Power Limit

Overview

In case the DC power supply of the Drive has limited output power (for example limited output I DC at a certain V DC level) this feature can be used to make sure that the output power drawn from the power supply is limited.

Use case: Battery powered system

A good example of such cases are applications where batteries are used, and their output currents should remain below a certain limit.

This feature does not measure the mechanical torque directly. Instead it is using the “computed electrical torque” and assumes that the “computed electrical torque” is equal to the real generated mechanical torque of the motor.

It calculates the rotational speed of the motor with values from the position sensor at the motor shaft.

Requirements

This feature works in CST, CSV, CSP, PT, PV and PP modes.

The following parameters must be properly set from the motor datasheet:

  • Motor torque constant

  • Position sensor configurations

  • Number of pole pairs

Detailed Description

Mechanical output power of the Drive in any condition is equal to:

P_{out} \big[ W \big] = \tau \big[ Nm \big] \cdot \omega \bigg[ \frac {rad} {s} \bigg]

where “τ” represents “Torque”, and “⍵ represents the “rotational speed”.

At any operational point, the Drive has access to rotational speed. As a result the limit of generated torque can be calculated as:

\tau_{limit} \big[ Nm \big] = \frac {P_{out,max} \big[ W \big]} {\omega \big[ \frac {rad} {s} \big]}

This limit is updated in each execution loop of the torque control, and it limits the absolute value of reference torque.

Note

By default the power of each Drive is internally limited to 110% of its maximum peak output power. As a result, this object will only be effective if its value is less than 110% of Drive’s maximum peak output power.