Re: [CAD_CAM_EDM_DRO] Digest Number 2381
Posted by
Eric Keller
on 2002-06-14 16:51:33 UTC
Peter asks:
dc, have the magnets in the stator case and windings
on the rotor. Thus they need brushes to get the
current to the rotor.
Brushless motors have the opposite arrangement of
magnets and windings. Thus no brushes. They are
three (or rarely, more) phase motors. The difference
between AC and DC brushless motors are almost
nonexistant. I have seen it explained as some have
sinusoidal windings so that the motor doesn't jump at
the transition between phases, but I'm sure there are
DC motors that are built the same way. Usually it
means that the feedback device will drive an amp that
requires a sinusoidal signal.
The price to be paid for the lack of brushes is that,
whereas PM DC motors speed is regulated by the dc
voltage across the motor, brushless motors need to be
commutated. Practially what this means is that the 3
phases are driven by a time varying voltage that is
somewhat like 3 sine waves at 120 electrical degrees,
or trapazoidal waveforms that are approximations to
the sine waves.
The feedback on a brushless motor can consist of a
resolver, hall effect sensors, halls with encoder, an
encoder with simulated halls, encoders with sinusoidal
tracks, and a lot of others, since there are effective
algorithms for sensorless drive, and thus you can use
any sensor to improve the sensorless algorithms. The
new motors coming out almost all have halls with
encoders.
Since I am not going to spend upwards of $1500us on
each axis drive, I stick to ebay. For most people, it
is best to buy matching motors and amps. The newer
the amp, the more generic it is, but again, the new
ones mostly cannot use a resolver for feedback.
proportional to speed. An encoder drives a counter
with a digital signal. You really only need an
encoder unless the amplifier is analog.
Eric
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> What is the practical difference between AC and DC,brush dc, commonly called permanent magnet dc, or just
> brush and brushless servos?
dc, have the magnets in the stator case and windings
on the rotor. Thus they need brushes to get the
current to the rotor.
Brushless motors have the opposite arrangement of
magnets and windings. Thus no brushes. They are
three (or rarely, more) phase motors. The difference
between AC and DC brushless motors are almost
nonexistant. I have seen it explained as some have
sinusoidal windings so that the motor doesn't jump at
the transition between phases, but I'm sure there are
DC motors that are built the same way. Usually it
means that the feedback device will drive an amp that
requires a sinusoidal signal.
The price to be paid for the lack of brushes is that,
whereas PM DC motors speed is regulated by the dc
voltage across the motor, brushless motors need to be
commutated. Practially what this means is that the 3
phases are driven by a time varying voltage that is
somewhat like 3 sine waves at 120 electrical degrees,
or trapazoidal waveforms that are approximations to
the sine waves.
The feedback on a brushless motor can consist of a
resolver, hall effect sensors, halls with encoder, an
encoder with simulated halls, encoders with sinusoidal
tracks, and a lot of others, since there are effective
algorithms for sensorless drive, and thus you can use
any sensor to improve the sensorless algorithms. The
new motors coming out almost all have halls with
encoders.
Since I am not going to spend upwards of $1500us on
each axis drive, I stick to ebay. For most people, it
is best to buy matching motors and amps. The newer
the amp, the more generic it is, but again, the new
ones mostly cannot use a resolver for feedback.
>Depends on the amp. A tach puts out a DC voltage
> Is there a difference between an encoder and a tach?
> Or is RPM just a calculated by the controller from
> the encoders feedback?
proportional to speed. An encoder drives a counter
with a digital signal. You really only need an
encoder unless the amplifier is analog.
> I've seen servos that don't look like they haveUsually they are, almost always with brushless motors.
> encoders on them. Are servos often sold with the
> feedback hardware?
Eric
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