300+ TOP Variable Frequency Drive(VFD) Interview Questions – Answers

1. What Is Sensorless Vector Control?

Sensorless vector control is a technique used in variable frequency drives to rotate the force vector in the motor without the use of a shaft position sensor. The goal of AC sensorless vector technology is to give the user “DC-like” control while making traditional speed or shaft position feedback from the motor unnecessary. Sensorless vector control removes a major source of complexity and potential for failure, such as tachometer feedback, while simplifying many AC drive installations. The sensorless vector algorithm must be tuned to match the characteristics of the particular motor being controlled by the drive. This improves torque performance at very low speeds as compared to typical volts per hertz control.

2. Does A Motor Need A Chassis Ground Connection?

Yes, for several reasons. Chassis grounding is used for protection in the event of a short in the motor that puts a live voltage on its housing. Motors and other components exhibit leakage currents that increase with aging and a grounded chassis generally emits less electrical noise than an ungrounded one.

3. What Type Of Motor Is Compatible With Inverters?

Inverter motor type must be a three phase AC induction motor. Preferably, you should use an inverter-grade motor that has 800V insulation for 200V class inverters, or 1600V insulation for 400V class inverters. For motor size, in practice it is much better to find the right size motor for your application; then look for an inverter to match the motor.

4. How Will I Know If My Application Will Require Resistive (dynamic) Braking?

For new applications it may be difficult to tell before you actually test a motor/drive solution. In general, some applications can rely on system losses such as friction to serve as the decelerating force, or otherwise can tolerate a long decel time. These applications will not need dynamic braking. However, applications with a combination of a high-inertia load and a required short decel time will need dynamic braking. 

5. What Is Ringing And What Happens To The Motor When I Place It More Than 50 Feet Away From The Drive Without A Line Reactor?

High voltage ringing (reflective voltage) occurs on all VFDs. With IGBTs replacing SCRs on smaller, more inexpensive drives, ringing has become more pronounced. This capacitive-coupling effect is caused by high speed switching (commutation). Distance greatly enhances the effect. Cables on the output side of the drive act like capacitors. When the IGBTs switch or produce the PWM output, the higher frequency part of the current will find a path through that capacitance. Think of the cable as a capacitor that increases in size as the cable length increases. Ringing is very detrimental to motors; it weakens the first stage rotor windings and shortens the life expectancy of a motor. This is especially true for motors under 10 hp.

Typically they are machine wound, have thinner coats of varnish, and have neither phase nor end paper (i.e., aggregate insulation properties). That’s why we always recommend a drive rated motor. Vintage and non-drive rated motors that are used with VFDs have a lifespan of unknown length. You simply cannot know what to expect from them. Long cable runs with ringing can add 10 to 15% to the drive’s current rating, causing the drive to trip out on excessive current. The capacitance will also cause a voltage drop that may cause speed performance problems on the lower end of the speed scale (increased current demand for demanded torque). A few general guidelines: 

1. Try to reduce distances as much as possible
2. Use an output line reactor on cabled distances greater than 50 feet
3. Always use a drive-rated motor.

6. Is There A Standard For Inverter Rated Motors?

NEMA MG-1, Section IV, Parts 30 and 31. Part 30 pertains to fixed supply motors; part 31 pertains to inverter supply motors. Always talk to your motor manufacturer for additional details. Note: the international standard is IEC 60034-17.

7. What Drive Power Cable Do You Recommend And Why?

Belden 29500 – 29507 cables are specifically designed for use with VFDs. Using the wrong cable can increase the detrimental effects from ringing due to capacitive coupling. Thermoplastic insulation, found in standard THHN cable, will break down over time if used to connect inverters to motors. The standing waves caused by pulse width modulation (PWM) will cause high voltage potential on single conductors. This renders corona (reflective/ringing voltage) in the air gaps between the conductors, which could break down and cause a system failure.

8. Can A Drive Replace A Softstart?

Yes. Softstarts are used for reduced torque starting and stopping of standard 3-phase induction motors (such as centrifugal pumps, compressors, ball mills, jar mills, fans, blowers and saws). If the motor is not adequately protected from sudden changes in rotational torque associated with starting and stopping, the current will be excessive to the system and the motor will surge, causing damage to all the equipment linked to it. Over the long term, this leads to increased over-current tripping and increased mechanical wear of gearboxes, clutches, transmission and conveyor systems. ANSI/IEEE Std. 141-1993 (Red Book) provides a comparison of different reduced voltage starting methods. An electronic softstart of VFD, although not listed, would have similar characteristics to the autotransformer starter.

