Installation considerations for IGBT AC drives

In the last four years, adjustable speed AC drive (ASD) manufacturers have migrated from bipolar junction transistor (BJT) semiconductors to insulated gate bipolar transistors (IGBTs) as the preferred output switching device. The advantage of IGBTs over BJTs is that device rise and fall time switching capability is 5-10 times faster, resulting in lower device switching loss and a more efficient drive. However, for a similar motor cable length as the BJT drive, the faster output voltage risetime of the IGBT drive may increase the dielectric voltage stress on the motor and cable due to a phenomenon called reflected wave. Faster output dv/dt transitions of IGBT drives also increases the possibility for phenomenon such as increased common mode (CM) electrical noise. Electromagnetic interference (EMI) problems and increased capacitive cable charging current problems. Also, experience suggests any pulse width modulated (PWM) drive with a steep fronted output voltage wave form may increase motor shaft voltage and lead to a bearing current phenomenon known as fluting. This paper provides a basic understanding of these issues, as well as solutions, to insure a successful drive system installation.