Variable Frequency Drives

Part 5a. Design Aids and Pitfalls.

By Ed Butts, PE, CPI

Figure 1. Voltage vs. time impact from a reflective wave.

As with control valves, variable speed devices (VSDs), including variable frequency drives (VFDs), have limitations in their application and operation and must be applied and used in water systems where their unique features can be used to their full potential but with minimal operational risks.

Occasionally, technology advances faster than the ability to fully comprehend the limitations and risks of devices as well as the possible negative impact from using them in the wrong application.

The current popularity of VFDs is primarily due to their ability to precisely control motor and pump speed, and therefore, provide better control of delivered capacity and head (pressure) from a centrifugal pump. They also provide high inlet to outlet electrical efficiency, reasonably easy installation, reliable and flexible operation, simple adjustment requirements, and have also realized continually lowered costs in recent years.

We begin a three-part series this month, though, on some of the design pitfalls, cautions, and application tips to be used when applying VSDs and VFDs to pumping systems. There will likely be some information that may be redundant to many of your designs, but if even one of these tips or warnings is helpful for a specific application, the effort will be worth it.

Application Tips, Limitations, and Potential Design Pitfalls

Figure 2. Cable length impact on VFD peak voltage.

Many of the following limitations and application considerations apply to variable frequency drives only while others may apply to all types of variable speed drives. All, however, should be considered when evaluating the possible use of any of these devices.

When choosing a VSD or VFD, there are some specific considerations which must be made that could otherwise be cause for concern. The following are nine things to consider.

#1 Consideration

Reflected waves are defined as over-voltages caused by the quick switching of insulated gate bipolar transistors (IGBTs) used in modern VFDs. Voltage wave reflection is a function of the voltage rise versus time (dv/dt) and the length of the motor cables.

The impact of reflected wave voltage, also known as transmission line effects or standing waves, can equally damage both motors and cables. It essentially mirrors the instantaneous waves that occur over time during hydraulic surges, specifically water hammer (Figure 1). When not diminished, these reflected waves place undue stress on cable insulation as well as motor insulation and windings. This ultimately causes damage and reduces the life of the equipment.

Figure 3. Projected insulation life based on cable length.

Most VFDs today operate on pulse width modulation (PWM) technology, with the inverter producing a continuous pulse square wave output rather than sinusoidal wave forms. Refer to the Engineering Your Business column in the July issue for more detail.

These voltage pulses are transmitted to the motor through the motor power cable. The cable has an inherent inductance that’s proportional to the length and size of the cable. High inductance, due to long cable lengths, increases the amount of time it takes to charge the inherent capacitance of the motor, which increases the amount of energy in the cables. This damage happens because the reflected waves create peak voltages higher than the motor and cable are rated.

Motors with a high surge impedance (i.e., lower HP) will generally endure a higher peak voltage than higher HP motors with lower surge impedance. Additionally, when the voltage pulses reach the motor, they encounter a higher impedance than in the transmission cable, causing the pulses to be reversed and reflected to the drive. Under certain operating conditions, a pulse from the VFD can add to a pulse reflected from the motor, resulting in a potential doubling of the voltage level.

Figure 4. Impact from reflected wave voltage on motor voltage.

Fundamentally, the greater the length of the cable (i.e., the distance between the motor and VFD), the greater the value of over-voltage, known in this case as the reflected wave. This creates a unique situation when VFDs are used on deep-well submersible pump motors or other motors with long offset cable lengths. Together with cable length, the rise time of the switching device also influences reflected waves and the amount of over-voltage (Figure 2).

If the turn-on time of the switching device is slow, the capacitance of the motor has time to charge, and discharge occurs in step with the switching device. But if a switching device’s turn-on time is rapid, the voltage applied across the cable increases with greater energy stored, resulting in a higher voltage.

This can have a direct bearing on the insulation’s life. The projected motor’s magnet wire insulation life is dependent on the switching frequency as well as the cable length (Figure 3). The voltage spikes that occur with reflected waves can also ionize the air that exists in the air gap between the motor windings and rotor, as well as the air between the cable insulation and cable jacket within a few seconds. This causes a partial discharge to occur and is known as a corona.

The point at which this ionization causes arcing across the motor windings or across the conductors in the cable is known as the corona inception voltage (CIV) (Figure 4). When the CIV is reached at the motor, the insulation in the motor windings can be damaged or even totally fail. When the CIV is reached for the cable, a short circuit or permanent damage to the cable can occur.

