Variable Speed and Frequency Drives

Part 5c. Design Aids and Pitfalls.

By Ed Butts, PE, CPI

We conclude our series on variable flow and head systems this month with a final look at design aids and pitfalls of variable speed systems.

The previous two columns each highlighted nine considerations. We will wrap things up with a look at 19 more items to understand. Let’s dive right in!

#1 Consideration

Motor bearing failure is one of the most frequent issues caused when using VFDs. The high frequencies during the rapid switching of voltages and transient spikes are induced into the motor rotor, generate circulating voltages, which build up a voltage potential between the rotor and stator.

This causes electric currents to run through the motor and discharge into and through unprotected motor bearings. This high shaft voltage is often indicated by internal frosting, pitting, arcing, or fluting through or across the motor’s ball bearings and races. Fluting damage is related to the characteristics of the PWM waveform, VFD programming and characteristics, and the installation.

These unmitigated shaft currents can turn polished motor bearings into non-functional and destroyed pieces of metal in a short period of time, but even VFD-rated motors are potentially vulnerable. This continuous mini lightning storm within the bearings can cause premature failure of the bearings unless provisions are made to prevent this occurrence.

Fortunately, these currents can be redirected or neutralized using a few different methods. Insulated bearings or VFD filters may be needed if damage from bearing fluting is to be avoided. Newer motors specifically designed and built for VFD operation now include built-in bearing protection, while retrofit grounding kits are available for many older motors to prevent possible motor bearing failures.

The motor manufacturer should always be consulted to determine the best method to use to dissipate bearing currents. Other viable solutions such as shaft grounding rings, grounding straps, ceramic bearings, insulated motor housings, and even coated bearing races are also available options, depending on the specific situation.

Shaft grounding (Figure 1) is recommended in NEMA MG1 31.4.4.3 as an effective means of bearing protection for motors operated from VFD supplied power. One grounding device is generally adequate to dissipate inverter-sourced shaft voltages, thereby protecting both bearings for motors as large as NEMA Size 6000 frames.

There are seven common techniques, along with variations of each, that can minimize or eliminate bearing damage caused by these ground currents:

1. Faraday shield
2. Line or load filters (Use sine wave dv/dt filters on submersible motors)
3. Insulated or ceramic (i.e., electrically isolated) housing with bearings
4. Grounding brush
5. Grounding ring
6. Use of non-conductive lubricant
7. Insulate the opposite drive end bearing and use a shaft current device to mitigate any bearing currents (Aegis system).

#2 Consideration

Proper grounding and bonding practices can be instrumental in minimizing motor related failures caused by PWM waveform characteristics and installation factors. Shielding the cable or wire between the motor and the VFD can also significantly improve these spikes as well.

VFD manufacturers typically provide detailed guidelines for line and load side filtering requirements and the proper grounding of the motor to the VFD and output cable routing. General grounding requirements of a VFD installation are illustrated in Figure 2.

Cabling manufacturers can provide recommended cable types, shielding, and sizes for VFD installations and other critical information concerning output wiring impedance and capacitance to ground. A true Earth ground loop is the most familiar and used grounding system, but all structures that are supplied by AC power use it primarily for safety reasons because a short circuit in an ungrounded system could be fatal.

Uncoated metallic well casings and commercial building structural steel are usually the best connections for a zero-voltage potential because both provide a low impedance path with steel embedded into the ground. In most electrical services, grounded connections and continuity are established by bonding the electrical system to the utility-neutral steel or copper water piping or embedded ground rods or rings.

However, installing a VFD system and relying solely on a true Earth ground may be problematic as well as ineffective. Depending on the impedance (i.e., resistance to AC current), some higher-frequency currents may not effectively pass back to the drive, and thus may increase voltages above the safe touch potential of 50 VAC. These stray currents can cause damage to anything within the VFD circuit.

This means the entire electrical system, motor, and VFD must be fully and effectively bonded and grounded with no resistance differential. For the portion of wiring between the VFD and the wellhead, it is recommended to use Type XHHW wire rather than THWN and threaded and coupled steel conduit or other metallic conduit with grounding continuity to create a complete and low resistance ground path rather than PVC.