9. Can I Run My Drive At Extremely Low Speeds?

Three major problems exist with both conventional methods of motor control and VFD control: 

1. When a motor is directly coupled and run at a low speed, it becomes very inefficient. You could go with a high torque motor that is significantly larger than the standard and would have the same results most of the time. 
2. Typical motors do not cool themselves well at extremely low speeds. The added heat buildup in the windings can cause premature motor failure. Check with the motor manufacturer for more details. 
3. The third issue is load inertia, which is larger than that of the motor. 

The effects of all of these things increase greatly with a sloppy transmission system. A speed reducer or gearbox should be employed to solve this problem. It will match the inertial changes and increase efficiency. Speak with a technical resource about sizing the gearbox to meet your application needs.

10. Are Variable Frequency Drives Phase Sensitive?

The input wiring is not sensitive to phase. If you were to change one set of input leads, the rotation would not change. The opposite is true for the output of the drive. It is sensitive to phase, so changing one set of leads to the motor changes the direction of rotation.

11. What Is A Line Reactor And What Will It Do For My Application?

A line reactor is a special type of inductor used on the line side or the motor side of a drive application depending on the specific circumstances. They are used on the line side of the drive to smooth inrush current, reduce noise, and to act as a buffer and protect the drive system. The line reactors have a 3% voltage drop based on impedance. This drop can be beneficial for drives on systems exceeding the rated input voltage of the drive. In this regard, the line reactor has been used to replace the much larger drive isolation transformer. Line reactors are used on the motor side of the drive to protect the motor by smoothing the drive output waveforms and by reducing ringing and capacitive coupling, especially with long cable runs between the drive and the motor.

12. What Is Grounding And What Reference Material Is Available?

Equipment or conductor-enclosure ground refers to connecting the non-current-carrying metal parts of the wiring system or equipment to ground. This is done so the metal parts which a person might come into contact with are at or near ground potential.

13. What Are Some Of The Common Reasons Why Motors Fail?

The EPRI Power Industry Study by General Electric in 1985 offered the following causes based on 6,000 utility motor failures: 41% were bearing related, 37% were stator related, 10% were rotor related and 12% were other causes. 

14. My Pump Is Losing Speed; Does The Drive Have A Problem?

Pumps are divided into three basic categories – centrifugal, rotary, and reciprocating. Take any of the three types and break them down further into four parts: the rotating element, the casing, the motor, and the drive. The rotating element has a shaft, sleeves, bearings and an impeller. The casing has a pump shell, wear rings, and shaft seals. The motor and drive are intuitive. Here is a basic remediation list: 

1. the belts are slipping (if applicable)
2. the impeller is worn
3. the impeller is loose on the shaft or the shaft is sheared
4. the casing is worn
5. the original motor was replaced with a slower model or with a greater amount of motor slip
6. the requirements have changed beyond the design specifications of the system. 

Using a manual tachometer, verify if there is a speed problem. If all of the above have been investigated and a problem still exists, then you may need to increase the maximum frequency of the drive. Note that anything beyond 60 Hz will result in a shift from a constant torque to a variable torque situation. There is a low probability that the drive will detrimentally affect the intrinsic speed of the system.

15. Is There A Reference For Application Help?

NEMA standards publication “Application Guide For Adjustable Speed Drive Systems.”

16. What Is The Difference Between Torque Control And Torque Limiting?

Torque control can be done with any of AutomationDirect’s PID-capable drives. This would be a closed loop system using torque as the process variable. There should be both a torque reference and torque feedback signal. The user would establish a torque setpoint and configure the drive to maintain that setpoint. Many customers have used torque sensors, load cells, and current transducers to establish an analog input to the drive. Ideally, a vector drive would work better than a volts/hertz drive.

A vector drive allows tighter speed regulation and better control in the lower speed range. Torque limiting can be done with AutomationDirect’s Hitachi SJ300 series drive. Instead of using an external torque sensor, torque limiting uses the drive’s internal current sensor. The operator would set the torque limit, and when that level is exceeded, the drive would act like a governor on a generator. The speed would be restricted until the appropriate level is again maintained. Mechanical devices, in addition to drives, are also employed for torque limiting, such as clutches, shear pins, gearboxes, etc.

17. What Makes Joliet Technologies Different, Compared To Other Variable Speed Drive Integrators?

As you can tell from our website, we are committed to providing as much information as possible regarding the products and services we provide. We have done our best to give you the information about the drives we use in our systems, so you can make clear and informed decisions for your particular application.

