VFD Interview Questions – Comprehensive Brochure for Engineers

1). What is a VFD, and what does it do in an electrical system?

A motor controller known as a variable frequency drive (VFD), often referred to as an adjustable-frequency drive (AFD), an adjustable-speed drive (ASD), or an AC drive, controls the frequency and voltage of an electric motor’s power source. The VFD may also regulate the motor’s ramp-up and ramp-down during start and stop, respectively.

VFDs are frequently put in electrical systems connecting the power source & the motor. The incoming AC power is converted by the VFD to DC power, which is then converted back to AC power at the desired frequency & voltage. The motor then rotates at the desired speed as a result of the VFD feeding it modified AC electricity.

A wide range of electrical systems can benefit from the flexibility and strength of VFDs by operating more effectively and reliably.

2). Describe the fundamental working principle of a VFD

A variable frequency drive (VFD) is a type of a motor controller that controls the frequency and voltage supplied to an electric motor. The variable-frequency drive’s operation is primarily determined by its changing speeds & soft start or soft stop features.

3). What are the primary elements of a VFD circuit and what do they do?

A VFD circuit’s main components are:

• Rectifier: A device that convert AC input voltage to DC voltage.
• DC bus: A capacitor bank which stores the rectifier’s direct current voltage.
• Inverter: A device that converts DC voltage to alternating current voltage.
• PWM controller: This device regulates the frequency & duty cycle of the output voltage.
• Filter: Removes high-frequency noise from output voltage.

4). In motor control applications, what are the benefits of using VFDs?

VFDs provide numerous benefits in the motor control applications, such as:

• Energy efficiency
• Enhanced process management
• Motor deterioration is diminished
• Longer motor life
• Reduced vibration and commotion (vibration)
• Improved security

VFDs are a versatile and potent instrument that can be used to enhance the performance, efficiency, & dependability of a broad range of electrical systems.

5). Explain the distinction between vector control  &  V/f (Volts per Hertz) for VFDs.

VFD Vector Control	VFD V/f Control
VFD vector control is an even more advanced motor control approach than V/f control. It uses input from the motor to more precisely manage the speed and torque. Vector control is frequently employed in high-performance, high-accuracy applications such as machine tools, robotics, & food and beverage processing.	VFD V/f control is a less complicated and less costly way of motor control. It maintains a steady voltage-to-frequency ratio, which maintains the magnetic flux in the electric motor constant. V/f control is frequently employed in low-performance applications such as fans & pumps.

6). What variables should be taken into consideration while choosing a VFD for a certain application?

The following criteria should be considered while considering a VFD for a certain application:

The form of motor being controlled must be appropriate with the VFD.

• Motor power rating: The VFD’s power rating must be equal to (or) greater than its motor’s power rating.
• Speed range: The VFD should be able to give the motor with the appropriate speed range.
• Torque requirements: The VFD should be able deliver the motor with the necessary torque.
• Duty cycle: The VFD should be able to manage the motor’s duty cycle.
• Environmental conditions: The VFD must be able to function in the environment in which it shall be installed.

7). How can a variable frequency drive (VFD) help in energy savings & motor protection?

VFD contribute to energy savings & motor protection by:

Motor speed matching to load necessities: This may reduce as much as 70% of energy consumption, particularly in applications wherein the load varies often, such as fans & pumps.

Reduced starting current: VFDs can lower starting current by as much as 80%, which can lengthen motor life and reduce electrical system wear and tear.

Overload protection, voltage spike protection, & other electrical problems: This may be able to prevent the motor from burning out, saving time & money on repairs & replacements.

8). What is meant by motor derating, & why is it essential when using a VFD?

Motor derating is the process of lowering an electric motor’s rated power output. When utilizing a VFD, it is critical to derate the motor since the VFD may cause the motor to function at higher temperatures compared to when it was powered straight from the line.

A number of factors can influence the quantity of derating required, such as:

• The motor types
• The type of VFD
• The outside temperature
• The motor’s duty cycle

Some of the factors explaining why motor derating is essential when utilizing a VFD include:

• To avoid overheating the motor
• To lengthen the life of the motor
• To increase the motor’s dependability
• To lessen the possibility of motor failure

9). Describe the most typical difficulties (or) issues that may arise with VFDs and how would solve them.

The following are examples of common problems or concerns that might occur with VFDs:

Overheating: If a VFD is not adequately vented or is operated at an excessive load, it might overheat.

Ground faults: Ground failures can arise if the VFD is not correctly grounded (or) if there is an issue with the wiring.

