Brushless DC Motor Drive(Full Control):
1) The synchronous motor which is used in its self-controlled mode is known as the Brushless DC motor.
2) The BLDC motor drive is an inverter fed self-controlled permanent magnet which has the polyphaser windings on the stator or armature and permanent magnet type rotor.
3) The difference between self-controlled and the synchronous mode is that in self-controlled mode rotor sensors are getting used however in synchronous mode there is no use of such sensors.
4) The power transistors in voltage fed inverters that drives the synchronous motor are controlled by pulses generated by the rotorsensors.
1) The time for which each transistor remains on and sequence in which they are turned on and off is also decided by the signals generated by the rotor sensors.
2) This gives an conclusion that the rotor always revolves around the angular speed and which is equal to the average speed of the stator field therefore it is free from problem of pull out which means falling out of the synchronization. And hunting, which can be observed in free running mode.
3) The characteristics of permanent magnet synchronous and self-controlled mode are same as that of the DC motor.
4) Due to permanent magnet there is no use of the brushes and sliprings; hence permanent magnet synchronous motor if operated in self-controlled mode then it is called as brushless DC motor.
5) Speed and torque characteristics are also same as that o the DC motors.
6) The transistors in the design circuit are switched in pairs which means two transistors will conduct one is from upper and one is from the lower one.
7) If we want to change the direction of the motor then we need to make lower transistors on and off more than once during their on interval.
8) Then the duty cycle of these pulses is made to change in proportion with the control voltage (Vc) to vary the speed of the motor.
9) The direction of rotation of motor can be changed by changing the phase sequence of the applied voltage to the armature or stator.
1) Dynamic braking or regerative braking can be possible in this type of the motor
2) In dynamic braking mode upper three transistors are turned off and lower transistors are made turned on completely.
3) This will dissipate the entire energy in motor inductance, into the power transistors and motor will quickly turned to rest.
4) In regerative braking the motor will act like an generator and feeds the stored energy back to the source.
5) The rotor sensors used for feedback are hall type sensors and they will need the external DC supply for their operation and they operate in the principle of the induction, to generate the square wave signals to feed back the information about the position of the rotor.
6) The driver circuits are basically the current drivers for the power transistors; as the lower three transistors receive the PWM signal and are turned on and off more than once during their conduction period, they are being switched at a higher frequency.
7) Therefore Sometimes the lower transistors are replaced by MOSFETs, in that case the lower three drivers should be totempole drivers.
8) The base driving waveforms for the six power devices connected in bridge configuration are shown in fig.
9) Note that the pulse width of the base driving signals for the lower three transistors is changed to change the speed of the rotor. The control circuits of the BLDC motors is available in the IC form and has following features:
1) Undervoltage lockout
2) Error amplifier for close loop servo applications
3) Cycle by cycle current limiting
4) Internal thermal shutdown
5) Fault detection and shutdown
6) Variable pulse width output for lower three transistors
7) Dynamic braking
8) Direction reversal