All-in-one MV drives use LV components to cut costs
At the recent SPS IPC Drives show in Germany, WEG unveiled a new range of medium-voltage drives which it plans to sell as complete systems integrated into cabinets, along with isolators, fuses, and multi-level feed transformers. This means that purchasers of the MVW3000 drives will not need to buy additional MV switchgear.
The new range spans voltages from 2.3–8kV and power ratings from 280–2,400kW. It uses a multi-level technology based on the series connection of three to ten low-voltage (690V) power modules with IGBT output inverters in an H-bridge configuration, depending on the output voltage. This arrangement makes it possible to achieve medium-voltage levels using low-voltage components – such as diodes, IGBTs and capacitors – thus cutting costs.
The mains power factor is above 0.95 over the entire motor speed range, without needing any extra harmonic filters or compensation capacitors. Efficiency, including the transformer, exceeds 95% over the entire speed range and is above 96% for load levels above 40%. The charging circuit for the multi-level power transformer ensures transformer core magnetisation without inrush currents and gentle charging of the DC link capacitors for the inverter stage. The almost sinusoidal output voltage and current reduce power dissipation, vibrations and torque pulsation in the motor.
Mains harmonic distortion figures for current and voltage, in accordance with IEEE 519, IEC 61800-3 and G5/4-1, are claimed to be “outstanding”. The drives comply with the limits in these standards, even in their basic configuration.
The power transformer allows the mains voltage to be adapted to the motor output voltage, and the common-mode voltage on the motor winding to be reduced. It also reduces common-mode currents through the motor bearings to extend bearing lives. The interfaces between the inverter CPU and the power stage for IGBT control, temperature monitoring, voltage feedback and current feedback, are implemented using fibre optics to improve noise immunity and to provide effective isolation between the control and power sections.
The drives protect motors against overloads, overheating and rotor blockage. The temperatures of the power stage and transformer are also monitored constantly. The drives can optionally support industrial communication protocols including Modbus, Profibus, Devicenet and Ethernet.
Johannes Schwenger, WEG’s head of product management of LV and MV drive systems in Europe, says that “the present voltage and power range is only the first stage in the product evolution. Higher voltages and power levels are already available upon request.”