Noise and vibration: If a VFD is not properly placed or if there is an issue with the motor or the VFD itself, it can produce noise and vibration.

Tripping: If a VFD detects  a ground fault, an overload, or similar problem, it will trip.

Failure to start: VFDs can fail to start if the power supply, wiring, (or) the VFD itself is faulty.

To troubleshoot VFD issues, perform these steps:

• Inspect the power supply
• Test the wiring
• Examine the VFD display
• Analyse the motor
• Check out the VFD cooling system

10). What causes harmonics in the VFD systems, & how can they be prevented?

The non-linear switching operation of the VFD results in harmonics in VFD systems. The VFD generates a non-sinusoidal current waveform when it turns on and off. The harmonics in this non-sinusoidal waveform are multiples of fundamental frequency.

In VFD systems, harmonics can lead to a number of issues, such as:

• Transformers, wires, and motors that are too hot
• Protection relays & other instrumentation failure
• Increased vibration and noise
• Decrease in power quality

In VFD systems, harmonics can be avoided in a number of methods, including:

• Using VFDs with integrated harmonic filters
• Putting in harmonic filters at the VFD output or input
• By employing a VFD with a greater pulse count
• Lowering the frequency of VFD switching

11). What safety measures have to be implemented when handling high-voltage electrical systems and VFDs?

Safety measures to follow when using high-voltage electrical systems and VFDs:

• Always turn off the system before making any repairs.
• Wear the appropriate PPE (personal protective equipment).
• Know the risks associated with using high voltage electricity.
• If not certified to work on high-voltage electrical systems, do not work.

12). How PID control apply to VFD applications?

PID control is a way of control that is utilized in a broad number of applications, one of which is the control of variable frequency drives (VFDs). PID control is a system that is both effective and versatile. A high level of accuracy in controlling of a motor’s speed and torque is attainable with the application of PID control.

13). What does VFD regenerative braking perform, and where is it effective?

A rotating motor’s kinetic energy is transformed into electrical energy during regenerative braking in VFDs, which then transfers that energy back to the power source. To achieve this, the motor is operated as a generator by the VFD.

Regenerative braking is helpful for applications where the load is constantly changing or where the engine needs to be quickly slowed down. This is due to the potential for regenerative braking to lessen energy loss.

The following are some typical uses for regenerative braking:

• Cranes
• Hoists
• Elevators
• Machine tools
• Electric automobiles

14). How should the cable size & length be chosen to connect a motor to a VFD?

The following criteria need to be taken into consideration when choosing the right cable size & length to connect a motor to a VFD:

• Motor current rating: The cable size needs to be capable of transporting the motor current rating despite overheating.
• Cable length: The voltage drop is influenced by the cable length. The voltage drops increases with the length of the cable.
• Temperature in the surrounding area: This factor has an impact on the cable’s ability to carry current. The cable’s capacity to carry current decreases with increasing ambient temperature.

15). How are variable frequency drives and soft starters distinctive from one another?

Both variable frequency drives (VFDs) &soft starters are devices used to regulate the speed & torque of electric motors. But there are some significant distinctions between the two equipments.

When using a VFD, the AC power source is first converted to DC power, which is then inverted back to AC power at the specified frequency and voltage. This enables the VFD to modify the power supply’s frequency and voltage in order to manage the motor’s speed and torque.

Soft starters function by gently raising the motor’s voltage over time. This helps in lowering the motor’s starting current and preventing overheating.

16). What does a VFD system’s braking resistor do, and when is it required?

In a VFD system, the braking resistor’s function is to dissipate energy created by the motor when it slows down. This energy is produced when the motor slows down or stops, and if it is not dissipated, it could lead to the VFD overheating.

A resistor attached to the VFD outputs serves as the brake resistor. The energy produced by the motor is lost in the braking resistor as temperature when VFD brakes the motor.

In VFD systems when the load is constantly changing or when the motor needs to decelerate quickly, braking resistors are required. This is due to the fact that not all applications are appropriate for regenerative braking, that is the recommended kind of braking in the VFD systems.

17). How should a VFD be set up and programmed for a particular application?

Collecting VFD & motor nameplate data:

This information serves as essential for VFD configuration. VFD and motor nameplate data includes voltage, power, speed, frequency, current, & service factor. This information is needed to configure the VFD to drive the motor safely.