#3 Consideration

Variable frequency drives are often rated for the applied voltage and amperage and not horsepower. The designer must ensure the unit has an adequate amperage rating to accommodate the largest expected motor current, especially if planning to run the motor at speeds or frequencies higher than the specified nameplate condition. In some cases, this may require moving up to the next largest HP or amperage rating.

#4 Consideration

When the application calls for a considerable amount of operation at full speed conditions, consideration should be given to adding a full voltage bypass contactor to a VFD. This can substantially lessen the operational hours and lengthen the life of the VFD.

If the motor is planned to operate on the VFD at all service conditions, the motor should be derated at full load between 3% to 10% to prevent overheating of the drive. In other words, the maximum design load on a 50 HP motor at full speed should be around 45 to 48 HP.

#5 Consideration

As electronic devices, a VFD and some VSDs can be particularly susceptible to a high-voltage situation or lightning strike. Adequate provisions to protect the unit from these surges should always be provided. This can often be in the form of additional grounding, reactors, filters, isolation transformers, a bypass in the form of lightning or surge arrestors, or fast-acting fuses.

#6 Consideration

In the appropriate size and motor rating, VFDs can also be used as phase converters to convert single-phase power to use with three-phase motors since most single-phase motors and other fractional horsepower ratings are not designed to be operated on a VFD (see Figure 3 for an example). There are several distinct limitations to this application and specific requirements related to oversizing. Therefore, the motor and VFD manufacturer must be consulted before applying a VFD to this use.

When used as phase converters, the single-phase line will draw 173% theoretical greater power than the motor amperage from each line. However, after adding internal losses and an overload factor, this value rises to approximately 185% of the motor’s full load current.

Therefore, for an added margin of safety, doubling the motor’s current is generally recommended, and is what should be planned and anticipated. Thus, for a 30 HP, 460 VAC, three-phase motor rated at 40 amps, the two single-phase line power currents should be estimated at 40 amps × 2.0 = 80 amps per line.

As previously indicated, the direct use of a single-phase motor with a variable frequency drive is extremely limited and restricted. In addition to the prior stated method of using a three-phase motor on a single-phase power supply, the other two methods are to allow the motor to operate as a two-phase motor (Figure 4a) and using the VFD as a single-phase device (Figure 4b). Although a single-phase motor cannot be simply connected to a VFD, both methods require modifications to the starting switch and start and run capacitance. This requires careful attention to detail and engineering oversight.

#7 Consideration

Motors with thermal overloads installed in the motor control may not operate properly with a VFD. The current carrying thermal overload is designed for normal sine-wave AC power. Operation on a VFD may cause nuisance tripping or not protect the motor at all as would be expected using utility power. Thermostats or thermistors installed in the motor and connected to the VFD may provide suitable thermal protection in this case.

#8 Consideration

Special considerations are required when multiple motors are to be powered from a single VFD unit. VFD manufacturers can provide guidelines for proper motor thermal considerations and starting and stopping of the motors.

Cable runs from the VFD to each motor can create conditions that will cause extra stress on the motor windings. Filters may be required at each motor to provide maximum motor life. When used for multiple motors, the chance for a motor to develop impedance mismatch is greater, thus increasing the chance of the reflected wave phenomenon. All motors must be the same size to maintain consistent speed control and each motor must be provided with individual overload and short circuit protection as per NEC-Article 430.

#9 Consideration

Vertical turbine and submersible pump motors operated on a VSD present other unique conditions that may require additional considerations by the user or design engineer. These include:

• The non-reverse-ratchet (NRR) function on a vertical turbine pump can interfere with motor rotation at extremely low speeds (below 300 RPM). This can cause locked rotor conditions and possible drive tripping.
• Unexpected or unacceptable vibration and noise levels caused by the torque-pulsation characteristics of the PWM waveform, a critical speed (frequency) condition falling inside the variable speed range of the process, or the added harmonic content of the PWM waveform exciting a system component may all occur.
• Operating pumps and motors at speeds above nameplate values (i.e., overspeed) can cause numerous issues. In addition to higher amperage, this can cause motor or VFD failure from overload, pump failure on vertical turbine pumps due to lineshaft bearings running at excessive speeds, possible pump cavitation, and probable rapid overload and resulting failure on any motor that was originally designed for full load conditions at the motor’s nominal design speed.
• Application problems related to the controlled acceleration/deceleration phase and torque of the pump and motor while operating on a VFD may impede the proper and accelerated rise of system pressure on a closed-loop water system.
• Any reduction of pump speed that may impact the downthrust reflected onto the motor may fall below the minimum axial thrust or cooling requirements of the motor thrust bearing, particularly the thrust bearing on submersible motors. Verify this will not present an issue with the manufacturer and never run below a motor’s minimum allowable operational speed.
• Water hammer during rapid pump shutdown can possibly result in a dangerous system hydraulic surge and damage a non-reverse ratchet (NRR).

