Re: [CAD_CAM_EDM_DRO] Can someone please explain this.......

Greg Jackson wrote:

>Roland Fresitad has quite a reputation, but I am more than a bit confused on
>the power supply engineering in his current article in "Home Shop
>Machinist".
>
>1)
>The transformer is .5 KVA with an input voltage at 110 VAC. This would
>suggest a continuous current rating of 4.5 amps. I would think that a slow
>blow fuse ( 5A or 6A) would be needed to account for cap inrush, but a
>continuous rating on the fuse should not exceed the continuous rating of the
>transformer. His article prescribes a 10A fuse.
>
>

It works, and he hasn't had a fire (YET!) A smaller, slow-blow fuse,
would be a MUCH
better idea. A smaller, fast-blow fuse and an inrush surge limiter
would probably be
the best design.

>2)
>The output rating of the transformer is stated as 64 VAC at 8A, consistent
>with 0.5 KVA. With the 1.414 factor to DC, this puts the DC supply at 90
>volts. Hello? Isn't the Gecko rated to 80 volts? Any back EMF on
>deceleration will take it even further. A rapid stop on this machine could
>drive it over 100 V if there is much mass moving around. I realize that,
>with such an undersized transformer, the voltage will sag when drives are
>moving, but it still gets back up there when there is no current being
>sucked up.
>

The transformer is rated at 64 V, but he doesn't note anything about how
that
number is specified. He doesn't state 64 VAC, RMS, or anything. I'm sure
it is not, really, a 64 V AC RMS, but something lower, that gives 64 V DC.

>3)
>The dropping cap is listed as 500K Ohm. Maybe I did my calculations wrong,
>but that looks like it will drop the cap from 90 volts in 11 hours. I would
>think 60 seconds would be a nicer number to go for.
>
>

Yup, 500 K Ohms on a 27000 uF cap is useless! You want it to dump the
stored
energy in a couple of minutes, at the worst. The time constant works out to
3.75 Hours, meaning it reduces the voltage to 37% of the original voltage in
that time.

>4)
>There are 3 drives in the system show, each rated to 7 A. While they will
>be current limited through induction at high speed and go into current fold
>back when at rest, there are actually times when all three drives will be
>operating at 7 A. Even if the DC bus is at a reduced voltage due to the
>limits of the transformer, we're still talking about at least 1500 watts DC
>being delivered through a 0.5 KVA transformer.
>
>

No, this is not true. These PWM (Pulse Width Modulation) drives are
essentially
power in = power out + losses. The internal losses in these drives is
in the tens
of Watts, maximum. The voltage dropped in the motor windings at idle is
very
low, around 2 V maximum. With 7 A rated per coil at 2 V, that is 14 * 2
=28 W.
So, the drives will need something like 38 W per axis. 38 x 3 axes is
114 W,
not 1500.

The emergency stop switches are usually designed for control circits,
not power,
and may not be able to break the current in some failure modes.

Jon