We currently have DC drives and motors used in our manufacturing plant,
rolling steel coils into tubes. Someone recently said that we should
convert to AC drives and motors.
What is the better way? Or Pros and Cons.
Lots of people claim all kinds of things. Without knowing his reasoning its
hard to agree or not with his statement.
There are advantages (and disadvantages) to using AC motors with vectors
drives over DC motor/drive combinations. I will summerize some of the pros
and cons as I see them (remember this only an opinion).
Better control of speed with AC vector versus standard DC with similar
AC motors can be somewhat smaller than DC motors for the same HP
No brushes on AC motors
Can use common DC bus configuration - might save you some money and give you
more options for dealing with regen energy
May create less havoc with your power system
Vector drive requires encoder or resolver feedback for best results. These
can be the weak link in a motor/drive combination.
DC motors are usually beefier, and can take more abuse, which in a mill is a
very real concern
AC motors may require external fans for cooling if run at low speeds for
long periods of time
DC drives can often give you 150% FLA for short periods of time, while this
would trip an AC drive
If you end up running motor wires any distance you can have all kinds of
issues that need to be dealt with using AC drives
If you need to deal with regen energy after a drive trips, you can do this
with a DC drive by shorting a brake resistor across the motor leads
I'm sure others can add to my short list. It all comes down to your
One point- AC machines are not as limited in HP size as DC machines-
(already covered under no brushes)
Another thing and this may be the deciding one. - If you have a system set
up with DC motors and these are doing the job- then -if it ain't broke-
don't fix it. Expansion or upcoming replacement "might" be best converted to
AC but it would be necessary to sit down and do a good engineering and
economic (they are interdependent) study and make the necessary choices
then- not because "someone says"