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Spindle speed incorrect.

Posted: Sun 28 Oct , 2018 21:26 pm
I have a problem with spindle speeds on my recently acquired Mirac.

If I select a spindle speed of say 1000 rpm, the spindle actually rotates at about 800 rpm (it wavers a bit) as indicated by the On Screen Display and an independent test tachometer using a reflective sensor checking the spindle speed. Throughout the speed range the actual speed is about 20% below that selected.

The speed encoder is driven from the spindle by a toothed belt and the pulleys look to be the same size. The machine has twin VFDs/invertors, one for the spindle and one for the tool changer turret. There are no speed over ride pots.

I'm not sure where to look to find the fault. Is there perhaps a parameter in the .opt file relating encoder pulses to spindle speed? Do the VFD's have to be set up? It's quite possible any memory retention batteries they have are flat and they're using default parameters. How are the VFD's driven from the software? 0-10VDC analogue or digitally?

Any advice would be greatly appreciated.

Thanks, Max.

Re: Spindle speed incorrect.

Posted: Sun 28 Oct , 2018 21:43 pm
by TDIPower
Hi Max, I have seen various posts over time on spindle speed issues (not sure which machines though). The problems have been from various issues, faulty encoders, VFD issues and spindle motors demagnetising.
Have you downloaded the wiring diagrams? I know the speed controller on the micromill, microrouter, starturn/mill, novaturn/mill of the late 90 to mid 2000 seemed to run the speed controller from the 0-10v from the control board. On the mills/routers, I don't recall seeing any speed feedback/monitoring although there is an encoder on my Starturn 5 lathe (British Encoders). I have seen a post where someone had issues with this encoder not functioning correctly.


Re: Spindle speed incorrect.

Posted: Mon 29 Oct , 2018 9:06 am
by Martin
Hi Max

It may just need adjusting on the control card.

Depending on which control you have the pot is located in different places.

If you now which control you have or can post a picture it should be straight forward.

Re: Spindle speed incorrect.

Posted: Mon 29 Oct , 2018 15:20 pm

I found a manual for the Combivert VFD online. I can confirm that it's controlled by an analogue voltage which is presented to terminals 17 and 18. This voltage varies depending on the speed selected.


Picture of control card attached. I believe it's a Baldor Eurostep? The only pot fitted is VR3. If this is the speed control pot, then I think I know the cause of the problem. The original control card fitted to the Mirac had suffered the usual problem of the battery leaking and corroding the tracks. I swopped it for one taken from a Triac which I've converted to USB control. The EPROMS were swopped between the old and new cards, but VR3 would have been set up to match the Triac.

If you can confirm that VR3 is the speed control pot I'll adjust it.

Thanks, Max.

Re: Spindle speed incorrect.

Posted: Tue 30 Oct , 2018 7:02 am
by Martin

Re: Spindle speed incorrect.

Posted: Tue 30 Oct , 2018 9:38 am
Thanks Martin.

I'll give it a try.

Re: Spindle speed incorrect.

Posted: Tue 30 Oct , 2018 22:27 pm
I've used the pot on the custom card to set the min speed at 100 RPM, but if I call for the max speed of 3800 RPM only 3000 RPM is achieved.

Is there a pot for max speed or will I have to reprogram the VFD?

Thanks, Max.

Re: Spindle speed incorrect.

Posted: Thu 01 Nov , 2018 13:42 pm
by Martin
It seems strange that both the top & bottom speeds are out.

Is the speed overide set at 100%?

If it is then you will need to set the top speed on the drive.

Re: Spindle speed incorrect.

Posted: Fri 02 Nov , 2018 21:40 pm
I've managed to fix it.

In the file FLSTEP.OPT is a line "highspindle 5000" this sets the maximum speed that can be selected from the keypad. When this speed is selected the maximum control voltage of 10V is sent to the VFD.

