Grain Silo S.W.E.R Solution
Terry Knight Electrical was approached by a customer to facilitate a tricky solution regarding a grain silo application. The problem was a remote silo with a single wire earth return (S.W.E.R) 480V supply but with 415V equipment; in particular, 6 motors powering augers, sweeps and aerators.
S.W.E.R is effectively a single-phase supply. It has been faithfully serving rural Australia for many years now. It is used extensively with over 200,000kms of line run across the country. Compared to our traditional networks, it is significantly cheaper to roll out with some lengths as long as 300km.
For all of its benefits however, there are some limitations. As in this case, the challenge was to power up three-phase motors.
A successful solution has been to implement the use of variable speed drives (VSDs). Some three-phase drives can operate on single-phase supplies or S.W.E.R systems. AC drives work by rectifying the AC supply voltage to DC voltage. The DC voltage is then inverted to variable voltage, variable frequency to run the AC motor. A single-phase voltage once rectified has a much greater DC ripple than a three-phase rectified voltage. Therefore, for a three-phase drive to operate satisfactorily off a single-phase supply, it needs extra filtering; with chokes and capacitors to smooth out the higher ripple and reduce the harmonics. This is generally achieved by selecting a larger drive which is critical.
As an example, if we were to have an 11kw three phase 415V load, the drive would have to be at least twice this rating, potentially using up to a 22kw VSD. Not all drive suppliers are able to achieve this outcome, so knowing the various technical options can save contractors time and money.
After sounding out other suppliers Terry chose Middy’s Ballarat West team to achieve a speedy delivery, and with the support of Hank Terpstra from the Industrial team, both contractor and end customer were pleased with the final result.
This article was originally featured in our quarterly Middy's MAG #33 Mar - Jun ’21 Edition. Read Latest Issue