Silent phase commutation in a three-phase brushless DC motor

Details Silent phase commutation in a three-phase brushless DC motor Silent phase commutation in a three-phase brushless DC motor

1999-06-30

2001-06-19

Nappi, Robert E. (Department: 2837)

Electricity: motive power systems

Limitation of motor load, current, torque or force

C318S132000

Reexamination Certificate

active

06249099

ABSTRACT:

TECHNICAL FIELD
The present invention relates to DC motors, and more particularly, to reducing the noise generated during phase commutation of a three phase, current controlled motor.
BACKGROUND OF THE INVENTION
Three-phase brushless DC motors have many uses, among which are as spindle motors for computer hard disk drivers, digital video disk (DVD) drivers, CD players, and tape-drives for video recorders. Such motors are recognized as having the highest torque and power capability for a given size and weight Compared to DC motors employing brushes, brushless DC motors enjoy reduced noise generation and improved reliability because no brushes need to be replaced due to wear.
FIG. 1
shows such a three-phase brushless DC motor
10
with three phases A, B, C having three coils
12
,
14
,
16
connected to each other in a Y-configuration at a center tap
18
. As is well-known, the coils
12
,
14
,
16
are part of a stator that causes a permanent magnet rotor to rotate. The first coil
12
(phase A) is connected to a supply voltage Vret by a first high-side transistor
20
and to ground via a first low-side transistor
22
and a sense resistor
23
; the second coil
14
(phase B) is connected to the supply voltage Vret by a second high-side transistor
24
and to ground via a second low-side ransistor
26
and the sense resistor; and the third coil
16
(phase C) is connected to the supply voltage Vret by a third high-side transistor
28
and to ground by a third low-side transistor
30
and the sense resistor
23
. Each of the transistors is an NMOS transistor as is typical. Represented in
FIG. 1
by voltage supply symbols are respective back EMF sources EA, EB, EC that are inherently induced by the permanent magnets of the rotor while the rotor is rotated.
This type of motor is driven by exciting its phases in a suitable sequence while always keeping two phases under power and leaving a third phase in tristate or floating with a high impedance (Z). For example, assume that initially the fist high-side transistor
20
and the second low-side transistor
26
are activated with high control signals on their gates while the other transistors are inactive. This results in a current IA through the first phase A having a value of +I, a current IB through the second phase having a value of −I, and zero current IC through the third phase as shown in FIG.
2
. At predetermined instances (t1, t2, . . . ) the driving of the phase switches so that current is driven through the phase that was previously floatin and one of the other phases is left floating such that the algebraic sum of the currents in the three phases are always equal to zero. In
FIG. 2
the driving sequence is as follows where the first letter indicates the phase of positive current flow and the second letter indicates the phase of negative current flow:
AB-AC-BC-BA-CA-CB.
In the instant of commutation from one stage to another (instances t1, t2, . . . ), if the current front were infinite, one would ideally find a system without perturbations. For example, at the instant t1, the phase A would maintain the current +I while the phases B and C would exchange the current flow, one from −I to 0 and the other from 0 to −I.
In reality, because of the presence of different time constants in the circuit, the commutation fronts of the two currents (IB and IC in the example of instant t1) would be non-ideal and non-synchronous. That is, the current IC increases more slowly than the current IB decreases. This translates into a variation of the current IA instead of the current IA remaining constant. The current variation generates torque ripple in the motor and much acoustic noise.
Analyzing the scheme of
FIG. 1
, it is possible to determine the reasons for the different commutation times in the two interested phases. At instant t1 (before the commutation) we would have:
VoutA=Vret IA=I
VoutB=0 IB=−I
VoutC=Vct IC=0
Vct=½ Vret
Given that the phases are out of phase by 120°, the electromotive forces driven will be instantaneously algebraically summed to zero. The commutation moreover happens at the instant t1 at the end of optimizing the torque ripple in the system. The back EMF in the three phases would have the following values: EA=E, EB=EC=−E/2, where E equals the maximum back EMF.
In the instant just after the commutation we would have:
VoutA=Vret
VoutB=Vret+Vbe (due to the current of coil B recirculating in the intrinsic diode of second high-side transistor
24
, where Vbe equals the drop across that intrinsic diode)
VoutC=0
Vct=⅔ Vret.
The back EMF values will remain instantaneously unchanged.
The voltage across the second coil
14
is therefore:
VoutB−(Vct+EB)=Vret+Vbe−(⅔Vret−E/2)=⅓Vret+Vbe+E/2,
while the voltage across the third coil
16
is:
VoutC−(Vct+EC=0−(⅔Vret−E/2)=E/2−⅔Vret.
The two voltages will therefore be significantly different, creating different time constants of charge/discharge of the two currents (IB will be reduced more quickly than C will be increased).
SUMMARY OF THE INVENTION
An embodiment of the invention is directed to a method and motor driver for driving a three-phase motor having first, second, and third coils. The method electrically connects the first coil to a first voltage reference and the second coil to a second voltage reference while leaving the other coil floating during a fist driving phase. During a second driving phase, the first coil is electrically connected to the first voltage reference and the third coil is electrically connected to the second voltage reference while the second coil is left floating. During a transition phase that immediately follows the first driving phase and immediately precedes the second driving phase, the second coil is electrically connected alternately to the first and second voltage references. By alternately connecting the second coil to the first and second voltage references and during the transition phase, the method causes the current through the second coil to reduce to zero at a slower rate than prior art methods. This enables the variations of the currents what the two phases in commutation, the second and third coils, to happen in a way that maintains their sum substantially constant. This reduces the torque ripple occurring during phase commutations and its accompanying acoustic noise.


REFERENCES:
patent: 4651067 (1987-03-01), Ito et al.
patent: 5097191 (1992-03-01), Bahn
patent: 5191269 (1993-03-01), Carobolante
patent: 5204594 (1993-04-01), Carobolante
patent: 5304902 (1994-04-01), Ueki
patent: 5569989 (1996-10-01), Acquaviva
patent: 5614797 (1997-03-01), Carobolante
patent: 5869946 (1999-02-01), Carobolante
patent: 6049181 (2000-04-01), Kalomeitsev

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