Sync RSG motor problems - help and thoughts needed

[newage]

Hi all
First post so go easy on me..... 

I`m running a 2hp single phase 2800rpm induction motor, with 100uf start cap and 30uf run cap, the motor has now gained a fault where every time i switch it on it blows the 13A fuse.
I`v had a mate put a mega across the windings and they all show as good (The motor does start to run for about half a second).



This Sync RSG had been working fine, I tested out the start and run caps and they seem ok but as they are cheep-ish items I`m going to swap them out for the same value but with
a higher voltage rating say 450V AC.

I have been looking at a soft starter from RS components (United Automation Soft Starter, 230 V, 1 Phase - RS Stock No.:330-5358).
If I were to go down this route what are peoples thoughts and should I remove the start cap but keep the run cap.

Cheers Mike T   

[MRMILSTAR]

Was this motor designed and built as a synchronous motor or did you modify a normal asynchronous induction motor by grinding flats on the armature? 2800 RPM sounds like an odd number.

[johnf]

100uf cap has a fault probably short

[newage]

The motor has been modified by milling flats on the rotor to make it Sync.

Cheers Mike T

[MRMILSTAR]

The motor has been modified by milling flats on the rotor to make it Sync.

Cheers Mike T

Grinding flats does reduce the motor's torque. If you cut off too much, the motor may have trouble starting.

[klugesmith]

Sounds like it used to work after rotor modification, then stopped working.

Capacitor failure is first guess.  Or switch failure, so the start winding stays energized.
But maybe the modification increased the running current, or starting current x time,
enough to make the windings get so hot that insulation failed somewhere.
Might take minutes or hours of operation for a short to develop.

Current measurement today might help, even if you didn't record the value in better times.

A short in the windings might be confirmed and located by doing a growler test on the stator.
Come to think of it, there are only 2 poles in the 3000 RPM (synchronous) stator.
How about making a connection to middle of run winding, between the two coils, and to middle of start winding.
See if the applied voltage (low DC or low AC) divides evenly between the two winding halves.
The AC test would be much more sensitive to a short that bypasses only a few turns.
Could do it as a bridge measurement: voltage between inter-coil connection and a two-resistor divider.

[newage]

Hi there

Yes indeed the SRSG has been running smooth and true for months.
I’ll pass the ideas along to my sparky mate who has the test gear.

The internal switch gear looks to be in good order, I’ll change the caps first (first one turned up today metal can 30uf 450v AC)

Cheers Mike T