# Current in a 3-phase inverter and a BLDC motor with back EMF

#### edgardm

Joined Aug 17, 2017
2
The 3-phase inverter is an electronic circuit that drives the BLDC motor by commutating the DC supply voltage to the three phases of the BLDC motor. The three-phase inverter has six power switches, two on each phase leg which are connected to the DC voltage source. Figure 3‑7 shows a three-phase full-bridge inverter. By activating a positive and a negative switch in two phase legs, the current flows through the transistor which conducts positive, pass through two phases of the BLDC motor and returns through the negative transistor.
My question is regarding the back current produced by the back EMF (Electromotive force). How is the current affected by the back EMF when the switches are not activated in a phase of the motor?
For example when S1 and S2 switches are activated, the current cross from S1 to phase-A of the motor through phase-C and by an S2 switch. What does it happen then with the current produced by the back EMF of the phase-B (eb)?

Many thanks in advance

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#### crutschow

Joined Mar 14, 2008
31,490
It goes through the diodes across the across the transistors.

#### porrapai

Joined Aug 17, 2017
2
correct. The current produced by the back EMF circulates through the flywheel diodes back to the DC bus. It requires a method for the dissipation of the returned energy, either in a resistive load or by a complex method of returning it to the power network via an active rectifier bridge usually composed of IGBT's

#### edgardm

Joined Aug 17, 2017
2
correct. The current produced by the back EMF circulates through the flywheel diodes back to the DC bus. It requires a method for the dissipation of the returned energy, either in a resistive load or by a complex method of returning it to the power network via an active rectifier bridge usually composed of IGBT's
Thanks for your answer. Assuming that the returned energy is dissipated in a resistive load. Is it correct to have the current in one phase as the below image?

#### porrapai

Joined Aug 17, 2017
2
Greetings edgardm.....yes the waveforms look right...I have the variable speed drives business several years ago but if I have some documentation regarding the subject I will post it