DC Spindle Motor Blows Fuses (Newbie)

You are welcome here.

Could you post a sketch? It will help us understand the problem better.

Hi Joshi,

Thanks for your reply, Do you mean a schemetic wiring diagram? I only have a hard copy drawing and unfortunatly no scanner so I cannot post,

I can try to photo the schemetic and upload as a picture maybe?

I do have the spindle drive (Contraves Varidyn) wiring and technical info on pdf, would this help,

Allen.

More info. will certainly help, understanding your problem better.

Hi JRaef,

Thanks for your reply, Sounds logical. I was kind of wondering if the RPC was a part of my problem.

Is there anything I can do rather than going to the expense of an even bigger drive, I am wondering if the spindle controler can be trimed to reduce the back emf? there are trim pots on the spindle drive, do you think this might help? or can I reduce spindle brakeing to stop the sudden in rush of current?.

The drive (Spindle drive) was original, so I am a bit confused as to why this may need changing?

Thanks again for your help, Allen.

Ps, Like your "fire extinguisher" comment!!

Hi JReaf,

I am pretty sure you have hit the nail on the head so to speak, I ran a few tests over the weekend and you are exactly right about energy being pumped back in to the line source, I done some voltage checks and the readings were very worrying under spindle braking? as you said the faster the spindle run, the harder the braking, the higher the voltages/current.

So now I have a problem, a bigger drive will probably cost more than the machine is worth.

Is there any way I can let the spindle stop/decelerate slowly? even allowing the spindle to run down on its own to a stop (I know this is not practical but time is not an issue).

You mentioned a drive with single phase input?, I guess you mean a VFD (variable frequency drive) would this work? can I hook this up to the control so spindle speeds can be adjusted via the control rather than from the vfd itself?.

Thanks for any advice/help, Allen.

Hello moonman;

Are you sure it is a DC 3 phase motor with armature and field? this sounds kind of odd.

I have seen three phase SCR spindle drives which blow the fuses if you interchange any two of the phases, they are very sequence sensitive; have you moved your machine from one outlet to another?.

An open braking resistor (attached to the heat sink of the drive) would cause a considerable bus voltage rise, with the subsequent overload.

Hi Yahlasit,

The wiring diagram shows a Armature and Field controled by a SCR spindle drive, (I am going to try to upload the schematics tomorow so you can get a better idea).

Your point about phase possitions is interesting, I am wondering if the wild leg from the Rotary Phase Converter is the problem?.

This is something I am not really clear about, I need to know what phase is best to have the wild leg on?.

You also mention a braking resistor? I need to do more investigations on that point?,

I will try to post more info tomorow, getting late here in the UK,.

Thanks again for your help, greatly appreciated to you all,

Allen.

A very plausible idea about an open braking resistor. Without this load to dissipate the back EMF during braking a diode (possibly MOSFET body diode) maybe turning ON thus making a lower impedance path during braking. Pop goes the fuse.

As someone else pointed out earlier, the fuse failing is preventing a more catastrophic failure from happening. Identifying what and where this electromechanical system has changed for this fuse to pop is hard to identify remotely. It can even just be that some drag is now missing or increased inertia is present in the mechanics. Thus nothing is wrong with the driver.

I have a problem with with my cnc machine, The machine is fitted with a DC spindle motor ( 3-Phase 6.5hp 4.8 Kw) driven from an SCR spindle drive unit (Contraves Varidyn).

I am having a hard time imagining a three phase DC motor. Did you mean:

I have a problem with with my cnc machine, The machine is fitted with a DC spindle motor driven from an SCR spindle drive unit (Contraves Varidyn, 3-Phase 6.5hp 4.8 Kw).

I bet that JRAEF got it, he sounds very plausible.

Assuming that JRAEF is correct, my suggestion is to turn off dynamic braking and build in a larger "wait" when changing speeds.....if possible!!!

Also, not changing spindle speeds in a job may work as well.......take a bit longer but....

Yes, most likely a four quadrant DC Drive with strong regeneration capacity for braking.

A simple test / cure would be to increase the deceleration time in your drive. This would reduce the peak current when reducing the speed rapidly. It might be set too short relative to the drive's capacity.

Also check the current limit on the same drive, somebody might have cranked it up to a level above the fuse rating.

Hi All.

Can I first say thanks to all for such a good response to my question, there are some very interesting and usefull points that give me something to work on,

I have done a bit more investigating today, it appears that the spindle motor is not original and has been replaced at some point? The original motor was 9.25 Hp 6.9 Kw this got me thinking, could the contraves (Spindle drive unit) still be set up for the more powerfull motor? and is now runniong a less demanding unit?.

Marcot mentioned the "current limit" may be high, this is possible as this may not have been adjusted to suit the less powerfull motor,....something I can check and adjust on th Contraves.

I think that JReaf has also hit on a problem, it is just how I can get over it, I think again as Marcot said reducing deceleration would do the trick,

K fry, well as for the DC motor question, I assumed that because the contraves is an SCR drive and the wiring diag shows armarture and field coils that the unit is a DC motor?.

I have included the schematics sheet 1 and 2 although these only show the machines main wiring, not the TNC or Contraves unit (have Contraves wiring diag if it helps)

Thanks again everyone, a great help!! Allen.

It is as good as useless to post such diagrams as CR4 (sadly) has an appalling quality for pictures and drawings, they are unreadable.......

It is usually best for people who want to have a detailed look, to email you via the CR4 email system, sending you a private email address that will allow quality drawings to be transferred.

Private email addresses should NEVER be posted on the open forum.....

The only other way would be to place the drawings on a free website (not one that requires membership or payments as many do. I do not know of one, sorry!), one that anyone can access easily, to allow anyone to download them and helping you further. You do need then to just post the web-link here....

The good point is that in either method, you will only get highly interested/qualified opinions. So do try one or the other.

The problem has been on CR4 as long as I have been here, probably longer......I do not expect any changes, but if one was made, it would be good if it was at least to allow attachments on CR4 email.......

Hi Andy,

Yes I see your point, I copied the drawings back from Cr4 to my Pc, and as you say useless!!, I did not realise that the drawings had been so distorted by posting on Cr4, sorry, it's a shame because I am new to this excellent website and the response to my questions have been very encouraging.

So now I will have to find a way to post without losing quality, I think the best way would be a link to a website or other?,

If anyone does want to see the drawings then perhaps e-mail would be best, and I am willing to e-mail to anyone with a genuine interest in my problem.

Thanks again for pointing this out.

Allen.

We mostly just try and help each other here, there are a few aggravating souls as well, you will soon get to know them....

Have a great day.