Some options for mitigating the possible effects from reflected waves include the following:

• Specifying inverter duty motors, which are capable of withstanding higher peak voltages. Inverter duty motors withstand higher peak voltages by using voltage spike-resistant insulation systems, which may include inverter-grade magnet wire, improved insulation ratings, and low heat rise designs.

These improvements allow an inverter duty rated motor to withstand an upper limit of over three times the motor’s rated line-to-line voltage, per National Electrical Manufacturers Association (NEMA) Standards MG 1 Part 30 and 31.

NEMA standard MG 1 Part 30 limits peak voltage to 1000 volts and dv/dt rise times to 2 msec (milliseconds), but a typical VFD using an IBGT inverter on a 480VAC power source can easily exceed these thresholds. NEMA MG1 Part 31 inverter duty motors have an improved insulation system that can withstand 1600 volts of peak voltage and a 1 msec rise time along with improved bearing shielding and lubrication for higher temperature operation.

Another solution is using a soft PWM drive that automatically adjusts its transistor switches to limit voltage spikes on NEMA MG1 Part 31 motors. Optional shaft ground rings for above-ground motors can also help mitigate potential bearing damage by providing a low-impedance
path between the stator and rotor.

It is important to note that reflected wave voltage can also impact the repeatability and signal reliability produced by electronic control and instrumentation. For a standard motor, a rated line and load reactor (Figure 5) is recommended for non-inverter duty motors greater than 50 feet from the VFD or inverter duty motors greater than 250 feet away from the VFD. Depending on the maximum withstand voltage, a load reactor will typically protect the motor up to 400 feet away from the VFD but should be placed as close to the VFD output as possible.

• Installing dv/dt output filters (Figure 6) to limit the peak voltage. These filters work by changing the voltage risetime and slowing pulse width modulation pulse transitions. Typically, due to the distance of drop and offset cable and degree of reflected wave voltage, submersible motor manufacturers will not warrant a 460V, three-phase motor without installation of an output filter.

The cost of a dv/dt filter is usually more than the cost of a reactor and is best to be installed as close as possible to the VFD for optimum effectiveness. Thus, these filters are typically placed in the VFD enclosure and provide greater motor protection compared to load reactors, in certain cases, up to 2000 feet away. It is generally recommended to install a dv/dt output filter on the load side when the VFD and motor are located over 100 feet up to 300 feet from each other; the actual length is dependent on the manufacturer and system.

Generally, when dv/dt filters are used, line reactors are not recommended and may exacerbate the problem. Note that dv/dt filters offer the following advantages for pump motors driven by VFDs:

• Reduce voltage spikes to below 1000 volts
• Slow down PWM dv/dt ratio by a factor of three
• Reduce common mode currents by approximately 30%
• Protect both the motor and the power cable insulation.
• Installing a sine wave filter to convert the rectangular PWM output signal into a smooth sine wave voltage, thereby removing the voltage spikes. Because this filter works by creating a sine wave, there is no limiting distance. Due to the increased cost, it is not recommended to use this filter type unless the VFD is located about 1000 feet away or more from the motor.
• Limiting the distance between the VFD and motor. By limiting the distance between the VFD and motor, costs can be reduced by not having to purchase ever increasingly more expensive filters, not to mention the costs saved from the length of cables as well as the increased cable size required due to voltage drop. Typically, the values shown in Table 1 are recommended based on HP and withstand and motor voltages for reactors and dv/dt filters.

#2 Consideration

Figure 5. VDF with line and load reactors.

Damage to submersible motor windings and bearings can be caused by the repetitive and rapid pulses applied to the motor from a VFD’s non-sinusoidal power-switching (on and off) circuitry. The many terms used to describe this phenomenon include harmonic content, parasitic capacitance, capacitive coupling, electrostatic buildup, reflected wave voltage, and common mode voltage.

Regardless of the term used, high peak voltages and fast voltage rise times can cause cumulative degradation of motor components including winding insulation, bearings, varnish, etc.

If the load impedance is higher than the line impedance, current is reflected in the reverse direction toward the VFD, creating high voltage spikes at the motor that can be twice as high as the rated DC bus voltage. Installations that require long lengths (more than 100 feet) of cable between the VFD and motor, such as submersible and long-offset motors, are especially vulnerable to this electrical problem and possible
motor damage due to a coupling effect between the motor conductors where the voltages can build to dangerously high values because of the inverter pulse reflecting from the motor back to the VFD.

