1. What is VVVF Drive?

VVVF Drive stands for variable voltage variable frequency drives. It is an electronic device used to control the torque and speed of the squirell cage motor by varying the voltage and frequency.

2. Why do we use VVVF Drive in cranes?

VVVF drives have various advantages like

• - stepless speeds - you can achieve any speed from 5% to 100%
• - avoid load swings - normally when cranes are operated the hanged load tends to swing, giving a threat of accident also controlling the same is difficult then apart from wastage in time. With VVVF Drive you can achieve reduced load swings
• Motor protection inbuilt -- all the motor protections are taken care of like overload, over current, single phasing, reverse operations etc
• Energy Efficiency : it helps in reducing the energy losses.

For more details refer our blog article on https://www.revacranes.com/advantages-stepless-speed-control-using-vvvf-drives/

3. What is DBR ?

DBR stands for dynamic braking resistance

4. What is the Role of DBR ?

Braking resistor is a must to use with a drive. When the brakes are applied, the drive provides braking torque to the stop the motor, and the motor in this case becomes a generator and produces energy. So a DBR of calculated value is used to absorb the backflow of energy from the motor during braking

5. What is closed loop circuit in vvvf drive

Closed loop is when an encoder is attached to the vvvf drive to give feedback to the drive with respect to the position of the motor. This feature is required in case there are precise predefined movements like in case of an elevator. In case of elevator the lift should stop at a predefined floors. In crane industry generally open loop is preferred, as the precise accurate movements are not required

6. VVVF drive does not function properly due to variation of incoming voltage, DC Bus voltage

VVVF Drives are designed to work with I/P power supply voltage variation from 380V to 480V without any effect in output voltage and frequency. Voltage fluctuations of (-15% to +10%) of rated Mains Voltage is acceptable.

7. An instantaneous voltage spike in the incoming AC supply may cause a high Bus fault and shutting off the insulated Gate Bipolar Transistors (IGBTs) of VVVF drives.

VFD has a built-in MOV (Metal Oxide Varistor) for protection against voltage spikes. Besides new generation VVVF Drives offer protection from OverVoltage,Under Voltage and Single Phasing.In a circuit without VVVF drive one has to put extra hard wares to facilitate these protections.

8. Can VVVF Drive work in ambient temperature is too high in peak summer and humidity varies from 70% to 100%, the excessive heat and dampness leads to premature VVVF drive failure.

Drive works up to 50° ambient temperature without any difficulty . We can use upto 60° with derating at 2% per degree rise in temperature. Drive works with 95% Relative humidity.The electronic PCB’s have a conformal coating to protect it from humidity and chemicals.

9. A large no. of welding plants are used in the shops, it causes fluctuation and imbalancing of supply system to the VVVF drive.

Reply: -15% to +10% in voltage fluctuations to the Drive Input is sustainable without any trouble.However in case fluctuations are severe than this, a 3 Phase Reactor at the input of Drive shall be required.Proper earthing also helps to nullify the effect.

10. Vibration and heat cycles are typical causes of loose connection and can lead to dangerous arcing, Arcing at the output can damage other components and cause over current faults in the VVVF drives.

Drives are manufactured and tested to sustain low intensity of vibrations. For high intensity , vibrations pads are provided in the panel to absorb vibrations. There is least possibility of arching at the O/P of drive as all devices are static wheras in conventional control there is a possibility of arching as Slip Ring Motor Control uses Carbon Brushes and Copper Rings. Further Drive generate Over Current Alarm in case of any eventuality of over current and protect Motor. This is a added advantage of drive.

11. The problem of supply voltage leads to damage to the capacitors. The leaking capacitors may lead to corrosion &/or damage traces on PCBs.

The capacitors of VVVF Drive are designed to sustain high voltage variation.Two capacitors of 450 V is put in series and thus Capacitor Bank is made.Rare possibility of failure due to supply voltage variation within limits.Further Drive has inbuilt OV/UV alarm on display and gives Relay O/P when problem in input supply voltage goes beyond 460 volts. We can use this Relay O/P

12. VVVF Drives maintenance is a problem and drives burns very quickly and the cost is too high for replacements

VVVF Drive is an electronic device and if not used carefully can lead to failures, we have been using drives in cranes since 1995 and there are drives which are working since last 30 years also without any problems. The main reasons for VVVF Drive failure are

• Braking resistance is not correclty selected or is not performing as required : incorrect value of the braking resistance may cause failures
• Ignoring the alarms and warnings : many times operators ignore the warnings and alarms and reset the drive to run the cranes, this ultimately causes failure of the drive
• Excessive Moisture : if there is excessive moisture in the panels it can cause failure of the drives, use space heaters in the panels
• Heating : proper cooling and ventilation is required for VVVF Drive, check the exhaust fan of the panel is clean and working properly.
• Electromagnetic Interference : there are lot of electromagnetic interferences in the plant which can lead to VVVF Drive failure, please check line filter is installed.

13. Can VVVF Drives be retrofitted in OLD Crane systems

Yes VVVF Drive can be fitted in OLD Crane systems and it is highly recommended to do install the same. Motors might also have to changed in some cases. But it is better to retrofit a new VVVF Drive system in the OLD Cranes and increase the life of the crane   For the advantages of VVVF Drives please refer https://www.revacranes.com/advantages-stepless-speed-control-using-vvvf-drives/