From the moment you contact us (and you can contact us in a multitude of ways, by phone, fax, e-mail, RFQ form or Ask a Pro form) you will realize that our sales staff and engineers have the knowledge and resources to provide the drive system solutions you need. We will work one on one with you to make sure all your questions are anwsered and all application issues are addressed. Our goal is to give you the best products & services at the best price, it’s that simple.

18. What’s The Difference Between A Non-regenerative Dc Drive And A Regenerative Dc Drive?

The difference between these two types of DC drives is explained on this page, DC Drive Types.

19. What’s The Difference Between A Variable Frequency Drive (vfd) And A Variable Speed Drive (vsd)?

A variable frequency drive (VFD) refers to AC drives only and a variable speed drive (VSD) refers to either AC Drives or DC Drives.

VFD’s vary the speed of an AC motor by varying the frequency to the motor. VSD’s referring to DC motors vary the speed by varying the voltage to the motor.

20. What Is The Difference Between A Vsd And A Soft Start Starter?

The difference is the speed that the motor can run, when at full speed. With a soft start starter, the motor is reduced voltage started, and then when the motor is at full speed, or a timing circuit has timed out, a “running by-pass contactor” pulls in and the motor continues to run at full base speed. With a VSD (Variable Speed Drive) or VFD (Variable Frequency Drive) (these are the same thing, just different names for the same unit), The motor will soft start, and you can vary the speed of the motor, by varying the output frequency from the VSD or VFD.

So, if you don’t need to vary the speed of the motor, once the motor is up to speed, then the correct solution is a soft start starter for that motor. If the process requires the motor speed to vary at anytime, the correct solution is a VSD / VFD.

The cost for a soft start starter is much less than a VSD or VFD.

21. Can A Variable Frequency Drive Replace A Softstart?

Yes, a variable frequency drive can replace a soft start starter, since most drives give you a current limiting ramp to start and stop your motor. BUT, if you do not need to vary the production speed of your motor, then the soft start starter is still the correct product for the application. This is because the cost of the soft start starter is less than the cost of a variable frequency drive.

22. Are The Variable Frequency Drives Phase Sensitive?

No, variable frequency drives are not phase sensitive. You can adjust the direction of rotation of the motor, through the software of the drive.

23. Can A Variable Frequency Drive Be Used For A Fan Application, Blower Application Or A Pump Application?

This is one of the best applications for an AC drive. In most fan/pump applications, there is a need to vary the output of the fan/pump. This is very easy to achieve with an AC drive. Plus you get the benefit of reduced energy cost of running the motor!!!

Suitability of VFD for pumps:
VFD’s are well suited to pumping applications. Many are manufactured to include motor speed/torque profiles specifically intended to optimize pump performance. However, there are some issues which must be considered before deciding on the use of VFD’s:

1. Suitability of the motors – the motors must be rated for inverter-duty to withstand the higher electrical stresses drives can impose. Some older motors were not built to withstand these stresses and may fail prematurely.
2. Distance of motors from VFD’s – the VFD’s will induce voltage transients on the load connectors. If these connectors are too long (manufacturers often suggest keeping motor lead length to 50m or less), the voltage at the motor terminals may increase to several factors above nominal, which can also cause premature motor failure.
3. VFD installation location and environmental conditions – this will determine the type of enclosure and conditions under which the VFD’s will operate. VFD’s which must withstand severe moisture, temperature swings, or dust can fail prematurely and must be protected by suitably rated enclosures.

24. How Can A Variable Speed Drive Improve Our Production?

The drive can be “tuned” to the application to achieve maximum output of the equipment. Sometimes this is slightly under the base speed of the motor, and sometimes, it is over the base speed of the motor. 

25. What Is The Percentage Of Operating Cost Reduction After Installing A Vfd?

This is a very open-ended one which depends on many factors – load, incoming supply voltage, utility costs, and several others. A good place to start customizing the inquiry for your use would be at ABB’s web site; they have tools for rough calculations for energy savings using VFD’s.

26. Can I Run A 230v, 3ph Motor With 230v, 1ph Input?

Yes. Joliet Technologies is pleased to announce that we have the solution to run most 230V, 3Ph motors with only 230V, 1Ph input! Call for more information.

27. What Is The Best Drive For My Application?

This is not an easy question to answer. Before we can give you a comprehensive answer to this question we will need to know more about your application and specifications. Simply call (815) 725-9696, toll free (866) 492-9888, fax (815) 725-9393 or e-mail us with your application and specifications and we will recommend the drive that’s right for you.

