Re: [CAD_CAM_EDM_DRO] PWM servo amps, and old servo motors

Brad Heuver wrote:

> I am looking for servo amps to replace the 1970's vintage ones on my
> tape-O-matic. The X and Y axis have Reliance 457665-MV 3/4 hp motors, type
> TR with 6 wires. They are 120V, with a 6.0 amp rating on armature, and a
> 0.65 amp field rating. I think the Z is the same, but its nameplate is not
> visible.
>
> Is a pwm amplifier one that will send high frequencies to the motor that
> will harm its insulation, vs other types of amps?

At the voltages used in these systems, that is not likely. I put L-C
output filters that reduce the PWM carrier to less than .1 V on my servo
amps, but most PWM servo amps don't have the filters. I've never heard
of the motors being damaged by it. On 440 V motors run on VFD's, that is
another story.

Before PWM, there were SCR phase-controlled amps, which had HORRIBLE
bandwidth of a few Hz. The CNC control had to be massively slowed down
to keep the servo stable. There were also linear servo amps, which were
usually water cooled. Don't even think of running these, they will eat
you out of house and home with electricity consumption.

>
> Looking at the Advanced MOtion Controls website, they say of their
> amps:
> "These amplifiers drive brush-type DC motors or inductive loads at a high
> switching frequency and can be configured in various modes and accept an
> analog ± 10V command signal." Their literature goes on to say that the pwm
> amps have a 22kHz switching frequency.
> Just looking to get a bit smarter in picking amps for conversion of the
> old tape-O-matic.

These amps will probably work fine. Does your machine have DC tachs
and encoders? If it has resolvers, that is a problem, and you'd have to
get an interpolator box or replace them with encoders for most new
controls.

Jon