YORK YK — VSD - Precharge - DC bus voltage imbalance

The panel reads VSD - Precharge - DC bus voltage imbalance. The manual lists this text in its cycling fault messages for the variable speed compressor drive and applies it to all model drives. The manual also prints the fault on its medium voltage drive list with a power cell reference appended — VSD - Precharge - DC bus voltage imbalance - XY — where X names the phase the power cell sits in (A, B or C); Y says whether that cell is the upper (U) or the lower (L) one within its phase.

This is a cycling shutdown raised by the variable speed drive because the DC bus voltage imbalance condition appeared while the drive was still in its precharge period. In the manual's words, This message indicates the same as the previous VSD DC bus voltage imbalance message, except the condition occurred during the pre-charge period. The imbalance is checked on all phases: throughout precharge, the gap between each phase's positive and negative bus voltage has to stay inside a band running from 0 VDC up to 248 VDC, which the manual gives as 15% of 1,650 VDC. If the gap stays outside that band for 3 ms without interruption, the drive responds with this cycling shutdown.

• While the drive was in its precharge period, the gap between the positive and the negative bus voltage on a phase grew past the permitted window and stayed outside it for a continuous 3 ms.
• The manual treats the underlying condition as the same one behind the VSD DC bus voltage imbalance message — the only difference is that it was caught during precharge, and the manual applies the condition to all model drives.

• Bus balance during precharge: the size of the gap between each phase's positive and negative bus voltage must stay between 0 and 248 VDC — the manual expresses this window as 15% of 1,650 VDC.
• Trip condition: the gap has to stay outside that window for a continuous 3 ms before the drive brings the unit down on this cycling shutdown.