Suppose a induction motor is connected through a inverter. Inverter is fed from a DC-DC boost converter. My question is: to boost converter how load current will look like? Will it look like DC or AC?
The Current will most likely flow in one direction as pulsating DC throughout the whole system. I'll assume a basic 3-Phase inverter and simple DC-DC boost converter for thinking this through. I also am assuming the converter is connected directly to the inverter's DC bus.
The current through the induction motor will be roughly sinusoidal cycling at what ever frequency is appropriate for the speed the motor is supposed to be turning. The current flowing out of the inverter's DC bus will be pulsating DC, cycling at a rate 6 times higher then the frequency of the inverters output (The increased frequency is due to the three switching branches feeding the motor's phases).
The current flowing out of the inverter's DC bus will lower the DC bus voltage. The DC-DC converter will act to prevent that dip in voltage. The converter does this by supplying the needed current to keep the voltage steady into the inverter. The only way to keep the inverter's DC bus voltage stable is too feed into it whatever current is flowing out.
Assuming the DC-DC converter is able too it will supply pulsating DC current into the inverter.
EDIT: The inverter output voltage will switch at whatever switching frequency the inverter's PWM frequency is set to, typically somewhere between 1k Hz - 40k Hz. The motor will act as a low pass filter due to its high inductance, which results in a lower frequency sinusoidal current waveform.
In short the inverter's output voltage will switch at the PWM's high frequency, and the current will more smoothly oscillate at 50Hz. The 50Hz 3 phase motor current will look like pulsating DC at 300 Hz to the DC-DC converter.
I hope this helps.