New Cromwell S800 MK3 Lathe

valleyboy101 said:
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
Click to expand...

The generator voltages are about as I expected: the load drops the output a bit. The motor armature voltages don't make sense. The notation in the drawing is peculiar but, knowing how a Ward Leonard works, I think I can follow it. In this mode the motor armature should be connected directly to the generator armature. You might see a few volts of drop due to dirty switch contacts and/or bad connections but the change from no-load to stall at the motor should be as large as or larger than that at the generator.
 
Hi John,
I did clean the contacts on the drum switch as they were quite stiff and dirty. All other contacts have been checked and are tight. I'll try reversing the motor and see if the other contacts on the drum switch make a difference.
Michael
 
valleyboy101 said:
Hi John,
I did clean the contacts on the drum switch as they were quite stiff and dirty. All other contacts have been checked and are tight. I'll try reversing the motor and see if the other contacts on the drum switch make a difference.
Michael
Click to expand...

Did you measure those motor armature voltages right at the motor right across the armature?
 
I measure the voltage at the brass terminal blocks on the motor and gen., where the wires exit to the internals of the motor/gen.
I checked the voltage drops between gen. and motor armatures and it is 6 V DC in both forward and reverse - with no load. 6 V is an awful lot, like +- 25%. I don't know if it enough to cause all of my problem, but it certainly contributes to it.
I think i'll re read the Ward Leonard descriptions on the internet.
Michael
 
valleyboy101 said:
I measure the voltage at the brass terminal blocks on the motor and gen., where the wires exit to the internals of the motor/gen.
I checked the voltage drops between gen. and motor armatures and it is 6 V DC in both forward and reverse - with no load. 6 V is an awful lot, like +- 25%. I don't know if it enough to cause all of my problem, but it certainly contributes to it.
I think i'll re read the Ward Leonard descriptions on the internet.
Michael
Click to expand...

I was thinking that maybe the braking resistor was not being switched out when the motor was running: that would account for poor torque. However, it also would result in a large drop in motor armature voltage when stalled. The fact that the motor armature voltage does not change when you stall the motor is odd. What is the condition of the brushes? (I'm grasping at straws. I wish I could get at the thing in person.)

Could you measure the voltage across the resistor? Should always be zero except when braking.

Does the drum switch have a wiring diagram on it? Perhaps that could be used to resolve the ambiguities in that very nonstandard diagram.
 
Hi John,
I think the braking resistors stay connected all the time. Maybe I'll try just disconnecting them and see what happens. No the drum switch does not have a wiring diagram, but I have figured out all its connections. The switch is a 4 pole - the first I've ever seen, nice touch to show a 3 pole in the diagram. As you say the wiring diagram does leave something to be desired, but for the cost of the manual it was the only really useful piece of information that I got. ( apart from the weight of the machine)
The commutators both look excellent. Good pressure on the brushes, no arcing, good undercutting, nice and smooth - almost like new.
After dinner I'll see what I can come up with re the braking resistors.
Michael
 
valleyboy101 said:
Hi John,
I think the braking resistors stay connected all the time. Maybe I'll try just disconnecting them and see what happens. No the drum switch does not have a wiring diagram, but I have figured out all its connections. The switch is a 4 pole - the first I've ever seen, nice touch to show a 3 pole in the diagram. As you say the wiring diagram does leave something to be desired, but for the cost of the manual it was the only really useful piece of information that I got. ( apart from the weight of the machine)
The commutators both look excellent. Good pressure on the brushes, no arcing, good undercutting, nice and smooth - almost like new.
After dinner I'll see what I can come up with re the braking resistors.
Michael
Click to expand...

The motor field discharge resistor should be hardwired and should not be disconnected. The motor armature resistance, if I read the diagram correctly, is shorted out by the switch in run mode. If that resistor is always in series with the motor and generator you won't get much torque. However, I'm still baffled by your motor armature voltage readings. Before you try disconnecting the armature resistance check the voltage across it, both no-load and stalled.

That machine ought to work well and give you good speed control with full torque.
 
I found my missing 6 V - the braking resistors have it between them.
Then I disconnected them and nothing, no life in the motor.
That and the missing 6 V lead me to believe that the resistors are wired in series rather than parallel.
However I don't really understand how that can be because, as per the wiring diagram contact A1 has one wire to the fuse and one to the resistor, AA2 has one wire to the motor armature and one to the other side of the resistor.
Back at it tomorrow,
Michael
 
Major Victory!!
I found the source of my lack of drive power. After a lot more ringing out of circuit and testing of resistances I found the culprit - a burnt contact on the reversing switch. It was on the pole that switches the armature and one side was still making contact, so when it was switched on it didn't connect across the switch as intended but through the resistors, giving the 6 V drop from gen. to motor. The motor now runs with lots of torque. I am embarrassed that it took so long to find the problem but at least it is found. Thanks for your help John and others.
Michael
 
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