#10 Consideration

Although use of a VSD can conceivably eliminate the need for various sizes of other pumps in an application with varying flow demands, a designer should often resist the impulse to arbitrarily use only one pump where one or two smaller pumps along with or without the VSD unit will provide a better overall system.

Beyond the obvious advantage gained through unit redundancy, the use of smaller, fixed-speed units will often provide smoother system operation and more reliable unit handoff as well as lessen demand on a single VSD unit. This will extend both the VSD driven motor and the drives service lives considerably.

While on the topic of variable demands, the so-called sweet spot for the highest induction motor efficiency is usually around 75% of a motor’s full load speed. Typically, a motor’s efficiency gradually lessens with progressively lower or higher loads. However, below a level of about 25% of full load speed, most motors experience a dramatic reduction in efficiency due to an inconsistent magnetic flux within the motor’s air gap.

If a significant percentage of operating hours are anticipated below the applicable lowest load, the system designer may alternatively consider designs that allow spreading the full load among multiple sizes of smaller units or bypassing flow to maintain a higher motor and pump speed. The basic concept is that units are activated only when needed to meet the requirements associated with their specific output and deactivated when the demand is satisfied. This is a valid and efficient operational scenario for many pumping applications.

#11 Consideration

These guidelines apply to typical and stable single source, full three-phase, wye-connected AC power supplies obtained from an electric utility. If an engine-driven generator or alternate power source, such as an open-delta or delta power supply, is to be used for a standby or primary power supply, an analysis must be conducted to address possible issues with the VSD and possible derating associated with this alternate power source.

#12 Consideration

Due to a low operating power factor, many power supply systems may require the use or addition of power factor correction (PFC), generally, by using oil-filled run capacitors placed on the line or load side. When used with a VFD, PFCs should never be placed on a VFD load circuit as substantial damage to the motor, VFD, or power system can result.

#13 Consideration

VFDs are generally not used to continuously run a motor at or above its 100% full load rating at a reduced speed unless the motor was specifically designed for this condition. Typically, a cooling fan is attached to the motor’s rotor; thus, when the motor slows down, so does the cooling fan and air flow. This means a slower-running motor producing a higher HP will likely run as a hotter motor, which will usually translate to a shorter life. That’s why using a VFD with an open dripproof (ODP) motor running into the service factor will generally void the motor warranty.

Typically, a motor with a service factor of 1.15 or more over the nameplate rating should be able to handle the additional heat unless the motor is continually running at slower speeds. If the motor regularly runs at a severely reduced speed and needs additional cooling, a constant speed fan placed to blow additional air across the windings can help to remove and dissipate this heat.

This operating condition will also eliminate any inherent overload capabilities that the motor may have and increase the risk of damage when operating under adverse operating conditions, such as low voltage or high ambient operating temperatures. The insulation class of the motor provides a general indication of its overload capability when used on a VFD. Thus, a Class H (125°C rise) insulation is preferred over the more typical Class B (80°C rise) and Class F (105°C rise) insulations. In any case, a VFD driven motor’s insulation system and rating should always be examined and verified for adequacy with the application.

#14 Consideration

To realize the full effectiveness of an automatically controlled variable speed drive, the process variable used for control, usually consisting of line or differential pressure, water level, flow rate, or other process variable must be carefully selected and obtained from the proper location.

If a variable speed control depends on signal feedback from a process variable as the control input, it is vital that the nature and stability of the process variable is understood and the selected location for the variable represents a true and accurate value of the intended parameter. For line pressure control, this is generally in the form of a gauge style analog strain-gauge transmitter that directly attaches to an appropriate location on the discharge line and sends the signal to the drive for speed reference and control.

