4.3.2. Motor limiters¶
Motor winding current and voltage limiters and motor shaft revolution speed limiters are provided to ensure safe operation. These limiters, if activated, will lead to gradual power and rotation speed decrease until the parameters being limited are within acceptable range. Motor limiters work with voltage and current values directly on motor windings, unlike critical parameters, which work with current and voltage at the controller input. Another distinctive feature of limiters is that they do not stop the motor and let the controller enter Alarm state; they merely limit voltage, current and motor revolution speed.
For DC motors:
- Max voltage is the nominal motor voltage. It defines maximum motor winding woltage. It is usually used to limit voltage growth when the motor or stage is jammed. Should only be used if maximum motor winding current is unknown. This parameter is used in PID regulator.
- Max current defines maximum engine winding current. It is usually used to limit current growth when the stage is jammed. One should set this limit based on maximum current which can be sustained by the motor without damage (primarily by heating).
- Max RPM is the maximum motor shaft revolution speed. Is usually used to limit revolution speed when working with reducing gears and other devices which have strict maximum speed limits.
One should be aware of the distinction between maximum motor current and nominal current. In general they may be different because of motor cooling features and operating conditions. Also, one should not mix up maximum current and starting current with stationary motor shaft.
Changing maximum motor voltage might disrupt PID regulator. For more information see PID-algorithm for DC engine control.
Maximum motor voltage may exceed nominal voltage, usually by 10-15%. If you are using a motor with low load and you need high motor speed, then you can increase maximum motor voltage.
Current limiter operation.
It is important to note that maximum current limiter does not react immediately when working with DC motors. When a higher than maximum current is detected in the motor winding, voltage supplied to the motor is gradually decreased until winding current is less than Max current. In the worst case of a rapid jam during high-speed movement motor voltage may decrease for at most 370ms. If the current limit is chosen right, motor should not overheat during this time.
If the Max current value is set too low, it is possible that DC motor will not be able to move under high load or high friction.
For stepper motors:
- Max(nominal) Speed is a maximum motor shaft rotation speed in steps per second. Current stepper motor speed is defined by Speed parameter (see Predefined speed rotation mode).
- Nominal current defines maximum motor winding current. This value cannot be exceeded due to characteristic stepper motor control.
In the XILab software limiter settings are described in sections Settings of kinematics (DC motor) and Settings of kinematics (stepper motor).