Thread: VFD and RPC.

a 15amp circuit only delivers 3600watts , the inrush currents on the 5000watt phase converter would be staggering , you need an electrician to dig up your back yard

Hi

From my understanding because it is a rotary converter and uses a motor to get the third phase, you would encounter inrush current on startup, this inrush current could very well be several times greater the rating of the motor.

There are several ways to reduce the size of inrush current, (assuming you currently are just using a standard switch), such as using a soft starter (which can cost a bit) but it does put less strain on the motor on startup which can reduce maintenance costs in the long run. It can reduce the inrush current by steadily ramping up the supply to the motor, rather than just going straight to full in milliseconds.

The whole idea of the Circuit breaker is to protect the wiring, so there are limits on how far you can change a circuit breaker without changing the wiring and without knowing what wirie size is installed. A type circuit breakers have very little tolerances for inrush so can trip easily on startup, as you move down the scale the amount of inrush tolerated increases. Type C (trips at 5-10 times on inrush) and D type (trips at around 10 - 20 on inrush) is not all that uncommon when it comes to motors in particularly three phase motors which don't have any soft starting mechanisms.


I highly recommend an electrician to look at your setup especially with something of this scale and so many variables which are unknown, he should be able to use a clamp meter and measure the inrush current and go from there.

I shouldn't think it would be hard for a sparky to rectify, but without being able to see all the variables it's hard to be concrete about anything.


All the best on your setup and I hope everything is smooth sailing from here

David


Disclaimer: This is not a how to guide, all electrical work should be carried out by a licensed electrical contractor

Originally Posted by SawDustSniffer

a 15amp circuit only delivers 3600watts....

How do you come to that conclusion?
A 15A OUTLET is not meant to be LOADED with more than 3600W continuously. A 15A circuit is usually protected by a circuit breaker of 20A, 25 or 32A are also common when the circuit is protected by an RCD as well. These circuit breakers will trip if that current is overstepped for more than a few seconds - the higher the current, the faster they will trip.
So a 15A circuit could be delivering 50 or more A for a very brief period.

I doubt that the inrush current for a 5kW RPC will be too great.
I also agree: talk to a sparky to ensure you get it right.

Joe

I had a similar problem and managed to solve mine by staggering the start up sequence.

In your case however, it looks as if you may need a soft start circuit. This allows a certain amount of current to flow to start the machinery up and then after a while allows the full amperage to flow.