Use of VFD with Flame-proof motors: Implications and Mitigation

Variable Frequency Drives (VFDs) are widely used in industrial applications, thanks to their ability to control motor speeds efficiently. However, their application with Flame-proof motors, especially in hazardous environments, presents unique challenges and risks.

Today, let’s look into these implications in detail. We'll break down the technical factors and key considerations for successfully integrating VFDs with Flame Proof motors, ensuring safe and efficient operations in challenging environments.

What is VFD?

VFD is an electronic device used to control the speed of an electric motor. It does so by adjusting the frequency of the power supply. To measure the speed of an electric motor, we use the equation:

Motor Synchronous Speed = (120 × Frequency) / Number of Poles

In this equation:

• 120 is a constant
• The number of Poles refers to the number of magnetic poles in the motor, which is fixed once the motor is manufactured
• Frequency is the only variable that can be adjusted to control the motor’s RPM (revolutions per minute).

By modifying the frequency, a VFD allows precise control over the motor’s speed, making it a highly efficient solution for various industrial applications.

Using VFD with Flame-proof Motors

Although VFDs provide various advantages, using them with Flame-proof motors—especially in hazardous environments where explosive gases or vapors may be present—brings about several critical challenges. Let's look at them one by one.

1) Harmonics & Motor Heating: Being silicon-based devices, VFDs introduce harmonics into the power supply. Think of harmonics as distortions that lead to additional heating of the motor. This is particularly problematic for Flame-proof motors, which are designed to operate within a specified temperature limit (often classified as T4, meaning the motor’s temperature should not exceed 135°C at any hot spot). The extra heat generated by harmonics can push the motor beyond its safe operating temperature, increasing the risk of a hazardous situation.

2) Reduced Cooling Efficiency: The cooling fan of a motor is usually mounted on the motor shaft at the non-driving end. When a VFD reduces the motor's speed, it also decreases the cooling fan's effectiveness. Since the fan speed is directly linked to the motor speed, any reduction in motor speed results in less airflow to cool the motor. This diminished airflow can worsen the heating issue, increasing the risk of overheating and
potential damage to the motor.

3) Hazardous Environment Considerations: Flame-proof motors are specifically designed for use in environments deemed hazardous due to the presence of explosive gases or vapors. When you combine the elevated motor temperatures caused by VFD-induced harmonics with the reduced cooling efficiency, you create a potentially dangerous scenario. This situation significantly increases the risk of fire or explosion, making it crucial to carefully manage the operating conditions of these motors in such environments.

Mitigation Strategies

Having seen the challenges of using a VFD with the Flame-proof motors, let's now delve into the mitigation strategies. To mitigate these risks, recommended strategies for using VFDs with Flame Proof motors are:

1) Use of a Higher Frame Size: Opting for a motor with a larger frame size can enhance heat dissipation, effectively reducing the risk of overheating. A bigger frame typically provides more surface area for heat to escape, helping to maintain a safe operating temperature.

2) Integration of PTCs (Positive Temperature Coefficient Thermistors): It's helpful to equip the Flame-proof motors powered by VFDs with three PTCs. These thermistors are embedded within the motor winding and linked to a thermal overload relay. If the motor temperature rises beyond safe levels, the PTCs activate the relay, cutting off the power supply to the motor. This proactive measure helps prevent damage and mitigates
the risk of hazardous situations arising from overheating.

3) Enhanced Winding Insulation: The motor winding should be treated with appropriate insulation to withstand the additional heat and stress caused by VFD-induced harmonics. This enhanced insulation helps ensure the motor operates safely under these demanding conditions.

Conclusion

Using VFDs with Flame-proof motors requires careful consideration and implementation of various technical measures to ensure safety and reliability. While these mitigation strategies are effective, they often come with trade-offs, such as longer delivery times and increased costs. However, the significance of maintaining safe operating conditions in hazardous environments cannot be overstated. By adopting the right approaches, it is
possible to harness the advantages of VFDs while effectively minimizing the risks associated with their use in Flame-proof motors.

Key Takeaways

• Opt for motors with larger frame sizes for better heat dissipation.
• Ensure motors are equipped with PTCs connected to a thermal overload relay for temperature monitoring.
• Utilize appropriate winding insulation treatments to manage the effects of VFD-induced harmonics.

By adhering to these guidelines, the integration of VFDs with Flame Proof motors can be effectively managed, ensuring both optimal performance and safety.

By: Manish Chandrani; +919825014977