The VFD has a parameter o2 which sets the frequency the VFD outputs when it sees 10V on its input. This reading must be multiplied by 30 to give the speed in RPM, as there are 60 seconds in a minute and the spindle drive motor has 4 poles. This parameter was set to 129, which when multiplied by 30 gives a speed of 3870 RPM which is pretty close to the maximum safe speed for the chuck which is 3800.

For correct operation the value in highspindle must match the speed calculated from the frequency set in parameter o2. The existing values were 5000 and 3870. I opted to rest the value in highspindle to 3800, because I didn't want to overspeed the chuck. Apart from that it's easier to edit the FLSTEP.OPT file than is to change a value in the VFD, which also requires a password.

Having adjusted the pot on the Custom card at 100 RPM I was able to obtain approximately the correct speeds throughout the range. There's very little torque at 100 RPM though.

So either the speed control of this machine has never worked correctly, or someone has changed the value in the VFD?

A question for Martin. Does the speed control use a closed loop? ie is the speed constantly monitored and the input voltage to the VFD adjusted to provide feedback, or does the system free run?

Thanks, Max.

Re: Spindle speed incorrect.

Posted: Sat 03 Nov , 2018 10:08 am
by Martin
Hi Max

It`s open loop.

As you say you will have very little power at low speed as the motor is running below it`s base speed.


Re: Spindle speed incorrect.

Posted: Mon 20 Jan , 2020 13:39 pm
by Adrian-CNC
Hi All,
My KEB Combivert died last week, so have to bite the bullet and bought a £50 Chinese VFD (2.2KW) just to be sure ;-) The Combiverts I believe are a weak spot on the machine.

I've only had the new VFD it on the machine for a couple of days, and have had to reprogram quite a few settings to get is close to working, (Password is not set by default) and I'm currently looking at the speed stability as it seems to wonder up and down at the slower speeds.

To get the Max RPM of 5000 out of it I have to change the max frequency to 170Hz which gives about 5300 RPM according to the machine. So I recon 5000 rpm is about 168Hz ish. (I've since limited the speed to 5000 after finding this out with a combo of scaling and frequency)

I've also had to play with the scaling (reprogram) in the VFD to get the programmed speed to match the spindle read out on the machine. this seems to be a bit iterative as the instructions that came with the VFD don't really help. (changing the frequency also seems to move it a bit)

Its good to know that the speed control is open loop as this rules a 'servo loop' issue out, so I must be just looking at the isolated supply and op-amp that converts the frequency out of the processor to the 0-10V analogue. Looking at the circuit the voltage supply for the op-amp is a bit crude, so I'm guessing any change in rail voltage can affect the speed control voltage output, so this might be part of the problem.

I had a quick play with the 6 tool changer as this all used to hang off the same combivert but with a change-over contactor.
I also believe that the Baruffaldi tool changer is 110V not 220v so will need to buy another VFD which should really be hard wired to the motor, its not good to disconnect them when the motor is running (So I'm told)

I could make the tool changer spin but not land on the correct tool requested so will need to have another go when the other VFD turns up. Also I'm not sure what the correct frequency should be, as at 50Hz it seems to run a bit slow, but it is all guess work at the moment.

As I find out some more I'll try and keep you posted.

Re: Spindle speed incorrect.

Posted: Sat 25 Jan , 2020 21:33 pm
When I originally started this thread I was using the Mirac with its internal PC which runs DOS. The parameters relating to spindle speed are in the file FLSTEP.opt.

I'm now using the Mirac with an external PC running VR Turning. The spindle parameters are in the machine parameters file which can easily be edited. Changes only take effect after you've closed, then reopened, VR Turning. The min speed was set to 10 and the max speed to 5000. However these parameters only control the voltage in the range 0-10 volts sent to the VFD, so no matter how I set them I can only get a maximum speed of
129 X 30=3870 RPM, the max frequency of 129 being set in the VFD.

