(this section get motor industry senior technical experts: WeiRongYi, Hu Junhuai, too, cui Jiang Xiongzhi technical support etc.)

1, overview of brushless dc motor

Permanent magnet brushless dc motor has many advantages, such as low interference (circuit has certain electromagnetic interference), long running life, good speed regulation performance, control method, the output torque, strong overload capacity, wide speed range, starting fast response, stable operation, high efficiency. Permanent magnet brushless dc motor has a lot of ac asynchronous motor, stepping motor and dc motor does not have advantages. It is widely used in office machinery, medical equipment, intelligent wear, computer, acoustics, ventilation industry, automatic control, instruments and meters, automobile, defense industry, etc., special mention is that in the computer, optical drive, hard disk, such as large DVD with very sophisticated forms of permanent magnet brushless dc motor, permanent magnet brushless dc motor is affecting people's lives in time.

As the miniaturization of controller, modular, before doing the larger controller can now be made smaller and some can do together and motor, the permanent magnet brushless motor so convenient to use and so comfortable. Many of the permanent magnet dc motor is increasingly replaced by a permanent magnet brushless dc motor. In DianJiJie, research, development of permanent magnet brushless dc motor is a kind of new trend. This aspect of the works and more than ever.

2, permanent magnet brushless dc motor, the relationship between mechanical properties analysis and calculation

Permanent magnet brushless dc motor and permanent magnet dc motor has some difference, motor stator and ac asynchronous motor stator, rotor and the stator principle of dc motor difference, similar ac motor power supply and power supply, power supply mode is also various. Therefore has not seen the writer introduce the mechanical characteristic of the brushless motor books, the formula of calculating motor no-load and load points calculation formula is very complex, but the motor is protean will not deviate from the main principle of the electromagnetic relationship. Whether can from complex mechanical properties to find out the regularity of brushless dc motor, the motor should be able to seize the inner link, so as to improve our understanding of the nature of the brushless motor. So the author from a new perspective to see the intrinsic relationships between mechanical properties of brushless motor.

We can see from the above introduction of various kinds of brushless motor, the calculation for the input power of the motor are visual reversing circuit and motor connected together as a system to consider the efficiency of the motor. Only plays a role of electronic commutation and the reversing system, in this case, the brushless motor is the mechanical commutator into electronic commutator.

From another point of view, the performance of the motor and the associated characteristic constant is very much like those of permanent magnet motor, whether can you think of permanent magnet brushless dc motor and dc permanent magnet motor mechanical characteristic curve of internal relations are the same? Whether can put the formula of mechanical characteristics of permanent magnet dc motor used in permanent magnet brushless dc motor? If possible, will be to our design, calculation of permanent magnet brushless motor has brought great convenience.

Table 1 90 permanent magnet brushless dc motor measured data

U (V)

I (A)

T (N.m)

N (r/min)

Po (W)

Pi (W)

Eta (%)

KE = 8.3 V/Krpm

24

1.5

0

2892

0.00

36.00

0.00

KT (N.m/A)

24

3.8

0.2

2774

58.09

91.20

63.70

0.0870

24

5.2

0.3

2716

85.32

124.80

68.36

0.0818

24

8

0.5

2600

136.13

192.00

70.90

0.0777

24

9.6

0.63

2544

167.82

230.40

72.84

0.0777

24

11.8

0.8

2464

206.41

283.20

72.88

0.0777

24

14.4

1

2393

250.58

345.60

72.50

0.0775

24

17

1.2

2311

290.39

408.00

71.17

0.0774

Table 1 in bold black and underline the point, as the maximum efficiency of the motor, which is just the maximum efficiency, near to the maximum efficiency point, so some calculation error, and I put into use is 10.19, also affected by the point calculation.

Maximum efficiency is 0.7288

The maximum efficiency point torque: 0.8 = 8152

The maximum efficiency point speed: 2464

Current of maximum power point: 11.8

Calculate the mechanical characteristic curve:

Ideal no-load rotation speed of the motor:

Motor no-load speed:

No-load current of the motor:

No-load torque of the motor:

Locked-rotor torque of the motor:

Calculate the locked-rotor torque of the motor:

The motor torque constant:

Motor electric potential constant:

(use the dragging method:)

Motor stalling a current:

The motor armature resistance:

The output power of motor:

The input power of the motor:

At any point on the motor speed:

We take take a bit of torque, such as table 0.5 for calculating its speed and computing speed difference:

The relative error:

The relative error:

We take take a bit of torque, such as table 1.2 for calculating its speed and computing speed difference:

The relative error:

The relative error:

Above to calculate the relative error of the calculated and measured values, the error includes point is near the maximum efficiency point only, with the maximum efficiency point between the error of the measuring point error and motor, etc.

From the above calculation and analysis, we think of it this way: permanent magnet brushless dc motor and permanent magnet dc motor of the mechanical characteristic curve of the internal relations are the same. So we use the book of the mechanical characteristics of permanent magnet dc motor is deduced relationship formula to calculate the permanent magnet brushless dc motor is completely feasible. So as long as know brushless motor related a few quantity, the way in which we can use the book introduces the whole brushless motor mechanical characteristic curve is very convenient calculation, it is also very happy thing.

3, the permanent magnet brushless dc motor torque constant potential and constant

Brushless motor (system) of constant torque calculation formula is:

Formula (1)

Is the electric motor operates when the electric current coil number of turns.

Is the total work of electric motor winding magnetic flux.

Brushless motor electric potential constant (system) :

(3)

In fact, (4)

When maximum efficiency of the motor from 0.65 ~ 0.85, the value is close to 1, the data are shown in table 6.

