How Does Closed Loop Control Work in a VFD?

So how does closed-loop control work in a VFD? We've already covered the basics of a VFD, its components, and how the open-loop volts/Hz control works. In this video, we'll talk about the more advanced closed-loop control for precise control of the motor speed and torque. Now before we get into the more advanced closed-loop control, let's do a quick recap of the open-loop control. You'll remember from our volts per Hertz video that as long as you keep the ratio of the output frequency and voltage constant you'll have a power that increases linearly with the frequency and a torque that is constant up to the motors rated frequency. This is great for more simple applications such as pumps or fans, but for more advanced applications such as spindle control or material handling you'll need the closed-loop control to precisely control the speed and torque of the motor. So in the open-loop volts per Hertz mode the drive outputs power to the motor and it just keeps outputting it outputting but it doesn't actually know what's going on at the motor. With the addition of an encoder, the drive can get speed feedback from the motor so it knows precisely what speed the motor is running at, which can then be incorporated in the drives control algorithm for the closed-loop control, Then you have the loop of power going from the drive to the motor and speed feedback going from the motor to the drive. Now let's break that down into a little bit more detail. So here you have a basic block diagram of the drive's closed-loop control. We'll start at the end at the motor. From there the encoder feedback is fed into the drive to be evaluated and adjusted if necessary. For example if you picture a motor in most cases the encoder will be mounted on the back side so the speed it's measuring is the true speed that the motor is spinning at. But in some cases there's a gearbox and the encoder is mounted on the output of that. The measured speed would be 20 50 or a hundred times less than what the motor is actually spinning at. In this case, the drive can evaluate and adjust for that ratio as necessary so the speed it's seeing is the speed that the motor is truly spinning at. From there, this evaluated and adjusted encoder feedback is fed into the speed controller. So once the feedback is evaluated, this actual speed is fed into the speed controller. In the speed controller the drive is comparing the command speed and the actual speed and constantly adjusting the output based on its PI controller. As a basic example of what that is, we have this graph here. The pink will be your command speed and the yellow will be your actual speed. The actual speed won't always exactly match your command speed. So the drive is measuring this error and constantly trying to adjust its output. This output is then fed into the current controller. The encoder evaluation is also fed into the motor model. The motor model uses the basic nameplate data of the motor to create a virtual equivalent inside the drive of the motor. Using this and the measured output voltage and current. The drive calculates what it thinks the motor speed should be. This is then also fed into the current controller. The current controller takes all of this feedback to calculate exactly what should be output to the motor to run at the desired speed and torque. Using the motors encoder feedback and the drives complex control algorithm, closed-loop control allows for a higher performing and more efficient motor operation. If you want a little bit more detail on how closed-loop control works and to stay up to date with what's going on here at KEB America, check us out at KEBamerica.com or like and subscribe to our youtube channel, or follow us on our social media accounts.

Channel: KEB America, Inc.

Views: 12,487

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Keywords: Closed Loop control, encoder feedback, motor control, motion control, v/hz control, basics of a vfd, Variable Frequency drive, KEB drive, KEB VFD, KEB F5 Drive, What is Closed Loop control?, Lightboard, Lightboard lesson, KEB America, drive control, speed control, current control, virtual motor model, encoder evaluation, VFD command speed, command speed vs actual speed, motor speed control, Inverter, KEB inverter, How it works

Id: DJWWWUmJ4pQ

Length: 3min 50sec (230 seconds)

Published: Fri Feb 15 2019