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Does a RPC help buffer inrush current to the load motor

Brian Campbell

Plastic
Joined
Dec 14, 2021
Hi Guys

I have almost finished building my RPC.

I have just found out our power supply company has a limit on starting motors direct online of over 4kw due to voltage drop affecting neighbors etc.

I have a 7.5kw lathe motor I would like to run. The RPC I am building uses 4 x 2.5kw motors that will be started using a staggered start to reduce in rush overload.

I was under the impression that when the 7.5kw motor was started the 10kw of motors already rotating would help buffer the the in rush drawn from the utility? with the motors acting like an electrical flywheel, am I mistaken?

The other thing to add is that I have 2 of the 3 phases and a neutral already present so am just using the idler motors to generate the third phase.
 

Karl_T

Cast Iron
Joined
Jun 13, 2008
Location
Dassel,MN,USA
Maybe a tiny bit of buffering, not significant.

Now a 7.5KW motor is pretty small compared to the 200 amp services common in the USA. Unless you have the very old 60 amp service, it should not be an issue.

If it is a problem, a VFD is your answer.
 

JST

Diamond
Joined
Jun 16, 2001
Location
St Louis
A "soft start" of some sort is usually required for larger motors by most powerco's. The size that is "larger" varies, in my locale it is about 5 HP, or just under 4 kW.

The VFD is not your only option. A soft start circuit works just as well on an RPC as it does on straight 3 phase powerco supply.

There may be more buffering that you might think. Much of that is due to being 3 phase, and thus having less current draw per leg than straight single phase. The power drawn through the generated leg is buffered a bit by drawing off stored rotational energy (not much unless you have a flywheel on the idler) and partly by the natural larger impedance of the generated leg.
 

BT Fabrication

Stainless
Joined
Nov 3, 2019
a motor is a large inductive load. so an inductor will slow down the speed of the electrons if its already loaded. if unloaded it will cause it to spike due to magnetic forces in the coils. a transformer works pretty decent at being a better preventer of in rush current as its basically a massive magnetic field of wire.
 

rons

Diamond
Joined
Mar 5, 2009
Location
California, USA
I have a 7.5kw lathe motor I would like to run. The RPC I am building uses 4 x 2.5kw motors that will be started using a staggered start to reduce in rush overload.

I was under the impression that when the 7.5kw motor was started the 10kw of motors already rotating would help buffer the the in rush drawn from the utility? with the motors acting like an electrical flywheel, am I mistaken?

The other thing to add is that I have 2 of the 3 phases and a neutral already present so am just using the idler motors to generate the third phase.
If you have already obtained the 4 little motors then my opinion of that is left out...

Your option is to use a small capacity motor to spin up the "1" idler motor using a belt or clutch-like device, maybe two friction plates that disengage.
 

johansen

Stainless
Joined
Aug 16, 2014
Location
bainbridge island
The only way to use the rpc motor as a flywheel is to put a cvt between a flywheel and the rpc and then pull energy out of the flywheel to over drive the rpc.

The rotor of an rpc slows down from say 7rpm less than synchronous speed to 20rpm less than synchronous speed within like.. 6 line cycles. The energy contributed in the difference in rpm is so low it would be difficult to measure.

That data comes from a 4 HP motor, 3600rpm.
 

JST

Diamond
Joined
Jun 16, 2001
Location
St Louis
That i true.............

BUT, the start current is a fast surge, much faster than 6 seconds. And, the load is nearly a short circuit, so the removal of energy (as heat) is fast.

A flywheel can help, due to the drive voltage to the RPC also dropping, so that energy input to the rotor is reduced, and stored energy can be more important. I did calculate it out once, and found that there was a reasonably significant contribution to the fast surge.

The difference was that without it, the rotor (lower rotating mass)was slowed significantly, while with it there was less slowing and somewhat more capability to provide the surge (higher rotating mass). Calculation is very dependent on assumptions, and it is not easy to verify the results.

It could be that the effect is less than the assumptions predicted.
 








 
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