I steadily come across applications of VFDs used for controlling pumps, fans and other machinery. I very often see theese applications having one feedback (analog output from the VFD) which is taken back into some DCS/PLC-system. This analog output very often represent the current measured in the VFD.
To be honest, I don't understand why one measures the current only.
For example, for a constant torque application the current will mainly remain the same over a wide operational range. So, if constant torque application revs from 40% to 120% of nominal speed of the motor, it won't show in the current measurement.
Considering a varying torque application for the same situation: rev from 40% to 120% speed, the current will indeed change. Apart from indicating a change in torque, the current does not provide any additional information.
For example, measuring speed and/or power must be more valuable, as I see it.
Anyone that might enlighten my thoughts about this?
Br,
Andreas
Although I do not understand the question well, the controller may contain a status estimator, so a minimum number of sensors will be used.
In most applications employing VFDs, the voltage is maintained constant. Hence, the current is a significant parameter that reflects power. When the torque is higher the current would be higher.
Marcel Nicola and Lahiru Laminda Abeysekara I'm sorry to say, but your answers does not answer my question.
For example, voltage is NOT maintained constant in most applications. By changing the frequency, the voltage output will change by the same ratio (constant frequency/voltage ratio) for applications within the area where the VFD is able to maintain this ratio (typically not at frequencies above nominal). And also, for constant torque applications current will remain the same over a significant operational area even if the speed is changed significantly - hence current is not correlated to power. However, I agree with the statement about current being a significant parameter when considering varying torque applications such as centrifugal pumps etc.
But still, I think measuring speed and/or power must be more valuable, as I see it. This would provide a more complete picture as to how the application is funcioning.