Oscilloscope Diognosis, Retrofit
About This Project

We attended site by request from the building manager on why newly retrofitted Soft Starter Units failed. The 3rd party installing electrician had installed this new technology on a previously standard motor starter system. The initial tracing of wiring led us to discover an additional 110 Vac Control Transformer located in a new Enclosure feeding the Chiller Control Panel. The Chiller manufacturer’s normal 110 Vac Control Transformer was still in circuit.
Looking at the oscilloscope traces of the outputs from each transformer showed that they were out of phase by 180 deg, giving a potential difference of 250 Vac RMS.
We decided to change the additional transformers primary tap-in’s to bring both control transformers secondary outputs in phase. This was successful but there was still a potential difference of around 11 Vac RMS between the outputs.
A decision was made to decommission the additional 110 Vac Control Supply in the new control panel and to tie into the standard 110 Vac Control Supply from the Chillers own Transformer. The failed Solid State Starter Trigger Boards were replaced. Our recommended modification proved successful over 10 consecutive starts.