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PaulKraemer

PowerFlex 70 w/ ControlLogix - relate RPM to Hz?

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Hi, I have an AB PowerFlex 70 EC drive with a 20A-ENC-1 encoder module with a 1024 PPR encoder. I also got the PF70 with a 20-COMM-E Ethernet/IP module so that it can communicate with my AB ControlLogix (CLX) PLC (through a 1756-ENBT) module. I want to set things up so that my CLX controls the speed reference to the PF70. I want my operator to enter a speed setpoint in RPM on my HMI. I want my CLX to send this setpoint to the PF70. I want the PF70 to control motor speed based on this setpoint (making use of the encoder). What confuses me is from looking through the PF70 documentation is that the only way I can figure out how to send a speed reference to the PF70 is in Hertz. I can't figure out how (or if) I can send it a setpoint in RPM. I have a vector duty / inverter duty 60 Hz motor with Nameplate RPM = 1740. For sending a speed setpoint to the PF70, can I simply assume a linear relationship between the frequency output to the motor (0-60 Hz) and the speed of the motor (0-1740 RPM)? For example, let's say I want to run at 1200 RPM...can I simply set my speed reference = 41.38 Hz (60 Hz * 1200/1740)? Assuming I have param 80 [Feedback Select] = encoder and I have my encoder type (quadrature) and my PPR set correctly, does this guarantee that my motor will run at 1200 RPM? Thanks in advance, Paul

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Paul I think you answered one of your questions already just by giving the math equation. If you want the user to input the speed in RMP, just put that RMP into a dint register, say speed_RMP. Input speed_RMP into a CPT instruction, for example: speed_RPM = 500 so (60*speed_RPM/1740) = 17.24HZ. Every time the user changes the speed in RMP the CPT Instruction calculated the new speed in HZ. The second question is yes the drive speed i.e. RMP is linear to the HZ given to the drive. This is based on the parameters in the drive for min max RMP of the motor and min max Hz of the motor.

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Thanks Steve, That helps. This is my first time trying to do speed control with an AC adjustable frequency drive. I guess I am just having a hard time grasping the fact that the speed of the motor (in RPM) will be such a simple linear relation to the output from the drive to the motor (0-60Hz) regardless of any other factors such as load. With your example where I want a speed of 500 RPM and your scaling calculates 60 Hz * 500/1740 = 17.24 Hz, will 17.24 Hz give me 500 RPM both when the motor is loaded and unloaded? It seems like with this method, I am not making use of my encoder. If an output of 17.24 Hz will always give me 500 RPM, why even use an encoder? With my limited understanding, I think with my setpoint of 500 RPM, the drive would have to run a PID loop to monitor the feedback from the encoder and constantly adjust the output Hz to the motor in order to maintain the speed setpoint (500 RPM) regardless of load and any other factors that may have an effect. Any light you can shed on this will be greatly appreciated - thanks again. Paul

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Paul: Today's variable freq. drives have stable speed regulation. By adding the encoder to the system and using closed loop vector control, the speed regulation not only gets better, but the drive can make the motor respond to changes in load. Today's drives, when operated either in open loop vector control (without encloder) or closed loop vector control (with encoder) can sense load changes and change the waveform going to the motor. Open loop (or V/Hz mode) will not regulate the speed as well and will not always detect changes in speed due to the load. I hope this helps.

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Thanks drforsythe, That helps. I think I'm starting to at least understand the concept of how closed loop vector control works with an encoder. I'm probably oversimplifying this, but if there were a purely linear relationship between Hz to the motor RPM and I wanted to run the motor at 1200 RPM, I would tell the drive to send 41.38 Hz to the motor. Due to load, there will be some slip and the motor might not really run at 1200 RPM. The drive would realize this (thanks to feedback from the encoder) and increase the Hz to the motor until the motor really was running at 1200 RPM. Please feel free to correct me if there is anything glaringly wrong with my simplified understanding, but if my understanding is somewhat correct, the only thing I am confused about is how to enter my speed setpoint? I am using a PowerFlex 70. From reading the documentation, it seems like there are many ways to enter your speed reference, but in all of the ways, you are basically entering it in Hz. This doesn't make sense to me when I want to enter a setpoint in RPM and have the PID loop in the drive make use of feedback from the encoder and calculate the necessary Hz to the motor in order to produce my desired RPM given the current load. Because there is not a truly linear relationship between RPM and Hz (because at different loads the same Hz will produce different RPM), I don't see how simply scaling my setpoint in RPM to a speed reference in Hz could possibly work. Thanks again - I really appreciate your help. Paul Paul: Today's variable freq. drives have stable speed regulation. By adding the encoder to the system and using closed loop vector control, the speed regulation not only gets better, but the drive can make the motor respond to changes in load. Today's drives, when operated either in open loop vector control (without encloder) or closed loop vector control (with encoder) can sense load changes and change the waveform going to the motor. Open loop (or V/Hz mode) will not regulate the speed as well and will not always detect changes in speed due to the load. I hope this helps.

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Paul: The thing to know is that the drive frequency determines the speed of your motor. When you set up a vector drive, one of the setup parameters is the nameplate frequency at base speed. (Base speed for motors made in North America is 60Hz). All of the drive's calculations are based on that relationship to the base speed. So, it does not matter to the drive what units you send as a reference speed, it will compare your speed reference in Hz to the motor base speed to what it is actually reading from the encoder. It will then control the output of the drive (voltage, amps and hertz) to change the motor speed. When I have done this in the past, I simply have a calculation in the plc that converts my user units (feet per minute, rpm, etc) to frequency in hertz. I then send that number (usually multiplied by 10) to the reference speed memory location in the drive. I hope this clarifies things for you.

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That would mean that a 40HZ reference will tell the motor to run at 1200RPM, even though the output frequency of the drive may be up to 41.38HZ (depending on load).

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