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Precautions for frequency converter

several problems in the application of frequency converter

the selection of braking resistance is also a problem that readers ask more. To sum up, it can be summarized in the following three aspects: (1) various data are consistent or close to the method of accurately calculating braking resistance, but it is not easy to calculate, especially the data of flywheel torque (gD2) of the drag system; (2) The calculation results of the approximate algorithm introduced in various materials are inconsistent and difficult to adapt; (3) The braking resistor configured according to the manual will also smoke or burn out. I don't know why

1 basic knowledge

1.1 speed reduction process of variable frequency speed regulation system

as we all know, in the variable frequency speed regulation system, the motor decelerates by constantly reducing the frequency. As the frequency decreases, the synchronous speed (the speed of the rotating magnetic field) also decreases, the actual speed of the motor rotor exceeds the synchronous speed, and the rotor winding is in the regenerative braking state due to the cutting of the magnetic line of force in the positive direction. The regenerated electric energy is fed back to the DC circuit to generate pumping voltage

from the perspective of mechanical characteristics, the process of deceleration by reducing the frequency is shown in Figure 1

(1) working state before speed reduction

it is assumed that the operating frequency of the drive system before speed reduction is F1, and the mechanical characteristics of the motor are curve ①; The load is constant torque, and the resistance torque is TL (for simplicity, it is assumed that the loss torque is included in TL)

at this time, the working point is point Q, and the electromagnetic torque TM of the motor is balanced with the load torque TL: TM = TL

(2) speed reduction process of the drive system

first, the frequency drops to F2, and the mechanical characteristics become curve ②. Because at the moment when the frequency just drops, the speed of the drive system has not changed due to inertia, so the working point jumps to q1'point of curve ② and enters the second quadrant. The electric motor is in a regenerative state, the electromagnetic torque is - value, and the speed of the drive system decreases along curve ②

when it drops to Q1 point in the first quadrant, the frequency drops to F3 again, the mechanical characteristics become curve ③, and the working point jumps to q2'point, entering the second quadrant again

and so on

the process shown in Figure 1 is greatly amplified, and the actual interval between each two gear frequencies is much smaller

from the above speed reduction process, it can be seen that each time the frequency decreases, the motor is only in regenerative braking state for part of the time, as shown in the shaded part in the figure. Therefore, the voltage fed back to the DC circuit is pulse type, which is the reason why it is called pump up voltage

1.2 factors related to pumping voltage

the magnitude of pumping voltage depends on the speed at which the rotor winding (squirrel cage bar) cuts the magnetic line of force in the positive direction. Specifically, it depends on whether the rotor can follow the frequency in time when the frequency (and thus the synchronous speed) decreases. In terms of mechanical characteristics, it depends on the position of the turning point during each frequency change, such as Q1, Q2 and Q3 in the figure

however, in the process of frequency reduction of the frequency converter, the difference between the frequencies of each two gears is constant, so the size of the pumping voltage is mainly related to the following factors:

(1) the flywheel torque gD2 of the driving system

if the flywheel torque is large, the actual speed of the driving system will decline relatively slowly due to the large inertia, and the position of the turning point during frequency transformation will move left, increasing the pumping voltage

(2) the shorter the speed reduction time tb

the faster the frequency decreases. The actual speed of the drive system has not been reduced much, but the given frequency decreases again. As a result, the position of the turning point during frequency transformation also moves left, increasing the pump voltage

in practical work, the speed of deceleration can be regarded as a relative concept related to the size of inertia. For example, the preset deceleration time is 10s, which may be too fast for a system with large inertia; But in a system with small inertia, it is obviously too slow. Based on this understanding, the relationship between the speed reduction speed and the pump rise voltage is shown in Figure 2: if the preset speed reduction time is long, as shown in figure (a), the position of the turning point during frequency transformation shifts to the right, as shown in figure (b). As a result, the pumping voltage is small and cannot reach the upper limit of DC voltage, as shown in figure (c). On the contrary, if the preset deceleration time is short, as shown in (d) in the figure, the position of the turning point during frequency transformation will shift to the left, as shown in (E) in the figure. As a result, the pumping voltage increases, exceeding the upper limit of DC voltage, as shown in figure (f)

1.3 loss torque

there are various losses in the drive system, such as friction loss, ventilation loss, etc. the loss torque formed by these losses will help the drive system slow down. From the perspective of energy, these losses will consume power. When the motor is in the regenerative state, it consumes regenerative power, so as to inhibit the increase of pump voltage. It is generally estimated that the loss torque is about 20% of the rated torque of the motor. That is, the loss of power can produce a braking torque of about 20% TMN (rated torque of the motor). When the inertia of the system is very small or the deceleration time is very long, the whole deceleration process is carried out in the motor state, as shown in Figure 3 (b). Therefore, the pumping voltage is 0, and the DC voltage is stable at the rated value udn. Figure (d) shows the test point of DC voltage

2 accurate calculation of braking resistance

2.1 basis for accurate calculation of braking resistance

(1) requirements of the drag system for the deceleration process

(a) preset deceleration time (b) deceleration process

(c) the pump voltage is 0 (d) the test point of the circuit

td - the time required for the speed to drop from N1 to N2, s

tl'- converted value of torque of angstron company headquartered in Dayton, Ohio, USA, nm

