DC brush motor can operate without current loop. There is no need for current loop, for example, when implementing speed Phase Lock Loop control.
There is a problem to run BLDC without current loop - the current of torque producing motor phase drops as much as 50% at the beginning of each commutation interval (every 60 electrical degrees) at least for low speed. Motor torque drops roughly in the same proportion. This torque ripple can't be tolerated for most applications and hence the need for BLDC current loop.
The most correct (not cheapest) way to build BLDC high performance current loop is to use 2 current sensors to be able to sense motor current in each phase - the current of torque producing motor phase is then selected to close the current loop for respective 60 electrical degrees interval. At the beginning of each commutation current loop recognizes current tendency to drop and boosts PWM duty cycle to stabilize current.
Low cost current feedback - one DC bus current shunt - compromises BLDC performance.
First of all, in the absence of commutation (normal operation as 2 phases are in conduction) DC bus current on average is motor current multiplied by PWM duty cycle (motor to DC bus voltage ratio). This issue can be fixed using DC bus current proper sampling and hold for PWM period.
Secondly, on commutation interval DC bus shunt does not provide the current of torque producing motor phase. Moreover, commutation behavior would differ for even and odd commutations.
There are claims that, based on in-depth understanding of BLDC commutation phenomena, commutation current can be stabilized without current loop by control means
