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Driving a Fanuc spindle with a CT Unidrive SP, Help needed!


Jan 21, 2022

I've started a retrofit of my 1989 VMC with Fanuc OM-B. i have previously converted a CNC late and now it is time to do the same with the mill.

The mill in question has a Fanuc 1S Spindle motor (A06b-0750-B100#3000

And i have done a lot of reading in different forums and when looking for a drive for a Fanuc spindle, pretty quickly the CT Unidrive SP's and Motion Guru pops up. So i bought a used SP1406 from eBay that i have started to configure, but i have run in to some problems:

I have tried both in open loop and in closed loop vector mode and in open loop when using the auto tune it runs but the power factor Pr 0.43 changes from default 0.850 to 0.997 and that feels way high. When running the auto tune in closed loop the tune goes successful but the power factor Pr 0.43 changes to 0.027 and some other parameters feels way off. I will list them further down. And after the auto tune i can't run the motor. no trips, and i can press run and hear it trying to spin the motor but nothing happens.

These are the parameters / values I've entered for my motor when setting up the drive in closed loop vector mode:

Pr 0.02 (Maximum speed): 8000 rpm
Pr 0.03 (Acceleration rate s/1000rpm): 2
Pr 0.04 (Deceleration rate s/1000rpm): 2
Pr 0.42 (No. of motor poles): 4
Pr 0.44 (Motor rated voltage): 200V
Pr 0.45 (Motor rated speed, base speed - slip speed): 2760 - 3000rpm (Tried both values)
Pr 0.46 (Motor rated current): 8.6A
Pr 0.47 (Motor rated frequency): 100Hz

In above parameters i have assumed that the motor rated voltage is 200V as it is on the label on the motor.

Also the base frequency i have calculated using a formula i found (frequency = (# of Poles * RPM) / 120) so in my case (4*3000)/120=100Hz.

And the rated current is also from the label (8.6A cont) but i feel it's quite high for a 1.5kW motor?

a few parameters that get changed from the auto tune in closed loop is these:

Pr 5.17 (Stator resistance): 0.400Ω
Pr 5.24 (Transient inductance): 1.768mH
Pr 5.25 (Stator inductance): 12.92mH
Pr 5.29 (Motor saturation breakpoint 1): 69
Pr 5.30 (Motor saturation breakpoint 2): 92
Pr 5.32 (Motor torque per amp): 655.35 N m A-1

I'm pretty sure i have entered something incorrectly and by that messing up the auto tune but i can't figure out what.

I have another regular 3ph induction motor with all the values on the nameplate and after entering those and running a auto tune that motor runs great so that is why i assume i have entered something incorrect on the Fanuc motor.

Any help / advice is very welcome!

Best regards Victor


Jan 21, 2022
I have done some more testing.

My guess was that some of the parameters i entered was incorrect.

And with above parameters it won't run after the auto tune.

I measured the voltage when running the motor from the original Fanuc controller and at 3000rpm it was about 100VAC.
So i changed Pr 0.44 (Motor rated voltage) to 100VAC instead of the 200VAC that is on the name plate.
Did a new auto tune and the motor runs very nice. almost as quiet as the original Fanuc driver. and with very low no load current.

is it likely that the 200VAC on the motor is max voltage and not rated voltage?

Also i got help and did some resistance and inductance measurements and we came up with 0.5Ω between phases, so the 0.4Ω that the auto tune comes up with is not so far off.

and for the inductance we used a scope and a 9v battery to do the measurements and we came to the conclusion that my motor should have 2.25mH inductance between phases. but the auto tune now with the 100VAC setting comes to 17.5mH and this sounds very high. any ideas?

Br Victor


Jan 21, 2022

Thanks for input!

I tried that yesterday.

200v rated voltage
266,66hz rated freqency

It works. Sounds good, runs cool.
Good power factor about 0.83 if i remeber correctly.

How will the constant torque from 0-3000rpm work now that i have rated frequency at max rpm (8000) ?

Br Victor


Hot Rolled
Dec 24, 2019
It probably won't get full power at 3kRPM. Why that didn't work I'm not sure; it implies the motor isn't intended for that many V/Hz. It's possible that the original drive was backing off the voltage under low load, but that seems unusually sophisticated for that era.