The selected speed differs slightly from the speed measured by the machine from the encoder pulses and displayed on the screen. I have no idea how accurate the displayed speed is. It occurred to me that I could use a Frequency Counter and Digital Multimeter to plot a graphs of selected speed against applied voltage and applied voltage against actual speed. Something to try when I have a spare moment!

As far as the ATC goes the turret shaft is driven from the motor by a worm gear. The turret shaft has a coarse thread on it and turret itself acts as a nut. Initially the nut unscrews and the turret moves away from the back plate until it unlocks. The turret then rotates to the correct position for the tool selected. At this point the motor is reversed causing the turret to move towards the back plate and lock to it. In locking, the turret activates a micro-switch and the motor stops a few seconds later. If the micro-switch fails to operate the motor will run stalled until a timer expires. At this point the motor is stopped and an error message generated.

The turret position and hence tool selected is encoded as four binary signals on four wires, so the software doesn't have to remember the turret's position. It can read it at any time.

Re: Spindle speed incorrect.

Posted: Sat 01 Feb , 2020 21:16 pm
by Adrian-CNC
When you get 3870rpm is there 10 volts on the analogue input of the VFD. if that is the case the scaling in the VFD is wrong. On My lathe 5000RPM is about 168Hz. (With the max 10V on the input)
When setting up the VFD I had to change the Max frequency P06 from 50Hz, and to get the Sxxx value to tie up with the achieved value the P73 Max External Analogue from 31440 to something like 55000 (I think it is a 16Bit number so max value is 65535), If important I can check the exact value later, but you can find if by a bit of trial and error, I also had to change P74 the Min External Analogue I reduced this to get the slower speeds in the range, but at 100 rpm it wont run at the moment. Now the achieved speed is pretty close to the requested value.
The Tool Post VFD now works, the problem was that the Frequency Ramp Up and Down was so slow be default that the that the software was over shooting the position, The frequency ramp is now at the Max setting and I also changed the stopping mode.
If anyone is interested I can post a wiring diagram and some pictures, along with the settings I've change.
The other problem I had with the speed was noise on the analogue signal which made the speed wonder up and down, I've fitted a 2.2uF capacitor at the analogue input of the VFD and this seems to have cured this. Currently I don't have the ferrite beads on the output wires to the motors so may need to add these back in as well.
The P numbers (Parameter Number) will be different for different VFDs. I don't know if I need to play with the Torque compensation, but I've only turned a little bit of brass since fitting the new VFD's.

Re: Spindle speed incorrect.

Posted: Tue 04 Feb , 2020 11:25 am
The maximum speed parameter sets the highest speed that can be selected in software be it DOS or VR turning. It is also the speed at which 10VDC is sent to the VFD.

In the case of my machine the maximum speed parameter was set to 5000 RPM, but the VFD was set up so that 10VDC corresponded with 3870 RPM.

Given that the maximum speed for the chuck is 3800 RPM I set the maximum speed parameter to 3800 RPM, but I will change this to 3870 RPM to give a slight improvement to the scaling.

The fact that the VFD was set for 3800 RPM cannot be a coincidence. Most likely it was set at the factory, but the maximum speed parameter was left at 5000 in error. Or given that this machine came from a school, the staff there changed the VFD setting to foil any attempt by students to overspeed the chuck, but neglected to change the maximum speed parameter.

IMG_20200202_192658.jpg (37.22 KiB) Viewed 5590 times

Re: Spindle speed incorrect.

Posted: Tue 11 Feb , 2020 22:36 pm
by Adrian-CNC
I think there must be different Chucks Fitted, I'm sure mine is a 'super Precision' and good for 5000rpm, My machine is also an older one with the 6 position tool, holder. The control card is the Syntel GSM? version.
I got the software from the Denford web site and the default was set to 5000 rpm. And had to replace the internal PC. But if the required max speed was lower, then the 0-10V scaling and max frequency would have to scale accordingly. The bigger problem I had was the ramp rate of the frequency for the tool changer and the slight variation in spindle speed for value less than about 2000 rpm.