Can anyone help me with ideas for driving a med/large dc spindle drive motor, please?

Hi Guys,

I am trying to find out some information about driving a 7.5Kw dc motor in a bridgeport interact 1 Mk2.

This is the plate on the same motor type (taken from an ebay listing)...

Attachment 28644



The main thing here is that i have never come across this type of motor before and it has rather confused me (i thought it was just a lumpy 3 phase until i read the plate!), as well as finding info on it seems to be pretty hard too. The original driver is not working and I am trying to find out if there is an alternative way to drive this type of motor but without understanding what i am looking at, it seems impossible!

Does anyone know of A) Data sheet for this type of motor (i can't find one online but maybe I am searching wrong!?)

or B) a resource where i can find more information on how this is driven


or C) (if we have any motor specialists around here) if it can run in different ways, like using an AC drive instead of a dc drive

Any help at all here would be greatly appreciated, even if it is just "pointing me in the right direction"

Thanks,

Mort

Wow, that's quite a motor - 4.7kW at 6000rpm. Problem is, you need to drive a current in both the armature (rotor) and the field (stator) and be able to vary current in one or both to achieve speed control. Simply connecting them in series won't be a solution, even if you had a controller to regulate the current.

If you can't mend the original drive, you may be best to fit a modern induction motor and matching VFD. Are you likely to be doing that sort of power at full speed? It would be fairly shifting material.

I did see a brief discussion about these somewhere very recently, and it seemed to be there were a few of these motors around, but the drives were rare due to them being the weak point, with no commonly available compatible replacements.

I'd be looking at replacing the motor, but I'm guessing these probably aren't a standard fitment, so options may be limited.

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Originally Posted by m_c

I did see a brief discussion about these somewhere very recently, and it seemed to be there were a few of these motors around, but the drives were rare due to them being the weak point, with no commonly available compatible replacements.

I'd be looking at replacing the motor, but I'm guessing these probably aren't a standard fitment, so options may be limited.

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I was kind of hoping I could figure out how to drive one without the existing driver. I have a lot of motors and drivers and such but this is a new system to me and i don't really even understand how it works so I have no hope of getting it moving!

Would love to swap it out for a better (read as: easier to control) one but it just isn't in the budget right now.

Thanks for the reply though!

Cheers,

Mort

Hi Muzzer,

No, i would imagine that even if i could get half that power from this thing it would happily do what i want it to do! It really confuses me that this thing is feed from 3 phase 400v into the controller but is a DC motor!!

I have looked into controlling various motors before from brushless dc (esc type controllers) to VFD controllers for the normal 415ish type three phase stuff, but this one is extreme (from my personal perspective) and even just knowing where to start is a real issue!

Cheers,

Mort

In theory, driving one is a easy.
You put some current through the field winding, then by controlling the voltage applied to the main winding (well actually armature), you control the speed.


The 150VDC is the easy bit, and could probably be done using something like a suitable KBIC SCR drive that could control the current.
The issue is the variable 400VDC at up to 20A needed to provide motion.

SCR drives are pretty common, but normally limited to a couple HP, as their main use now is basic speed controllers running from single phase.

You could possibly get away with one small SCR drive set to give the required field current (the current control is more important than the voltage), then use another 240VACinput/180VDCoutput SCR drive for the armature, but that would limit the possible speed, to under half the rated speed.

There is also aspect that reducing the field current/strength increases speed for any given armature voltage, so it's a bit of a juggling act.
I think in normal use, whoever specifies/installs the motor will decide what speed they'd like, and will fix the field current to give their desired operating speed.

If "only half the power" would be fine for you, then there's maybe an easy option. Use a 230V:115V stepdown transformer/rectifier/smoothing cap for the field winding, and a PWM regulator run off rectified 230V AC for the armature - last time i looked for one these they were available as an off the shelf module from a few suppliers. This will only give you 325V tops on the armature but hey that's still 4000...4500 rpm and at least 75% power.

Years ago I had a controller for one of these wound field motors that had no active controls in it. The stator had a fixed DC current generated by a bridge rectifier connected to the full mains voltage. I don't recall if there was a resistor or if the winding had an inherently high resistance - obviously you can't use its inductance when dealing with DC. The rotor current was controlled by a rotary knob. This turned either a variac or a rheostat, then the output was connected to the rotor via another bridge rectifier. The speed regulation of these machines is reasonably good in open loop, so it was adequate for the application which was moving a bogey on a large PTFE pipe convoluting machine.

