Average Current Sensing Empowers Blower Motors

Jun 1, 2007 12:00 PM
By Ramesh T. Ramamoorthy, Senior Applications Engineer, STMicroelectronics, Schaumburg, Ill.

Click here for the enhanced PDF version of this article including diagrams/and or equations.

Brushless dc (BLDC) motors are widely used in industrial and home applications, such as in HVAC blowers, because of their efficiency and cost advantage. Furthermore, sensorless control of BLDC motors is preferred because of the even lower cost this method provides. The operating speed range in such applications does not warrant sensors either. However, accurate control of torque for blower motors is critical from a performance standpoint, as the air-flow rate is characterized based on torque.

Torque is directly proportional to the average motor current and hence current loop is crucial to the performance. Linearity of this loop is influenced by the faithfulness of current feedback. Most simple methods based on shunt-current sensing are not linear, whereas other approaches are expensive.

However, a new technique overcomes these limitations by sampling the dc-link current at the center of the pulse width modulator (PWM) on-time. This technique has been implemented in a control chip that provides sensorless control of a BLDC motor.

Approaches to Torque Control

A BLDC motor driven in the conventional six-step method resembles a brushed-dc motor, except that the winding positions are reversed and the commutator is replaced by an inverter. Hence, one might think of regulating the average dc-link current for torque control, but this will result in regulating power instead of torque. Because at a constant dc-link voltage, regulating average dc-link current will only regulate power output. This will lead to the motor current (torque) varying inversely with its speed, depending on applied motor load. Any effort to compensate the average dc-link current data with the duty cycle to obtain the average phase current can be impaired by noise-filter time constants, usually rendering this option ineffective.

The dc-link current does not reveal winding currents during the PWM off-time, so a designer could choose to monitor all three winding currents continuously and build a regulator. To do so would require two current sensors to monitor any two-phase currents. While the third-phase current can be reconstructed from these two, the cost of the sensors makes this option expensive.

A third option would be to regulate the peak current per PWM period. Though this method is inexpensive and easy to implement, it is not linear. During the PWM on-time, at lower duty cycles where speed and back electromotive force (BEMF) are small, the phase current rises much faster than at higher duty cycles where speed and BEMF are large. The same peak currents per PWM period represent different average currents for different duty cycles. An intuitive geometric approach reveals this (Fig. 1). A typical variation in average current versus duty cycle at a given peak-current reference is shown in Fig. 2.

Want to use this article? Click here for options!
© 2007 Penton Media, Inc.