In long distances, a 460-volt output can build and exceed 1000 volts at the motor. These applications are particularly susceptible to operational problems and resulting premature failure and must be carefully considered during system design.

#3 Consideration

Figure 6. A dv/dt filter.

In addition to harmonics, motors operated with VFDs are much more vulnerable to electromagnetic interference. Electromagnetic interference are errant audible electrical signals, also known as noise, caused by one or more drives located in a common area. This problem can result in interference to radio or TV signals, leading to complaints from nearby residents and other electrical users, and may require the use of specialized (sine wave or dv/dt) filters or other methods to control any severe noise issues.

These electrical signals can appear in control systems and produce undesirable effects in the system such as communication errors, reduced equipment performance, and malfunctioning equipment.

Mitigation measures include installing electromagnetic interference (EMI) filters, ensuring a low impedance ground system, installing VFD shielded cable, and installing a common mode choke. Proper grounding of the motor to the VFD unit is critical to prevent noise problems and possible stray voltage. In some cases, the use of shielded offset or drop cable conductors with submersible motors may be necessary.

Failures of VFD units or motors often occur simply because the devices may operate at different ground potentials. Again, it is critical that all devices be at the same ground potential and tied together into a true and common earth ground.

Various protection methods, such as load reactors and filters, exist to control and limit electrical noise, but each VFD installation must be evaluated individually before using any of these methods. Generally, unshielded cable distances between motors and VFDs should be limited to those shown in Table 2.

#4 Consideration

As indicated in the table, use of VFDs with submersible motors must be carefully performed. Submersible motors must not fall below the minimum allowed speeds (i.e., operating frequencies) that are needed to provide adequate bearing lubrication. Therefore, this minimum operating speed must always be observed.

Typically, this value corresponds to a minimum operating frequency of 30 Hz (50% speed). In addition, submersible motors depend on a minimum water velocity traveling past the motor to provide adequate cooling. Although lowered motor speeds will result in lowering of the motor horsepower, the minimum required cooling flow rate or velocity past the motor must always be observed.

Finally, each motor manufacturer has separate requirements and guidelines when applying VFDs to their motors, such as minimum and maximum frequency, as well as allowable types of VFDs, filters, and cable. These guidelines should be fully applied before application of the motor.

#5 Consideration

Considerable amounts of heat are generated from multiple components of the VFD during the power conversion steps from AC to DC back to AC. The VFD switching frequency can be lowered to reduce heat generation in the drive but lowering the frequency will increase the creation of harmonics and the audible noise heard from the motor.

The primary method for preventing overheating of the equipment is using heat sinks and fans to dissipate the heat through vents in the enclosure. If possible, it is ideal to place VFDs inside a climate-controlled electrical enclosure or room.

However, there are multiple vendors that can provide VFDs with an air conditioning unit. The HVAC system will need to account for the added heat generation as VFDs can quickly raise the indoor ambient temperature, particularly with higher horsepower (more than 100 HP) units. It is wise to reach out to VFD manufacturers during design to determine the expected heat generation and best mitigation methods.

#6 Consideration

Knowing the precise speed range required by the load is important since most general-duty induction motors can only accommodate a 10:1 speed turndown ratio without overheating and degrading the motor’s winding insulation life. Basic inverter duty rated motors are generally rated for a 20:1 speed turndown ratio when combined with scalar (V/Hz or V/f) VFD control methodologies.

For many load types, this degree of turndown ratio is more than adequate. For example, with a pump rated for 1000 GPM, it generally does not make economic sense to try to control that same pump below 50 GPM or a 20:1 ratio. If the application really requires this flow reduction, and depending on the manufacturer, motors rated for use with vector VFD drives can often accommodate up to a 1000:1 or 2000:1 turndown ratio. If you need extremely high turndown ratios while maintaining high levels of low-speed torque, this often means using a VFD with vector control.

#7 Consideration

Many water systems, especially deep-well pumping systems, require a fixed amount of minimum operating head needed to simply elevate the water to the surface or ultimate destination. This minimum head, which is often referred to as the static lift or head, reflects the system’s minimum required operating head, and thus, the resultant minimum pump speed that must be programmed into the drive to ensure a constant delivery of water to the surface or discharge point.

This value of minimum speed varies between well and pump applications and can also vary significantly in any single well due to the seasonal or yearly differences that often occur between static and maximum (deepest) pumping levels. If a drive is permitted to lower the motor and pump speed below this minimum setpoint, the pump will no longer be able to generate the head needed to deliver the water to its destination, resulting in a shutoff flow condition that, if not immediately corrected, will rapidly lead to pump or motor failure.

