[quote=", post:7, topic:14472"]
But when my PC runs on UPS, I hear motor cycle engine like noise coming from the speakers. In speakers, AC is also converted into DC. On my LCD monitor display, sometimes, blur lines appear when it is on UPS. So, why this happen? Modified sine wave current is the reason. I checked the frequency of Homage UPS current via digital multimeter, and it was showing 1.8 kHz value when UPS was providing current from backup batteries, and 50 Hz when it was on WAPDA. Any idea!
Well, your audio system must have a power supply (may be internal or external adapter). If this power supply is based on conventional transformer, then there will be HUM sound in the speakers if input mains in not sinze wave.
Your LCD monitor, though has a switched mode power supply, might mis-behave if
(1) this power supply has power factor correction
(2) if some part of incoming AC is tapped to derive a 50Hz signal ( or a time base for some circuits inside).
As regards the frequency shown on digital multimeter, 1.8KHz is a wrong figure and the reason is that low cost digital meters are calibrated / designed to measure frequency of sine wave only (this saves money on internal hardware of multimeter). A true RMS meter will show correct reading.
Have you noticed that APC (a well respected name in computer UPS) and many others make modified sine wave UPS for computers and millions of units are sold every year.