What's the difference between open loop control and closed loop control? Simple, it's like driving a car. When you are driving your car, you give the car more gas to go up a hill, and less gas when going down a hill. In this example, you are changing the amount of gas that is propelling the car up or down the hill. This example can be applied to open loop control in blowers. When you add resistance to the blower it will work to 100% of the voltage provided. In other words, it will max out the current Limit to provide 100% of the voltage input into the system.
On the other hand, closed loop control is like using cruise control. When you are taking a long road trip across the country you don't want to fatigue your foot by constantly pushing the gas pedal, so you decide to set your cruise control to 65MPH. By setting cruise control, the car knows how fast to drive. Closed loop control is like cruise control for your industrial blower’s motor. It can be factory programmed to include specific speed command. Most blower manufacturers can program your desired speed command into the blower.
Which Type of Blower Will Work Best for Me?
When you are considering a blower, you’ll need to know how it will work with your existing system or the system that you are considering for purchase. One thing you’ll want to consider is open loop or closed loop control? If you plan to control the blower using an external controller, an open loop controlled blower will be your best option. By using open loop control, you’ll be able to control the speed of the blower with your external controller or parameters set in your system.
Although this seems like the best option, a closed loop blower provides you with options for different speed commands, allowing you to have a pre-programmed ready-to-use blower. Blowers can be programmed to provide the exact specifications necessary to achieve the performance specified.
Now, let’s get technical...
Open Loop Control or Motor Voltage Regulation
In open Loop regulation mode, the user command voltage determines the percentage of line voltage applied to the blower motor. For a 10V full scale control voltage signal, 2V represents 20% of the Line voltage applied to the motor; 4V represents 40% of the Line voltage applied to the motor, etc. The blower calibration potentiometer can be used to adjust the full-scale command voltage. For example, turning the potentiometer clock wise for SV full-scale command voltage would result in 2V representing 40% of the Line voltage applied to the motor, 4V represents 80% of the Line voltage applied to the motor, etc. The blower motor follows its natural response according to the blower motor torque speed curve and fan loading conditions. Thus, the blower naturally speeds up as air restriction increases (fan torque decreases).
When the fan is heavily loaded at free flow resulting in high motor torque demand, the motor current Limit protection function limits the current to the motor to a safe Level by clamping the motor voltage to a Level below 100% motor voltage. The motor current Limit protection function overrides the user command for 100 % motor voltage, limiting the commanded speed range of the blower.
If the blower is calibrated at full air restriction, full scale command voltage represents 100% Line voltage, and the blower control ability is only restricted for blower operating conditions that result in motor current Limit. This will result in Limited controllability at high flow conditions.
If the blower is calibrated deep in current Limit (no air restriction), then full-scale command voltage represents the current Limit clamping voltage, which is well below 100% Line voltage (- 40%). This will limit the maximum achievable sealed speed (pressure) and power of the blower. A calibration point midway may exist to achieve the desired compromise between range of control ability and blower maximum performance.
Closed Loop Control or Fan Speed Regulation
In this closed Loop regulation mode the control automatically adjusts the motor voltage to maintain a commanded fan speed. In this regulation mode the blower fan speed will remain constant as air restriction to the blower changes. The blower will follow the user speed command if the blower is physically capable of achieving the commanded speed.
Information provided by Ametek DFS