In part I and part II of this blog series, we talked about ‘blind’ startup of a motor, which is an easy approach but has certain limitations. If the application can’t accept the blades going back and forth (as in ceiling or pedestal fans) or reverse spinning is prohibited (for example HDD motor or VCM), you should consider initial position detection (IPD).
IPD is the method to detect the motor position before it spins up. Because back electromotive force (BEMF) related methods are not available before the motor starts, we need to find another motor characteristic that is a function of motor position. Motor winding inductance (saturation) meets this requirement.
The equivalent circuit of the motor winding is an inductor in serial with a resistor. The magnetic field in the winding increases when the phase is driven with current.
The material of the winding core, the silicon steel, has a saturation point where the magnetic field can’t increase as fast as the current increase. The saturation point is determined by the magnetization curve (BH curve) of the substance.
The permanent magnet of the brushless DC (BLDC) motor generates a magnetic field in the winding core that can superimpose to the magnetic field generated by phase current.
Here is a simple experiment to verify this principle:
1) Turn on the switch. The 12V power supply starts to charge the inductor.
2) Turn off the switch when the inductor current reaches 1.6A.
3) Capture the current waveform.
4) Put a permanent magnet close to the inductor.
5) Provide an extra magnetic field to the inductor core.
6) Repeat the test.
Here is the result:
With the magnet nearby, the inductor is saturated and the current increases faster.
Also, the stronger permanent magnet we choose, the shorter time the current reaches the threshold.
We can apply this concept to the motor.
Sequentially apply voltage across two of the three motor phases this way: VW WV UV VU WU UW. When the current reaches the threshold, the voltage across the motor is stopped. Measure the time it takes from when the voltage is applied until the current threshold is reached. The time varies as a function of the inductance in the motor windings.
The state with the shortest time represents the state with the minimum inductance. The minimum inductance is because of the alignment of the north pole of the motor with this particular driving state.
In order to achieve an accurate IPD result, several parameters should be properly selected.
1. IPD current threshold
2. IPD clock
3. IPD release mode
Does this answer all of your questions on how to start-up your BLDC? What other BLDC questions can I answer?