Diodes or SCRs? It's Either Or

While reading Cory Lamerande’s article on harmonic distortion (March issue), I found a contradiction. The text under Figure 5, on page 74, (reproduced above) states: “Assume the diodes are SCRs, and you have a circuit found in most AC drives.” On page 76, the second paragraph begins: “In most AC drives, the input rectifier uses a diode bridge.” Lemerande can’t have it both ways. Most of today’s PWM

EC&M Staff

Oct. 1, 1998

While reading Cory Lamerande’s article on harmonic distortion (March issue), I found a contradiction.

The text under Figure 5, on page 74, (reproduced above) states: “Assume the diodes are SCRs, and you have a circuit found in most AC drives.”

On page 76, the second paragraph begins: “In most AC drives, the input rectifier uses a diode bridge.” Lemerande can’t have it both ways. Most of today’s PWM drives (up to 300 hp) use a diode bridge. The higher hp VFDs use SCRs on the front end.

Philip Radin, P.E.
Bergen County Utilities Authority
Little Ferry, N.J.