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Because there are 3 phases. Unlike a brushed DC motor, it doesn't have a common ground.
On a BLDC motor the ground "shifts" between a pair of phases during rotation.
You have to remember that these 3 wires can't be connected directly to a 2 wire battery.
Instead, the battery is connected to the (+) (-) leads on a complex electronic circuit called ESC which commutates the motor. The 3 wires of the motor connect to the output side of the ESC.
Very nice animations and explanations about stepper- and BLDC motors.
The stepper motor animation from Nanotec with diverse setting options and connection diagrams has a new
simplified user interface and provides a good insight into the functionality of a stepper and BLDC motor.
I doubt that rajeshbhanushali123 was talking about stepper motors.
I think he was talking about main drive brushless motors with an electronically commutated input circuit that feeds the phases - Like those used in hobby applications.
A stepper motor is mechanically more complex and has a different principle of operation. Usually, it isn't capable of very high speeds and inappropriate as a "main drive motor" to power a veichle.
A stepper may be used as an alternative to a closed loop servo.
In terms of electrical machines, stepper and BLDC motors are both PMSM (permanent magnet synchronous motors respectively machines). This involves several common properties. The usual control methods are however different, also the preferred number of phases and pole pairs.
We still don't know for sure what the OP means with three wires BLDC. He may refer to a three phase motor, or a motor with built-in electronics and an additional control or monitor cable.
Most brushless motors have three leads (all of them that I have seen), one for each phase. However, I note that some commercially available motor controllers also have a common. Note the attached pic for the TI 11873 evaluation module (DRV11873). Why is this done and how does one treat the common lead when controlling a motor with three leads?