In Industrial Automation, Variable Frequency Drive is one of the most used devices for controlling the motor. VFD is an electronic equipment which varies the speed of the motor by varying its frequency. VFD finds its use in many applications due to this. It would be interesting to know that there are many jobs available in the market which purely caters to knowledge of VFD.
So, many times, candidates face unexpected questions during interviews, which they face difficulties in answering. So, it is necessary to have a general overview of types of questions asked during interviews.
VFD Questions
In this post, we will see some general and unusual types of questions asked during interviews related to VFD.
Difference between a Soft starter and a VFD?
VFD can vary the frequency of a motor, but a soft starter cannot do it. Yes, they both have the same function of smoothly starting or stopping a motor by varying its acceleration and deceleration time, but a VFD only can vary the speed of the motor. A soft starter cannot do it.
What are the Loads used in VFD?
There are two types of loads used in VFD – constant torque and variable torque. In constant torque, the torque remains constant irrespective of the speed. Some general examples are drum-hoist, conveyors, and traction drives.
The torque mostly varies based on the load capacity, and not on speed. In variable torque, the torque varies based on speed. Some general examples are centrifugal pumps, blowers, and fans.
Here, when running at low speeds, the power required for load is also less and thus, the toque is also less. The same condition applies at high speeds which causes higher torque.
What type of motor is compatible with VFD?
 Motor manufacturers give a name called inverter rated in their nameplates. This means they are compatible with VFD. If you see names like CT (constant torque), VT (variable torque), PWM (pulse width modulation), then this means they can be run with a VFD.
Which is the main semiconductor responsible for the operation of VFD?
IGBT (insulated gate bipolar transistor) is the semiconductor used in VFD operation. This is because they have the highest switching speed among all semiconductors and it can be done multiple times at a great frequency, as compared to other ones like BJT, Triac, and MOSFET.
Can a VFD convert a single phase to three phases?
Yes. But remember that it cannot convert single-phase AC voltage to a voltage higher than that, which many will think when reading the question itself.
VFD converts it into a DC voltage through a single-phase rectifier circuit and then inverts the DC voltage into 3-phase AC supply through a three-phase inverter bridge. The motor will get its output accordingly, but it will be not a 440V motor. It is a low-voltage three-phase motor.
What are the main elements of a VFD?
The main elements of VFD working are – the rectifier, DC bus, inverter, PWM controller, and filter. First, the AC voltage is converted to DC voltage through a rectifier, which is stored in a capacitor bank or DC bus.
The inverter again converts DC voltage to AC voltage, and the output voltage is modulated by varying the pulse of the waveform through IGBT. The filter will remove high-frequency noise from the output voltage.
 What are the factors to consider while choosing a VFD?
Some general factors to be considered are motor power rating, torque requirements of the load, environmental factors and conditions, speed range required by the motor, and motor duty cycle.
What use is of motor derating in VFD?
Motor derating means choosing a motor at a lower rated power output than the VFD input power and this helps in avoiding motor overheating, increasing the motor’s reliability, reducing motor failures, and lengthening the life of the motor.
Is PID function in VFD?
Yes, the PID function is available in VFD. You can connect analog inputs to the VFD and tune PID parameters inside the VFD program, to vary the motor speed. This is the same that you do in a PLC, thus eliminating additional PID programming in a PLC.
Can a VFD control multiple motors?
A VFD can control multiple motors subject to the following three conditions – the motors must run at the same speed, failure signals should also be taken from multiple motors (this is because VFD is one and it is not possible to detect failure in which motor), and VFD must be upsized as compared to the motors (this means the HP rating of VFD must be higher than HP ratings of total motors combined).
Are communication protocols available in VFD?
Yes, common PLC communication protocols available for data exchange in VFD are Modbus RTU, Ethernet, Can Open, Profinet, and Profibus.
What is the encoder checking function in VFD?
Some motors have an encoder connected with it for monitoring actual speed. This encoder signal is taken into VFD and the VFD checks whether the motor is running at the desired speed by checking encoder counts. Many functions for encoders are available in VFD for properly controlling the motor.
Which of the following loads is used with VFD – resistive, capacitive, or inductive?
The inductive load is used with VFD, as motors require a certain amount of current to establish a magnetic field before they can start running, and VFD does this job by varying acceleration time. Also, the output waveform of VFD is not suitable for capacitive loads due to its high frequency.
What is DC injection braking in VFD?
DC injection applies DC current into the motor during the stop condition, which opposes the motor rotation and stops the motor quickly. But it must not run for a long time, as it can damage the motor windings after prolonged use.
What are the popular VFD brands in industries?
The most used names in the VFD market are
- Danfoss,
- ABB,
- Yaskawa,
- Schneider,
- Siemens,
- Rockwell,
- Fuji,
- Toshiba,
- Delta,
- Mitsubishi, etc.
In this way, we saw the generally asked questions during the interview for VFD.
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