Troubleshooting an old DC variable speed drive

[DickDV]

agarb, you need a new DC drive. Discard the old one (send it to Liefmotif).

Your motor is a compound wound 40hp motor with 230V armature and unknown field voltage and current.

I would buy a 230V non-reversing, non-regenerative DC drive with an adjustable field regulator.

Before you dismantle the old drive, check the current in the F1 to F2 circuit. That is the shunt field and note the highest current you see among all of the different speed steps. It might change between steps and it might not but we are interested only in the highest current. Also, be sure to note the polarity between F1 and F2.

Buy and install the new drive being sure to preserve the connection between one of the A leads and one of the S leads. The remaining A and S leads are your armature connections to the new drive terminals labelled A1 and A2.

Connect the F leads to the corresponding terminals on the drive and set the field regulator for the lowest current or voltage. Power up the drive and slowly adjust the field regulator up until you have the same current and polarity flowing between F1 and F2 as you noted before for maximum current.

Energize the armature circuit and note motor shaft direction. If reverse, stop, power down, and reverse the connections on A1 and A2 on the drive. Do not change the F connections or the motor junction box connections.

This should give you a properly running motor with better torque at all speeds. If you find that you can't quite get to full speed, set your field regulator current down slightly until full speed is reached at full armature voltage.

Be very careful that the armature is not energized with no field current or very low field current or an overspeed run-away condition can occur. You will likely explode the armature if that happens and the motor will be ruined. There is significant personal hazard when this happens as well and we wouldn't want you to end up in the same state as the motor!

Good luck!

 

[leitmotif]

OK now that we are switching from "fix what you have" to replacement.
1. What is the machine? What does it do?
2. What is power supply to existing controller? I assume DC - is that off a main DC bus for the whole shop or an individual rectifier set fed with AC - What is AC phasing and line voltage?
3. How old is the motor and what is its condition - does it have lots of years left in it?
4. That is an old looking machine - from one of the photos it appears to have drip oiling - almost not seen on current era machines. What is overall condition of machine and is it worth putting more money in? Does factory support spare parts?
5. Do you have 3 phase and what is it delta or wye and what is voltage?
6. If delta is it grounded?

DICK DV Are you suggesting replacing with a another identical updated step starter or a solid state drive? I think either is a good option - from the sounds of things the existing unit is just getting near end of life ie "overload not used anymore".

BUT can he replace with a like unit - do they even make one or is solid state only option?

IF SOLID STATE DC DRIVE
If machine is fed off a DC bus you will need one designed for that.
IF you decide to feed the drive off 3 phase you may need an isolation transformer.

In addition to what Dick said
I would measure resistance F1 F2 A1 A2 S1 S2 and temperature of motor at rest. Then paint these values on the motor for future referance include date and the temperature. It may be worth some time to trace wires back and ensure F1 and F2 are fed from same DC as rest of motor and whether it is long or short shunted and up or downstream of starting resistors. I think it would also be worth some time to trace all and make a drawing of where everything hooks together so that you know what you have to enable future equipment selection.

There is nothing wrong with DC - it has worked well for eons. There are some machines where it is preferable to AC drive with a VFD.
GOING ON TANGENT
It may be worthy of some time to evaluate replacing motor and drive with 3 phase motor and a VFD.

Dan Bentler

 

[agarb]

leitmotif

This is a verticle boring mill (picture attached).

Power to existing controller is 250 vdc from a motor/generator set that is dedicated to this machine. Not sure age of motor, etc. I am not really too involved in maintenace of our equipment. I have no idea about the overall condition of the machine, but seriously doubt we have any plans to replace it. Just patch it together and keep running. We do have 3 phase available, 460 delta, ungrounded. Everything else in our plant runs off of this.

The maintenance guy that I need to talk to is off today, so I need to touch base with him Monday. I'm hoping I can determine which contactor is bad and get it replaced (assuming I can find a spare). The ideal short-term goal is to repair this drive so we can run product and then long-term either replace the drive with an equivalent or replace both the motor and drive with AC versions. Unfortunately, if we repair this drive nothing further will get done until it breaks again.

