Benefits of IGBT Switching Speed in Elevators

PWM VFD operations need great switching speed which can be attained by using IGBTs (insulated gate bipolar transistor). Switching on and off several thousand times a second is one of the main attributes of IGBTs. A VFD IGBT can turn on in less than 400 nanoseconds and off in about 500 nanoseconds. It is composed of a gate, collector and an emitter. When a positive voltage (typically +15 VDC) is applied to the gate the IGBT will turn on. This is same to closing a switch. Current will flow between the collector and emitter. A VFD IGBT is turned off by removing the positive voltage from the gate. During the off state the IGBT gate voltage is generally held at a small negative voltage (-15 VDC) to restrain the device from turning on. IGBTs are used as power devices by all recent VFDs. These devices make it possible to reduce annoying audible noise by using switching frequencies beyond the audible range. Unfortunately, VFDs using IGBTs, present a high potential for generating RFI - Radio Frequency Interference. Fast switching in these devices generates sharp-edged waveforms with high frequency components that generate more RFI. The most probable complaint is interference with AM band radios 500-1600 kHz. However, sensitive computers, medical equipment and other interference-sensitive devices sharing the same power buss could experience significant interference. In extreme conditions, the VFD itself can experience electrical noise interference. If elevator machine room equipment is not properly laid out and correctly wired, the electrical noise propagated by the elevator VFD system can intervene with the elevator controller. The switching speed, simple control and overload withstand of the IGBT currently make it a component of considerable interest.