[Andy Fritz]

Hi,

This old beast from -78 has been working great up until a month ago;
I powered up, hit spindle forward and incresed RPM slowly. Got a field loss alarm, re-booted to try to clear it and got field loss and fuse alarm. Two of the clearup's (thyristor fuses) and two FCF2-20 fuses had popped.

I replaced all fuses and tried again. This time I only got Fuse alarm and only one of the FCF2-20 fuses popped.

I dont have a manual for the drive so dont know what spec resistance is for the SCR thyristors, but at least the individual SCR's dont show open or short.

I have cleaned up the drive and bought a replacement drive control PCB and a Thyristor exciter module, just in case.

I have meggered the motor which is DC shunt type and get 40 - 50 M ohms between armature and frame. On a Fluke DMM I get around 3 Meg ohm between field winding and frame but when meggered at 500VDC I only get 0,03 M ohms. I have not read anywhere what insulation resistance is acceptable for a DC motor field coil, but would suspect this is way too low. Chime in, please?

Now I have pulled the motor off to clean it up and have a closer look inside. I have only pulled apart small permanent magnet DC servos before and this one is 155 kg.

Is there something I should watch out for?

Oh, if anyone has a service manual for Yaskawa Varispeed 505MT or a wiring diagram for Mori Seiki SL3, preferably with Yasnac 2000B control I would be happy to pay for a pdf copy.

Mori and Yaskawa dont have parts or documentation any longer which is very understandable. Many give the advise to scrap the machine, but I really like it:-)

All the best,

Andy

[underthetire]

Should be above 200mohm. Get the motor cleaned.

[Andy Fritz]

Should be above 200mohm. Get the motor cleaned.

Thanks! Do you mean 200 mohm between armature and frame, field winding and frame or both?

Do you know if the tach has to be removed to get the armature out? If so, does it need to be aligned using a scope or does it not matter?

Andy

[underthetire]

Just between winding and ground(frame). Winding to winding varies on size /brand of motor, but all windings should match.

[Al_The_Man]

Thanks! Do you mean 200 mohm between armature and frame, field winding and frame or both?

Do you know if the tach has to be removed to get the armature out? If so, does it need to be aligned using a scope or does it not matter?

Andy

If it is like the Fanuc motor fitted to the Mori's with a 6, the tach is easy to take off, it does not require alignment when replacing, just observe original polarity.
In fact often you can remove it still wired and lay to one side.
Al.

[underthetire]

If its just a tach, just put it back on. A tach only gives 0-usually 10 volts depending on motor speed. Clean it while you have it off. Polish the armature with scotchbrite or emery cloth.

[Andy Fritz]

Thanks for the advise guys!

I got the tach and its commutator removed, but the pulley on the buisiness end won't budge.

Do you think it would be ok to leave the motor horizontal while pulling out the armature? I worry about damaging insulation on the stator, armature or both. Maybe I worry too much??

There is definitely something wrong with the field coil; the motor name plate says 90V and 1.8 amp for the field coil. That would give a resistance of 50 ohm, but it measures to 9,75 Kohm taking 3- 4 minutes to climb.

Andy

[Al_The_Man]

You should be able to draw the armature out horizontally, the windings are protected to a degree by the lamination's, have a few hands there to help.
It is not as bad as a P.M. motor where as soon as the shaft is free of the bearing the armature slams up against the magnets.
Once the armature is out you should be able to determine which field coil is open?
Al.

[Andy Fritz]

Thanks Al!

My idea is to slide in a large sheet of teflon paper between stator and armature so there is no direct contact. For that to work I need to remove either of the bell ends. The front pulley is stuck on the shaft so I will focus on the rear bell end.
I have tried to hit the shaft rear end wth a brass mallet while the front bell end is released from the frame, but nothing gives. What is typically used in this situation? Something to cool the shaft?

Andy

[Al_The_Man]

Usually bearings are not in that tight, does the front end seem to come off part way up to the pulley?
If so check for set screw in the pulley and try a puller on the pulley maybe?
Al.

[Andy Fritz]

Hi Al,

There is a set screw on the pulley and that is loosened. The problem is a stuck nut in fron of the pulley. Anyway I managed to get the armature out. As you said the bearing in the rear was not THAT tight:-).

I have never seen a dirtier motor than this. The varnish on the inside of the frame is gone just behind the field coils and the carbon dust layer on the bottom is 5 mm deep.

I think the best would be to remove the field coils to see what is going on. Are they sensitive?

Let's see if I can learn something: The field wires are marked K and J and these are the ones I have been checking resistance across. However, there are clearly 4 coils inside. Are they all in series and J and K are just the ends?