(2) calculated value of braking resistance

when calculating the braking resistance, the lost torque (20% TMN) should be subtracted from TB. Root (3)

, where: RB - calculated value of braking resistance,

udh - allowable upper limit value of DC circuit voltage, V

tmn - rated torque of motor, nm

when calculating formula (3), it should be noted that:

① about the upper limit value of DC voltage udh

when the three-phase line voltage is 380V, according to the national regulations on the upper limit value of voltage fluctuation, there are:

udh 38021.2=645v

but in most frequency converters, the action voltage of braking unit is flame retardant. PP material in the household appliance industry should win the recognition and trend of users, udh=700v

② about the motor speed before speed reduction NM1

during the operation of production machinery, NM1 often changes and is unlikely to be determined. In the actual calculation, the rated speed NMN of the motor can be substituted

3 approximate calculation of braking resistance

although the method of calculating braking resistance introduced above is relatively accurate, it is also quite troublesome. From the perspective of practical application, it is not necessary. Therefore, the operating instructions of many frequency converters give some approximate calculation methods, and some directly provide the specifications of braking resistance for users to choose. Several main approximate calculation methods are introduced below

3.1 approximate calculation methods

the more representative approximate calculation methods collected by the author mainly include the following:

method 1 (see the instructions of VLT5000 frequency converter (Danfoss, Denmark) (4)

the "fund" is not for profit: PMN - the rated capacity of the motor; The constant 478801 is determined according to the calculation of udh = 850V. If you want to reduce the udh value, you can reduce it proportionally

when the braking resistance is selected according to the calculation result of formula (4), the obtained braking torque TB is:

tb 1.6pmn (5)

method 2 (see the instructions of Mingdian VT230S inverter (Japan) (6)

in the formula: the constant 593 is for 400V inverter; TB is the required braking torque

method 3 (calculated according to the data provided in the instructions)

when the current through the braking resistance is equal to 50% of the rated current of the motor, the obtained braking torque is approximately equal to the rated torque of the motor, which is summarized as follows: (7)

(8)

then TB TMN (9)

3.2 the value range of braking resistance

in order to reduce the resistance level of braking resistance, The braking resistance with the same resistance value is often provided for several motors with different capacities. Therefore, the difference of braking torque obtained in the braking process is large

(1) value range of braking torque

tb = (0.8 ~ 2.0) TMN (10)

(2) value range of braking resistance (11)

it can also be seen that the size of braking resistance is allowed to change within a certain range

The value ofb is roughly as shown in Figure 6 (b), which is actually the inversion of the resistance temperature rise curve shown in Figure 5. As can be seen from the figure:

① if the deceleration time TB is less than 0.3s, then take B = 11

② if the deceleration time is 0.3s

③ if the deceleration time TB is> 20s, the value of B decreases slowly because the temperature rise curve of the resistance begins to approach the stable temperature rise

2) the value of B during repeated deceleration

the working condition of repeated deceleration is shown in Figure 7 (a), and the acceleration and deceleration of the drag system are repeated

set: TB - time required for each deceleration,

tc - time required for each deceleration cycle,

then:

① if tb/tc 0.01, take B = 5

② if 0.01

③ if tb/tc> 0.03, the value of B begins to decrease slowly

(4) rough correction

according to the statistical results of the relevant data provided in the instructions of each frequency converter, the value range of B is roughly as follows:

PMN 18.5kw - B = 5 ~ 9

PMN 22KW - B = 2.5 ~ 4

5 precautions when selecting braking resistance according to the instructions

as mentioned above, in order to reduce the series of braking resistance, The braking resistance with the same resistance value and capacity is often provided for several motors with different capacities. For example, in the specification of Ansen TD3000 series frequency converter, the meaning of the motor capacity (a) not repeatedly reducing speed (b) the value of B

is required

for those with larger motor capacity and the same braking resistance in the same grade, the ratio of braking torque to rated torque is relatively small for those with larger motor capacity. In some occasions where flywheel torque is large and rapid braking is required, or where potential energy needs to be released, such as lifting machinery, the above braking resistance may not meet the requirements, so the braking resistance with a smaller resistance value can be considered

for example, when the capacity of the motor is 37KW, if you need to increase the braking torque, you can choose 10 braking resistance, then:

ib = 70A IMN TB 2tmn, the braking torque is large enough

5.2 about the adjustment of resistance capacity

(1) calculation of correction coefficient

the energizing power of resistance is determined by equation (14): (2) correction of correction coefficient

according to the curve shown in Figure 6 (b), if the deceleration time TB is less than 8s, B = 8.17 is available. In most cases, 8s deceleration time is sufficient

in the case of repeated deceleration, according to the curve shown in Figure 7 (b), when B = 8.17, it can only be used when tb/tc ＜ 0.01. In practical work, it can be estimated as follows: first, estimate the time required for each deceleration, and set TB = 4S, as long as the time of each cycle TC <400s = 6.7min, it is available. Under most working conditions, this is sufficient. However, for some occasions that require frequent inching, it is necessary to estimate separately

hoisting machinery

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