How to calculate the output power of a Servo Drive

How to calculate the output power of a Servo Drive

Power in a Y-wired motor

The following diagram shows the electrical model of a Y-wired BLAC motor:



The power in any motor can be calculated as the sum of each phase power (product of RMS current and RMS voltage). In the case of a phase-balanced motor:

In a Y-connected motor,
  • The phase current (Iph) is the current in RMS seen at the Servo Drive output terminals.

  • The phase voltage (Vph) is defined as:


where Vph−ph is the RMS voltage difference between two output terminals of the Servo Drive. The use of the Space Vector Modulation (SVM) allows the use of all the DC-bus voltage as line voltage amplitude. 

where D is the applied duty cycle.
Therefore, the power delivered to the motor power can be defined as:

Power in a Δ-wired motor

From the point of view of a Servo Drive, the power provided to a Δ-wired motor is exactly the same. However, the voltage and current values are shared different within the phases:


In a Δ-wired motor:
  • The phase current (Iph) is not the current seen at the Servo Drive. On the contrary, it is defined as:

where Iine is the RMS current seen at the Servo Drive output terminals.
  • The phase voltage (Vph) is the RMS voltage between is the voltage difference between two output terminals of the Servo Drive, defined in (3).

Therefore, the total power in a Δ-wired motor results as:


Which, from the point of view of a servo drive, is equivalent to (4).

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