Saw Running Slow or Just Quiet?

[Bill Adamsen]

I have a larger Italian made Bandsaw that I was running for a long time (several hours?) continuously today and it seemed like the machine seemed to "get quieter." I don't have any sort of blade velocity gauge to reference, that would have helped. It did not seem to lose any power, though the job I did was not very demanding.

I popped the cover off the 3-phase load center and legs between the breaker and the saw were reading a steady voltage and current. But this baffles me. The voltage coming in to the RPC (single phase) is 243.7 volts between the legs ... 120v & 124v for each leg (L1 & L2) to ground. Unloaded (no tool on) the 3-phase made leg (T3) to each leg are the voltages... L1-T3=262, L2-T3=271 and L1-L2=243.9. Loaded (and by any of the operating machines) those drop to approximately L1-T3=240, L2-T3=252, L1-L2=242. All within a reasonably small percentage of each other. This is well within the range specified by the manufacturer.

The current on the Bandsaw was L1=4.7a, L2=11.4a and T3=10.2a. So while the voltages are normal, the current readings are baffling variable to me. I don't have a prior current recording to refer to. Should I be concerned about this? I guess I can go out and check the current readings on other tools running.

[update] Just checked two other machines with similar kilowatt motors and the amp draw is very close on both machines and within about 25% overall ... L1=9.4, L2=12.5 and T3=12.8. The variation on the bandsaw is 40%+. It could be connections ... will check tomorrow. Interestingly, nothing got hot on the motor or bearings today during work. It seemed to be running along smoothly. Too smoothly.

[Bill Space]

Hi,

The three currents in the legs should be about the same. Why different? Not sure.

First check all connections in the leg with the lowest current.

After that? Motor problem?

Realize this is not much help, but for sure the motor phase currents should be about the same for a three phase motor being supplied with about equal phase voltages.

Since you have measured equal voltages at a point between the RPC and the motor, the issue is not likely with the RPC.

If Phase to phase voltage is equal when applied to a three phase motor, the phase currents to the motor legs should be close to equal too.

So the guess would be an issue in the leg with the low phase current. Something between the motor terminals and the RPC terminals.

Of course shorted windings within the motor could also be an issue...But check the easiest things first.

[Bill Adamsen]

Question
Can anyone tell me from the attached nameplate, what the RPM might be for that motor?

This scenario bothered me quite a bit last night. First thing this morning I pulled out an older "log" (where I stupidly hadn't done current readings), went to the shop and checked to make sure all connections were good, that all the L1,L2,T3 connections were running correctly from RPC to the machines, and took readings (attached). I'm not happy about the current imbalance on any of the machines ... especially the bandsaw. The RPC manufacturer's troubleshooting guide says "Q: if the three-phase currents are not equal then A: if the load comes up to its rated load and none of its three legs exceeds its nameplate FLA rating, there is no problem." Based on that, there is no issue. I'm not convinced.

[John Lanciani]

1700 rpm under load if you are running it at 60 Hz. (It will be closer to 1800 idling)

[Bill Adamsen]

So that symbol "G/l" literally means "Giri (revolution) under load" ... I should have put more effort into looking that up. Thanks John!

[John Lanciani]

revolutions per minute. (1') Induction motors are typically rated under load, at idle it will be running at almost synchronous speed - 1800 rpm for a 4 pole motor; http://www.oddparts.com/acsi/defines/poles.htm