DickDV - would you care to recommend and manufacturers and/or specific product lines that I should look at? Perhaps something like this: http://www.dodge-reliance.com/pdf/catalog/custom_classic/max_pak_plus_dc_drives.pdf?

 

[leitmotif]

AGARB

Where are you located?

I think
1. That machine is probably well worth keeping. Todays metallurgy is better but I think the old time castings were better or at least more massive to overcome metallurgy problems. It may not be as precise or repeatable accuracy as CNC but with the correct operator can do as well.
2. I think if you can do it patching and cobbling the existing DC controller is the way to go unless you do not need the machine for say a month.
3. Either way I would use the month to evaluate replacing the DC drive and what to replace it with either solid state DC or 3 phase motor and VFD.
4. Do not scrap contactors, resistors and the MG set please. I am interested in electric vehicles and may need a MG either for generating 400 VDC or as dynomometer.

Dan Bentler

 

[DickDV]

If some understanding of the method of operation of the old drive can be had, repairing the old drive would be possible and maybe even cheapest if labor time and machine downtime is free.

Otherwise, a replacement DC drive with an isolation transformer would be the next best thing in terms of immediate cash outlay.

Optimally, a 460VAC powered AC drive with a new motor would be preferred. The present system complete with MG set is horrifically inefficient and unreliable. If it were me, I'd spring for the AC option. As long as the motor is on hand, the drive conversion wouldn't take 48 hours. And the machine looks to be worth the investment.

As for DC drive recommendations, Baldor-Reliance, Control Techniques, and ABB are all viable products. Pick one with good local support. Nothing else matters more.

 

[agarb]

The label on the existing motor gives a speed range of 400 to 1200 RPM. So to upgrade to a new AC motor and drive, is it really just as simple as dropping in a new 40HP, 6 pole, inverter duty motor/drive, or are there other considerations that must be made to account for the different motor types? I don't know much about selecting motors for various applications.

I guesstimate that a new 40 HP motor and VFD would run in the $10K range plus some labor to install... sound reasonable?

I kind of want to try to repair the thing myself, just so I can learn something new, but I need to talk to my boss and maintenance guy after the long weekend so we are all on the same page.

I don't think machine downtime is an issue (yet). We have another, even older, vertical boring mill that can be used. This one used to run off a central driveshaft down the center of the plant, but was retrofited with AC motors years ago.

Thanks for all the advice so far.

 

[leitmotif]

Agarb

I would guess 10 K is pretty close but I would use 7K for VFD and 4 to 5 K for motor. Six pole sounds like it would fit your needs well. I think I would go up to 50 HP to match and or give a little more torque than the DC.

If you eliminate the MG set you will definitely have to extend the AC feed. I would repull with new wire. you may need to change the AC breaker - maybe even downsize since you are no longer driving both MG and machine motors. Allow 1 to 2 K??
At 400 RPM you will be at 30% speed which does not require a separate motor cooling fan but I would install one anyway allow 1K for this.

I assume all feeds and speeds ratios are set via gearboxes and all are driven with the one motor. You may not need an encoder for motor RPM but if you go vector drive then it may be a good idea ie tells the VFD the true story on motor RPM. One advantage would be that it would be good for a motor RPM tach.

Dynamic brake resistor do not forget them. The nice thing with VFD is you can get near instantaneous stopping. This is another feature you may want to consider in your E stop circuit. Add a crash reverse switch which will allow rapid reversal of motor in case you jam a bit - check with VFD mfr for this - this is hard on geaboxes so make it an emergency type thing. I would allow 1K for resistor. Normal stops can be programmed in for 3 to 5 seconds.

Since this is a mill/drill I do not think vector drive is mandatory (it has advantage of controlling for motor slip on heavy cuts) but it would sure be nice if you are doing casting with porosity or odd shaped parts ie hogging otherwise you could get away with a planar drive. Think I would go vector just to ensure even speed thus better cutter life.