Proper VFD parameter selection:

The VFD parameters govern its operation. This contains motor type, characteristics, VFD control mode, & protective settings. The motor type parameter instructs the VFD what motor to drive. Motor types require various control strategies;therefore, this is crucial. Motor parameters include voltage,  current, frequency, power, & speed. These settings must be adjusted correctly for the VFD to control the motor. Control mode dictates how VFD controls motor speed. VFD control modes include open-loop, closed-loop, and vector. The VFD protective settings safeguard the motor and VFD. These settings cover overvoltage, undervoltage, overcurrent, and thermal protection.

VFD programming:

After selecting VFD parameters, program the VFD. The VFD’s keypad (or) a computer & programming software are used for this. VFD programming software set up the VFD for application by entering its specifications.

18). Describe the concepts of VFD motor protection features such as overload and over-temperature protection?

VFDs include a number of motor protection features, such as overload and over-temperature protection. These characteristics are intended to protect motor from damage if a problem or abnormal condition occurs.

• Overload prevention keeps the motor from overheating. Overloading the motor might cause it to overheat & eventually burn out.
• Overheat protection keeps the motor from overheating. This is significant because overheating can harm the insulation & other components of the motor.

19). Explain the principle of motor slip and how a VFD addresses or affects motor slip?

Motor slip is the discrepancy between the rotor’s actual speed and the stator’s synchronous speed. The rotational magnetic field of the stator rotates at the synchronous speed. The rotor’s actual speed is not always as fast as its synchronous speed.

The load on the engine contributes to slip. The slide increases with increasing motor load. The layout of the engine also has an impact on slip. Different motors are made to have different levels of slip.

The following methods that a VFD addresses (or) influences motor slip is:

• It might speed up the motor.
• The motor speed can be managed using it.
• The motor starting (initial) current may be decreased.

20). What problems might VFDs cause with motors with low or variable load?

VFDs can cause numerous problems with low- or variable-load motors, including:

• Motor overheating
• Motor insulation failure
• Motor vibration and noise
• Reduced motor performance.

21). What makes a VFD reduce mechanical stress on a motor & other system component?

In a number of ways, a VFD minimizes mechanical stress on a motor & other system component:

• Enhanced setting up and shutdown
• Decreased speed fluctuations
• Vibration reduction
• Noise reduction

22). Discuss the function of line & load reactors in the VFD systems.

At the power supply & the VFD, line reactors are mounted. They serve to reduce input current harmonic distortion & protecting the VFD from voltage spikes & transients.

At the VFD & the motor, load reactors are mounted. They serve to reduce output current harmonic distortion & protecting the motor from voltage spikes & transients. They are also helpful in improving motor performance and lowering noise & vibration levels.

23). what does “harmonic distortion” mean with regard to VFDs and how is it minimized?

When there is harmonic distortion, the AC voltage (or) current waveform is not sinusoidal. Instead, it has extra frequencies that are multiples of the basic frequency and are referred to as harmonics.

The switching operation of the VFD results in harmonic distortion. Sharp edges with harmonics are produced on the waveform when the VFD turns on and off.

In VFD systems, harmonic distortion can be reduced in a number of ways, including:

• Line & load reactors are used.
• Utilizing a low switching frequency VFD
• Utilizing a VFD with a harmonic filter built in
• Use a multiple-pulse VFD.

24). Describe the ramp-up and ramp-down processes for a VFD application.

Two essential features of variable frequency drives (VFDs) are ramp-up and ramp-down.

• Ramp-up – process of starting a motor at a low speed and progressively increasing the speed.
• Ramp-down – process of progressively reducing a motor’s speed until it comes to an end.

In order to prevent damage to the motor & other system components, ramp-up and ramp-down are both crucial. VFDs assist in lowering the starting current and torque, which can lead to overheating and mechanical stress by progressively increasing & decreasing the speed of the motor.

25). What function do braking choppers and resistors perform in VFDs?

In VFDs, braking choppers & resistors are utilized to dissipate the additional energy that is created by the motor as it decelerates. This energy is produced when the motor slows or stops, and if it is not dissipated, it may cause the VFD to overheat.

• Braking choppers are the electronic switches that redirect extra energy to a braking resistor. The electrical energy is subsequently converted into heat by the brake resistor.
• Braking resistors are often constructed of metal or ceramic and are intended to endure extreme temperatures.

Braking choppers & resistors are commonly employed in VFD applications when the motor must be swiftly decelerated or where the load changes regularly. This is due to the fact that regenerative braking, the preferred technique of braking in the VFD systems, is not appropriate for all applications.