Different methods for servo drive specifications

A servo drive is commonly used in robotics. Synapticon’s drives are used with three-phase permanent magnet (PM) motors, including brushless-DC (BLDC) motors and permanent magnet synchronous motors (PMSM).

There are many different control standards for three-phase PM motors, including:

  • Sinusoidal commutation

  • Block commutation (six-step method)

  • Field-oriented control (FOC)

When selecting the correct motor for an application, it is important to understand the application for which the motor is required. Once this is determined, motor specification datasheets are often referred to, which include data on continuous torque, continuous current, torque constant, etc. Choose the best drive option based on the information listed.

Note: Synapticon drives are based on the FOC standard.

Motor Specification Datasheets

When reviewing a motor specification datasheet, it is important to note the unit types that are listed on it, including the following:

  • Torque constant (Kt) is the ratio between "the current inside motor windings" and the "torque generated by the motor".


  • IRMS is the "root-mean-square" of a sine waveform, and a way of measuring the effective value of an AC (alternating) current.

    IRMS is a standard unit used to measure maximum phase current.
    For example, for Circulo 9 , the maximum phase current is 60 ARMS.

  • Ipeak sine is the peak value of the sine curve.

It is important to note that on other manufacturers’ datasheets, instead of IRMS, the sine amplitude current or “active current” may be listed here. This value may be the Ipeak sine value, which provides a different measurement than IRMS to measure phase current.

Attention: When Ipeak sine values are used instead of IRMS, the drive may behave in an unexpected way. Motors or other parts may be damaged as a result.


Figure 1  -  Peak and RMS values graphical depiction

Because different methods can be used to communicate the drive’s capacity, the following equations are helpful to determine the proper units for the rated motor current and to compare data among different specification datasheets. Use these equations when you need to determine a specific value.

Conversion between units

To convert between units of current, use the equation below.

Here, the RMS current is converted to the peak of sine current:

To convert between units for constant torque, use the equation below.

Here, the RMS value of Kt is converted to the peak of sine value of Kt: