non linear loads

[sparkmantoo]

can someone give me a good definition as to what a nonlinear load is? i believe that it has to do with the noise that is encountered on electronic equipment, such as computers, electronic ballasts, etc. most loads i think are linear. please clarify.

 

[bphgravity]

Re: non linear loads

A load where the wave shape of the steady-state current does not follow the wave shape of the applied voltage. This is per Article 100 Definitions of the NEC 2002.

 

[mkbuck]

Re: non linear loads

Electronic equipment is powered by DC voltages that are rectified and filtered from AC sources, with diodes and capacitors. These power supplies are often referred to as switching power supplies.

The nature of a diode is that it conducts current flow in only one direction, when the forward bias voltage exceeds .9 volts.

By using four diodes connected in a bridge circuit, we are able to rectify an AC voltage (current) to a pulsating DC voltage (current) of approximately the same RMS value. A capacitor opposes a change in voltage by attempting to hold a charge. The capacitor(s) (is) are charged when the diodes are forward biased and draw their charging current from the AC source.

The voltage (charge) stored in the capacitor(s) servers the DC load when the voltage of the AC sine wave drops below the value of the DC voltage.

The diodes of the power supply are reversed biased by the DC voltage across the capacitors when it is higher than the voltage of the AC sine wave.

Given these basic facts, the power supply only draws current from the AC line when the voltage of the AC sine wave is higher than the DC voltage across the capacitors. Thus the current wave form does not follow the voltage sine wave, hence the term "nonlinear".

This causes the current drawn from the AC source to start and stop abruptly. This sudden stop in electron flow, which is to say current flow, causes a shock wave effect that creates reverse flow current waves at multiples of the fundamental frequency.


MKB

 

[Ed MacLaren]

Re: non linear loads

An example of a simple device that causes a non-linear load current is a standard lamp dimmer.

The sketch below shows the waveforms with the dimmer set at about half brightness.

Ed

 

[dereckbc]

Re: non linear loads

MKBUCK, the DC power supply you described is a linear power supply rather than a switch power supply. A linear power supply uses transformer at the input to step-down and isolate the AC voltage, then rectifies and filters as you describe.

In a switcher the line voltage is directly rectified and filtered at 300 + VDC which feeds a power oscillator operating at 25Khz or higher. This relatively high frequency allows the use of a small, lightweight and low cost transformer. The output is then rectified and filtered again. The control circuit steers the power oscillator so that it delivers just the right amount of energy needed, and little is wasted as in a linear power supply..

In either case, harmonics are generated by the electronic equipment consuming power rather than the electrical distribution system itself. There are ways to deal with it, but it cannot be prevented, only diverted or isolated.

 

[bennie]

Re: non linear loads

Dereck and ED: Doesn't the current wave form in a non-linear circuit flatten out in relation to the voltage wave form, instead of turning on and off?

[ March 24, 2003, 02:35 AM: Message edited by: bennie ]

 

[Ed MacLaren]

Re: non linear loads

Doesn't the current wave form in a non-linear circuit flatten out in relation to the voltage wave form, instead of turning on and off?

There are many kinds of "non-linear" circuits. The standard incandescent lamp dimmer circuit is not a rectifier. It is basically just an electronic switch that is timed on early or late in the cycle.

The waveforms above are for a simple incandescent (pure resistance) lamp load.
The amount of "flattening" or phase displacement of the load current would depend on the load characteristics.

Ed