[chuckd87106]

I am working on an old Pratt & Whitney star-turn 1800 lathe. The problem with the machine is that the turret will not turn for changing tools. I've found some faulty relays and rectifier, and also discovered the control circuit was wired incorrectly. The PLC appears to be sending the proper signals to the relays, it energizes one relay for forward indexing and energizes the other for reverse indexing. I've developed a diagram that I believe will serve the purpose of a motor controller for this SepEx type motor. I've used all the parts that were all ready there which was a transformer, fuse, high power adjustable resistor, two rectifiers, and two industrial relays with 4 NO contacts and 2 delayed contacts (1 NO and 1 NC). One of the existing relays (700-N400A1, 700-NT) is setup for an “on delay” while the other is set up for an “off delay”, but my circuit requires both be set up for an “on delay”. I'm concerned that there is a reason for the “off delay” contacts, but I cant determine what the reason may be. I configured both relays as “on delay” to allow the current through the armature to be limited during start-up. I also configured the other non-delayed contacts as an H-bridge to change the direction of the current through the stator windings. I'm fairly confident in this circuit but would appreciate any feedback concerning its operation, I've attached the circuit diagram. I also realize that I should use some kind of over current protection (other than the fuse) to protect the motor, is there a particular type of solder-pot or overload I should use with a DC motor? Thanks.

[Al_The_Man]

One thing to watch is when switching the field, that the field is not opened up while the armature is energized, otherwise runaway or speed up can occur.
How bit is the motor?
Al.

[chuckd87106]

Thanks for the reply All. I think its a 3/4 hp motor. I am switching the field and armature at the same time. I'm intrigued how the motor could continue to run. Don't you need current flowing in the field winding for a magnetic field to exist? And without the magnetic field, how can there be attraction or repulsion of the armature?

[Al_The_Man]

Intuitively you would think so, but the attached shows it in detail, it is only when the armature is rotating that disconnecting the field can be dangerous, this is why many large DC motor controllers have field loss protection that shuts the drive down.
Field weakening is also practiced in order to increase RPM, but at the cost of torque.
The Shunt Motor Open Field
Al.

[chuckd87106]

Thanks for this link Al. Its good information to know, I had no idea.