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How to ensure reliable motor operation with variable frequency drives

A variable frequency drive (VFD) is the industry’s standard technique for controlling the speed and torques of induction motors. To ensure reliable operation of motors with VFDs, the user must consider various measures and best practices.

By Cole Casteel, PE, and Taha Mohammed, PE January 17, 2025
Courtesy: ABB

 

Learning Objectives

  • Understand the fundamental components and operations of induction motors and VFDs.
  • Recognize the key factors for selecting motors and VFDs.
  • Grasp important factors for VFD and motor installation.

Variable frequency drive insights

  • VFDs are gaining widespread popularity for driving motors in industrial and commercial facilities due to their efficiency.
  • It’s important that the motor characteristics and application are coordinated with the driven equipment during the motor and VFD selections.
  • Monitoring the temperature can help identify VFD problems before they do too much damage.

Induction motors use the principle of electromagnetic induction to convert electrical energy to mechanical energy to rotate or turn the motor shaft. And although a variable frequency drive (VFD) can be integral to efficient motor operation, there are many factors to consider before you install one.

VFDs can be used to adjust the frequency and voltage of the alternating current (ac) power applied to the stator and then control the speed, torque and power of the motor. There are two main control methods that VFDs use to control the operation (speed, torque, power) of an induction motor: vector control and scalar control. While the vector control provides more precise speed control, it is more complex and adds additional feedback devices monitoring the shaft rotation. Thus, the most common and widely control method used is scalar control, also known as volts per hertz or V/f.

The main components of a VFD (see Figure 1) are:

  • The ac-dc converter, which converts the incoming 60 Hz ac signal to direct current (dc) using rectifiers or insulated-gate bipolar transistors (IGBT).
  • A dc link that smooths the dc signal using capacitors.
  • A dc-ac converter that takes the dc and converts it back to ac at the desired voltage and frequency using pulse width modulation (PWM) technique with IGBT transistors.
Figure 1: Variable frequency drive block diagram. Courtesy: CDM Smith

Figure 1: Variable frequency drive block diagram. Courtesy: CDM Smith

As energy saving and more operation controls are desired, VFDs are becoming more widespread for driving motors in industrial and commercial facilities. When using VFDs, there are extra factors and considerations to be taken for a reliable facility and system operation. Understanding and applying these factors will help prolong the lifespan of the motors and VFDs as well as minimizing shutdowns due to unexpected equipment failure.

The following are key factors and best practices to consider when selecting or using motors with VFDs.

How to coordinate VFDs with the driven equipment

For the equipment to operate reliably and properly, it is essential that the motor characteristics and application be coordinated with the driven equipment during the motor and VFD selections. In addition to voltage and phase compatibility of the motor and VFD, the VFD needs to be compatible with the motor and the driven equipment.

One main characteristic is the torque application, for example centrifugal fans and pumps are variable torque application and a normal duty VFD is adequate, while conveyors and positive displacement pumps are constant torque application and require heavy- or severe-duty rated VFDs, which have higher overloading capability.

An additional application to be discussed is the lowest speed the load will be operating at and making sure the motor turndown ratio can accommodate that application and can be safely operated at that low speed without overheating and compromising the operations. For example, a 10:1 turndown ratio for a 3,600 revolutions per minute (rpm) motor is 360 rpm.

Another important item is the motor full load amperage (FLA) and making sure the VFD can provide equal or greater current than the motor FLA. The motor horsepower should not be used to select the VFD and the motor service factor should not exceed 1.0.

Choosing the proper environment for VFDs, motors and drives

Like with anything else in industrial facilities, such as structural supports and pipes or other electrical equipment, motors and drives must be suitably rated for the environments in which they are installed. Process areas in industrial facilities can be subject to physical damage, sprayed or standing water, high humidity, dust, corrosive chemicals and extreme temperatures, all of which can damage or rapidly degrade VFDs containing sensitive electronics.

The VFDs can be protected from their environment with a properly rated National Electrical Manufacturers Association (NEMA) enclosure, such as 4X stainless steel. However, these enclosures come with their own drawbacks. They have larger footprints, add additional cost and make it harder to remove excess heat generated by the VFD. For these reasons, it is recommended to install VFDs in dedicated, climate-controlled electrical rooms (see Figure 2).

