1Overview – VFD Motor Controller
The variable frequency drive (VFD) motor controller has been proven to be suitable for shipboard low voltage and medium voltage application. The variable speed electric propulsion is very popular due to the full range of speed and torque controls. The medium voltage variable speed electric propulsion can achieve high propeller power a with full range of speed controls.
The VFD motor controller produces electrical noise. The standards recommend a wide range total harmonic distortion (THD) with limits of from 5% to 8%. The author recommends the shipboard power system THD should not be more than 5%. This 5% THD is allowed when all other design attributes are fully complied with.
There are situations when the THD can be up to 8%, but that distribution system must be isolated from the other system so that the electric noise is managed within the dedicated system.
The 5% harmonic contents can create a dangerous situation affecting other low voltage equipment and control circuits. The design engineers must understand the THD limits for the specific design and should consider these as a system level harmonic management approach.
It is highly recommended that one manufacturer VFD be used and the manufacturer guidelines for harmonic management, VFD cable selection and VFD cable termination should be followed. Otherwise the coordination with multiple VFD suppliers can be challenging.
The following requirements will be discussed as to the use of VFD:
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