# DC motor control response time

#### ASWIN VENU

Joined Sep 14, 2016
1
I'm working on a project which uses two motors which spin at slightly different speeds which are derived from a reference speed which may change very frequently. I plan to use separately excited dc motor. Due to the rapid variation of rpm it requires a fast speed of response, say a settling time of .1 to .5 seconds.My questions is if that is possible to attain such high speed of response? If so I would like to know what method(s) to use. I intend to use a 1Hp motor and the rpm ranges upto 1500.

Thank you.

Joined Jul 18, 2013
28,069
It depends greatly on the load and type a mechanical transmission, inertia etc , by 'separately excited' are you using the term in the accepted sense of separately excited field?
Max.

Last edited:

#### MrAl

Joined Jun 17, 2014
10,927
I'm working on a project which uses two motors which spin at slightly different speeds which are derived from a reference speed which may change very frequently. I plan to use separately excited dc motor. Due to the rapid variation of rpm it requires a fast speed of response, say a settling time of .1 to .5 seconds.My questions is if that is possible to attain such high speed of response? If so I would like to know what method(s) to use. I intend to use a 1Hp motor and the rpm ranges upto 1500.

Thank you.
Hi,

Controllers can be designed to do just about anything, but part of the problem is what the motor can take without a problem. There are usually limitations.

As you probably know, to get current to flow in an inductor fast you need a high voltage. Also, rotational inertia of the motor and load act almost like extra inductance so you need an even higher voltage to get the speed up to par quickly, and braking action to get it to slow down quickly.
How fast this can happen depends on what the motor can take as far as input voltage and that also means what voltage you have available. If you have say a 12v motor and you only have 12v available, then dont hope for too much unless that is enough to get the motor up to speed quick enough. And again since that depends on the motor and load parameters you need to know the motor and load parameters in order to predict what you can do with this system.
I can give you an example but it wont help unless you have the actual data for your system. To get that you probably have to look some things up and take some measurements.

The whole physical law at work here is F=m*a in the rotational sense. To get a particular acceleration you need enough force (F):
a=F/m

and the higher the force the higher the acceleration (a). In a motor the force comes from the current, and the current comes from the applied voltage. So to get a higher acceleration you need more voltage. If you cant apply more voltage then you are stuck with whatever acceleration you get from the voltage you can get. You could reduce the mass (m) also, but usually you cant do that because the motor and load have a fixed mass that cant be changed.