Family 3

How to choose a Servo Drive for your retrofit

After having examined, in the previous post, how to figure out what servomotor we are using, what kind of data we need and where to find them, we are now looking at how to use that information to choose the right servo drive.

The essential parameters needed to choose a new servo drive are:

  • Type (DC, Brushless, Induction)
  • Output current and voltage
  • Servomotor power supply voltage
  • Speed signal

Let’s see how to link these data to the motor parameters.


This is the most obvious characteristics: a servo drive should be chosen according to the type of servomotor you need to control: the choice will be among servo drives for DC motors, Brushless (synchronous) or Induction (asynchronous) motors.

In the description of all ALTER servo drives you can find an indication of whether the servo drive is more suitable for “axis” or “spindle” use.

Output voltage and current

  • Output voltage of the servo drive must be compatible with the one specified in the servomotor data.
  • Output current of the servo drive must be equal or slightly higher than the rated current specified in the servomotor data. In case the servomotor is intended to be used for short amounts of time in operational modes different from the nominal one (S1), it is necessary to know also the current requested for these modes and size the converter accordingly.

Power supply voltage

  • Line supply voltage and frequency must be compatible with the servo drive characteristics (power supply, DC Bus)
  • Operating frequency: 50 or 60 Hz? ALTER servo drives can operate with both these supply frequencies just by changing one simple setting.

Speed signal

The signal type accepted by the servo drive must be compatible with the transducer type mounted on the motor (e.g. resolver, encoder).

Once the servo drive model that suits your needs has been chosen, in some cases you can add some optional features: to stick to the retrofit field, you can start from the functions of your old servo drive and cross-check those of your new model.

Having the servomotor information listed in the previous blog post and following the suggestions above, in the products section of ALTER’s website it is possible to choose the right product for you. The navigation of our website is designed to guide you through the choice of the perfect servo drive for your desired application.

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