about the voltage input for ac motor

[thomas33]

hai all,i am doing 3 phase ac motor drive.Now i have a problem that the input voltage for drive is 440-volt AC and it should be rectify to dc and after removing the ripples the voltage should be increased and i have to give the voltage to igbt's emitter and the output for igbt is nearly 650volt.I heard that the input of the motor should get 440volt only.Guy's can you please give me some information for rectifying this problem.

 

[Wesbflyer]

Since the output of the IGBT is still DC (pulsed) you need to convert this to AC again.
Are you building a variable speed drive? You might take a look at some of the current manufacturer's product data to see how they implement the concept.

Wes

 

[Wesbflyer]

I am going to guess that you are not familiar with the prinicples of AC electricity.

First, the nominal 440V AC system is rated according to the RMS value of the system voltage, which is aproximately 70.7% of the peak value. This means that in a 440 Volt system, the peak voltage applied to the load (motor) is around 630 volts. This corresponds very closely to the filtered DC voltage you will see in the controller between the rectifier and the IGBT switching circuit. A motor rated for 460VAC is designed to live with these peak voltages ( 630 to 680V) as a normal condition.

Consequently, The motor does not see excessive voltages, at least when operated at it's design speed (60 hz). The PWM nature of the output of this controller produces an applied voltage to the motor with a constant peak value, but with a varying duration. The result is the motor sees this applied voltage as not much different from a sine wave power supply.

However, to maintain the torque performance of a motor that is operated at speeds less that the 60HZ rated speed, you will need to reduce the voltage to maintain the ratio of voltage to frequency that the motor is rated for. For a 460V motor designed for 60 HZ, this ratio is around 7.67:1. (460V / 60 HZ).

I also assume you are working on this just for your own amusement. There are ready made, all the bugs worked out, complete with warranty controllers available for about what the parts would cost.

Have fun!

Wes