 

Figure 2: Variable frequency drives installed in environmentally controlled room. Courtesy: CDM Smith

Figure 2: Variable frequency drives installed in environmentally controlled room. Courtesy: CDM Smith

Equipment heat degradation

When it comes to electrical and mechanical equipment, one of the biggest reasons for degradation of equipment is heat. The power electronics that make up VFDs generate heat and if this heat builds up within the VFD enclosure, the components can be damaged, operate inefficiently or cause the VFD to shut itself down for protection, forcing equipment downtime. Most VFD enclosures are equipped with fans and air filters to ensure the flow of clean air across the components.

As with home air filters or on-facility heating, ventilation and air conditioning equipment, the air filters will become clogged with particulates and dust, inhibiting airflow. Even when the VFDs are installed in an air conditioned space and the heat cannot escape the VFD enclosure, the damage will be done.

It may seem like a small thing, but changing the air filters on VFD enclosures regularly, as well as verifying the functionality of the fans and ensuring a clear space around the vents can extend the longevity of the VFDs.

When a VFD is installed in a harsh environment, it is important to remember that the NEMA-rated enclosure only provides protection when used and maintained appropriately. For example, a NEMA 3R outdoor enclosure protects the VFD from rain, but if the door is left ajar, it may as well have a NEMA 1 enclosure rating. Enclosures that provide stronger protection (3R, 4X, 7), tend to have more and heavier duty latches and bolts to keep the doors closed.

These are the areas where the environment can do the most damage to the VFD, so it is crucial for the longevity of the drive to make sure the doors stay closed, ensuring the integrity of the enclosure is maintained.

The importance of disconnecting contacts

Safety disconnecting means are required for each motor to be located within sight of the motor location, per NFPA 70: National Electrical Code Article 430.102. However, a code exception is included that allows for the elimination of the motor disconnect if it is impractical or would introduce additional hazards, with an informational note clarifying that motors associated with VFDs meet this condition.

Despite the exception bypassing the need for a separate disconnect, many facilities’ operational staff still prefer to have them, as they can provide a safer working environment allowing technicians to open the disconnect and maintain visuals on the disconnect while they service equipment.

A consideration when including local motor disconnects for motors driven by VFDs is to include auxiliary “break-before-break” or “early break” contacts within the disconnect switch to connect to the VFD and send a signal to the VFD immediately to shut down before the switch is opened. When the motor load is abruptly removed from the load side of the VFD while running, transient voltage and current spikes are created that can damage the transistors in the drive.

Rarely, the damage can be rapid and catastrophic, destroying the drive, but more likely the surges will wear down the VFD electronics, lowering their lifespan. The addition of these auxiliary contacts allows the drive to shut off its output immediately before the load is lost, saving it from unwanted transients. For existing installations without early break contacts, it may be worth stopping the VFD before opening the local disconnect (see Figure 1).

Gauging VFD and power quality

Harmonics in electrical systems are high-frequency sinusoidal currents that get added to the main power wave at multiples of the power frequency (60 Hz). They are created when ac power is converted to dc, which is the first stage of a VFD.

The concern with harmonics is often on their upstream effects, such as increased heating of transformers, nuisance tripping or issues with the electric utility provider. With VFDs, there are also concerns with power quality downstream. As mentioned above, the ac output of a VFD is constructed from the dc bus by PWM, rapidly turning the output transistors on and off. The high-speed switching interacts with the inherent inductance and capacitance of the cable feeding the motor and the motor itself to create what are known as standing waves or reflected waves. The standing waves cause the cables and motor to experience a higher voltage than normal, sometimes higher than the rating of the insulation, causing premature breakdown of the insulation.

There are multiple causes and symptoms involved with power quality issues from the VFD outputs, so there are multiple tools to address them and the best ones will depend on the situation. To minimize reflected waves, it is best practice to keep cable runs between the VFD and the motor as short as possible.

