Variable Frequency Drive – what is VFD?
Variable Frequency Drives (VFD) are not exactly a soft start. They obtain this function, but they have a much bigger scope of work. The basic principle of use VFD is to control torque during the operation of the machine, by a change of frequency. The main difference compares to soft-start, the frequency can be changed in time with full regulation. A good example onboard is the seawater cooling pump, which has to operate constantly. Diesel generators need cooling all the time, even when the vessel stays in port. Depends on the sailing area different cooling inapplicable. For this reason, the perfect solution for these pumps is the use of VFD. The pump can operate constantly with different capacities of cooling, without switching on/off pumps.
Variable Frequency Drives in practical examples
Another very popular example where can be found Variable Frequency Drives onboard is the reefer container. VFD starts to be very popular in this kind of equipment because of Maersk. Starcool reefer containers, designed by the Maersk sub-company are equipped with VFD. Reefers operate constantly during long sea passage. Compressors don’t need to start and stop as it happens in other types of reefers. To be accurate, Daikin also starts to produce reefer with VFD, however, Maersk was first.
What is VFD with example is described above, of course, there are much more examples where VFD can be used onboard, like cranes, pumps, etc. But more important is how they work. So VFD is built by 3 main parts. 1st part is a rectifier.
The three phases enter the VFD and connect to the rectifier. The rectifier consists of multiple diodes connected in parallel. Diodes allow electricity to flow only in one direction. 2nd part consists DC link. This is capacitor is responsible for filter ripples from voltage. 3rd part is an inverter, build with 6 sets of IGBT and diode. (which work as electronic switches with controllable direction and frequency of output.)
How are build Variable Frequency Drives
Hearth of VFD controllers are IGBT’s, they are insulated-gate bipolar transistors and can be replaced by Power MOSFET. Power MOSFET will be used for voltage rating below 1kV, to all circuits above will be used IGBT. GBT or MOSFET can be switched to an upper and lower status. To turn on the IGBT we have to provide a few Volts to a gate. After get the voltage IGBT conducts in the direction of the arrow. In a controlled sequence, we start and stop IGBT’s to prevent a short circuit to the DC link. By proper operation of transistors, the three-phase current is built in the motor. By operation of transistors, we control current. This on and off time is called PWM ( Pulse Width Modulation). Switching on and off looks like on the graphic below. Switch of IGBT’s several thousand per second, (in from 4 to 16kHz) will build nice current. But what happens during the change of current? Here is coming the diode build in parallel to IGBT. The function of diodes is to provide a current path by switching on. This is the way they are named commutation diodes.
For an easy explanation for people not related to electronics, You can say that diode is working as a check valve, capacitor like a filter, and MOSFET like a valve.
Conclusion about Variable Frequency Drives Onboard
What are the biggest benefits of Variable Frequency Drives? Most important is reduced energy consumption. Reduced energy means also reduced cost of operation. Shipowners like very much every possibility to reduce fuel consumption. Increased control level, basically full control of RPM will extend equipment lifetime. The cost of maintenance e-motors and devices connected to them will go down. Smooth control means no hammering shock during start and stop.
What are the biggest disadvantages? VFD’s are very accurate and complicated devices. They are expensive, so costs and advantages have to be balanced before the decision. In case of any issue with VFD, will be difficult to repair it on board, most probably will be needed a professional service for it.