What options do you have for starting a large motor? Which is most cost effective? How can you maximize reliability? These are just some of the questions Five Star can help answer.
Recently a client came to us with a desire to replace a 15 year old VFD that was becoming a reliability issue. The application was very critical as this motor had to run or their process was down. Replacing the VFD was a no-brainer but what else could they do? Five Star evaluated an Across the Line bypass and a Reduced Voltage Soft Starter. The ATL option was quickly ruled out as the transformer feeding the motor just did not have enough capacity to start the motor directly. The team was leaning towards a RVSS in parallel with the VFD. Five Star used our proprietary motor starting analysis tool and revealed that the transformer could not even supply enough current to start the motor with a RVSS. The only way to start this motor was with a VFD.
There are ways to make a VFD more reliable. In the example cited, automatic cell bypass allowed the drive to continue to operate with one or even two cells bypassed. This automatic cell bypass is unique to the Siemens VFDs which Five Star provides. With proper engineering, a VFD with a bypassed cell can still make full voltage and provide the current necessary to operate a motor at full speed. Alternatively it may be acceptable to operate the motor at a reduced maximum speed when a cell is bypassed which can save a little on the upfront cost of the VFD. It all depends on the application and at Five Star, we pride ourselves in being able to consult on your application and recommend an optimal solution.
Another client had a number of motors which needed to be started on a relatively weak utility system. Variable speed was not required although the application of VFDs would start the motors without difficulty. After some analysis, Five Star recommended that Static VAR Compensation be considered as an alternative to VFDs. SVC is essentially very high speed PF correction that maintains the power factor as close to unity as possible throughout motor starting or other operations which impact power factor. T-Star is the SVC manufacturer that Five Star works with. T-Star’s microprocessor PF controller measures PF in real time and makes corrections 30 times per second. This client was able to start all of his motors with a combination of across the line starts and soft starts coupled with the SVC solution. The installed cost was considerably lower than applying VFDs to each motor and had the added benefit of reducing utility cost.
As the above examples show, Five Star Electric can assist our clients during conceptual design through implementation. We also have our own staff of field service technicians who help our customers maintain their systems. Please call us anytime we can be a resource to you.