Sensor less Control of the BLDC Motors

Sensor less Control of the BLDC MotorsFrom Near-Zero to High Speeds

ANAND KUMAR.K

ABSTRACT

This paper presents the theory and implementation of a novel sensorless control technique for the brushless dc (BLDC) motor. The proposed new sensorless drive method solves the problem of the sensorless BLDC motor drives at very low speeds. It provides a highly accurate and robust sensorless operation from near zero to high speeds. For this purpose, an approach, a new flux linkage function is defined, that is speed-independent. The validity of the proposed method is verified through both simulation and experimental results and discussion.

PROPOSED SENSOR LESS COLTROL METHOD

• Most popular and practical sensorless drive methods for BLDC motors rely on speed dependent back EMF.

• Since back-EMF is zero or undetectably small at standstill and low speeds, it is not possible to use the back EMF sensing method in the low speed range.

• Estimated commutated points are shifted by 30(deg) from zero crossing of back EMFs have position error in transient state.

• Practical minimum speed of conventional sensorless drive is around 10% of the rated speed.

To overcome the above drawbacks, a novel method based on a new speed independent function is proposed.

proposed method does not rely on back-EMF; hence need of external circuit for sensing terminal voltage has been removed.

Principle of the Proposed Method • The general voltage equation of one of the active phases is given by

………. ( 1 )

Where is the active phase voltage is the phase resistance is the phase current is the rotor position

is the total flux linkage of the active phase n is the number of phases

• For three phase BLDC motors, the total flux linkage of phase A is

………( 2 )

Where is flux linkage due to permanent magnet that is attached on the rotor

Based on the characteristics of the SMPM type of BLDC motors, the flux profile can be simplified as (3) along with the following assumptions:

1) the motor is operated within the rated condition and hence the saturation effect due to current level is neglected;

2) the leakage inductance is negligibly small and hence neglected;3) iron losses are negligible;

………( 3 )

Substituting (3) into (1) gives ..…….. ( 4 )

So, ………….( 5 ) For balanced wye-connected BLDC motors, ………… ( 6 )Using (6),the (5) is simplified as ………. ( 7 )

The last term in the voltage equation is so called back EMF, and the term is redefined as follows

……… ( 8 )

Where is the flux linkage function that is changed only by rotor position.

The line-line voltages are as follows,

……… ( 9 ) ……….( 10 )

……….( 11 )

Where ω is the instantaneous speed is the line-line flux linkage function that is changed only by

rotor position

Now we define a new function, ,as …… ( 12 )

…… ( 13 )

Since the , function itself has one to one relationship with rotor position, it possible to use this function for position estimation. But the instantaneous speed terms that is unknown for dynamic operations, is

required to calculate the function.

DERIVATION OF SPEED INDEPENDENT POSITION FUNCTION

To eliminate instantaneous speed term ‘ω’ that cause trouble in using the , function for position estimation. One line-line function is divided by another line-line function, and divided new speed independent function is named

For example,

… ………..( 16 )

G’ FUNCTION WAVEFORM

• The standard commutation instant is when the functions are changed from positive infinity to negative infinity as in Fig(1).

G FUNCTION AT EACH MODE

• When the function reaches a predefined threshold value, the motor is commutated. The threshold value is defined based on the current rising time and desired advanced angle. It is noted that the commutation signal can be generated at the peak point that is the most sensitive part of the function.

• Since the waveform of the function is identical at the entire speed range, it can be characterized at steady state in a look-up table, and used as a position reference for sensorless operation at all speeds.

• The position is estimated by the look up table of the characterized function.

• To control currents, a simple PI or hysteresis can be used for the proposed sensorless method.

• The 60(deg) rotor movement is enough to detect the commutation instant using the position estimation equations in Table I.

• After the first detection of the commutation point, both torque and speed control is possible using the estimated speed from time duration of each commutation point.

SIMULATION RESULTS

DSP-BASED IMPLIMENTATION AND EXPERIMENTAL RESULTS

ESTIMATED ERROR OF THE PROPOSED METHOD

CONCLUTION

With the proposed method, the minimum speed of the sensor less operation has been improved to be around 1.5% of rated speed. This technique makes it possible detect the rotor position over wide speed range from near zero to high speed. Also the proposed approach provides a precise commutation pulse even in transient state because of speed independent characteristic of the function.