Table 5

Motor maximum efficiency

0.65

0.9624

0.7

0.9733

0.8

0.9888

0.85

0.9939

General motor's maximum efficiency within 0.7 ~ 0.85, so the above formula can approximate:

(5)

Look from our another train of thought:

A motor work is set voltage, if no-load speed of the motor is determined, the motor is basically:

(6)

Or the maximum efficiency of the motor is probably know, the motor can be accurate to determine:

(7)

From the formula 1-22, so:

(8)

(9)

(10)

If some needs accurate:

(11)

This formula from potential constant Angle the practical formula of motor effective number of conductors,

In the actual work, design professionals focused on brushless motor's voltage constant, we can also use this formula to calculate the effective number of turns of the motor.

(12)

4 Y - equivalent winding brushless permanent magnet dc

Square wave driven permanent magnet brushless dc motor of Y - the relationship between the equivalent circuit and ac three-phase motor winding, the conclusion, there have been many discussions and verified that each phase of the triangle winding circle number is star winding times, winding conductor cross-sectional area is times:

(13)

(15)

Is speaking, these two different winding connection in the same square wave brushless motor drive, the no-load speed should be the same, because the two motor winding connection is different. So the internal resistance of the motor, voltage bridge arm, power resistance and reversing the resistance pressure drop is not the same. But the characterization of their basic performance of the motor voltage constant torque and constant should be the same

In fact because the internal resistance of the triangle is small, working current is big, drive crystal tube pressure drop is big, on the motor voltage rather small, features soft instead.

Because of the motor voltage constant torque and constant uniqueness in mechanical properties of the motor, the permanent magnet brushless dc motor of the classics in product specification commonly used measurement of motor line voltage constant conversion of the electric potential of the constant and motor torque constant to represent the basic mechanical properties.

If two motor each phase winding, the same connection is different, so y connection of voltage should be delta connection times when they unloaded speed should be equal. If y connection of 36 v DC motor, so pick up into a delta connection, the voltage should be as follows:, because the triangle features soft, with the working voltage of 24 v as a triangle, although the no-load speed higher, when the load can be similar and the performance of the connection of the triangle.

Five permanent magnet brushless dc motor wire back emf constant and electric potential is constant

In practical work, because motor no-load speed is close to the ideal no-load rotation speed, but the maximum efficiency point of no-load speed and motor, so from a motor no-load speed calculation is error. Motor generator factory method for this line back emf constant.

Method is introduced as follows:

With motor brushless motor shaft, the brushless motor rotor rotating at a constant speed, such as 1000, measuring two fuses the voltage of brushless motor, the brushless motor in 1000 when two opposite electric potential, the back emf for each turn of ratio is a constant, called brushless motor back emf constant:

(16)

Star or triangle line voltage constant and brushless motor system based on the relationship between electric potential constant:

(17)

Because when brushless motor as synchronous generator to produce electric potential, the waveform is alternating sine wave, and if we are using the multimeter measure the effective value of the waveform, the amplitude for this must be The Times, and we made with the amplitude of the dc voltage input after electronic commutation brushless motor, the generator to produce electric potential amplitude and motor input voltage values are the same.

At this moment we don't consider brushless motor electronic commutator various forms and brushless motor driven power at work within the loss and pressure drop, at this time of the motor voltage constant is the only, then the motor torque constant is also unique, so the motor voltage constant as follows:

(18)

Motor of electric potential is constant and line voltage constant relations, star and triangle are the same.

See from the above analysis, the generator method to determinate the motor voltage constant, then converted to motor torque constant to represent the mechanical properties can be representative of the mechanical characteristics of the brushless motor groove (drive) without any loss.

Permanent magnet brushless dc motor (6 potential constant and motor torque constant

If we talk about the power of the permanent magnet brushless motor is dc power supply, drive transistor is MOS tube, the motor load is very small, so the transistor on the pressure drop is small, if ignored, the waveform is typical of square wave, so the brushless motor drive is equal to the permanent magnet dc motor commutator, regardless of the electronic commutation of the motor, motor current, the inductance and so on of motor and permanent magnet dc motor of the different factors, so in general sense, the use of brushless motor is measured after the line voltage constant electric potential of the motor constant and the corresponding motor torque constant of an association:

Motor line = potential constant voltage constant (19)

Motor electric potential constant torque constant (20)

If potential constant use, torque constant use, so their relationship is:

(21)

Is not our to far-fetched to the motor line voltage constant and the voltage constant and the motor torque constant relates in together, in fact, in the basic conditions, the above formula design brushless motors have great help to us.

Because of a brushless motor drives, so its mechanical characteristic curve is not an ideal straight line. Is a curves upward curve, as shown in figure 49.

Figure 49

Its characteristic is brushless motor in light load when the speed is relatively high, overloading when speed is low. About brushless motor in the characteristics of light load and overload are soft. So with motor generator method to calculate the potential of motor constant of the motor torque constant is error. Because of brushless motor racing is higher, the motor is relatively small, then the calculation of the motor is relatively small, look from the figure 8-10-49 brushless motor curve in the middle of the line feature is hard, then the value of the motor and calculated from the motor of the calculated value is larger than the motor, sometimes vary widely. But brushless motor and the mechanical properties of series motor, brushless motor's mechanical characteristic curve is much flat.

So the motor torque constant should not be used: best, should use 1 to 2 points between adjacent numerical calculate the motor torque constant, so that we can accurate.

Table 8

U (V)

I (A)

M (N.m)

N (RPM)

Pa (W)

Pi (W)

Eta (%)

kt

Kt and the first point calculation

Kt with adjacent point calculation

48

0.76

0

3365

0.00

36.48

0.00

0

48

4.9

0.5

3251

170.21

235.20

72.37

1231.52

1231.52

48

6.4

0.7

3219