It was a very simple controller that worked fine until finally the insulation on the ancient motor windings broke down and the torque vanished.

Quote:

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Originally Posted by Muzzer

Years ago I had a controller for one of these wound field motors that had no active controls in it. The stator had a fixed DC current generated by a bridge rectifier connected to the full mains voltage. I don't recall if there was a resistor or if the winding had an inherently high resistance - obviously you can't use its inductance when dealing with DC. The rotor current was controlled by a rotary knob. This turned either a variac or a rheostat, then the output was connected to the rotor via another bridge rectifier. The speed regulation of these machines is reasonably good in open loop, so it was adequate for the application which was moving a bogey on a large PTFE pipe convoluting machine.

It was a very simple controller that worked fine until finally the insulation on the ancient motor windings broke down and the torque vanished.

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I have 2 similar things sitting in the workshop, made by my late father for his watchmakers lathe & a mini pillar drill which were powered by Croydon 230V DC shunt wound motors - they use a variac to vary the armature current, the field current is set by the winding resistance which is in the high hundreds of ohms IIRC.

Quote:

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Originally Posted by Voicecoil

If "only half the power" would be fine for you, then there's maybe an easy option. Use a 230V:115V stepdown transformer/rectifier/smoothing cap for the field winding, and a PWM regulator run off rectified 230V AC for the armature - last time i looked for one these are available as an off the shelf modules from a few suppliers. This will only give you 325V tops on the armature but hey that's still 4000...4500 rpm and at least 75% power.

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Nice idea. You'd need to check if the field winding has enough resistance to limit the current to the required level but it looks as if it might be about right.

Hi Guys,

Thanks for the ideas, it is a great help! So I have a 230-110 transformer and the parts to rectify that after the drop down. I also have capacitors big enough to smooth this out. That part SEEMS to be simple enough (famous last words).

The pulse width modulation regulator on the other hand has me scratching my head again, i'm afraid....Can anyone give me a link to a module that might give me an idea of what i'm looking for?

I have used PWM with smaller motors using a micro controller to product the pwm and an H-bridge to control the power. Is that basically (although a much bigger version) what i am looking for?

Also, can i use a nice big shunt resistor to limit the current for the field winding, if there isn't enough resistance already? I have some pretty large ones knocking about from a servo motor braking circuit.....

My biggest fear here, if i'm being honest, is that i do something daft and damage the motor....It is not something I can afford to "scrap". If i can get all the parts together, can i "test" this with lower power to make sure i have a working solution before I go "full" power? Or is there any advice for this to make sure i don't do any serious damage?

Thanks again for all the help, any advice is greatly appreciated.


Cheers,

Mort

Do you have any DC bench power supplies? 30 - 60V at perhaps 10A? That might be enough to get it spinning.

You probably don't need to worry too much about smoothing the voltage at this stage, so a variac and bridge rectifier might suffice.

To check what current the field winding will draw, firstly check it's DC resistance with a DMM, then apply the law of Mr Ohm to get the current - this will give you an idea whether you need to add anything in series for safety.
As far as PWM drive modules go, there used to be a guy from Hungary on fleaBay who made some good ones at a reasonable cost (around £100 or so) though I can't find him at the mo - COVID issue I wonder??? - we swapped a few emails and he knew his stuff. Otherwise go to Inverterdrive.co.uk, you will find a selection of drives there. The principle is just as you have described only bigger - you'd be talking of between 2 and 4 paralleled 600V+ MOSFETs top and bottom, or a pair of big chunky IGBTs. Provided you have a big variac there shouldn't be any reason why you can't test the setup at a lower voltage - and if you had a real monster you could even use it for speed control!

Hi Guys,

I will follow your suggestions as best i can!

I need to get started by figuring out the connections. The panel is marked up as three phase with 6 brass bolts for connection, which i why i thought it was a three phase motor in the first place (i guess it's a standard fitting rather than one made for each type of mill). Once i have figured out the three connections I will start on the power supplies. Will start on about 60v up to about 110v (depends on what i can get to work from the pile i have stashed).

