DC Motor Problems

cooling cooling cooling!! what size and type are you working with?

What type of DC motor? Is it self cooled or force cooled.What are the field and armature voltages and currents ?base speed?pl be more specific to answer ur ?

heeranandan:

I hope you intended to say 75% of the motors rated full load rpm and not 7.5%.

At 7.5% your motor is nearly stalled and drawing excessive current.

1-If it's direct coupled (motor/load) you need a bigger motor or reduced load.

2-If the load is gear or pulley coupled to the motor, reduce the primary gear/pulley size or increase load gear/pulley size.

Good luck!

Donzi

Good points, thus far... especially (#1) cooling(!)... and, further to post-2, more info is needed before a complete answer can be given. [There are SO many different types of DC motors.]

If this is a self-excited unit, to which you refer, there exists a critical speed, below which the necessary excitation voltage will not build up.

As already requested, please feed all the facts that you can, in order to minimize the need for "speculative" answering.

Best wishes ~

It is a seperatly excited DC shunt motor used as Pinch roll in re-rolling mill provided with ABB DC drive. Field supply is constant where as armature supply is variable as required. during rolling heavy section DC motor rpm is 118 and armature voltage is 33V and current draqn is 165 ampere

Apologies for not being on the same 'wavelength', here... (just as in another recent post, I must admit that I've been away from immersion in this field for awhile)...

Your description throws the proverbial curve-ball at me; I assume that you are referring to a Compound-Wound machine (in "shunt" machines, the excitation fields are connected across the armature... 'yes'? ... if not, do you know whether it's a short-shunt, or a long-shunt connection...?)

I'll break-out my refresher materials when I get home, and see if something jumps out at me, other than what's already suggested <cooling!>.

Best Regards ~

Sounds like cooling is still your big concern, Do you have a blower attached to this motor? Is the armature rated for 500 volts? field around 300?, Is the current load within name plate ratings? If so it will do the work as long as you keep the heat out of it,Its a constant tork machine at really any speed. Do you change the speed to keep up with the roll ? field weakening? Its still all about the heat being removed.