Also, the motor tag says "shunt" which should mean the field is shunted between the armature A and H wires, but there is no continuity between armature and field. All shunt motor diagrams I have seen sugggest there should be. Is this because the shunt is made in the spindle drive?

Andy

[Al_The_Man]

There is nothing special about the field coils, just orient them correctly when replacing, all coils should be in series, shunt field just means the motor field current is fed separately from the armature, with a series motor, the armature current also flows through the field, this is why a series motor should never be ran off load as it has the characteristic of running away due to the field getting weaker as off load rpm/BEMF increases.
A P.M. motor is automatically a shunt motor due to the field separate from armature current.
Al.

[Andy Fritz]

Thanks for the explanation Al. Very good info.

Andy

[Andy Fritz]

Let's see...

The four windings have bolts through the frame to keep them in place which I removed, but only #1 and #3 could be loosened. 2 and 4 seems stuck.
I also noticed that the varnish on the frames inside is actually in good condition so if I knock the stuck windings out I will likely scrape the varnish off.

Before doing that I just want to be sure there is something wrong with the field winding.

Any idea what #1 and 3 are? They look very different from 2 and 4.

There are two wires labled 1B and 2B. I think they are for something temperature related. It is hard to see where the wires go as they are bundled up with all other wires with varnish on top. Any chance some type of broken thermistor could affect the resistance of the field coil?

Am I calculating expected field resistance at 50 ohm correct using 90V and 1,8 amps from the plate or is the plate info just there to show what amps and voltage should be fed to armature and field to get 3600 rpm?

There is a direct short between the face of the stators where some varnish is missing (2 and 4) and the frame. Is that normal?

Cheers,

Andy

[Al_The_Man]

The two small poles are called commutating or compensation Interpoles and one reason is to reduce arcing on the brushes.
There is another method used by offsetting the brushes on the comm, but this turns it in to a uni-directional motor!
The fact the core of the poles contact the stator does not matter, as long as the winding does not, of course.
The two wires are a temperature sensor.
Your field current calculation is correct.
The higher the field current will produce higher torque, but the rpm will be limited by the supply voltage, to increase rpm something called field weakening is used, lower the field current and the rpm will rise, at a sacrifice of torque.
Al.

[Andy Fritz]

Thanks again Al!

I knocked the poles out and can now clearly see where one of the field winding insulation has burned through with matching burn mark on the frame.

I guess if I cut through the insulating mesh, epoxy etc. I should be able to free the copper coils one turn at a time.

I searched youtube for motor rewinding videos, but it looks like it is mostly toy DC motors or smaller AC motors being rewound by non-pro's.

Have you done this? If you would not touch it, there is little point for me to attempt it:-)

There are some motor rebuild shops in Bangkok, but it would cost quite a bit to ship it return and high chance of paying top dollar to get a (big) paper weight back.

Andy

[Al_The_Man]

I have re-wound coils like that in the lathe, if the coil former comes off the pole when it is removed then this is the best way, make an arbor to carry the former in the lathe and using a drum of enameled wire cable slowly fill the former up to where the old coil came to, (saves counting turns).
Al.

[Al_The_Man]

Remember to wind on same way and observe same connection polarity.
https://www.google.ca/search?q=coil+...w=1364&bih=921

http://www.ecplaza.net/product/auto-...3-2149354.html
Al.

[Andy Fritz]

I think I will start with a quick and dirty fix connecting the broken wires together just to check resistance. Of all those turns I think if 3 or 4 gets bypassed it should still give me roughly the correct resistance. If it does, I know the other winding is ok.

Andy

[Andy Fritz]

Hi Al,

I trying to get my head around winding resistance calculations and I have a feeling I am missing something elementary. I would assume that the resistance in a copper winding is constant as long as temperature is fixed and no coils are contaminated, shorted or open. The thyristor drive can increase winding resistance, but not reduce it.

If the above is true and based on 90V and 1,8 amp I calculate how many meters of wire I need to create a resistance of 50 ohm, how can I then get a field resistance of 10,84 ohm (from name plate; 1150 rpm, 90V, 8.3 amp)?

Another reason I am asking is that I would think the same applies to the armature so if name plate armature voltage is 210 with 44.5 amps at 3600 rpm should I expect to see 4.72 ohm between two commutator bars 180 degree apart?

If so how can the armature resistance be reduced to 4.35 ohm if running the motor at 1150 rpm (200V/46 amp)?

I am trying to determine if also the armature needs rewinding.

Andy