VFD speed control error

First, speed error is generally due to changes in torque demand. In an induction motor, this error is mostly slip. So the question becomes, how well does the drive compensate for torque induced slip speed changes. With a good vector drive, this can get down in the range of one-tenth of motor slip without an encoder. If you need better than that, an encoder is required. Note here that the error is a result of torque changes. If your torque doesn't change, you won't have much speed error to start with. Second, in some applications, especially those involving web products and tension control, cumulative error is just as important as actual error. For example, even if you are very accurate with actual error, if it is all negative or all positive, eventually you are going to have too much or too little tension. No encoderless system will assure non-cumulative error. For that you need an encoder. Third, speed reference error is often overlooked. That is error either in the speed signal going into the drive or error in the drive translating the input command into an actual output speed. Usually, the majority of this error is due to the analog input terminal analog-to-digital conversion. A 10 bit resolution A/D input will not be nearly as accurate as a 14 bit resolution input. This is a matter of purchasing a drive with the input resolution adequate for the intended purpose.