VARIABLE-frequency drive (VFD) retrofits for HVAC fans and pumps have been around in the GCC region for many years; however, over the past six years or so, with the regional as well as worldwide goals to reduce carbon emissions, save energy and “go green” intensifying, there has been a proliferation of companies installing VFDs on all types of HVAC equipment, including chillers, and the move often has resulted in premature motor failure. This article will provide some basic understanding of why the motors fail with VFDs installed and steps that you can take to avoid these motor failures.

The first question that ought to be asked is whether the motor you are applying the VFD to is able to have a VFD installed on it or not. For instance, a single-phase motor should not have a VFD installed on it.

Motor winding insulation

When considering a VFD on a three-phase motor, the insulation rating of the existing motor must be Class F or higher. Due to their internal functions, VFDs have been known to cause high-frequency voltage spikes in the motor windings. Class F or higher will help protect your motor windings. Check your existing motors’ nameplate. It will indicate “Inverter Duty” for motors compatible with VFDs. If the nameplate is missing or damaged, you ought to contact the Motor OEM for verification that the existing motor is fit for VFD operation.

 Bearings

The aforementioned high-frequency voltage spikes can cause damage to more than just your windings. AC motors operated by VFDs use pulse width modulation (PWM) to control the speed of the motor. This means that there are common-mode voltages, which are capacitively induced onto the shaft of the motor and can discharge in the motor’s bearings, causing electrical discharge machining (EDM) pitting, frosting and fluting damage, which result in unplanned downtime and repair costs. In addition, larger motors over 100 HP (75 kW) and medium-voltage motors may also have high frequency circulating currents, which can also cause EDM pitting, frosting and fluting damage. In addition to this, bearing lubrication will break down, eventually damaging the bearings, as well.

Speed ratings

Since VFDs vary the frequency to manipulate the AC motor’s speed and torque, they can run a motor outside of its rated speed; but that doesn’t mean you should do so.

When running your motor at speeds lower than the manufacturer’s rating, the cooling system’s capability is decreased. If you intend to run your motor lower than base speed, an auxiliary cooling system may need to be installed.

When running your motor at speeds higher than the manufacturer’s rating, the motor attempts to draw additional power from the VFD. This power draw can lead to overload situations and other critical damages.

Lead length

In your typical VFD-motor circuit, the cable length should not exceed 50 feet. However, sometimes, there is no available mounting space for the VFD in the vicinity of the motor. If you have a lead length of over 50 feet, you will need to install additional filters (load reactors or DV/DT) to mitigate voltage spikes.

What to do when retrofitting a motor to VFD operation

Some motors are specifically designed to run on VFD power, while others need a few components added to the system to be compatible. In that context, it is important to ensure that you understand your motor’s capabilities and limitations before installing a VFD.

1. Proper high-frequency (HF) grounding of VFD-driven motor systems is vital to prevent earth-level discontinuities among system components. It is especially critical in applications involving a motor and coupled equipment that are not mounted to a common baseplate. In such cases, effective HF grounding of all system components is necessary to equalise the potential between equipment frames and to prevent ground loops between the motor and coupled equipment. Widely recognised as the most efficient path to ground for high-frequency currents, grounding straps are recommended by major motor and drive manufacturers. High-Frequency Ground Straps (HFGSs) ensure a very-low impedance path to ground from the frame of the motor for the high-frequency currents generated by VFDs.
2. Shaft Bearing Rings conduct harmful shaft voltages away from the bearings to ground. Voltage travels from the shaft through the conductive microfibres, through the housing of the ring, and through the hardware (or conductive epoxy) used to attach the ring to the motor, to ground. The HFGS is a braided cable used to lower the impedance between the motor’s frame and earth ground. Shaft Bearing Rings provide a safe path for damaging VFD-induced currents away from the motor’s bearings to the motor’s frame. HFGS bonding straps complete the path from motor’s frame to system ground.
3. Inductive absorber cores act as a common mode choke by absorbing the damaging high frequency noise associated with VFDs, so you can maximise equipment reliability.
4. Measuring the shaft voltage on VFD-driven motors will provide you with valuable information to determine if there is a potential risk of bearing damage from electrical bearing discharges. Surveying and documenting shaft voltage readings and waveforms will assist in determining the appropriate mitigation or solution. The best time for shaft voltage measurements is during initial start-up in new or repaired motors operated by the VFD. Shaft voltage measurements should be incorporated into preventive and predictive maintenance programmes and may be combined with vibration analysis, thermography or other services.

As VFDs are without question a valuable tool in reducing energy in chillers and HVAC equipment, it’s important to ensure that all that possibly can be done, is done to protect motors from the damaging effects of VFDs and reduce premature motor failures.

Dan Mizesko is President, Dalkia US Chiller Services. He may be contacted at dan@uschillerservices.com.