Changing Stepper motor by servomotor

Good morning.

I have a machine tool equipped with stepper motors. I want to change them by servo motors to improve the precision and speed.

But I am confuse, the torque of the existing stepper motors are indicated in N.m (Newton.meter). When I read the characteristics of the servo motors I do not read any torque in N.m but a power in W. (sorry for the units, I am European)

My questions are:

  1. For the servo motor what is this power in W ? is it the mechanical power on the shaft of the servo or the electrical power of this servo ?

  2. If it is the mechanical power of the servo, How calculate and translate the torque of existing stepper motors into the power I need for the new servo motors I will need

  3. How to select the encoder and drivers I will need for these new servomotors.

If someone(s) can help me about this subject, I will be very happy. Thanks in advance

I think you would be better asking this question on a forum that deals with CNC - for example When people here talk about servos they usually mean the things that control radio control model aircraft.

The definition of a Watt is 1 N m /second meaning a Force of 1 Newton moving a distance of 1 metre in 1 second. (1 Kg force = 9.81 Newtons)

Torque is often described in Newton metres, but in this case the metre is the radius at which the force acts. If the motor turns at 1 rev per second the circumference of the circle will be 2 Pi metres or 6.28 metres. So a Torque of 1 Nm applied at 1 rev/second would be 6.28 Watts. (I hope that's clear, and that I haven't made a stupid mistake).

I have no idea whether the Watts measure for your servo motor is the electrical input or the mechanical output. It could be either. And I have no idea how efficient the motor might be. I would be very surprised if torque data is not available from somewhere.


Thanks for the advice Robin2, have a nice day

A stepper motor is designed to be accepting its maximum current all the time. But, a DC motor rating is not as straightforward. As long as you don't exceed its absolute maximums, you can also choose a variety of voltages, and current limit combinations. But, you will generate heat, and must keep that from building up beyond its limits. That is why the wattage rating.

Also, note that you can choose between an occasional burst of power, or a continuous lower-level amount of power.


A proper industrial servo motor will come with a whole raft of specifications - and a
datasheet. What motor is it? They are usually a lot more expensive than steppers.

I suspect the OP has gone away.