Level transmitters often use a submersible, gauge pressure, or acoustic analog transmitter and flow measurement units generally utilize an analog signal routed from a transmitter mounted to or from the flowmeter itself.

In all instances, selecting the proper location for installing a transmitter is critical for accurate and linear signal outputs, and thus effective speed control. For example, mounting a pressure transmitter directly next to or on a pump, check, pressure regulating or throttle valve will expose the device to the potential line surges and varying pressures common to such a location while level transmitters placed adjacent to a sump inlet, pump suction, or other region of entrance or exit turbulence like surface vortices or cascading water can be exposed to the same type of hydraulic disturbances.

In both cases, the use of pressure snubbers for line pressure or an isolated stilling well for level transmitters may be needed to quell these disturbances and provide a clean and linear output signal to the drive.

Another example where the improper location of a controlling signal device can cause problems is with the signal transmission between the control device and the drive. Since the signal generated for control is almost always electrical in nature, a varying control signal occurring from excessive signal transmission distances, unstable supply voltages, excessive loop resistance from inadequate wire size, too many analog instruments in the loop, or routing with AC conductors or other sources of interference can easily distort the current or voltage signal that is used to convert the process variable to a meaningful control parameter.

Generally, DC analog signals are designed to operate with less than 1500 ohms of total loop resistance and one-way wire distances should be limited to 50 feet with no smaller than #16-gauge copper conductors used. Due to the lower conductivity and potential presence of damaging oxidation at terminals, aluminum wire should never be used for control signal conductors.

#15 Consideration

When using a VSD with a deep-well pump (vertical turbine or submersible) in a sandy well over 200 feet in depth, maintaining a minimum vertical (i.e., uphole) velocity of 2.5 FPS is necessary to prevent the settling of sand into the upper pump stages. If this minimum velocity is not maintained, heavier sand or other debris can settle back into the upper pump stages and discharge case, resulting in grinding and eventual destruction of the pump.

#16 Consideration

It is usually more advantageous to select the pump so that the system head curve intersects the full speed pump curve slightly to the right of the best efficiency point or window in order that the pump efficiency will first increase as the speed is reduced and then decrease as the pump drops further in speed.

If possible, designing the pump and VSD speed range so that the efficiency at the minimum operating speed is no less than 60%-75% of the best efficiency point is desirable. This can also extend the useful range of variable speed operation in a system with primarily static head. The pump’s manufacturer should be consulted on the safe operating range of the pump.

#17 Consideration

Significant improvements are continuously being made in VFD technology. For example, the expanded application and use of active front end (AFE) VFDs often provide numerous advantages over conventional drives in that they produce equivalent sine wave outputs. These “clean power” VFDs do not generate a high rate of voltage rise nor transient; thus, an existing three-phase motor and branch circuit conductors can be wired directly to the VFD power output without incurring added heat loss, rapid motor or cable insulation breakdown, or the need to use filters to dissipate voltage transients.

As the converter/inverter unit produces equivalent sine wave outputs, it produces less harmonic distortion than IEEE recommendations, generally less than 5%. Therefore, there is no need to add filters on the power input side to protect the VFD, cables, and power supply transformers. These drives are currently available in multiple supply voltages with horsepower ratings from 5 HP up to 300 HP.

Another example is the potential use of wide-bandgap (WBG) transistors, which is changing many uses of power electronics, including VFDs. In the future, this can present a viable alternative to standard variable frequency drives, particularly in applications with high switching losses and high temperatures. WBG semiconductors offer multiple advantages, namely more than an 80% reduction in switching losses and use at higher operating temperatures.

These advantages can result in higher switching frequencies with lower losses, simplified cooling, operation at a higher temperature, and smaller passive components for filtering. With this solution, the industry may get an option that corrects common VFD harmonic issues while simultaneously offering a smaller footprint and permitting higher operating temperatures.

Although this technology is not expected to be commercially available until at least 2025, this change may be closer than it appears as new WBG-based VFD architectures with integrated AFE and sine-wave types of filters are now being showcased and delivering on the promise of a true sine wave VFD.

