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AC current calculations

Created by: PISCEAN 2010 at: 1/21/2013 4:26 PM (7 Replies)

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1/21/2013 4:26 PM	Rate (0)
PISCEAN 2010 Posts: 8092 Rating: (884)	Hello Experts, How to calculate AC current when DC rectifier voltage & current parameters are know? e.g. DC recifier voltage is 500 Volt, DC current is 300 Amp. Through IGBT this DC is converted to AC. So I would like to know the formula which shall help to calculate AC voltage & current at output. Since this topic is related with converter system, I have selected this product conference to post my query. Thanks in advance. DEEP.
PISCEAN 2010 Posts: 8092 Rating: (884)
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1/24/2013 1:17 PM	Rate (0)
PISCEAN 2010 Posts: 8092 Rating: (884)	Hello Experts, In case of DC drives, max output DC voltage is 1.35 times the input AC line voltage. Like this in case of inverter, when input DC voltage is known, is there any formula to calculate maximum output AC voltage? Thanks in advance. DEEP.
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1/29/2013 5:50 PM	Rate (0)
Deleted User Posts: 767 Rating: (69)	Hello PISCEAN 2010, The ac current for a drive depends largely upon the amount of motor torque needed for the load to turn the motor shaft. There are two cases according to the type of motor (synchronous servo motor or asynchronous induction motor): 1. For synchronous servo motor (i.e. permenent magnet) the amount of ac current can be estimated by the following linear equation: I_drive = I_motor = Torque_motor / k_Tn k_Tn is torque constant for the servo motor (which is available from the manufacturer data sheet) 2. For an asynchronous motor the amount of ac current can be estimated as well but the equation is much more involved. I_drive = I_motor = Sum of the rated motor magnetizing current PLUS the amount of current needed to achieve the amount of torque producing current needed for the load. In both of the types of motors above manufacturer motor sizing software can calculate the amount of current needed for the load. One example would be SIZER for Siemens drives which is available for download from Siemens Service and Support pages. Best Regards, Deputy
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1/30/2013 9:50 AM	Rate (0)
PISCEAN 2010 Posts: 8092 Rating: (884)	Hello Deputy, Thank you very much for your kind reply. I am very much delighted after I read your reply. The reason is, atleast somebody read my post carefully, gave a thought & replied. So its great of you. Now, please excuse me, since I couldnot post my query correctly. My question is related basicaly with DC link voltage & IGBT current. What you explained is correct & true for motor application. But what if, the application is MF (medium frequency, variable in between 200 Hz to 900 Hz)power source? In this case the parameters measuered & displayed are 3 phase AC input voltage & current, DC link voltage & current. I would like to know out put of IGBT current & voltage. How to calculate it? I do hope, now, the query is much clear. Thanks & regards, DEEP.
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1/30/2013 3:49 PM	Rate (0)
Deleted User Posts: 767 Rating: (69)	Hello PISCEAN 2010, Following would be my best estimate of representative numbers and calculations which I think would at least give you good direction toward your data. Power (dc link) = 500 Volts * 300 Amps = 150,000 Watts = 150 kW Voltage (AC Three Phase) = Voltage dc link / 1.35 = 500 / 1.35 = 370 volts Power (AC Three Phase) = 150,000 Watts = 1.73 * 370 volts * I_AC I_AC= 234 Amps Deputy
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1/31/2013 4:31 AM	Rate (0)
PISCEAN 2010 Posts: 8092 Rating: (884)	Deputy Hello PISCEAN 2010, .... Power (dc link) = 500 Volts * 300 Amps = 150,000 Watts = 150 kW Voltage (AC Three Phase) = Voltage dc link / 1.35 = 500 / 1.35 = 370 volts Power (AC Three Phase) = 150,000 Watts = 1.73 * 370 volts * I_AC I_AC= 234 Amps Deputy Hello Deputy, Once again thank you for paying attention & sharing the information. Above calculated values i.e. 234 AC Amp, 370 volts AC etc are of input side or of IGBT out put side? Its not clear. Please help. Regards, DEEP.
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1/31/2013 3:42 PM	Rate (0)
Deleted User Posts: 767 Rating: (69)	Hello Piscean, These values would be on output side of IGBT's. Deputy
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1/31/2013 4:07 PM	Rate (0)
PISCEAN 2010 Posts: 8092 Rating: (884)	Deputy Hello Piscean, These values would be on output side of IGBT's. Deputy Hello Deputy, If this assumed to be true, will the effect of frequency varition affects the out put current & voltage? Because in this power source, out put frequency varies in between 200 Hz to 900 Hz.
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