If this minimum condition comprises most of the total head, the allowable reduction of speed will often not be sufficient to even justify the expense of a drive. For this reason, a measure of reserve head should be incorporated into the VSD speed adjustment setting. Generally, adding a safety factor of at least 15% of the maximum operating head will cover most of these instances.

In addition, although VFDs are most popular for use with submersible pumps, most other types of VSDs are generally limited to vertical turbine and above-ground pumping units. When considering using a variable speed drive of any type for a water pumping application, the designer must verify that the unit can transmit the required torque and horsepower from the motor to the pump under all service conditions. This is particularly important for positive displacement pumps and deep-well vertical turbine pumps when considering torque transmittal drives, such as hydraulic or eddy current couplings.

Refer to the Engineering Your Business column in the WWJ May 2024 issue, Table 2, for typical values of capacity and head and horsepower reductions for centrifugal pumps based on the degree of speed reduction.

With a VFD, a minimum speed is also required to ensure adequate cooling of the motor and that lubrication will always occur regardless of the load or speed. This is especially important for fan cooling, oil-lubricated motors, and submersible motors. This situation must be considered on a case-by-case basis, but its importance cannot be ignored or overstated.

#8 Consideration

VFDs and some VSDs provide optimized motor starting characteristics and can start and accelerate motors to full speed rapidly while drawing only 100% to 150% of the motor’s full load current or amps from the line. This ability to start at close to normal full load amperage is important if the power supply cannot withstand or deliver the typical six to seven times the full load amperage (FLA) as the instantaneous inrush current that a standard across-the-line (ALS) starting device draws or even the 250%-350% FLA of an electronic soft-start device.

VFDs do this by modifying the magnetic flux of an induction motor. This occurs since the magnetic flux is directly proportional to the voltage and inversely proportional to the frequency. By keeping the flux constant, the inrush current does not normally exceed the FLA rating of the motor although the full starting torque is maintained. This is a significant improvement over the soft-start method, which has significant voltage drop concerns and cannot start a motor under a fully loaded state, such as with a piston or reciprocating positive displacement pump.

#9 Consideration

Severe line-current voltage unbalance can also cause VFD nuisance tripping and third-level harmonic currents, which are unusual for phase-to-phase nonlinear loads. This situation can cause the overload protection device to trip when VFD line currents become unbalanced because there is a resulting excessive current in one or two of the phases.

This is common with open-delta (two transformers), three-phase power systems and can happen even though the average current of the three phases is well below the VFD’s current rating.

To identify a possible problem, the VFD’s line current can be measured during normal operating conditions to verify whether there is an overcurrent in any phase. Voltage or current unbalance and related harmonic voltage or currents can be determined by using the methods illustrated in The Water Works column, “Understanding Pump Drivers,” which was published in the January 2019 issue of Water Well Journal.

If the measured value on any phase is not greater than the VFD-rated line current and the reason for tripping is likely due to an upstream current unbalance relay, increasing the trip setting of the relay to a maximum of 30% may compensate.

One of the benefits of VFDs is that the line side voltage unbalance or the current unbalance does not reflect on the motor side. Therefore, the unbalance trip setting may be revised without any concern for damage to the motor.

Next, if the voltage is unbalanced by more than 2%, rebalancing the line voltages may help by relocating as evenly as possible all single-phase loads across all three phases or relocating (rolling the leads) unbalanced loads or correcting all overloads within the system.

In the end, an AC line reactor or isolation transformer at the problem VFD may be needed to reduce the line currents and harmonic distortion as well. The reactor should be sized to carry the VFD’s full load current and have an impedance rating anywhere from 2% to 6%. These ratings describe the expected RMS voltage drop across the reactor at rated current.

Choosing the optimum rating between 2% and 6% can be a problem. If you choose a higher rating, the higher voltage drop will give you the greatest amount of current balance, but you’ll sacrifice the responsiveness of the VFD. Starting with 3% impedance for 100 HP or less is often recommended; larger HP units may require up to 5% impedance.

______________________________________________

This concludes part one of this three-part miniseries on possible design pitfalls involving variable frequency drives and Part 5a of this five-part series on variable flow and head pumping systems. Next month, we will continue this discussion on possible VFD system hazards and application issues.

Until then, work safe and smart.

Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at epbpe@juno.com.