You can bet you will need to change the motor foundation but you are a machine shop so that should not be so bad. Call it 1K

Couplings should not be a problem and even if you do have an oddball machine input shaft you can always bore out a coupling half. Allow 500

Controls (start, stop, E stops, speed pot will more than likely have to be fully upgraded so call that 2 to 5 K depending on if you can do in house or job out. Technically you will have to bring all machine controls up to current standard including NEC, NFPA 79 etc - install a disconnect (does not necessarily have to be fused) near the machine. Dont forget OSHA stuff either.

I would put the VFD in a splash proof cabinet and install good filters and fan. Metal chips and cutting fluid just naturally seem to gravitate to a VFD. About 500 ??

Add another 2 to 5 K for a couple "while we are at it" type projects.

NO top entry conduit in any cabinets. If you can get there bottom feed only. All conduit fittings leak - I dont care if they do have gaskets.

I'm gonna let Dick tell you about driving VFD on ungrouned delta. I think he will recommend a delta to wye transformer. Whoops another 2 to 5 K

I tend to make my guesstimates high - people seem to be less mad when bids come in lower.
Dan Bentler

 

[DickDV]

Yes, I would say that Dan is going to be a bit high on the cost side but, as he says, that's the safe side.

I would definitely use a sensorless vector drive and am partial to ABB's ACS800 for this kind of service. If you get one in a Nema 12 Enclosure, you won't have to deal with the box, fan, etc. And the drive cost is much less today: about $3800.

In my view, a TEFC motor is good enough. No need for an auxiliary blower. If the motor is belted to the machine, drop the sheave size on the motor shaft about 10-15% and run the motor overspeed to make up for the speed loss. And, I would stay at 40hp on the motor and a heavy-duty rated drive (that's one with 150% shortterm overload, often called constant torque rated).

While I thoroughly detest floating delta power supplies, a modern AC drive will run from it ok. Just be sure to disconnect the CE filters as per the instruction manual.

Sounds like my kind of project! Wish I was there. Shucks, Dan, you might as well be there too! That way, you can take the old stuff home with you!!

Good luck.

 

[leitmotif]

Yes, I would say that Dan is going to be a bit high on the cost side but, as he says, that's the safe side.

I would definitely use a sensorless vector drive and am partial to ABB's ACS800 for this kind of service. If you get one in a Nema 12 Enclosure, you won't have to deal with the box, fan, etc. And the drive cost is much less today: about $3800.

In my view, a TEFC motor is good enough. No need for an auxiliary blower. If the motor is belted to the machine, drop the sheave size on the motor shaft about 10-15% and run the motor overspeed to make up for the speed loss. And, I would stay at 40hp on the motor and a heavy-duty rated drive (that's one with 150% shortterm overload, often called constant torque rated).

While I thoroughly detest floating delta power supplies, a modern AC drive will run from it ok. Just be sure to disconnect the CE filters as per the instruction manual.

Sounds like my kind of project! Wish I was there. Shucks, Dan, you might as well be there too! That way, you can take the old stuff home with you!!

Good luck.

Thanks Dick - like many maintenance guys all the engineering and selection was all done for me - all I had to do was keep it running.

I would like to take a crack at it and scrounge up neat stuff too.

Just need to get told where to go.

Dan Bentler

 

[agarb]

Making progress

I have made some progress on repairing this old starter.

Attached is portion of the schematic which I traced to make it more legible.

Essentially, it was determined that both 2A and 4A are not automatically pulling in to bypass the starting resistor.

I determined that just a small tap on 2A causes it to pull in, so I shimmed the spring to make it slightly tighter and it now seems to be working OK.

4A is a different story. It takes quite a bit of pressure to manually shift it. I checked the coil resistance and control voltage and they seem to be OK. I disassembled it and couldn't find any signs of physical binding, so now I am on a search for a replacement.

Can anybody explain the function of FF?

