You're asking a very important question about VFD operation at low frequencies and high torque loads. Here's a breakdown of the issues and considerations

Yes, exceeding the FLA, even for a seemingly short period, can be risky to the motor.

While the blower can help, it doesn't completely negate the issues. Here's why

1. Thermal Limits and the FLA:
FLA (Full Load Amps) is a thermal limit, not a theoretical electrical limit. It represents the current the motor can withstand continuously while maintaining its rated temperature rise within its insulation class. Exceeding the FLA causes the motor windings to overheat faster than the cooling system (including the blower) can dissipate the heat. Insulation Degradation

The insulation around the motor windings is susceptible to degradation from excessive heat. This degradation is cumulative. Every over-temperature event shortens the motor's lifespan. Even if the motor doesn't immediately fail, repeated overheating will eventually lead to insulation breakdown and a short circuit.

2. Cooling Considerations at Low Frequencies



Reduced Fan Effectiveness

At 6 Hz, the motor's internal cooling fan is spinning at a significantly lower speed (10% of its normal speed). This drastically reduces its ability to cool the windings.

Constant Torque vs. Constant Horsepower

You're correctly thinking about constant torque applications. At low speeds with constant torque, the power decreases proportionally to the speed, but the heat generated by the current through the windings remains a concern. The power equation you used is accurate (P = IVsqrt(3)PF), and you are correct that the lower voltage requires increased current to deliver the same torque.

External Blowers Help, But Aren't a Perfect Solution

An auxiliary blower (forced air) can improve cooling, especially at low speeds. However, it doesn't necessarily mean you can continuously exceed the FLA. The blower's effectiveness depends on its CFM (cubic feet per minute), the motor's enclosure type (TEFC, ODP, etc.), and the ambient temperature. You need to calculate the actual cooling capacity.

3. VFD Overload Protection and Limitations



VFD Overload Protection Is NOT Foolproof

Most VFDs have overload protection settings that will trip the drive if the motor current exceeds a certain level for a specific duration. However, these settings are often based on a thermal model of the drive, not the motor.

Inaccurate Motor Parameters

If the VFD's motor parameters (e.g., thermal time constant, resistance, inductance) are not accurately configured, the overload protection may not trip quickly enough to prevent motor damage.

Slow Response

Even with good settings, the VFD's overload protection may have a slight delay before tripping. This delay can be enough to allow the motor to overheat significantly, especially during prolonged overcurrent conditions.

4. Other Potential Problems



Harmonic Distortion

VFDs can introduce harmonic distortion into the motor current. These harmonics increase losses in the motor, contributing to additional heat. The lower the frequency, the more pronounced the harmonic effects can become.

Cogging/Torque Ripple

At very low frequencies, the motor's torque can become uneven (cogging or torque ripple). This can cause jerky motion and put additional stress on the motor's bearings and connected equipment.

What to Do When You Need High Torque at Low Speed

Here's a hierarchy of solutions, from best to worst, considering cost and complexity:1.

Oversized Motor

The best solution is to use a motor that's significantly oversized for the application. This allows the motor to deliver the required torque at low speed without exceeding its FLA. This will be the most expensive at first.2.

Gearbox

A gearbox allows the motor to run at a higher speed (and therefore higher frequency) while delivering the required torque at the output shaft. This is a very common and effective solution.3.

Motor with Forced Cooling (and Monitoring)

If oversizing is not possible, use a motor with a dedicated forced-air cooling system (e.g., a blower that runs independently of the motor's speed). More importantly, invest in temperature monitoring directly on the motor windings (e.g., RTDs or thermistors). Connect these to a protection relay or the VFD to trip the drive if the winding temperature exceeds a safe limit.4.

Improved VFD Settings and Careful Monitoring

As a last resort, if none of the above are feasible: Ensure the VFD has accurate motor parameters entered. Adjust the overload protection settings to be more aggressive (i.e., trip sooner). However, be very careful, as nuisance tripping can occur if set too sensitively.

Closely monitor the motor current and temperature during operation. If the motor current exceeds the FLA or the temperature rises rapidly, reduce the load or increase the speed. Consider using a higher switching frequency on the VFD to reduce harmonic distortion (but this may increase losses in the drive itself).

Derating the Motor

You might need to derate the motor's nameplate power and FLA. This means treating it as if it were a smaller motor, even though it isn't. This is a conservative approach.

In summary: Operating a motor significantly below its rated frequency while drawing current exceeding its FLA will* reduce its lifespan. The severity of the damage depends on the magnitude of the overcurrent, the duration of operation, and the effectiveness of the cooling system. A properly sized motor and/or a gearbox are the preferred solutions. If that's not feasible, focus on improving cooling and monitoring the motor's temperature directly. Never rely solely on the VFD's overload protection without careful testing and verification. Flag for review