28. Can I Convert My Dc Drive Application To An Ac Drive?

Yes, You can convert your DC drive system; to an AC drive system by replacing both the motor and the drive. Consideration must be given for the torque and speed range of the DC motor, when converting to AC. You will need to consider using a “Vector” AC drive for this conversion.

29. Will A Variable Frequency Drive Or Variable Speed Drive Save Me Money?

Yes, the AC drive can save you money on operating cost, when the load and motor speed can be varied. By reducing the speed of the motor, you will consume less energy, which will reduce your electric bill.

30. What Is The Cost Of A Vfd?

Cost varies widely based on manufacturer, voltage, ampacity, enclosure type, torque requirements, quantity of analog and digital inputs/outputs, and communications options.

31. Should I Replace My Old Analog Equipment With New Digital Equipment?

DEFINITELY!!! For so many reasons. . . difficult and costly to get spare parts, Hard to find Tech. Support / serviceability, no communications, little flexibility, etc.

32. Will Joliet Technologies Train Our Staff To Operate And Maintain Our Equipment?

We can develop a training program that is tailored to your companies’ needs and location. So, when it makes sense, we can train your personnel in our facility, or bring the classroom to your factory, for hands on training.

33. What Filters Are Used For The Vfd?

Line-side filters are sometimes installed to reduce EMI, although modern PWM drives are much less likely to generate significant noise.

On the load side, output chokes can be installed to reduce drive generated harmonics in the motor leads. Many drives have some output filtration already installed and harmonics are generally not a problem as long as wiring is correctly segregated, properly grounded, and the motor leads are not excessively long (typically less than 100 meters).

34. What Is The Best Drive Manufacturer For My Application?

Our sales engineers are knowledgeable of what products are available from well-known manufacturers. They will recommend the best drive for you based on your application and specifications. There maybe several products that will meet your specifications.

35. What Are The Maintenance Requirements Of A Vfd?

Under normal operating conditions, VFD’s generally require little maintenance once properly set up. Spare parts can be obtained readily, and some are replaceable by maintenance personnel. In other cases, qualified service technicians are required.

36. Is It Possible To Use The Same Soft Starter To Start Two Mv Induction Motors?

Yes, it is possible to start multiple motors from a single soft-start. Typically, the motors are started sequentially and are identically rated. In a typical MV application, additional contactor cabinets are connected and control-tied to the soft-start. Once the first motor comes up to speed, the soft-start “Switches” to control the next motor in the sequence.

In the case of only two motors, some soft-starts are manufactured with dual settings to permit starting of motors that have different starting characteristics, but actual motor output during ramp-up may not differ much, being limited by the inherent performance characteristics of the soft-start.

Multiple simultaneous starting of motors requires that the motors be identical and that the soft-start be sized for the total load and supplied with supplemental cooling.

37. What Is Exactly The Difference Between Ac And Dc Capacitor In Functionality And Manufacturing? Why A Dc Capacitor Is Required To Connect Across The Alternator Terminals?

In essence, a “capacitor is a capacitor”, although there are differences in design based on topology, type, and materials of manufacture. Sometimes capacitors will carry a dual rating, for AC and DC – the AC rating for a capacitor of the same capacitance is typically much lower than its DC rating (often at least 3-4x less since AC max voltage peak-to-peak is ~2.8x rms), to accommodate the significant heating internal to the cap caused by frequency/alternating polarity. Some manufacturers manufacture AC caps which are essentially two DC caps in series back-to-back; these limit current flow through either cap but their internal resistance is somewhat high, which can lead to eventual break down due to heating as well.

Use of a DC cap across alternator terminals is often done to filter alternator whine (noise) caused by voltage ripple, the “AC” component of rectified DC output. When the capacitance is properly installed and matched to the frequency of the noise, it provides a low-impedance path to “divert” the voltage ripple to ground.

38. What Is The Problem If I Use A 15hp Vfd For A 1hp Induction Motor Speed Control? I Set Data Of The 1hp Motor In The 15hp Vfd, Is There Any Problem Using It?

The issue with a drastically over-sized drive is controllability, especially at lower speeds. Accuracy will suffer, because the drive thyristor firing angle will be so low that it will be difficult for the drive software to manage. And if you are concerned about motor protection you will have very little because the normal operating ranges of the motor protection parameters (overload, over-current, etc.) in a 15hp drive are centered on 15hp motor parameters; they are ranges, of course, but you’ll be way out on the low end of these ranges and response will suffer.