Armature current spiking problem in a BLDC motor and solution: A case study

Key features of the electrical machine used in a flywheel energy storage system are, higher efficiency, higher speed, capability to be operated as motor and generator alternatively. Brush less DC (BLDC) machines has all these features. This machine is used for storing and retrieval of energy stored in the flywheel. To build Flywheel Energy Storage (FES) system, a suitable power converter/controller is required to be designed which meets the requirement of driving BLDC motor while storing the energy and boosting the voltage while harvesting the energy from the flywheel. This power converter/controller is known as Bi-directional Converter (BDC). Such a system was designed, built and tested. The controller uses signals from rotor position sensors for generating the triggering pulses to the power devices. During the testing of the system huge spikes were observed in the motor armature current and the motor could not be taken to desired rated speed. The motor could be run up to only half of the rated speed (i.e. 7500RPM). In depth analysis of the problem was carried out, root cause of the problem was identified, solution was found and implemented. The motor was made to run up to the full speed of 15,000RPM.