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How much dc bus voltage do I need in a 7.5HP, 220V, 3ph inverter motor drive ?

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yzou_ua

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I have a 7.5HP 220V induction motor driven by a 3ph inverter by space vector PWM control.
I need to choose dc bus cap. but but before that how much dc bus VOLTAGE I need? Is there any rule of thumb to have a rough range ??
Thx a lot!
 

mtwieg

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At the minimum you'll need 311V, but generally you'll want more than that, so probably closer to 350V. I know that for 240V drives, bus voltages around 370-400Vdc are common.
 
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yzou_ua

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Thank you, mtwieg. The minimum limit is 311V... Is there a maximum limit then??

At the minimum you'll need 311V, but generally you'll want more than that, so probably closer to 350V. I know that for 240V drives, bus voltages around 370-400Vdc are common.
 

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In an inverter drive you are trying to synthesise a 230V sinewave (311V peak), if you go for a high Bus voltage you just make the pulses narrower. So for a design, say 230 -10% (low input mains) giving required output, but actually running on a high mains (+10%), would give you a 20% headroom on the bus, allowing for switching device losses (10V?) would give 311 +20%= 373, + switching losses = 383 bus volts worst case. Write your own spec and figure it out, it looks like 400VW caps would not be out of place though.
Their value?, thinking about the above if there is a lot of ripple on the bus, when the volts are low the pulses can be made wider, so if the regulator circuit is fast enough, mains ripple can be made to "disappear", how ever when you are having low incoming mains, the bus voltage will already be low so ripple will make it even lower, so you simply run out of voltage when the output pulses are as wide as they can be. Going for gigantic capacitors to reduce ripple will lead to very high inrush currents and so its a compromise.
Frank
 
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mtwieg

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The maximum limit will be determined by switch and capacitor limitations. You want to have at least 10% headroom on the capacitor voltage and 25% headroom on the switch voltage.
 
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