You've identified a discrepancy between what you expect and what the drives are reporting, and you're right to question it. Here's a breakdown of potential reasons and troubleshooting steps, covering both drive settings/behavior and the mechanical system

I. Potential Issues & Troubleshooting Within the Drives/Electrical System:
Calibration and Accuracy of Drive Power Output Readings



Drive Calibration

Drives use internal models and measurements to calculate power output. There could be inaccuracies in these models. Some drives allow for calibration of current and voltage readings. Check your PowerFlex 525 manual for calibration procedures and parameters.

Measurement Resolution

The resolution of the current, voltage, and power factor sensors within the drives can affect the accuracy, especially at lower power levels. Even a small percentage error in each measurement can compound.

Firmware Errors

Less likely, but a bug in the drive firmware could lead to incorrect calculations. Check the Rockwell Knowledgebase for any known issues related to power output calculations in your specific firmware version.

Drive Control Mode (SVC vs. Other Modes)

While you're running SVC, ensure all the parameters associated with SVC are correctly configured for each motor. Small differences in motor nameplate data entered (especially impedance values, saturation factors, etc.) can significantly impact SVC performance.

Consider Sensorless Vector Control (SVC) Limitations

SVC relies on motor models to estimate torque and flux. The accuracy of these estimations diminishes at low speeds and light loads. Since the power outputs are quite low (only ~10% of motor nameplate) relative to the motor ratings, the estimates are likely to be less accurate.

Try Different Control Modes for Testing

Temporarily (for testing purposes only) try a different control mode like Volts/Hertz (V/Hz) on one drive (Blower 4, perhaps) and see if the power output changes significantly. Do this carefully and only for a short period, as V/Hz control is less efficient and accurate than SVC. This can help isolate whether the issue is specific to the SVC algorithms.

Remember to return to SVC afterward. Power Factor Calculation



How is Power Factor Calculated?

The drive firmware calculates power factor using internal measurements of voltage and current. Make sure the power factor calculation method (if configurable) is consistent across all drives.

Power Factor at Low Load

Power factor tends to be lower at light loads (which you're currently running at). Small variations in the current waveforms (due to harmonics, etc.) can disproportionately affect the power factor calculation at these low power levels.

Harmonics



THD (Total Harmonic Distortion)

Check the THD of the voltage and current on each drive's input. High harmonic content can distort power factor calculations and drive performance. This requires a power quality analyzer. Consider adding line reactors if THD is high.

Wiring and Connections



Loose Connections

Check all wiring connections to the drives, motors, and incoming power. Loose connections can cause voltage drops and affect current readings.

Grounding Issues

Ensure proper grounding of all equipment. Ground loops can introduce noise and errors in measurements.

II. Potential Issues & Troubleshooting Within the Mechanical System (Load Sharing and Blowers)



Drive and Motor Sizing Mismatch



Oversized Motors

You're running at a very low percentage of the nameplate power. Electric motors operate at the best efficiencies and power factor between 50% and 100% of their rating. As the loading falls to a much lower percentage, the efficiency and power factor falls as well. It may be that the difference in HP and loading isn't enough to have a significant impact on the power consumption at this operating point.

Belt Drive Issues



Belt Slip

Excessive belt slip on any of the blowers would cause the motor to draw less power than expected.

Belt Tension

Incorrect belt tension can also affect load sharing.

Pulley Sizes

Double-check the pulley sizes on each blower and motor. Are they exactly as designed to achieve the desired speed ratio and airflow? A small difference can shift the load significantly.

Belt Condition

Check belts for wear, cracks, and proper fit. Worn belts slip more.

Blower Characteristics



Blower Curves

Review the blower performance curves (pressure vs. flow). If the blowers have slightly different curves, they might not share the load equally even with the same motor speed.

Blower Condition

Inspect the blowers themselves. Are there any obstructions or damage to the impellers that could affect their performance?

Airflow Restrictions

Are there any restrictions in the ductwork leading to each blower that could affect airflow and load sharing?

PID Tuning and Dynamics



PID Loop Dynamics

The PID loop is maintaining constant pressure. It's possible that the PID parameters are such that it's quickly adjusting the drive outputs to maintain pressure, masking any differences in the load.

Drive Response Time

Ensure that the response time of each drive is similar. If one drive responds much faster to changes in the PID output than the others, it might be carrying a disproportionate share of the load in the transient response, but this would not be reflected in the average values you are seeing.

Load Sharing and Mechanical Coupling



Common Manifold Dynamics

The behavior of the shared air manifold can be complex. Resonance, turbulence, and other effects could lead to uneven pressure distribution and load sharing among the blowers.

Mechanical Resonance

There may be mechanical resonances that favor the operation of certain blowers at certain speeds.

III. Troubleshooting Steps


Verify Nameplate Data

Double-check the motor nameplate data (HP, voltage, current, speed, power factor, etc.) and ensure it's correctly entered into each drive's parameters. This is crucial for SVC operation.2.

Isolate and Test Individually

If possible, isolate each blower system (mechanically or electrically). Run each blower individually at different speeds and pressures. Record the drive power output, current, voltage, and power factor for each blower. This will help identify if there are differences in the blower or motor performance.3.

Measure Mechanical Torque (If Possible)

The ideal way to verify load sharing is to directly measure the torque being applied to each blower shaft. Torque sensors are expensive and require specialized installation, but they provide the most accurate picture of load distribution.4.

Monitor Trends Over Time

Record the drive parameters (power, current, voltage, speed) over a longer period (hours or days) to see if there are any trends or fluctuations that might indicate a problem.5.

Consult Rockwell Automation Support

If you've exhausted the above troubleshooting steps, contact Rockwell Automation technical support. Provide them with detailed information about your system, drive configurations, and the data you've collected. They may have specific knowledge about potential issues with the PowerFlex 525 and the SVC control mode.

Key Takeaways



Don't rely solely on drive-reported power output for accurate load sharing assessment. Drive calculations are based on models and measurements that can be inaccurate, especially at low loads. Focus on the mechanical system. The behavior of the blowers, belts, and ductwork is crucial to understanding load distribution. Direct measurement of torque (if possible) is the most reliable method for verifying load sharing.* Consider having the motors tested with a motor analyzer. This can reveal discrepancies in performance if the motors are from different manufacturers or have been rebuilt.By systematically investigating both the electrical and mechanical aspects of your system, you should be able to pinpoint the cause of the discrepancy and optimize load sharing. Flag for review