Thread: Cost of running RPC setup?

Hello ALL, im curious as to the amount of power a,say, 5HP idler with 5HP load is going to cost, can anyone estimate KW hours for running said load for 7 Hours a day?

A bare 5 hp idler is a bit small for running a 5 hp load motor.

But the real power consumption for a 5 hp idler is dwarfted by the
load motor. My 5hp idler uses between 220 and 240 watts running
unloaded, depending on how it is measured. That's about 1/3 hp.

Jim

The operating cost of running an RPC (as distinguished from the capitalized cost of same) is fairly low.

The RPC is acting as a rotating transformer, and the cost of running that rotating transformer is attributable to the windage losses in the rotating machinery plus the copper losses in the windings.

IOW, not very much.

However, if you are on a demand meter, your costs could be significantly higher as you would also be charged for VARS attributable to the generally low power factor of an RPC, for which you can compensate, within reason, by applying PFC caps.

Youd use correction caps to balance the 3phase as well? Can i assume a balanced to within 1% idler would negate any significant vars charges?

If running multiple load motors, the size of idler is suggested to be 1.4x the largest load motor, and the RPC can be tapped for up to 300% its idler FLA if not heavily loaded or started simultaneously,acording to this website.http://www.phase-a-matic.com/PDF/RTN-2009.pdf


Colin

If you have a multi-meter you could put an amp clamp on the feed wires and see how much juice the RPC is drawing with various loads.

Iv got a cheap one but only does DC current, will invest in one cos i need to size thermal overload for idler, which one source suggests to be 58% FLA of motor under utilisation catagory AC-3? http://cr4.globalspec.com/thread/113...ar-delta-motor

1) amp clamp meters measure the sum total of reactive
and real currents. As such they are a poor measure of
the bearing and windage losses on unloaded idlers, which
have a very poor power factor.

2) tuning the idler with capacitors, to improve its power
factor, will NOT change the cost to operate it, when it
is being run from a residential service, which only meters
for real power, NOT vars.

Jim

Do these losses reduce once load is applied on the idler, ie the motor is more efficient when loaded? Gents thank you

There are real losses in the idler, basically windage loss and bearing drag. Those don't
change when the load motor is being run, they are in addition to the power consumed
by the load motor, as it is doing work.

Jim