Q: I am using one of your BLH series brushless DC gearmotor and driver for a parts feeder. I wish to use an external potentiometer to control the speed. I read that 20 KΩ resistor is required. Do I have to use a 20 KΩ for the external potentiometer? What happens if I use a potentiometer with a different value/rating?
A: Depending upon the rating of the potentiometer, it will act differently. The way a potentiometer works is it changes resistance as it is turned clockwise or counterclockwise. For a 20 KΩ potentiometer the resistance goes from 20 to 0 KΩ as it rotates clockwise. Therefore, when the potentiometer is rotated completely clockwise, we create an instance of zero resistance. At zero resistance, the full voltage is applied to the driver which allows for the highest speed to be achieved. When looking at the BLH series, for example, the speed range is 100-3000 rpm when using a 20 KΩ potentiometer. You can reference the graph below for a visual representation of the potentiometer setting vs the speed of the motor.
For the x-axis, it shows the "external potentiometer graduation" between 0 and 100. For this potentiometer, 0 represents 20 KΩ resistance and 100 represents 0 KΩ resistance. Going further, the 20 could represent 16 KΩ resistance, 40: 12 KΩ , 60: 8 KΩ, and 80: 4 KΩ resistance.
**Please note this are theoretical values when in reality turning the potentiometer does not always result in a linear speed increase.
As previously stated, the graph shows the usage of a 20 KΩ potentiometer. When a smaller rated potentiometer is used it reduces the speed range. For instance, if a 10 KΩ potentiometer was used instead, then the speed range would be reduced to roughly 1300-3000 rpm. Here, turning the potentiometer completely clockwise still generates zero resistance and we achieve our 3000 RPM speed. However, there is not enough resistance to achieve a slower speed than 1300 RPM. Going back to the graph, the 10 KΩ resistance would be referenced at the 50 graduation.
When a larger rated potentiometer is used, a smaller percentage of the clockwise turn is used to adjust the speed. If a 40 KΩ is used with the BLH series, then the speed range would still be 100-3000 rpm; however turning the potentiometer half a clockwise turn would do nothing. It isn’t until passing the half way point that the motor would run at 100 rpm. Then, as the potentiometer continues to turn clockwise the speed would increase. A full clockwise turn would still result in a maximum speed of 3000 rpm. This can be seen at the lower region of the graph. At 20 KΩ of resistance or above there is too much resistance and therefore not enough voltage is provided in order to generate any speed. Only at the 13 graduation (roughly 17.5 KΩ resistance) does the motor start to spin.
In summary, when a 20 KΩ potentiometer is recommended for the product, it is because it will provide the most optimum setting for the speed as it is large enough to cover the complete speed range, but not too large where rotating the potentiometer would either result in no speed from the motor, or a larger difference in speed.
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