Back to my 5 hp lathe motor

[1937 Chief]

About 2 or so weeks ago I posted about my 5 hp already made rotary phase converter. Every thing was marked as the power in, and 3 phase out. All I had was a three phase idler motor to supply 3 phase. There was some discussion, if the 3 hp idler was big enough. I powered it up. the idler motor started ok, I had 230 3 phase coming out. Good so far. My lathe motor started, but not immediately. There was second delay, and a growling noise probably from the motor starter. relay. After the motor got up to speed the noise stopped. I have heard a lot of motors start, and they were right now, no noise. So with all that information, is it possible, my 3 hp idler motor is not big enough? It was nice to hear my Rivett lathe come to life, after being delivered several years ago.. Thanks for any help, Stan

[J Tiers]

Check to be sure that the POWER to the various relay coils etc does not come from the generated leg. If the wiring was all laid out for you, it should not, but it is a s well to be sure. If it is, the voltage may drop and cause anything from some chattering to a failure to start.

One exception..... if there is a "voltage (potential) relay, it must be connected to the generated leg to sense when the idler is up to speed.

However, a similar effect can occur if the idler is too small. The start of the load motor may be slow and involve some humming. In some cases the start circuit can re-engage, which is bad for it, and can cause humming and growling.

How big is the lathe motor?

And what kind of Rivett do you have?

[1937 Chief]

The lathe motor is 5 hp. Stan

[J Tiers]

Normally, a 3 HP idler would be considered too small to do well with a 5 HP load motor. It may start it, but not in a very fast and snappy way, most likely.

Of course the HP is not directly concerned, but the HP implies things like current capacity, and impedance. So higher HP will have lower voltage drop when used as an idler.

The 3 HP is trying to supply 5x the normal draw of the 5 HP motor, during a start. that's a big ask... Obviously a motor with lower impedance will do it better. That is why the usual ratio is to have the idler 1.5x the load motor for a single load. For multiple loads, then any others that are running but not under much load, will act as supplementary idlers.

[SVS]

Chief-Please refresh my memory-Does your lathe have a clutch?

From HERE, I think the “box” has a potential relay which seeing low voltage when spindle motor tries to start, closes (loudly and repeatedly) putting the start capacitors back on line.

(My GWM brand 6-10hp “box” did same thing running my Cincinnati Vercipower mill if spindle clutch was engaged in a high gear with cold oil. It starts a 7.5hp feed motor, then 10hp spindle motor(20hp originally) then an electric over hydraulic spindle clutch engages.)

I agree idler isn’t big enough, AND your “box” isn’t big enough to run one that IS. So, if lathe has a clutch, ditch the idler motor, hook box direct, and see what happens.

If motor must start loaded, this isn’t gonna work.

[ezduzit]

You need a 10 hp RPC for a 5 hp lathe.