Electric Motors/d/c motors


QUESTION: I need to replace a d/c shunt wound motor with a 90V permanant magnet. there nis currently a capacitor in parallel with the armature, and a damping resistor in series with the field. what do I need to do with these two items?

ANSWER: Kim you are using the same drive for the perm as the shunt motor?

It is possible to do if the parameters and form factors allow dependent on the load,    

But if you use a shunt controller on the perm motor  then there should be a field loss relay or protection,   how are you going to defeat that?

If your old motor circuit had cap across the arm  that was for that arm not the new one,   don't need it,    damping resistor is for the old motor not the new one, either,

Best way  and I am not sure what or how you are controlling the new motor, but the best way is a perm mag controller that matches the new perm magnet motor,   done,,,,,,,,,,,,,,,,,,,,

If you are somehow trying to use the old controller,   even with the field supply and how to defeat that,     if you can somehow get 90 volts or less [dependent on speed torque needs]  if it will handle the voltage and current   the other devices were for the benefir of the old shunt motor,  the new perm motor wont care,      it is designed to run off either pure dc  modulated dc  whatever type you have,    

Bottom line,  all the old control devices belong to the old motor,   the new perm motor can be run with 90 volts      get a rectifier and variable resistor,  if you need speed control  and FUSE IT,    build your own,m     you don't say what size we are dealing with but I am guessing  if the old was shunt, somewhere around 1 HP?  Less maybe?

No big deal to control a perm motor,   reversing all that, can be done with a drum switch or a toggle wired for reversing,    

you might shoot me some more details on what controller,   sixes amps,  all that and let me look all that over,         the resistor for the old motor on the armature is to kill the back emf when the fields collapse,      the inline resistor  could be for several reasons again depending on the load    speed changes   reversing  so on,          

send me a little more about the whole system  and we will go from there,    Dc Motors

---------- FOLLOW-UP ----------

QUESTION: Good Morning, and happy new year to you and yours. First off thank you for your speedy reply, second, forgive me for not including all the information. The motor I am replacing is a shunt wound 1/20 HP that is being controlled by a Gordon PC-250 Reel control drive. The reason for the change is there is not enough torque in this motor to overpower the brake that controls the tension. It is only 13 lbs. the replacement PM motor is 1/4 HP with 45 lbs. of torque. The drive specs. say the controller is good for up to 1/2 HP with either a shunt wound or PM motor. Problem is that when the unit calls for speed it constantly blows the fuse. (even with the motor disconnected). I cannot locate the schematic for this controller, and was just wondering if I had miswired the the motor or if something had to be removed or added to the circuit.

ANSWER: So if you fire up the controller,  which I assume only controls the motor,    no other devices,  there is a problem with the controller,   or it is a long shot, but the voltage to the controller could be way off tolerance,       

I would check the incoming voltage at the terminals of the controller,   why,  voltages can change from utilities at anytime of day or year        

I have measured voltages in residential, commercial or industrial locations as much as 25% off tolerance for many reasons,      

But for a controller to simply blow a fuse with no motor on the outputs, it is a 90 plus percent chance your controller,  or a wire from  or going to is grounded,  or the board is faulty,  something is raising the current and it is obviously not the motor,    as it is not connected,  

Now I assume you mean not connected to the controller not the load,    that is important and needs clarified,     

Was this setup running fine for a period of time and no problems?

Or is this a new setup or new to you?

If it had operated in the past,  and the fuse is blowing in the drive[controller]  all of a sudden  the simplest test is the incoming voltage        so simple it is often overlooked,   

Once you are satisfied the voltage is within tolerance,   then look for any obvious ground,  chaffed wire,  any darkened spots on anything visual,  which might indicate a carbon path or burn,     also  Check the fuse size and make sure somewhere down the road a smaller fuse size did not get installed,          

These are simple no cost tests that HAPPEN ALL THE TIME<      

But let me know if disconnected means from the controller or from the load,      will make a huge difference in where we go next    

If supplied with 115 US typical single phase voltage   the hot side should have  NO circuit with any part of the controller frame,  the neutral might    but not the hot,    if it does,  the controller has a ground fault somewhere,    if no circuit to frame   it still has to have some sort of false load it is seeing to blow the main fuse,  which I assume in a 1/20 hp there is only one fuse in the controller,         

If this is a new unit or new to you, the drive could have a voltage switch, which would be some sort of toggle or dip switch that would allow either 115 or 220 operation,    check for that also,       

It really sounds like you have a controller problem not a motor problem,     

For a test   at least on the new motor,     we could build a test set for the compound motor but it gets involved         if needed we will      but to test the perm motor,    at 1/20 HP the amp rating has to be very low   and I believe even Radio Shack carries a cube rectifier up to maybe 10 amps,   4 prongs  AC IN on two     DC out on  two,    the motor is rated at 90  you will be applying over voltage to
the motor for a short time but not enough to hurt anything   the incoming AC if within normal tolerance at around 115,  compared to the 90 as the motor is rate for,   is not a lot  and if the motor has a service factor    of say  1.15   the data plate designation for service factor could be something like   S.F.   1.15   or   1  or 1.20   that service factor would allow for a constant over load of whatever the factor is,   so that can be applied to the incoming voltage also,  

If the perm motor operates on the test voltage    put an amp meter in series  or get a clamp on DC amp meter,   and check     if it is a little high no problem    you just confirmed the controller is bad,  and it is likely a shorted or grounded or both  rectifier in the controller,  normally the first thing to fail,       it might be made of diodes  as it is a dual use controller,

Also on dual use controllers there is some method of dealing with the field supply  if not there would be live DC on those leads while not in use,   so normally they have a way to turn off the field side supply       to use on perm motor,         some more things to just check out     

let me know after you make some of these checks and we will go from there


here is a link to the specs of the controller    



parts for your controller  


---------- FOLLOW-UP ----------

QUESTION: Just wanted to let you know that your suggestions for correcting my problem were extremely helpful. It appears that the problem was the capacitor that was tied in parallel with armature. I wish to again thank you for speddy reply and execellent advice.  best wishes in the new year

Well great you got to the problem,  was this cap tied in the controller circuit?   If so it would make sense why the controller fuse would blow with no motor attached,       if it was shorted and in the controller feed,    it could be a direct short   with no motor attached,   

Likely got weak due to brake load as you explained,     and good it was the cap,  as the OEM Parts lo0ked to be high dollar,      I was just getting ready to print off the links from the parts link  and start drawing it out,    

As it is a dual use controller    it is probably needed for both shunt and perm motors,   it has to deal with back emf,      not much else a cap can do across an armature but stop any back feed,    

And if it failed and did not open  it would draw a bunch of current   right from the controller,    so it makes sense,          

Good for you and good job of trouble shooting,      that looked like a pretty intense drive for the size  with lots of bells and whistles       

Glad to hear you have the problem solved and now SHOULD Have a spare motor if needed,       

You should check the brushes to the comm,  and the SPRING TENSION    even with new brushes if the tension is gone   problems will be not far down the road,        

Also if the commutator needs dressed, be sure to undercut the mica  and remove any side mica on the inside walls of the slots       if the comm does not need machined,  then do not remove the browned or dark area on the comm if you try to use the old motor,     that dark area is normal and provides a slick surface for the brushes to ride on    

Past that it sounds liie you found it,      and done       good job  

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Three phase/ AC DC single phase motors, controls, any problems or failures, motor installation, performance issues, connections. All other electric motors/gearboxes/apparatus. Specialty repair concerns, obsolete motors and solutions. Other mechanical or specialty equipment. See my profile under Home/electrical at this site


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