Added length of cable increases the inductance and capacitance, also increasing the magnitude of the reflected waves. The high-frequency noise carried by the cables creates electromagnetic interference (EMI) that can affect nearby analog signals runs with power cable, like pressure or level transmitters signals. Using multiconductor, shielded VFD cable, especially when installed in cable tray or PVC conduit, will make sure those adjacent analog readings are not impacted by the EMI generated in the VFD cable.

With the prevalence of VFDs, industry leaders and motor manufacturers have designed motors with more robust insulation to be used with VFDs, as described in the NEMA MG1 standard and are labeled as inverter-duty.

The VFD output also induces stray currents in the rotor that discharges through the shaft and damaging bearings, causing vibrations bearing failure. To prevent stray currents and the unnecessary vibrations, heating and damage they cause, motors should be equipped with shaft grounding straps, insulated bearings or both.

Whether some of these extra measures are necessary will depend on individual circumstances, such as the VFD manufacturer and technology used, facility layout, motor size and process criticality. Proper protection will curb the negative effects from the PWM output of the VFD and extend the life of the motor.

Other filtering equipment such as sine wave and DV/DT filters may be used to eliminate transients between the VFD and the motor and protect the motor windings from voltage spikes. It is important to consult the VFD and motor manufacturer for recommendations on the proper filtering selection based on individual applications and setup.

Monitoring and protecting VFDs and motors

Similar to VFDs, heat buildup is an issue for the motors. The flow of electrical current is resisted by the motor windings, converting the electrical energy to thermal energy. In a motor, a fan blade is attached to the rear of the shaft to expel hot air while the motor is spinning. This kind of motor construction is called totally enclosed, fan-cooled (TEFC) (see Figure 3) and it works well to remove excess heat from the bearings and stator at rated speed.

Figure 3: Totally enclosed, fan-cooled motor on variable frequency drive with winding thermal protection and safety disconnect. Courtesy: CDM Smith

Figure 3: Totally enclosed, fan-cooled motor on variable frequency drive with winding thermal protection and safety disconnect. Courtesy: CDM Smith

However, when used with a VFD to reduce the speed of the motor, as the fan is attached to the shaft, it will spin slower, which reduces the effectiveness and allows heat to build up. Generally, it is not recommended to operate TEFC motors below 25% of rated speed without additional cooling or verifying the rating of the motor.

For motors driven by VFDs especially, monitoring the temperature can help identify problems before they do too much damage. The most basic method is to install thermostats constructed from bi-metallic switches around the stator windings. As the two distinct types of metals heat up, they expand at different rates, eventually breaking contact, letting the control circuit know the motor is getting too hot.

However, this discrete signal occurs only after reaching the setpoint and provides no additional diagnostics. Another method is using resistance temperature detectors that continuously vary their resistance as the temperature changes, which can be sensed and monitored remotely, giving more opportunity for proactive intervention to extend the life of the motor (see Figure 4).

Figure 4: Cutaway view of an induction motor. Courtesy: ABB

Figure 4: Cutaway view of an induction motor. Courtesy: ABB

If the facility has a supervisory control and data acquisition system that allows the networking of VFDs via Ethernet or fiber, the additional VFD parameters, signals and statuses can be remotely monitored, such as real-time voltage, current, power, output frequency, motor speed, motor torque and runtimes. This kind of data is valuable for operators and maintenance to ensure the health and longevity of their equipment.

Routine VFD and motor maintenance considerations

All equipment deteriorates over time, so it is crucial to test and maintain it regularly to ensure it remains in good condition. Performing proper maintenance is another key item that enhances the reliability of motors/VFD operations. This includes preventive maintenance and visual inspections, cleaning filters and vents from dust and debris. Motor and VFD inspections include checking for proper ventilations, unusual noises and smells, corrosion and excessive vibrations. Some preventive maintenance measures include applying lubrication, tightening connections and replacing parts.

For detailed maintenance and testing procedures, consider following the manufacturer’s instructions and adhering to the recommended maintenance guidelines from the InterNational Electrical Testing Association and NFPA 70B: Standard for Electrical Equipment Maintenance.

 

Taha Mohammed, PE, and Cole Casteel, PE, are electrical engineers with CDM Smith.


Author Bio: Cole Casteel, PE, and Taha Mohammed, PE, are electrical engineers with CDM Smith.