You can explore the most commonly asked VFD interview questions, accompanied by detailed expert answers to help you grasp key concepts and effectively demonstrate your knowledge. Whether you’re a seasoned professional or new to the field, our resource is designed to support your success by covering essential topics, technical details, and practical insights.

what is braking system in vfd

braking system of vfd refers to the process of slowing down or stopping an electric motor quickly and efficiently. The motor continues to run after the VFD receives a stop command. The braking system in a VFD helps prevent damage to the VFD hardware components and avoids unnecessary system faults.

how many types of braking system in vfd

Dynamic Braking

Regenerative Braking

DC Injection Braking

* Mechanical Braking

Flux Braking

* Mechanical Braking VFD Not controlled directly.

What is dynamic braking or DBR?

Dynamic braking is a technique by which energy regenerated by a motor is dissipated as heat in braking resistors. when VFD received stop command power to the motor is cut off and the load torque is more than the motor torque, the motor acts as a generator and the generated energy is dissipated across the braking resistors. The resistors used for this purpose is known as dynamic braking resistors. Dynamic braking is commonly used in applications where rapid deceleration is required.

What is Regenerative Braking?

Regenerative braking allows the energy generated during deceleration to be fed back into the power supply rather than being dissipated as heat. This method is more energy-efficient than dynamic braking and is used in applications where energy conservation is a priority.

what is DC Injection Braking?

DC injection braking involves injecting a direct current (DC) into the motor windings after the VFD has stopped supplying AC power. The DC current creates a stationary magnetic field, which opposes the rotation of the motor, thus bringing it to a stop. This method is used for precise stopping and holding of the motor position.

what is Mechanical Braking?

Mechanical braking involves physically applying a brake to the motor shaft. While not directly controlled by the VFD, it is often used in conjunction with electronic braking methods to ensure a complete and reliable stop, particularly in safety-critical applications.

what is importance of braking in vfd?

In processes where quick stops are necessary, such as in conveyors, elevators, or cranes, braking is essential to bring the motor to a halt swiftly and safely. Dynamic or regenerative braking is often used in these cases.

In emergency situations where the motor must be stopped immediately to prevent accidents or equipment damage, braking is crucial. This is common in machinery that poses safety risks if not stopped quickly.

In applications that require precise positioning, such as CNC machines or robotics, braking is used to stop the motor accurately at a specific point. DC injection braking is often employed for this purpose.

Braking is also used to hold a motor in position after it has stopped, preventing any unwanted movement. This is critical in applications where maintaining a specific position is essential.

In situations where a load is being lowered, such as in hoists or elevators, braking is used to control the descent and prevent the load from accelerating due to gravity. This ensures smooth and controlled movement.

In applications involving high-inertia loads, like large fans, centrifuges, or flywheels, braking is necessary to bring the motor to a stop within a reasonable time frame. Without braking, these loads could take a long time to come to a stop naturally, leading to inefficiencies or safety hazards.

braking can be used to prevent the motor from exceeding safe operating speeds.

What is plugging Braking method?

Plugging is the braking method in which a reverse torque is generated by applying reverse voltage or phase sequence to the motor. Plugging is the fastest braking method since it drives the motor to reverse no matter what the running speed of the motor is.

can we use plugging braking method in vfd?

When a plugging is started; the controller changes the polarity of at least one phase. When this occurs the polarities of applied phase voltage and back EMF is opposite and the current is limited only by phase resistance and inductance. The phase current, only limited by internal impedance of the immediately rises at a huge rate and a very high reverse torque is generated. This high current both stresses the motor windings and switching elements of the controller. If the motor is running under highly inertial load, prolonging plugging may even lead to melt down of motor structure due to excessive heat generated by windings.

Plugging puts down all electrical stress to the motor itself, heating up the motor. As mentioned before, if braking period is long, high temperature rise may even damage to the motor. Therefore, the motor’s operating conditions should be closely monitored to apply a safe plugging.

which type of communication protocol are used in VFD

communication protocols enable VFDs to communicate PLC, HMI, SCADA systems, and other industrial devices, ensuring smooth and efficient control of motor speed and other parameters. Variable Frequency Drives (VFDs) used following protocol for communication

• Modbus RTU
• Modbus TCP/IP
• Profibus DP (Decentralized Peripherals)
• Profibus PA (Process Automation)
• EtherNet/IP:
• Profinet
• BACnet
• Powerlink
• CAN Open
• AS-Interface (Actuator-Sensor Interface)
• DeviceNet

it is possible to control multiple motor by using vfd

A variable frequency drive (VFD) can be used to control multiple motors in some applications provided the right design considerations are made and appropriate protection is provided for each motor. following Methods to Control Multiple Motors with a Single VFD:

• Parallel Operation
• Sequential Control
• Master-Slave Configuration

which type of protection are required to run multiple motor in single vfd?

if we use a single vfd for multiple motor than we must follow some guideline

• Add FLA of all the motors and add 20% (to accommodate lower leakage inductance) to select the VFD size
• In multiple motor applications, NEC 430, part III, requires individual motor overload protection, which is (thermal) overload function, on the load side of a VFD.
• all motors be identical or similar in rating and load type. Protection devices like fuses or circuit breakers should be used for each motor to protect against individual motor faults.
• Ensure proper cable sizing and type to handle the combined current and voltage drops.
• The VFD should be capable of handling the inrush current and continuous current demand of all motors.

How to choose cable size to connect vfd and motor

Proper cable sizing is essential to handle the combined current and to prevent voltage drops. to select cable for vfd we use following consideration

• Determine the Full Load Current (FLC): The Full Load Current (FLC) of the motor is typically provided on the motor's nameplate or in the motor datasheet.
• Derating Factor for VFD Operation :Cables connected to a VFD may need to be derated because of the harmonics and additional heating effects introduced by the VFD.A typical derating factor ranges from 1.1 to 1.25 depending on the quality of the VFD and cable insulation.
• the Length of the Cable: we know that Longer cable more voltage drop. Excessive voltage drop can reduce the performance of the motor and cause overheating.
• Check Environmental Conditions :Cables are rated for a specific temperature. Ensure that the cable insulation type (e.g., PVC, XLPE) is suitable for the operating voltage and environmental conditions.
• Select the Appropriate Cable Size :select the cable size from standard tables (such as those in the National Electrical Code (NEC) or other relevant standards) that meets the current rating and voltage drop criteria.

What is Dc Bus voltage in VFD?

dc bus voltage is 1.414 x the rms line voltage, for example a 480V ac drive, the dc bus should be ~678V dc. A dc voltage value that is too low can cause the drive to trip. At the cause, the mains input voltage is probably too low, or the input sine wave might be distorted by flat topping.