First, let's look at an example:
This is the built-in PR mode of our CL57RS drive. In the example above, when the motor is set to run in speed mode, the operating speed is determined by the speed register. The hexadecimal speed value 01 2C corresponds to the decimal value 300.
So, if you want the motor to run at 0.1 RPM, how should you set it? You will find that a decimal value cannot be directly converted to hexadecimal.
For the Modbus RS485 protocol, due to the way hexadecimal values work, the value written in the command must be an integer. In other words, 1 is the minimum operating speed that can be written. In this case, modifying the register value alone will not work. Let's look at some alternative methods:
1. Change the Unit of the Speed Register
Some manufacturers provide a parameter for switching the unit of the speed register, such as 1 RPM or 0.1 RPM. Without changing the RS485 command, you can achieve decimal-level speed control simply by changing the unit. The previously set integer speed value will then be reduced by a factor of 10.
Of course, most RS485 drives on the market do not provide such a parameter for switching the unit.
In the example above, 6203h is the address of the speed register. Find its unit setting in the manual, as shown below:
As you can see, the unit is 1 RPM, not 0.1 RPM. Therefore, this method does not apply to our CL57RS.
2. Mechanical Gear Reduction Ratio
If the minimum speed that the drive can control is 1 RPM, and the unit cannot be changed through the parameters, you can consider adding a reduction mechanism externally, such as a gearbox. The reduction ratio can be selected according to your requirements.
For example, if you want a speed resolution of 0.1 RPM, you can choose a 10:1 gearbox. The motor outputs a stable speed of 1 RPM, and after reduction, the gearbox output shaft will provide a stable speed of 0.1 RPM.
Similarly, if you need a lower speed, simply choose a reduction mechanism with a higher reduction ratio.