September 2019

51 www.drivesncontrols.com September 2019 operation where the load is increasing or swinging rapidly. Connecting additional line reactors at the input to the drive only increases source impedance and should be avoided. Due to the high capital cost of standby generators, you need to consider the capacity required to support the total VSD load carefully before ordering because mistakes can prove very costly. Best practice is to oversize the generator capacity by three or four times to reduce its percentage source impedance to a point where the drives would be expected to operate without problems. For some applications, it may be necessary to oversize the generator by more than three times, and the required capacity will increase further if other kinds of non-linear loads are connected in addition to the variable-speed drives. If the generator capacity cannot be increased to the size required to support a given VSD load, a 1:1 ratio isolation transformer with phase-shifted secondary windings connected to the generator output could be considered. Typically, such a transformer would have two secondary windings, one connected in star and the other in delta, giving a 30-degree electrical phase shift. The installed VSDs would also need to support the connection of a second bridge rectifier which would then connect to the delta winding of the transformer, while the other rectifier is connected to the star winding. The newly configured VSD then becomes a 12-pulse VSD with significantly reduced peak currents. However, this option would normally be a last resort because the cost of the transformer and its installation could be far higher than the cost of upsizing the standby generator. In all cases, the generator manufacturer and the drives supplier – along with the suppliers of any other non-linear loads with significant input current – should be consulted before specifying the generator capacity. n A practical example Consider the example of a standby generator that is required to supply an installation in the event of mains power failure. The loads connected to it are: 2 x 15kW, 400V, 3-phase centrifugal pumps, operated by two six-pulse VSDs 22 x 1.1kW, 400V, 3-phase fans, operated in parallel by four six-pulse VSDs The total VSD loading is: (2 x 23kVA) + (4 x 9kVA) = 82kVA (The actual VSD capacity can be obtained from the supplier or manufacturer.) The minimum generator capacity to support the VSD load would be: 82 x 3 = 246kVA Various other loads, such as lighting and heating could add 4kVA. The minimum generator capacity required to support the total site load would therefore be: 246 + 4 = 250kVA smart plastics igus ® Caswell Road Northampton NN4 7PW Tel 01604 677240 Fax 01604 677242 sales@igus.co.uk plastics for longer life ® Industry 4.0: smart plastics eliminate downtimes Intelligent products give a maintenance alert ahead of time and can be integrated into standard predictive maintenance; smart plas- tics allow your equipment to run continuously as well as reducing maintenance costs. igus.co.uk/smart Avoid unplanned downtime