 

[leitmotif]

Good drawing - dont understand all your symbols but your drawing is for you to comprehend so not a big deal.

Relay FF allows for full shunt field strength during start. From your drawing it appears to sense armature current such that when the motor is up to speed (out of starting resistor sequence) and armature current is low it then cuts in shunt field resistance ie reostat to allow for speed control with reostat. The only problem with this setup is when someone forgets to reset reostat to zero (low??) resistance then when the resistance is "cut in" the motor will speed up draw a whole bunch of current and either cause FF to short out shunt field resistance again - I have seen the breaker trip on a large multi stage centrifugal pump when the reostat was not reset to the "start / low??" position.

I suppose your effort and time repairing relays means I cannot count on you as source for DC relays - dang and double dang.

Dan Bentler

 

[agarb]

Dan,

I traced the symbols just as they appear on my 1941 version schematic. I think the strange looking symbols are just their representation of N.O. contacts with an associated time delay.

The reason I asked about FF is that as the motor accelerates up to speed, (during the period that 1A-4A are being activated) is that it will click on/off several times, always accompanied by a really nasty, long-burning arc. An arc that sometimes seems to want to keep burning until as small draft extinguishs it. I've seen it burn as long as 5 seconds.

Dan, I do appreciate all your help. Obviously the long-term viability of this is very questionable. Maintenace/management needs to determine the best route forward. To answer one of your old questions, I am in OH.

I did find out that the motor has not been rebuilt or worked on in the past 23 years (how long our maintenance guy has been here). I talked to somebody at Baldor/Reliance who said they wouldn't even put a new solid state drive on this motor without having the motor rewound with modern insulation.

An upgrade to AC is also more difficult than just replacing the 40 HP motor/drive because the rest of the motors on the machine are also DC and run off the same MG set.

For now, the work-around is to whack the 4A with a broom handle every time they start the motor.

 

[leitmotif]

Re 4A would not be the first time I have seen relays "incentivized" to actuate.

The FF relay needs some attention. It may be possible to replace it with a electro mechanical or even solid state relay and time it with a timer to cut in the reostat say 5 second after starting resistance has run thru.

Another good option to be carefully checked out may be solid state relays ie FF and the starting resistance relays but if you allow for starting current ie the main line relays it is gonna be expensive.

With larger equpment the cost for motor control just aint chump change.

If you go with a solid state DC drive and rewind the motor you for sure are gonna be far beyond the cost of replacing all DC equipment with AC. Check with motor shop to see if they can just clean and dip - got some doubts about that but it is worth a five minute call.

Keep me in mind if you scrap the DC setup please. Just PM me.
Been kinda fun helping you out - you are welcome.

Dan Bentler

 

[Bitmore]

If the system is as old as you say, when was the last time the motor armature was serviced. I used to work for Square D and did allot of field work on their Rectivar DC drive and D.C. motors, the largest was 30,000 HP, when they removed the armature, I walked through the coils (Hylsa Aceros Planos, Monterey, Mexico).
The MAIN REASON for poor motor response was attributed to poor armature condition. Get a local motor repair service to "UNDERCUT and POLISH the ARMATURE" and of course after that a "hi pot" test, put in new brushes and I'll bet you anything you will notice more power and response..

bitmore..

 

[Bitmore]

A quick check I always performed is to get a good view of the armature segments under the brushes, remove the brushes and look at the segments on the armature. If they are scored or grooved they need to be "Polished". Then get a view of the end of the segments in the armature, if any of them look like a wave or have a curl on the side, they should be "Undercut". Undercutting removes the copper segment waste deposits and deformation between the actual segment lands. If the spacing between the segments is reduced by copper deposits the motor will use more current, have sluggish response, and most likely will trip current controls or overheat current limit resistors. The segment lands on the armature should have a sharp definite edges, this will greatly increase HP and response. I had one experience when I cut my finger by grabbing the comutator just after it had been "Undercut and Polished" what a rush, my fingers had cuts on them with equal spacing.