Can a VFD run an asynchronous motor?

Variable Frequency Drives (VFDs) are widely used in industrial and commercial applications for controlling the speed, torque, and direction of AC motors. The most common type of motor paired with a VFD is the asynchronous (induction) motor¹, often referred to as a squirrel-cage induction motor. These motors are the workhorses of industry, offering robust, cost-effective, and reliable performance. Due to their design, asynchronous motors naturally lend themselves to speed variation via frequency and voltage adjustments provided by a VFD.

Can a VFD Run an Asynchronous Motor?

Short Answer: Yes, absolutely. VFDs are most commonly used with asynchronous (induction) motors, and this pairing is at the core of modern variable speed drive applications.

Detailed Explanation:
Asynchronous motors operate on the principle of a rotating magnetic field produced by stator windings energized by AC power. The rotor tries to "catch up" with the synchronous speed of that rotating field but never quite does—hence "asynchronous," as the rotor speed lags the stator field’s synchronous speed by a certain percentage called slip². By controlling the frequency and voltage of the AC power with a VFD, you effectively regulate the speed of the rotating magnetic field. Since the motor speed is directly related to the frequency (Speed ≈ (120 × Frequency) / Number_of_Poles), decreasing or increasing the frequency through the VFD precisely controls motor speed.

Key Points:

• Induction motors are robust, cost-effective, and widely available.
• They handle variable frequency supply well, making them ideal for VFD use.
• Through proper V/Hz control ³or more advanced vector control methods, a VFD can maintain optimal torque and efficiency across a wide speed range.

Can We Use VFD with a Synchronous Motor?

Short Answer: Yes, but it is more complex than using a VFD with an asynchronous motor.

Detailed Explanation:
Synchronous motors, unlike asynchronous motors, rotate at a speed directly related to the supply frequency without slip. They typically require a constant field excitation (either from permanent magnets in permanent magnet synchronous motors (PMSMs) or from a DC excitation in wound-field synchronous motors). While VFDs are traditionally associated with induction motors, modern drives and advanced control algorithms (like Field-Oriented Control (FOC))⁴ allow the VFD to manage synchronous motors as well.

Considerations:

• Control Complexity: Running a synchronous motor via VFD often involves more sophisticated control strategies (vector control or sensorless vector control) to manage torque and flux.
• Feedback Mechanisms: Some synchronous motor applications may require encoders or resolvers for precise speed and position control.
• Application Specific: High-performance applications like servo drives, CNC spindles, and precise positioning systems often use synchronous motors with specialized VFDs (commonly termed inverters or drives in those contexts).

Which Motor Works on Asynchronous Speed?

Asynchronous speed refers to a motor’s shaft speed not matching the synchronous speed of the stator’s rotating magnetic field. Induction motors⁵ (whether single-phase or three-phase) are the prime examples of motors that run at asynchronous speed.

• Synchronous Speed: Determined by frequency and number of poles. For example, at 50Hz and 4 poles, synchronous speed = (120 × 50)/4 = 1500 RPM.
• Asynchronous (Induction) Motor: The rotor can never truly reach synchronous speed because if it did, no relative motion would exist to induce rotor currents. The difference in speed (slip) enables torque production. Thus, at 50Hz nominal, the motor might run at 1450 RPM instead of 1500 RPM, making it asynchronous.

What Are the Limitations of VFD?

While VFDs offer significant advantages—improved process control, energy savings, soft starting, and reduced mechanical stress—they also have limitations:

1. Harmonics and Power Quality Issues:
   VFDs introduce non-linear loads to the electrical network, causing harmonic currents⁶ that can distort the voltage waveform. Mitigation measures include line reactors, harmonic filters, or active front ends.

2. EMI/RFI Noise:
   Rapid switching (PWM) can cause electromagnetic interference⁷. Shielded cables, filters, and proper grounding help minimize this.

3. Voltage and Current Limits:
   Drives have maximum current and voltage ratings. They cannot exceed line voltage or supply more torque than their rating. Above base frequency, torque typically decreases as we enter the constant power region.

4. Thermal and Environmental Conditions:
   VFDs and motors may need derating in high-temperature or high-altitude environments. Dusty, corrosive, or humid conditions require appropriate enclosures and cooling strategies.

5. Complex Programming and Setup:
   Modern VFDs are feature-rich but require careful parameter setting, tuning, and possibly additional feedback devices for stable, efficient operation.

Can You Run Any Motor with a VFD?

Short Answer: Not every motor is ideal for use with a VFD, but most three-phase AC induction motors are excellent candidates.

Details:

• Three-Phase Induction Motors: The best match due to their design and construction. Their speed is inherently tied to frequency.
• Single-Phase Motors: More challenging due to starting windings, capacitors, and reliance on fixed-line frequency for starting torque. Certain single-phase motors are not suitable for variable frequency operation.
• Synchronous Motors (PMSM, Brushless DC): Can be controlled by VFD-like inverters with vector control or field-oriented control. Often used in high-performance, high-precision applications.
• Universal or Series Wound Motors⁸: Used in household appliances (drills, vacuum cleaners). They can be speed-controlled, but not typically with standard VFDs due to their commutation and DC/AC hybrid nature.
• DC Motors: Need DC drives, not AC VFDs, because their speed control is achieved by varying DC voltage or current, not AC frequency.

In general, standard three-phase induction motors and specialized AC motors designed for inverter duty⁹ are best-suited for VFD control. Check compatibility with the manufacturer if in doubt.

Visual Diagrams

Figure 1: VFD Controlling an Asynchronous Motor

The VFD takes in fixed-frequency AC, rectifies it to DC, and then inverts it back to AC at a variable frequency and voltage, allowing the induction motor to run at speeds above or below 50Hz (or 60Hz).

Figure 2: Speed vs. Frequency for Asynchronous Motor

Base Freq. Above Base Freq.

Conclusion

Yes, a VFD can run an asynchronous (induction) motor, and in fact, this is the most common and ideal pairing. Asynchronous motors naturally adapt to changing frequencies and voltages, making them perfect for variable speed control.

• Running at Higher Frequencies: Possible, but motor design and mechanical limitations become critical. Modest increases (like 50Hz to 60Hz) are often acceptable if the motor can handle the increased speed.
• Synchronous Motors: VFDs can also run synchronous motors with more complex control algorithms.
• Not All Motors Are Ideal: While three-phase induction motors are the best candidates, single-phase motors or certain specialized motors may not respond well to VFD control.
• VFD Limitations: Consider harmonics, EMI, power rating limits, and environmental constraints.

In summary, while a VFD gives you tremendous flexibility, ensuring you have the right motor type, operating conditions, and carefully tuned parameters is key to reaping the full benefits of variable frequency control.

References

• [NEMA MG 1](https://blog.ansi.org/ansi-nema-mg-1-2021-motors-generators/ "NEMA MG 1")10 - Motors and Generators
• IEEE Std 1566 - Performance of Adjustable Speed AC Drives
• Siemens, Yaskawa, Rockwell Automation technical literature and application notes.
• Motor manufacturer documentation for speed limits, overspeed capabilities, and inverter-duty ratings.

Disclaimer: Always consult the motor and VFD manufacturer’s documentation and, if needed, a qualified engineer before operating motors at frequencies other than their rated values or selecting a motor for VFD use.