Fuji Frenic mini FRN0047C2S-2U single phase supply amps riddle

I've been using this VFD for about 3 years to run a Mazak 18x80 manual lathe from a single phase supply.
The Mazak has a 7.5 hp motor, so I'm running the VFD, as Fuji recommended based on the max load current of 27 amps with a single phase supply.
The VFD shows a current draw of 7.5 amps (230v) with the spindle running at 400 rpm with no cutting load.
With a 2 mm cut at a roughing feed rate it typically show 9+ amps; so I'm really not putting much load on the machine.

I've just bought an old Colchester 21x80 lathe with a 15 hp motor. Based on my experience with the smaller Mazak I felt I'd be able to use the same VFD and keep an eye on the amps to make sure I didn't overload the VFD.

The Colchester has a clutch to stop/start the spindle whereas the Mazak has direct connection between motor and spindle.

The VFD shows 19 amps (230v) with the Colchester motor idling, and up to 20 amps with the clutch engaged at 400 rpm or so.

I'm trying to understand why the Colchester is showing almost 3x the amps with spindle running, and no cut, compared to the Mazak.

The Colchester spindle turns by hand with some drag.
When the clutch handle is moved to stop, the spindle coasts down suggesting not much drag.
The Colchester motor is 15 hp, whereas the Mazak is 7.5; but that is the potential power, and I doubt there is much difference in load when spindle is idling.
I bought a clip on ammeter yesterday, but it didn't work when tested on a toaster, shoot!

The only thing that worries me is the "new to me" lathe came wired for 400+ volts. This is a European 9 wire motor. So I followed the instructions for 230v. The labeling of the 6 & 9 wires was not clear, so despite my best efforts I may have switched them. I'm not sure whether the motor would run with the possible error or not.

My plan now is to get a new clip on meter to validate the VFD current reading, and check each leg separately.

So do you think I'm right to think the "idling" current should not be much different ?
Remember the Colchester motor is simply turning the clutch shaft when idling. There is apparently a brake combined with this clutch, but the spindle turns by hand.

19 amp current draw is a lot of power !

I'm looking for any help please to understand what's going on.
Thanks in advance.
Bob

I've just checked the amps using a clipon ammeter. All three legs are each drawing 20 amps with the motor idling. Same reading as VFD meter.
The motor turns easily by hand.

Surely a motor idling shouldn't take that much current ?
Bob

Seems about right to me.

Most induction motors pull around 40% to 50% of full load amps when idling. However, the power factor when idling is low, as low as 0.1, so the actual "power" involved is low.

A 7.5HP motor is UL rated 22A full load. The 7.5A draw at no load is closer to 33%, but still believable.

A 15HP motor is UL rated at 42A full load. If it draws 20A at no load, that is 47% of full load, still within reason.

And, remember, it is NOT a "lot of power", because the current is drawn at a low power factor.

Thanks JST.
I've looked at data for new 15 hp motors and it shows similar info for no load.

I put the ammeter on the input to the VFD and found 8 amps.
So that threw me again.
Input to VFD is 8 amps at 230v single phase.
Output from VFD is 20 amps at 230 volts 3 phase ?????????

So are you saying the reason the VFD output shows so high is the low power factor ?

Does that mean that as I load up the motor the power factor will increase, the VFD output amps will stay about the same while the VFD input amps will increase ?

Thanks in advance.
Bob

Sort-of. The input amps will go up eventually, the power has to come from somewhere, after all. The way a VFD works will disguise the change a little. (I am assuming the VFD stays set to one frequency).

So the high motor current at idle is why the VFD output current is high at idle. The VFD input current is related to the actual power that it has to supply, so it is lower.

Input current to the VFD occurs at full voltage, and is closer to being directly related to actual power (the input has a power factor also). Since the motor current is at a low power factor, the actual power is low, and the VFD needs only to draw actual power from the line, so current is low into the VFD.

There are two "uses for" current in a motor. One "use", which remains constant, is the magnetizing of the iron. That stays the same regardless of the power level, and depends only on the voltage. The other "use" is to power the load.

The magnetizing current is at a very low power factor, maybe 0.1 or so. The directly load-related power is "actual power", and is at a high power factor (theoretically 1.0).

The combination of magnetizing current and power current is the input current. But that combination ends up at a power factor between the two. You could almost look at the difference between the idle current and the full load amps (FLA) as being the actual power needed to run the motor. So idle would be zero output, but draws the idling current, and 10% power output uses 10% of that current difference between idle and FLA, at a net power factor that is a bit higher than idle. It can look as if the output is supplied by a change of power factor, but in reality the two currents are relatively independent.

Motors can easily have a net power factor of 0.6 at or near full power. Some are higher. Newer motors often will have the power factor at full power listed on the data plate.

Thanks Mr. JST.
That helps me understand much better.
Bob