds that information back to the controller, effectively turning
the stepper into a brushless DC motor.

Les



R Rogers wrote:

> /// From reading about steppers and servos it seems that steppers merely move when told to and do not sense a load and draw more current to overcome it, they skip and lose position instead when confronted with a higher torque requirement. Steppers are all about holding stationary position and not big on traversing to a new one. Servos on the other hand sense a load and draw more amperage to overcome the load and thus hold position at higher torque requirements. Is this accurate? My Bridgeport originally had steppers on it. Only 2 of the three worked reliably and one of them skipped incessantly. During the retrofit I switched to servos and there is no comparison. Rapids of 120 ipm and never loses position. I sold the steppers on Ebay for what the servos cost from the same. An even swap with a huge gain.
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> Ron
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> /////