How is speed adjusted with a potentiometer?
On the KS250 and PT500 the motor speed is set with the pot on the board; on the PT500 this pot is labelled SPEED and there is a separate RMP pot for the ramp. The ST42 has no speed adjustment, only direction and on/off control. The adjustment must be made with the motor under load and in real operating conditions.
The potentiometer gives the driver a reference level; the driver in turn sets the average voltage it applies to the motor according to this reference. What the user has to do is calibrate this reference to the real needs of the application.
Adjustment order:
- Start the pot from a low position and run the motor with the real load.
- Increase it gradually until the movement is smooth and uninterrupted; an adjustment made unloaded falls short once the load is applied.
- Measure the current at the same time; as the speed rises so do the current and the heating.
- Since speed and ramp are separate, set the speed first and the ramp time afterwards; moving both together makes it harder to follow the result.
- After finishing the adjustment, mark the pot position with the setting pot label kit supplied with the product, so you notice any loss of position in a vibrating environment.
Caution: on a DC motor, reducing the speed reduces the average voltage applied, and this limits the torque that can be produced. If you try to lift a heavy load at very low speed, the motor draws a high current without being able to start and heats up. Size the system according to the torque requirement at the lowest operating speed.
If you want to use an external reference instead of a manual pot, the KS250's IN5 and the PT500's E analogue input are available for this purpose.
Full topic: DC Motor Driver Selection Guide