Am having problem with the spindle revs on this machine .

I fitted a Sprint 340 i DC drive unit quite a while ago in place of the lenze controlller which had gone pop .

All was fine for a couple of months and I had no detectable problems . It then began to fluctuate in speed but this could be "tuned out " using a range of adjustments in particular the IR compensation pot. It soldiered on for a while until the fluctuations returned accompanied by some apparent light knocking which seemed to be coming from the back end of the motor, ( or possibly from the Choke which is a 40 mH unit fitted as standard) . There also appeared to be significant arcing at the motor commutator in time with the knocking

I should add that this can be intermittent but often shows itself when the machine has been on for 20 mins or so.

I took advice from the suppliers of the DC drive unit (https://inverterdrive.com/group/DC-T...0-Isolated-1Q/)
(they assured me that these drives either work or go bang, they do not behave in this way)

On their advice I carried out extensive checking to ensure that the mains supply was not being interrupted . Checked all mains connections, replace any crimps which seemed dubious , isolated the drive from the bob relay and used a simple link to put it into the "On" position, and fitted a pot in place of the 0-10v analogue input . I then ran it essentially bare except for the control signal from the pot .

It was not long before the problem returned. At a speed of about 600 revs I was seeing a steady control input of about 2.6v but the output was fluctuating wildly around 45v , on occasions dropping to zero . The spindle speed on the computer display was also fluctuating
I concluded that the erratic output was causing the knocking which I felt I was hearing from the choke , along with the arcing at the commutator . The supplier assures me though that the problem is external to the drive unit.

The same motor connected to a 42 v regulated PSU runs sweetly at steady 42.3v and steady 650 revs showing on my computer screen.

Is it possible the choke is faulty? Can I test it ? Can I leave it out of the circuit to see if it runs better without? Indeed , Is it needed anyway? Can I replace it with something different ?
Is there anyone out there with a spare one lying about ?(I dont see any on Ebay)

If I try to run a program it will run relatively sweetly ( slight fluctuation )on straight cuts , but on thread cutting it will just drag the spindle down to a stall.
Likewise on parting.

Clearly the machine is useless until I can get this sorted .. Any thoughts folks ?


No doubt there will be suggestions to go to 3 ph motor and inverter but there is no room. This motor sits in a space 125mm wide /deep
The nearest 1/2 hp motor I can find is 145 Diameter , 3/4 hp which I understand would be correct for the machine is 165 so neither will fit.