You're on the right track, but let's break down the situation to make sure you understand all the factors

Your understanding is partially correct.

Output power IS a critical factor for the motor windings, but it's not the only factor. Think of it like this: you can lift a 50-pound weight slowly or quickly. Both are 50 pounds (analogous to power), but lifting it quickly requires more instantaneous effort (analogous to current).Here's a more detailed breakdown: Power (kW)

This is the rate at which the motor is doing work. Your motor nameplate says it can handle 11 kW continuously. You're currently at 10 kW, which is good from a continuous thermal overload perspective. However, this is only part of the picture.

Current (Amps)

This is the flow of electrons through the motor windings. This flow creates heat due to resistance. Excessive current, even if the overall power is seemingly within limits, can cause localized overheating within the windings, leading to insulation breakdown and motor failure.

Voltage (Volts)

The voltage applied to the motor affects the magnetic flux. A significant undervoltage, combined with high current, can stress the motor. However, in your case, 331V is likely not too bad of a voltage from the VFD for a 460V motor, and the motor is probably seeing that voltage is close to its rated load.

Why the 21.8 A Exceeds FLA Despite Being Under Power Limit



V/Hz Ratio and Motor Loading

This is the most likely culprit. VFDs control motor speed by adjusting both voltage (V) and frequency (Hz). The ratio of V/Hz is crucial for maintaining proper magnetic flux in the motor.

Lower Frequency

If the motor is running at a lower frequency than it was designed for (e.g., less than 60 Hz in a 60 Hz system), the VFD will likely reduce the voltage to maintain the proper V/Hz ratio.

Increased Current

To produce the same output power (10 kW) at a lower voltage, the VFD must increase the current. Think of it as needing to push harder (more amps) when you have less leverage (lower voltage).

Efficiency

Motor efficiency varies with load and speed. At lower speeds and/or under certain loading conditions, the motor might be less efficient. This means it needs to draw more current to deliver the same output power.

Drive Derating

The VFD may derate its output at certain frequencies or temperatures. This means that the VFD might not be providing a "true" 331 volts.

The Potential Problem

The sustained 21.8 A exceeds the motor's 18 FLA. While the 10 kW is below the nameplate power, the elevated current puts the motor at risk of overheating over the long term, especially if you're operating near the motor's temperature limits, or in a dirty environment where the motor cannot cool itself properly.

What to Do


Verify Your Measurements

Ensure your VFD is accurately calibrated and displaying correct values. Use a clamp meter to double-check the output current.2.

Check V/Hz Ratio

Consult the motor and VFD manuals to determine the appropriate V/Hz ratio for your application and verify that the VFD is configured correctly. If you're running at a significantly lower frequency, the motor is being overloaded.3.

Optimize Motor Control Settings

Experiment with the VFD's motor control parameters, such as:

Motor Nameplate Data

Double-check that the VFD is programmed with the correct motor nameplate values (HP, kW, voltage, FLA, frequency, RPM). This is critical.

Slip Compensation

This can help maintain torque at lower speeds.

Boost Voltage

Be very careful with this, but a slight voltage boost at lower frequencies might help, but

only if done within the motor's and VFD's limitations. Too much boost will saturate the motor core and cause even higher current.4. Examine the Application



Required Speed Range

Is the motor required to operate at these lower speeds frequently? If so, the motor may be undersized for the application at these speeds.

Load Profile

Understand the load profile. Is it constant, variable, or intermittent? A variable load might allow the motor to cool down during periods of low demand.5.

Motor Temperature Monitoring

Install temperature sensors (PTC thermistors or RTDs) within the motor windings and connect them to the VFD or a separate monitoring system. This provides direct feedback on winding temperature and allows you to trip the drive if temperatures exceed safe limits.6.

Consider a Constant Torque Motor

If you need to operate at low speeds frequently, a "constant torque" rated motor might be a better fit. These motors are designed to deliver rated torque even at lower speeds.7.

Upsize the Motor (The Likely Best Solution)

While more expensive upfront, upsizing to a 20 HP motor (appropriately sized to the VFD) is likely the safest and most reliable long-term solution, especially if the 15 HP motor is consistently operating near or above its FLA. Ensure that the 20HP motor's FLA is below* the maximum current output of the VFD. Make sure the motor is properly sized to the load.

In Summary

While the output power is below the motor's rating, exceeding the FLA for extended periods is a risk. Investigate the reasons for the high current, optimize VFD settings, monitor motor temperature, and strongly consider upsizing the motor if operating conditions demand high torque at lower speeds. Your primary focus should be on keeping the current within the motor's nameplate rating while meeting the application's needs. It will likely be cheaper to replace with a larger motor than burn out your current motor and have to pay for downtime.

Disclaimer: This information is for general guidance only. Always consult the motor and VFD manuals and qualified professionals before making any changes to your system. Improper configuration can lead to equipment damage or personal injury. Flag for review