Current VFD technology also employs sophisticated methods for detecting and responding to detrimental operating conditions such as overcurrent, ground faults, and thermal overloads. These methods use advanced algorithms and realtime line side monitoring capabilities, enabling faster and more accurate reaction and protection compared to older VFD systems.

These additions to VFD safety features reflect a trend toward more integrated, proactive, and data-driven approaches to safety. The focus is not just on responding to hazards but anticipating and preventing them, which aligns with other trends in industrial safety and system automation. Modern units can optimize energy use and harmonic generation based on the load requirements. These algorithms dynamically adjust operating parameters, such as output voltage and frequency, to align with the driven load, resulting in reduced energy consumption and greater overall efficiency. This is a noticeable shift from older VFDs that operate at fixed or less adaptable parameters.

#18 Consideration

In addition to the multiple advantages in using a VSD, there are a few distinct disadvantages to consider:

• The first and most obvious disadvantage is that a VSD-controlled pump cannot provide the same function as a control valve at a no flow or near zero flow condition. In most pumping systems, a control valve is generally a better flow control choice when the system’s flow
  rate must or can drop below 50% of the full speed rated capacity. In addition, they can and generally do serve as a shutoff valve and for backflow prevention (i.e., check valve).

If a throttling valve is not used, the process designer will have to consider using an automated auxiliary valve on the pump discharge that will close when a no flow condition is required. Silent check valves can also be considered as a replacement for an inline throttling or control valve.

• The speed of response must also be considered as it is not possible to change the speed of a pump instantaneously using a VSD, whereas a properly selected control valve can be quickly repositioned and can modify the flow rate almost immediately. When using a VSD, ample response time of the device must be incorporated into the design to allow enough time to cause a change in performance.
• In well-designed applications where the lowest process or system demand may already closely match the full load capacity of the pump and motor, the use of a VSD would only add to the overall system losses and complexity described above as well as add unnecessary cost for a likely unneeded device. This is always a prime consideration for systems with constant head applications, such as raw water, dedicated reservoir fill, and constant output applications such as dewatering or transfer/lift pumps. In these examples, use of a reduced voltage or soft starter for motor starting is often the better selection.
• Lastly, when using a VSD for energy saving purposes, the efficiency losses associated with the VSD (1%-3% is common), the motor, power transmission means (lineshaft or electrical cable), and the pump must be considered as individual elements of the overall system efficiency. Failure to adequately factor all parasitic losses and individual efficiencies will result in distorted system efficiency values.

#19 Consideration

The projected life of a typical VFD is generally around 10,000 to 12,000 operating hours, which translates to between six to 12 years when exposed to normal service conditions of 1200 to 2000 operating hours per year (an average of three to six hours per day).

The yearly operating hours become especially important if the drive is expected to operate continually or for a significant number of hours each year, where the life may drop to only 12 to 18 months.

The wide variance in the operational life is mainly due to environmental factors and heat exposure. VFD electronics, semiconductors, and related components, such as circuit boards, thyristors, filters, and capacitors, have a finite operational life which starts at around four to five years. Even highand low-voltage non-electronic equipment, such as cooling fans, terminals, and fuses, possess a finite service life. Thus, these components can start to fail within just a few years of service, particularly if exposed to high levels of ambient heat, excessive cycling, overload, or inadequate cooling air and will eventually need to be replaced.

The operating life of a VFD can often be extended up to 25,000 to 30,000 hours by ensuring a clean, dry, and dust-free supply of adequate ventilation air through the cabinet to cool the components. In some cases, derating a drive may also provide an extended service life.

Preventive maintenance and replacement of selected components can also greatly extend the service life. By combining preventive maintenance kits with an effective planned maintenance program, VFD users can plan and budget service more easily, while getting the most out of their VFDs for up to 15 to 20 years.

Finally, selection of a variable flow and head or speed device should be conducted using a logical approach with consideration of all relevant factors. The flowchart shown in Figure 5 can assist designers with this effort for variable speed drives.

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This concludes our series on variable flow and head systems, and I hope it has been beneficial for you. Likely, the greatest advancement and future applications in variable frequency drives will be the matching of VFD technology to permanent magnet and other types of specialized motors. This concept will be discussed in next month’s edition of Engineering Your Business.

Until then, work safe and smart.

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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.