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1. A variable frequency drive (VFD) may be used for control of process temperature, pressure or flow. This application of VFD is without the use of a separate controller. Suitable sensors and electronics are used to interface the driven equipment with the variable frequency drive (VFD, frequency inverter).

2. Maintenance costs of the controlled system can be lower, since lower operating speeds result in longer life for bearings and motors.

3. Eliminating the throttling valves and dampers also does away with maintaining these devices and all other associated controls.

The main benefits of variable frequency drive (VFD, frequency inverter) are

1. to improve the efficiency of the motor-driven equipment through matching speed to changing the load requirements, thus saving energy consumed.

2. to allow accurate and continuous process control over a wide range of speeds, thus improving the process.

On the other hand, the closed loop control system utilizes feedback to measure the actual system operating parameter being controlled, such as temperature, pressure, flow, level or speed. In the control system, the feedback signal is sent back to the controller, where it is compared with the desired setpoint. The controller develops an error signal that initiates corrective action and drives the final output device to the desired value.

Through closed loop control of AC motor, the result is stable, accurate, dynamic speed and torque control from standstill through to maximum speed.

In the open loop control system, no feedback loop is employed. And system variations, which cause the output to deviate from the desired value, are not detected or corrected  by the system. Thus open loop control system has no feedback or error-detection process. The communication between the controller system and the motor is one way for open loop control system.

in the area of motor control, open loop control provides the simplest way of controlling AC induction motors, as the speed of AC induction motor is dependant upon the frequency of the supply. A basic means of control is simply to vary the voltage and frequency to achieve the desired motor speed. The main disadvantages of this kind of control are poor precision and poor low speed torque.

It's said that 40% of all power in Europe and North America is consumed in  the buildings. The biggest share of the energy is consumed in heating, ventilation and air conditioning (HVAC) applications.

In China's power consumption, the power consumption by the buildings accounts for around 32 percent. Yes. The biggest share of that is in heating, ventilation and air conditioning (HVAC) applications too.

The situations of power consumption is similar in other countries and areas. Thus the energy savings potential in HVAC applictions is large.  It is crucial to use all means available to reduce the energy consumption in the building, especially in HVAC applications.

Most variable torque drives have PID capability for fan and pump applications, which allows the drive to hold the setpoint (the programmed input value) based on actual feedback from the process, rather than relying on an estimation.

A transducer or transmitter is used to detect process variables, such as pressure levels, liquid flow rate, air flow rate, liquid level. Then the signal is sent to a programmable logic controller (PLC). Programmable logic controller (PLC) communicates the feedback from the process to the drive. The variable speed drive (VSD) uses this continual feedback to adjust itself to hold the setpoint.

Most variable frequency drives (VFDs, frequency converters) utilize Volts/Hertz type control (V/F control), which means they provide open-loop control operation. These drives are unable to retrieve feedback from the process, but are sufficient for the majority of the applications, where variable speed drives (VSDs, AC drives) are applied.

But on the other hand, AC drives (frequency inverters, variable speed drives) can be programmed to run the AC motor at a precise speed, to stop at a precise position, or to apply a specific amount of torque. This is often required for some process, such as material handling, textile.

In fact, modern AC variable speed drives are very close to the DC drive in terms of fast torque response and speed accuracy. But AC motors are much more reliable and affordable than DC motors, thus making AC motors far more prevalent. 

Open-loop control

1. shows an open-loop action (controlled chain);

2. can only counteract against disturbances, for which it has been designed; other disturbances can't be removed;

3. can't become unstable - as long as the controlled object is stable.

Closed-loop control

1. shows a closed-loop action (closed control loop);

2. can counteract against disturbances (negative feedback);

3. can become unstable, i.e. the controlled variable does not fade away, but grows (theoretically) to an infinite value. 
 

The product of world class strong performance closed loop vector control AC drive (VFD, frequency inverter, VSD).