I have a question that has been bugging me since I started working in a different factory. They seem to use an awful lot of DC motors, even really big ones i.e.--over 30 hp. Is there a reason why DC is better than AC for certain things? Most of the equipment is 10 to 20 years old, does this explain things any? I know VFDs are fairly new technoology, but still I wonder.

DC motors are better in applications where very wide speed control range is needed, particularly when lots of torque is needed at low speeds. Speed range can be adjusted from zero to full rated speed, or even higher with field weakening.

Lots of old DC motors are still in service, after having the old Ward-Leonard MG set controllers replaced with modern SCR drives.

NJ wirenut
I might have agreed with you 10 years ago (maybe even 5). The new technology used with AC drives is much better you can get DC like performance from the new drives. In my plant we have several large drives 300 & 400 hp the run just as well as the DCs and you don’t have the maintenance issues!

Even with todays VFDs I do not believe you can get the same high torque at the lowest RPMs with AC motors.

Admittedly most applications do not require great torque at the lowest speeds.

Isn't this the reasons trains still use DC traction motors?

You need to look at cost as well. If you are looking at a large layout of variable speed machines, especially large ones, VFD prices start to add up.

Comparing the cost of a 300-600HP drive to the maint costs of a DC machine gives you a very long time to get any ROI.

Now, if you only have one or two applications for this, the cost of setting up a DC power system may be the offsetting factor.

Just a thought.