06-05-2017, 10:56 PM
Give difference between 120 and 180 conduction modes on all points
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difference between 180 and 120 degree conduction modes
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Give difference between 120 and 180 conduction modes on all points
08-05-2017, 10:57 AM
The 180 degree mode and the 120 mode of operation 120 are two different strategies for switching the electronic power switches to obtain the desired AC output waveform. In the 180 degree operating mode, if you divide a 360 degree cycle into 6 60 degree intervals each you will have 3 devices driving at each interval
If I name the upper leg switches as 1,3,5 and the lower leg switches as 4, 6, 2 the switch board would be- (180 degree mode)
0-60 degree- Switches 1,6 and 5 are on
60-120 degree - Switches 1, 6 and 2 are on
120-180 degree - Switches 1, 3 and 2 are on
180-240 degree - Switches 4, 3 and 2 are on
240-300 degree- Switches 3, 4 and 5 are on
300-360 degree- Switches 4, 5 and 6 are on
Inference - Each switch will drive for 180 degree interval and for each 60 degree interval three switches are on and three switches are off.
For 120 degree operation mode-
Each switch is on for a 120 degree interval on each cycle and two switches are on simultaneously.
0-60 degree- Switches 1 and 6 are on
60-120 degree- Switches 1 and 2 are on
120-180 degree - Switches 3 and 2 are on
180-240 degree - Switches 3 and 4 are on
240-300 degree- Switches 5 and 4 are on
300-360 grade 5 and 6 switches are on
Inference - Each switch will conduct for the 120 degree interval and for each 60 degree interval two switches are on and four switches are off.
Efficiency and loss analysis- (Neglecting switching losses, as it will depend on the type of electronic power device used - MOSFET, IGBT, BJT ETC.)
Loss of power in electronic power switches can be classified as: loss of state, loss of conduction and loss of switching. The loss out of state (the switch is in deactivated state) the flowing current is very low and therefore the loss of off state is very small. The conduction loss will be Von (voltage drop in state) * Ic (current carrying capacity) which in high current applications is considerably greater than out of state losses.
Inference (High current application)
Loss of driving> Loss of commutation .................. (1)
180 degree mode (each 60 degree interval) - 3 turns on, 3 turns off
Loss (180) = 3 * Loss of driving + 3 * loss out of state ...... (2)
120 degree mode (each 60 degree interval) - 2 turns on, 4 turns off
Loss (120) = 2 * Loss of driving + 4 * loss out of state ...... (3)
Therefore, from (1), (2) and (3) if it is a high current application
The 120 degree mode would have less loss than the 180 degree mode.
For low current and high voltage applications, the difference between conduction loss and out of state loss is not significant and therefore can be neglected.
Based on THD (Total Harmonic Distortion and Load Power Factor) -
For delay and forward loads if the p.f is more than 0.9- Select the 180 degree mode
For ohmic loads: select the 180 degree mode
For pure inductive and capacitive loads - it will not matter how the resistive damping effect on harmonics is insignificantly small and therefore you can fire them in 120 degree mode or 180 degree mode.
You need to give three pulses for the 180 degree mode while you need to give two pulses in 120 degree mode for the circuit for each 60 degree interval and therefore for the sake of simplicity one would prefer the 120 degree mode.
Read more about THD analysis to understand which is best according to your application.
The 150 degree mode is now preferred a few days because of THD lower than 120 degrees and 180 degree mode in some applications.
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