A Variable Frequency Drive (VFD) is a motor control device used to achieve one simple major goal: controlling the speed and torque of AC induction motors. Apart from this there are a number of secondary benefits of the unit: protection of the motor (especially by archiving a soft start/stop of the motor) and the possibility of remote monitor/remote control. Other names used for the same device are: Adjustable Speed Drive (ASD), Adjustable Frequency Drive, Frequency Inverter and Variable Speed Drives (VSD).
The Variable Frequency Drive contains the following three main units:
Control unit – the brain of the entire drive, responsible of the entire unit logic, from collecting the input control data (usually set by the operator via a keypad) to the generation of the PWM signals (Pulse Width Modulation) required for the Main Circuit units.
Power Unit – this unit is just a translator between the low power Control Unit and the high power Main Circuit. On small power Variable Frequency Drive this unit is usually merged with the Control Unit.
Main Circuit – the high power part of the Variable Frequency Drive. This contains three main parts:
Rectification module : conversion of AC input voltage (single phase or 3-phase) to DC via a Power Bridge
DC Bus – leveling of the DC voltage via large capacity electrolytic capacitors
Output Inverter – The muscle of the unit containing the IGBTs bricks (Insulated Gate Bipolar Transistors)
When a unit is not working, we strongly recommend that the diagnostic and repair process to be performed only by qualified technicians with experience in the field of electrical equipment. There are serious dangers if unqualified persons try to disassembly and troubleshoot the Variable Frequency Drive caused by the high potential power stored into the DC Bus even after the disconnecting the unit from the main power line.
Our team of highly qualified technicians has a large experience dealing with Variable Frequency Drive units, minimizing the time required for finding and repairing all the problems of the unit. The repair procedure usually contains the following steps:
Disconnecting the Variable Frequency Drive from the motor and checking the connections between them. Sometimes the connection between the motor and the drive are degrading in time (mainly because of the heat cycles and vibration) and could generate (together with other factors) problems in normal operation of the assembly;
Checking of Rectifier Diodes and IGBTs – in other words checking of the Main Circuit inputs and outputs which are the most common failure of the Variable Frequency Drive. These checks are mandatory before the power up of the faulty units to avoid the possibility of further damage of the unit and to minimize any potential risk of electric shock. Specialized meters are used for this check to determine not only if the parts are still running well, but also to determine if their electrical characteristics are still in the manufacturer specifications.
Last, but not least, if no problems are found until now the unit is power up and the Control Unit and Power Unit are checked to pinpoint the problem. Depending of the level of complexity of the Variable Frequency Drive this step could (and most of the time does) involve the need of a large variety of testing and measuring equipment.
When the root cause of the failure was found the customer is informed about the state of their unit and also receive all the information about the repair procedure and the cost estimation.
Whenever possible, the original drive parameters are saved and after the repair they are restored into the unit, so the customer could use the unit directly without the need of reprogramming.
We have gained our experience in repairing Variable Frequency Drive in time, and have seen a large variety of units with more and more horsepower during the time. A good indicator of the quality of our services is the warranty return of the repaired units which is almost negligible. Therefore we will issue a full warranty for all the repaired units for a period of 12 month.