SOLUTION

The starting characteristics of the BLDC motor are similar to DC series Motor. During starting of the BLDC motor both the Speed and Back EMF is zero.

Since the armature winding resistance is small, the starting current will be large in a short period of time. It may reach several times or more than ten times the normal operating current. Within the allowable range, the large starting current is helpful to the acceleration of the rotor so that the motor can quickly start even under full load.

Moreover, the armature current increases rapidly in the instant of starting. Thus, the electromagnetic torque is much larger than the load torque so that the speed increases rapidly. Consequently, the back-EMF will increase so that the growth of the armature current becomes slower until it reaches the maximum. Then, the armature current begins to decrease. The decreased current will lead to a decrease of the electromagnetic torque, so the rising acceleration of the speed becomes smaller. When the electromagnetic torque and load torque achieve the dynamic balance, the speed will stay in the rated value, i.e. the BLDC motor will maintain steady-state operation.