Next up will be getting or making a PWM drive. I have a dozen 47N65C3 MOSFETs (650v 47A continous, 141A spike) and will try to make up a simplified PWM for a bit of control. I am not sure i even have the skills to get this to work but at least I will be learning something new and therefore it is never a wasted effort. The interdrive ones are great i reckon, but again, budget doesn't strech that far right now....possibly something to aim for if I don't get this working as is, but that might be a while down the line.

I don't have a variac that would come even close to large enough for this application so i will have to figure out that as we go. Or maybe find one on the "bay" as it would be a great addition to the bench anyway.

For now, I have already learnt a lot, thanks very much for your help and advice. Any further thoughts or advice is gratefully recieved!

Cheers,

Craig

Have a look here; https://inverterdrive.com/group/DC-T...Input%7c230Vac

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Originally Posted by EddyCurrent

Have a look here; https://inverterdrive.com/group/DC-T...Input%7c230Vac

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Thanks for the link but I just don't have the budget for this at the moment. I guess the motor would need something around the size of a Parker SSD 512C but at £400 it just isn't practical right now.

Cheers,

Mort

If I have a spare mo. I'll have a think about what would be the best driver for those things - BTW, do you want reversing on the spindle?

Thanks Voicecoil,

Yes, i want reversing but i am aware that this is an added complication that can be addressed at a later date. If there is an easy way to do it, great. If not, I can live without it.

Cheers,

Craig

Those Infineon FETs are likely a good choice, I've found their devices reliable in SMPS applications. They do however have a pretty huge input capacitance so you'll need a driver chip with plenty of beans like the ONSemi FAN7191. Going back to my original suggestion, rectifying & smoothing the mains to give ~325V @ abou 5KW will need some biggish capacitors which will draw hellish charging current spikes from the mains - using smaller caps that don't charge all the way when under load might make Powergen rather happier, but will of course reduce your available power - on which subject does the Bridgeport have pulleys connecting the motor to the spindle? Obviously at lower voltages you will get lower motor RPM.

Quote:

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Originally Posted by Voicecoil

Going back to my original suggestion, rectifying & smoothing the mains to give ~325V @ abou 5KW will need some biggish capacitors which will draw hellish charging current spikes from the mains

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I genuinely don't know the answer to this: Would it not be worth trialling without caps at first - the inductance of the motor windings will provide a hefty current stabilisation without throwing energy into expensive high voltage, high capacitance, high ripple caps?

Quote:

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Originally Posted by Voicecoil

on which subject does the Bridgeport have pulleys connecting the motor to the spindle? Obviously at lower voltages you will get lower motor RPM.

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It has a belt drive which i believe is a 1.5-1 ratio. I very much doubt I would be trying to drive this at anywhere close to it's max output and I can easily overcome most of the downsides of this just by reducing my DOC and better calculating the speeds and feeds. Hell, if i can get 30% of the normal max power it would turn this from a beautiful giant doorstop into a machine again!!

Quote:

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Originally Posted by Voicecoil

Going back to my original suggestion, rectifying & smoothing the mains to give ~325V @ abou 5KW will need some biggish capacitors which will draw hellish charging current spikes from the mains - using smaller caps that don't charge all the way when under load might make Powergen rather happier, but will of course reduce your available power.

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To be honest I am going to have to figure this out as i go along, it is deeply outside of my comfort zone. I work with big AC all the time but DC, of this kind, is a mystery to me (but getting clearer as i look into it more). Getting this running is first on the list so I will give this a go and put fuses into the line so i don't do any damage. I have a few different caps i can use and i can trial a few different configurations to see how it is effected. Then I will further look at the control side, when i can get this to work.

Quote:

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Originally Posted by Doddy

I genuinely don't know the answer to this: Would it not be worth trialling without caps at first - the inductance of the motor windings will provide a hefty current stabilisation without throwing energy into expensive high voltage, high capacitance, high ripple caps?

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I will try this too and let you know how it goes, so long as i don't do any permanent damage I am up for trying anything!

First things first though, i will have a bit of time tomorrow to do some measuring and calculations then will see where I am!

Thank you for all your help.

Mort