New Cromwell S800 MK3 Lathe

The lathe is suppose to be capable of 200 - 2000 in direct drive and 30 - 200 rpm in back gear. If there were other users around I could ask them, but it's a fairly rare lathe so I haven't heard from any other users. It does seem like a bit of a stretch for it to run at 170 V field and only 40 V armature voltage and still develop good torque at 200 rpm.
Michael
 
valleyboy101 said:
Hi All,
The lathe is all back together and runs nice and smooth and should be even better once it is on a solid floor rather than 2x6's and pipe rollers. There is only one problem - the drive motor has very little torque at low speeds. I have checked the electrical wire by wire and corrected a couple of problems. However as it stands now the generator is delivering correct 170 V DC to the motor field and the motor armature voltage varies by speed as it should. So I guess the only solution is to take the motor out and have it checked out by a motor shop. Not really looking forward to removing the heavy under mounted motor and even less so re installing it. Anyway off to the shop ASAP. I attached some picture of the lathe below, just a couple of paint touch ups to do.
Michael
Click to expand...

How are you varying the armature voltage? DC motor torque is proportional to armature current. If you are using a rheostat to vary armature voltage you can't expect much low-speed torque.
 
Thanks John,
The field runs at 170 V DC and the armature varies from 40 to 220 V DC depending on where the potentiometer is set to vary the speed. At 200 rpm the armature voltage is about 40 V DC. It is a Ward Leonard design drive system. I agree that I shouldn't expect much torque from such a low armature voltage. However the system is wired as per the wiring diagram and I can't believe that it was originally such a torquelss wonder. The manual brags in a restrained British manner about its torque at all speeds.
As I said previously I honestly don't believe that there is a thing wrong with the motor. It is a shunt wound motor.
Any suggestions?
Michael
 
valleyboy101 said:
Thanks John,
The field runs at 170 V DC and the armature varies from 40 to 220 V DC depending on where the potentiometer is set to vary the speed. At 200 rpm the armature voltage is about 40 V DC. It is a Ward Leonard design drive system. I agree that I shouldn't expect much torque from such a low armature voltage. However the system is wired as per the wiring diagram and I can't believe that it was originally such a torquelss wonder. The manual brags in a restrained British manner about its torque at all speeds.
As I said previously I honestly don't believe that there is a thing wrong with the motor. It is a shunt wound motor.
Any suggestions?
Michael
Click to expand...

That's really interesting. I've read about Ward Leonard systems but I've never actually seen one (I'd love to have a chance to play with one). It should give you full torque down to quite low speeds. Something is wrong. Can you post the wiring diagram? Here is a link to the Wikipedia article that explains how it works http://en.wikipedia.org/wiki/Ward_Leonard_control

BTW weakening the field of a large DC motor will speed it up, not slow it down. If you interrupt the field while the motor is running it can speed up so much it explodes.
 
Wisconsin isn't that far away. Seriously, I just tried to scan the electrical diagram but my computer has lost its driver for the scanner so i'm in the process of reinstalling it. I'll post it as soon as I can.
Michael
 
Sorry, but I'm having absolutely no success inserting a wiring diagram. I can upload it, but no way will the scan (or any of my previous pictures) insert the into the post.
I'll keep trying.
Michael

Scan1.JPG
 
valleyboy101 said:
Sorry, but I'm having absolutely no success inserting a wiring diagram. I can upload it, but no way will the scan (or any of my previous pictures) insert the into the post.
I'll keep trying.
Michael
Click to expand...

Seems to have worked this time: I see a diagram. Is that exactly how the machine is wired? In any case, assuming correct wiring my suspicions fall on the rectifier. What does the DC motor armature voltage do when you stall the spindle? What does the AC motor do? Does it slow down?
 
No the AC motor never slows down. Tomorrow I'll check the rectifier voltage under stall conditions. The lathe is wired the same as the diagram, with the exception of cutting out the 3 phase. The rectifier seems to put out its rated voltage. I tried over 10 times to post the wiring diagram, but after I wrote my apology for not being able to - well there it was. I guess it is the mystery of the internet.
Michael
 
valleyboy101 said:
No the AC motor never slows down. Tomorrow I'll check the rectifier voltage under stall conditions.
Click to expand...

Please also check the DC motor armature voltage and the DC generator voltage under both stall and no-load.

The lathe is wired the same as the diagram, with the exception of cutting out the 3 phase.
Michael
Click to expand...

Exactly how did you do that?
 
OK here are some readings taken at 240 rpm under no load and stall conditions, all in V DC.

No Load Stall
Gen. Field 12.6 1 2.6
Arm. 33.8 30

Motor Field 189 189
Arm 26.7 26.7

Hope they shed more light for you than they do for me. Now I'll go study the wiring diagram some more.
Michael
 
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