Type of Document: Article
Applicable for: Robo G4 products, motor drives
Date of Issue: 28/11/2023
Amps Limit Function
Roboteq drives have the capability to limit the motor current to a safe value configured by the user, thereby protecting both the motor and the product itself. This is achieved by reducing the PWM in order to regulate the motor current. This current limiting function is known in Roboteq drives as 'Amps Limit'. Since the drive will be in limit mode, the motor maximum power (both speed and torque) will be affected.
Amps Limit Threshold
The maximum allowed value for the motor current is derived from three configurable functions:
1. Amp limit configuration
2. I2T Configuration
3 Maximum Motor Output Power configuration
Each of these three functions can set a distinct threshold for the amp limit value. The threshold that will ultimately be applied will be the smallest of the three.
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Amps Limit Configuration
If there is not any other active limitation, amps limit will limit the current directly to the user configured value.
I2T confiuguration
I2T works based on the motor's peak current, nominal current, and allowed time (for the peak current). Once the I2T protection is triggered, the current will be limited to 80% of the nominal until the I2T integrators deflate. The figure below showcases an example where the I2T configuration results in a current limitation threshold that is smaller than the configured threshold for amp limits.
Max Power Configuration
The maximum Motor Output Power configuration sets the current limit threshold according to the following formula:
Max Power (W) = Max Speed (rad/s) * Max Torque (Nm)
Max Power (W) > Speed (rad/s) * current (A) * Torque constant (Nm/A) =>
current < Max Power (W) / ( Speed (rad/s) * Torque Constant (Nm/A))
From the formula, it is evident that for proper current limitation at any given speed, both the Torque Constant and the Max Power must be set correctly.
The figure below showcases an example where the motor current is being limited to a value much lower than the amp limit threshold configured directly by the user. This can occur unintentionally due to incorrect configuration of the Max Power or Torque Constant.
