This value is then added to the proportional term of the control algorithm to achieve the true PI controller output. This total sum of error is then multiplied by the proportional gain and divided by the integral time constant. A rough estimate can be calculated by adding up all of the boxes between the command and actual value curves. The integral control portion is more difficult to calculate because rather than simply subtracting the actual speed from the command speed, you must calculate the integral of the error from time zero to the current time. The shaded area (red) is the difference between actual and command speed over time. Because of this, a smaller value will have more effect on the control. The other thing to note about the integral term is that it is dividing by the integral time constant. Thus, the integral control will still be affecting the output if there is a built-up long-term error, even if there is no error at that specific point in time. Importantly, the integral control considers the complete integral sum of the error rather than the current size of the error. These two parameters influence each other, so it may take some trial and error in tuning the control to get them at their ideal values when first starting up a system. Integral control still uses the proportional controller gain but adds the integral controller time constant, which is also programmable in the drive. The integral control is used to eliminate long-term error and offset in the system. The second step of the PI controller is the integral control, which adds a little bit more complexity to the control. Graphs showing VFD command speed (Black) vs.
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