Can you connect three phase AC voltage directly to motor?

Physically yes, but its not to code. The motor that is connected to a three phase system has to have undervoltage protection and thermal protection to be complient to the electrical code. This is done with a three phase motor contactor. By using a contactor you have the ability to start and stop the motor. The overload protection is also combined with the contactor so if the motor gets into an overload state the contactor will automatically disconnct it from the supply. Also if there is a power failure when the motor was running the motor will be unable to start up again on resumption of power. The start button has to be pushed to re start the motor, (this is the under voltage protection). This is why the supply voltage is NEVER directly connected to a motor.