DrollTroll sez:
"Do multiple small rpc's have any inherent advantage over 1 big one--other
than easier starting, more economical operation?"
AFAIK, there is no theoretical reason why multiple small RPCs won't perform as well as one large one
providing the HP count is the same. However, like most things electrical and mechanical, efficiency
favors the larger package. The disadvantages of a single, large unit would include the possible
problem of the supply source being robust enough to provide the high surge current necessary to
start.
"Are five 2hp units in fact as powerful as one 10hp unit?"
Yes. Power is power, no matter how many increments the total is divided into. In some cases, it
might be an advantage to start multiple units in sequence to avoid the large surge current required
by a single unit of the same HP. Bringing the idler up to speed with a pony motor before throwing
the switch, is one way to start larger RPCs.
"Is it better to cap each smaller rpc, or just cap the net collection?"
Considering cost, It might be more economical to use individual caps in a collection of idler motors
There could be availability and cost issues with a single large cap. Otherwise, it's a wash.
"Is delta/wye an issue? One preferable to the other? OK to mix?"
Most 3-phase motors in the sizes you would consider for RPC idler motors are apt to be wye wound
machines. Mixing delta and wye motors in RPC's is probably possible, but well outside my range of
experience. Because of phase-voltage difference between delta and wye, a mixed unit might be
difficult to balance.
"If current pathways are complicated in multiple delta wound capped motors,
it must be really crazy in wye-wound."
Current pathways would be very similar. That is to say the 3rd leg connections to caps would all
come from the same electrical points on respective motors.
Post by Robert SwinneyThere is no synchronism between the speeds of idler and load motors in a
RPC. Speeds of each are a
function of the mains frequency, usu. 60 Hz. When a load is a applied,
neither idler or load can
slow down below their specif slip speeds. In a RPC, currents circulate
between 3-phase idler and
3-phase load in a complex fashion. There is no separate "generator" and
"load" in a RPC. Current
flow from the mains can be traced from one side of the line through 2
windings each of both the
idler and load. Thus, there are two 3-phase motors connected across the
single-phase mains each
running as a single-phase motor.
The above is not quite true because the unconnected 3rd leg acts as a
transformer winding within the
rotating mass of copper and iron. If that path is completed the third leg
will be energized by
transformer action. This accounts for the well-known phenomena that
enables a 3-phase motor to
start and run if another 3-phase motor is running as a single phase
machine across single-phase
mains.
It is the 3rd leg current that is augmented by capacitance in a RPC.
Current flow analysis of an
idler and load motor would show series currents tending to flow through
the balancing capacitance(s)
in both directions. That, of course, is impossible. However, when when
considered as a network, it
can be shown that series capacitance is used to tune (series resonate) the
"bidirectional" 3rd. leg
current bringing voltage to a value similar to the other 2 legs.
Do multiple small rpc's have any inherent advantage over 1 big one--other
than easier starting, more economical operation?
Are five 2hp units in fact as powerful as one 10hp unit?
Is it better to cap each smaller rpc, or just cap the net collection?
Is delta/wye an issue? One preferable to the other? OK to mix?
If current pathways are complicated in multiple delta wound capped motors,
it must be really crazy in wye-wound.
--
DT
Post by Robert SwinneyBob Swinney
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