Okay, let's troubleshoot why you're unable to control the speed of your drive via Profibus, despite other commands working. Here's a breakdown of common causes and troubleshooting steps, tailored to the information you've provided

1. Data Mapping and Scaling:
Critical

This is the MOST likely cause. You're sending a value over Profibus, but the drive isn't interpreting it correctly as a speed reference.

Drive Parameters

You must check the drive's parameters related to:

Analog Input Configuration

Where is the speed reference coming from? Often there's a parameter that selects which analog input (or digital comm input, in this case Profibus) is the source for speed control.

Scaling Parameters

How does the drive interpret the incoming digital value? Is there a scaling factor that needs to be applied? Many drives use a scaling system where a value of `0` equals 0 RPM, and a value of (for example) `10000` equals the maximum motor speed. If you send a value over Profibus that isnt in range, it may not be getting the proper value

Maximum Frequency

What is the configured maximum frequency for the drive? This sets the upper limit of the speed control.

Minimum Frequency

Similar to maximum frequency, what is the lowest frequency and the drive?

PLC/HMI Code

Ensure your PLC/HMI code is sending the correct data type and scaling.

Data Type

Is the speed value being sent as an integer, real number, etc.? Make sure it matches what the drive expects.

Scaling

The value you send from the HMI might need to be scaled to match the drive's expected range. For example, if your HMI shows speed in RPM (0-1800), you might need to scale it to a value of 0-10000 for the drive. A simple formula: `Drive Value = (HMI RPM / Max RPM) Max Drive Value`.

Example

Let's say your drive expects a value between 0 and 10,000 to represent 0-1800 RPM. If you want the motor to run at 900 RPM, you need to calculate and send the value: `(900 / 1800) 10000 = 5000`.

2. Profibus Configuration



Data Length/Format

Double-check that the data length (e.g., 16-bit, 32-bit) and format (e.g., integer, floating-point) of the speed control word in your Profibus configuration matches what the drive expects. A mismatch here will cause the drive to misinterpret the data.

Profibus Address

Verify that the Profibus address used for the speed control word in your PLC/HMI code is correct and matches the address configured in the drive.

GSD File

Make sure you're using the correct GSD (Generic Station Description) file for your specific drive model. An incorrect GSD file can lead to incorrect data mapping and communication issues.

Profibus cycle rate

Is it possible the PLC is not sending information fast enough for the drive to see it?

3. Drive Operating Mode



External Reference Enabled

Confirm that the drive is configured to accept an external speed reference (i.e., from Profibus). There might be a parameter that selects the source of the speed reference (internal potentiometer, analog input, communication bus). If it's set to "internal potentiometer" (or similar), it will ignore the Profibus value.

4. Faults and Inhibits



Drive Faults

Check the drive's display or diagnostic LEDs for any active fault codes. A fault might be preventing the drive from responding to speed commands. Clear faults as appropriate and investigate their causes.

Inhibit Signals

Ensure that no inhibit signals (either hardwired or via Profibus) are active that could be preventing the drive from changing speed.

Parameter Lock

Some drives have a parameter lock feature that prevents modification of certain parameters. Make sure this isn't enabled.

5. Motor and Drive Compatibility



Motor Nameplate Data

Verify that the motor nameplate data (voltage, frequency, current, RPM, horsepower) matches the drive's configuration. Incorrect motor data can lead to unexpected behavior.

Autotuning

Has the drive been autotuned to the motor? This process helps the drive optimize its control parameters for the specific motor.

6. Wiring

While you say other commands work, double-check the Profibus wiring connections between the PLC/HMI and the drive. Loose or damaged connections can cause intermittent communication problems. Pay attention to proper grounding and shielding.

7. "Direct Replacement" Considerations

While the drive may be a "direct replacement," it likely has default parameter settings. These settings are unlikely to match the exact settings of the old drive, particularly the Profibus configuration and scaling parameters.

Backup (If Possible)

If you had a backup of the old drive's configuration (parameter settings), try to apply it to the new drive (if the drive models are identical).

WARNING

Be extremely careful when restoring parameters from a different drive. Incorrect parameters can damage the motor or drive.

Troubleshooting Steps


Parameter Scan

Systematically go through the drive's parameter list, paying close attention to the parameters mentioned above (analog input configuration, scaling, maximum frequency, operating mode, Profibus settings). Document all the settings.2.

Test with a Simple Value

In your PLC/HMI code, try sending a known speed value to the drive (e.g., a value that should result in 50% of the motor's maximum speed). Monitor the drive's display or diagnostic LEDs to see if it recognizes the value and responds accordingly.3.

Monitor the Profibus Communication

If you have a Profibus analyzer or diagnostic tool, use it to monitor the communication between the PLC/HMI and the drive. This can help you identify any errors or inconsistencies in the data being transmitted.4.

Simplify the System

Temporarily disconnect any unnecessary components or devices from the Profibus network to isolate the problem.5.

Contact Technical Support

If you've exhausted all of the above troubleshooting steps, contact the drive manufacturer's technical support for assistance. Provide them with detailed information about your system, including the drive model, PLC/HMI model, Profibus configuration, and the steps you've already taken to troubleshoot the problem.

Example: Siemens SINAMICS Drive

If you are using a Siemens SINAMICS drive, the following parameters are particularly important: P700

Command source (set to "USS via CB" or "PROFIBUS")

P1000

Frequency setpoint source (set to "USS via CB" or "PROFIBUS")

P2000

Maximum frequency

P2001

Minimum frequency

P2003: Reference frequencyBy carefully reviewing these parameters and ensuring that your PLC/HMI code is sending the correct data, you should be able to resolve the speed control issue. Good luck! Flag for review