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How many EC fans can I control with a 0-10 Volt signal?

Input impedance EC motor
Every power source has a limit, also the source of a 0-10 Volt signal is limited. Sentera expresses the limit of a 0-10 Volt signal as minimum load resistance in the datasheets. E.g.: Analogue output: 0-10 VDC / min. load resistance 1 kΩ When the minimum load resistance is connected to the 0-10 Volt output, the output current will be at its maximum value
The current depends on the input impedance of the connected EC motor(s). This value is mentioned in the technical specifications of the EC fan or can be requested from the EC fan manufacturer. The input impedance is expressed in Ohms or in Kilo Ohms. 1 Kilo Ohm = 1000 Ohm.
 Impedance of EC motors
To calculate the maximum current, apply Ohm's law: U [Volt] = R [Ohm] x I [Amperes] 
Let's just assume that impedance (Z) and resistance (R) are identical to simplify things. This means that:  I = U / Z
If the EC fan has an input impedance of 1.000 Ω this becomes: 10 VDC / 1 kΩ = 0,01 A = 10 mA.
When the control signal is 10 Volt and the EC fan has an input impedance of 1000 Ω, a current of 10 mA will run through the wires of the 0-10 Volt signal.
How to calculate the combined impedance of multiple EC motors?
If you want to regulate multiple EC motors with a 0-10 Volt signal, you must connect the 0-10 Volt signal in parallel to the EC motors. Each 0-10 Volt input of the connected EC motors has a certain input impedance (Z1, Z2, etc.). To calculate the maximum number of EC motors that can be controlled by the 0-10 Volt signal, we must calculate the total input impedance Zeq of all connected EC motors. The total or equivalent input impedance (Zeq [Ω]) can be calculated using this formula:
calculate combined impedance

An example: When 3 identical EC motors - each with an input impedance of 600 Ω - are connected in parallel, their combined equivalent impedance is: 1/Zeq = 1/600 Ω + 1/600 Ω +1/600 Ω = 3/600 Ω 
Thus, their combined impedance Zeq is 200 Ω. 
When the minimum load resistance of the 0-10 Volt signal is 200 Ω and cable lengths remain limited, it will be possible to control the 3 EC motors simultaneously via this 0-10 Volt signal.
The advantages of digital communication
Controlling fan speed through an analogue signal has limitations. On the one hand, the number of EC motors that can be controlled simultaneously is limited (minimum load). On the other hand, the cable length is limited. The longer the cable, the higher the resistance. This extra resistance of long cables will affect the 0-10 Volt signal and the fan speed.
When EC motors are controlled via Modbus RTU communication, these restrictions are no longer necessary. Modbus RTU communication was developed to transfer data in a reliable way in industrial applications and in harsh environments. Cable lengths up to 1.000 m are no problem. 

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