In article ,
PrecisionMachinisT wrote:
"DoN. Nichols" wrote in message
...
In article ,
larry g wrote:
[ ... ]
One of the drives is rated for 220V single or 3-phase. Should I use
these
drives fed from the single phase, or should I put them on the 3-phase
side
of the RPC? I'm leaning toward using them downstream of the RPC so that
the
VFD's are not derated. Is there any electrical reason not feed them from
the RPC?
The only reason that I can think of depends on how well balanced
the RPC is -- at the load which the VFD and its load produces. If the
balance is not petty good, you will likely get the same poor
distribution of current through the rectifier diodes at the input to the
VFD that running from single phase produces.
The rpc is ( hopefully ) currently for balanced for the average inductive
load that is expected to be placed upon it.
This includes the converter and the average motor load ( inductive ) this is
a resonant circuit.
The VFD is not a resistive load. Nor is it an inductive load.
Instead, the input view of the VFD is a three-phase bridge rectifier
feeding into a *big* electrolytic capacitor.
The result is a current spike whenever one of the peaks goes
above the voltage in the capacitor (plus the forward voltage drop of two
arms of the three-phase bridge). The result should be a spike of
current near the peaks (positive and negative) of each phase relative to
the others. Normally, unless the output of the VFD is quite heavily
loaded, the spikes are of very short duration.
The problem, in my mind, is that if the voltage of the three
phases is not well balanced, you will get your current spikes only
between the two arms which represent the highest voltage, and it will be
little different than running it from single phase.
So -- I feel that you should assume the derating will be needed,
since while things may be well enough balanced at the moment, if the
value of the tuning capacitors shifts, it will be running in
single-phase mode anyway. Better to pay for the extra capacity now than
to discover that your VFD has been taken out by a drift in the balance
of the RFC.
If the system is up to this, it would follow that further resistive loading
( as in cnc equipment ) simply increases current draw, no ???
CNC equipment is probably a mix of inductive loading (any
motors, including the spindle motor if you aren't running that from the
proposed VFD), and the spikey capacitive loading from the power supplies
running the computer circuitry, the stepper motors (if that is what you
are using), or the servo motors.
My system supplies to several cnc machines, there is little reactive loading
of motors other than the coolant pumps.
So -- it is mostly seeing spikey capacitive loading, and
probably some VFDs (perhaps invisibly) built into the CNC machines to
run the spindle motors -- unless those are DC motors.
It seems to work fine, there is 50 hp of idler on a 100 and a 70 amp feeder,
to 4 cnc machines, rated average 40 amps each.
Quite a set of equipment -- and a more serious idler than in
most HSM RPCs, so you might get good enough balance to benefit the VFDs,
but I still think that it is safer to assume that you'll (sometimes)
need the derating that the vendor suggest for single-phase operation.
Good Luck,
DoN.
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