28 Apr
2005
28 Apr
'05
5:55 p.m.
All,
On 4/28/05, Paul Koning <pkoning at equallogic.com> wrote:
>>>>> "Simon" == Simon Fryer
<fryers at gmail.com> writes:
[...]
Simon> All the solid state phase converters I have
come across use a
Simon> three phase H bridge to generate three phases from a common
Simon> intermediate DC rail. The incoming voltage is converted to DC,
Simon> switched, PWM synthesising a sinusoidal at the required
Simon> frequency.
Simon> The PWM works pretty well for induction motors. I would expect
Simon> big iron to have some issues.
Why? I would expect power supplies to be every bit as tolerand of
distorted waveforms as motors are, even assuming that these converters
produce major distortion.
Two areas of initial concern.
1) Transformers in linear supplies seem to be very well suited for
50/60Hz. Trying to feed them with a square wave with frequencies of up
to 30KHz + would probably not do them a lot of good. They may be able
to handle it just fine, but then again they might not.
2) Recovery times for the diodes in the rectifier. I initially though
this would be a problem. The datasheet for a 1N4001 specifies
capacitance at frequencies of 1MHz. I guess the 30KHz + of a phase
converter is well within the stride of rectifier diodes.
Simon> Depending on how the three phases are used
for the big iron,
Simon> it may be possible for all three phases to be wired the
Simon> same. That is electrically connect all three phases together,
Simon> to a single phase supply. This can only be done if each phase
Simon> is used independently of the others (separate
Simon> transformers/SMPS). If there is anything in the machine that
Simon> wants all three phases - find a nice clean sinusoidal source.
Motors will -- disk drives for example. The larger disk drives (DEC
RP04 class, for example) use three phase motors. They also are picky
about the phase order being right, otherwise they spin on the wrong
direction which is not a Good Thing.
MMMmmm, out of phase wiring! I have heard of people who keep three
phase extension leads wired out of phase to solve these sort of
problems.
Some machines just take the three phases and split them out to
multiple single phase devices. Kind of like having a local three
single phase distribution boards at the back of the rack.
Simon
--
------------------------------------------------------------------------
"Well, an engineer is not concerned with the truth; that is left to
philosophers and theologians: the prime concern of an engineer is
the utility of the final product."
Lectures on the Electrical Properties of Materials, L.Solymar, D.Walsh