Loop Control: Open Loop Versus Closed Loop Blowers

What's the Difference between Open Loop Blowers and Closed Loop Blowers?

What's the difference between open loop control and closed loop control? In simple terms, open loop control is like driving a car. As you drive, you give the car more gas to go up a hill, and less gas to go down a hill, changing the amount of gas that is propelling the car up or down the hill. This analogy can be applied to open loop control in blowers. When you add resistance to the blower it will work up 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. If you are taking a long road trip, you don't want to fatigue your foot by constantly pressing the gas pedal, so you set your cruise control to 65 mph. With cruise set, the car calculates 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 commands. Most blower manufacturers can program your desired speed command into the blower.

Which Type of Blower Will Work Best For My Application?

When considering a blower, you’ll need to know how it will work with your existing system, or a system 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 is your best option. By using open loop control, you’ll be able to control the speed of the blower with your external controller or by using parameters set in your system.

Although this seems like the best option, a closed loop blower can also provide 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

(Also called: 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, and so forth.

The blower calibration potentiometer, a device that limits the passage of electrical current, can be used to adjust the full-scale command voltage. For example, turning the potentiometer CW 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. The blower motor follows its natural response according to the blower motor torque speed curve and fan loading conditions. 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's current limit protection function restricts 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's command for 100% motor voltage, limiting the commanded speed range of the blower. (For operating conditions that do not result in current limit, the control ability of the blower speed is unrestricted.)

If the blower is calibrated at full air restriction, full scale command voltage represents 100% line voltage; the blower control ability is restricted only 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 (with 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 midway calibration point may exist to achieve the desired compromise between range of control ability and blower maximum performance.