Method of ac motor soft start

IPC classes for russian patent Method of ac motor soft start (RU 2422977):

H02P27/04 - CONTROL OR REGULATION OF ELECTRIC MOTORS, GENERATORS, OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS (structure of the starter, brake, or other control devices, see the relevant subclasses, e.g. mechanical brake F16D, mechanical speed regulator G05D, variable resistor H01C, starter switch H01H; systems for regulating electric or magnetic variables using transformers, reactors or choke coils G05F; arrangements structurally associated with motors, generators, dynamo-electric converters, transformers, reactors or choke coils, see the relevant subclasses, e.g. H01F, H02K; connection or control of one generator, transformer, reactor, choke coil, or dynamo-electric converter with regard to conjoint operation with similar or other source of supply H02J; control or regulation of static converters H02M)

H02P1/52 - by progressive increase of frequency of supply to motor

H02P1/30 - by progressive increase of frequency of supply to primary circuit of motor

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Method of ac motor soft start / 2422977

In method of AC motor soft start, the motor is connected to a frequency converter, frequency start of the electric motor is carried out until the previously specified value of the frequency converter output voltage, which is less than the rated value of the electric motor supply frequency, afterwards the electric motor is disconnected from the specified frequency converter and is connected directly to the power supply network.

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FIELD: electricity.

SUBSTANCE: in method of AC motor soft start, the motor is connected to a frequency converter, frequency start of the electric motor is carried out until the previously specified value of the frequency converter output voltage, which is less than the rated value of the electric motor supply frequency, afterwards the electric motor is disconnected from the specified frequency converter and is connected directly to the power supply network.

EFFECT: reduced required capacity and cost of the applied frequency converter.

9 cl, 2 dwg

The invention relates to electrical engineering and can be used to implement soft start both synchronous and asynchronous electric motors that operate a variety of mechanisms.

For modern motors, asynchronous and synchronous, the dominant is asynchronous start with a short-circuited starting winding.

However, when direct asynchronous start, i.e. when connected directly loaded motor to the mains, there are significant inrush currents - 5÷8 I_nom.and more (I_nom.- rated motor current), which remain at this level up to 80-90% n_Mr.(n_Mr.- rated engine speed), and then decrease.

These currents, which at the beginning of the start purely reactive, create significant planting voltage power supply diagrams and considerable electrodynamic forces in the windings of electric motors, the current-carrying cables, which leads to their deterioration and, ultimately, to failure.

In addition, many synchronous motors critical is the dissipation in the starting winding, which upon completion of the start almost running.

In this regard, the dimensions of this winding optimize, namely for the case of direct start-up from the network settings winding choose that is they way that upon completion of the start temperature of the conductors was close enough to the maximum allowable for these conductors and in contact with these items.

Since the direct starting of motors is accompanied by large currents and severe temperature for starting winding, the number of direct launches for electric motors medium and high power is minimized specifications number of the order of 100÷200 a year.

But the real operation conditions, the requirements of energy saving (for example, the use of pumps only at night to pay for electricity for the overnight rate) require much greater flexibility and, in particular, the removal of restrictions on the number of launches of electric motors, including the most powerful.

Therefore, instead of direct starting motors use the so-called "soft start" [1], the essence of which consists in that the windings of the motor from a special source serves voltage whose parameters (mean value and/or amplitude and/or frequency and/or phase, and/or shape) different from those of the mains supply voltage three-phase network, and "untwist" the motor up to full speed, making sure that the current does not exceed a predetermined value, after which the motor is connected directly to the power three-phase set is, and a special source continuum or disable.

One way to smooth start is the so-called "frequency start [2], consisting in that the first motor is fed from the frequency Converter, the frequency of the output voltage of which change from zero to the supply frequency.

However after reaching the frequency Converter output frequency value of the network frequency Converter is switched off, and the engine is connected to the mains. To exclude the transition process with a throw of current often before disconnecting the frequency Converter phase with the network.

When the frequency start, compared with the direct start, considerably decreases the amount of heat dissipated in the windings of the motor.

However, to implement such a necessary start the frequency Converter from mains supply and with a capacity that would be provided during start-up torque that is equal to the sum of the static and the required dynamic (which provides the required acceleration) of points in each point of the transition process.

The cost required in this case, frequency converters is almost equal to the value of frequency converters for speed control during operation of the engine. Usually it is considered to be prohibitively large for devices smooth PU is CA. Although in some particularly important cases, such as the turbo-gas pipelines, and use this solution.

The present invention is the task of creating a method of frequency start AC motors, which would have all the advantages of the known method and it would use the frequency Converter is reduced, compared with the above capacity, which would have a low voltage and, respectively, lower cost.

The problem is solved in that in the known method soft start AC motor in which the motor is connected to the frequency Converter, to produce frequency start the motor to the predetermined frequency of the output voltage of the frequency Converter, after which the motor is disconnected from a specified frequency Converter and connect directly to AC power, made the improvement consisting in that the specified preset value of the output voltage is less than the nominal frequency of the motor power.

Stage acceleration with power from the frequency Converter fails at the frequency of rotation, when the value of the starting torque of the motor according to the characteristic of the induction start is sufficient to generate the, to the values of the torque to complete the process successfully run this speed to n_nom.when plugged in.

It is known that the value of the frequency Converter depends on the voltage at its output, which, at the same power depends on its maximum frequency. Since the maximum frequency used in the claimed method of the frequency Converter is less than the nominal frequency of the supply voltage of this Converter may be less than the supply voltage, which significantly reduces the technical requirements for the frequency Converter and, therefore, its value.

The required dimensions of the power Converter of this type, which should work units of seconds, can be several times less power and run the motor.

Thus, the inventive method is suitable for soft start both synchronous and asynchronous AC motors, provides the first part of the start - when powered from the Converter frequency - low frequency inrush current, the use of a frequency Converter with reduced, compared with the mains supply voltage and, correspondingly, low cost.

It is preferable that the specified preset value of the output voltage is Oia was less than 80% of the nominal frequency of the motor power.

Further improvement of the proposed method is that over a direct connection to the power grid, the electric motor is connected to the mains through the reactor for a time, during which the rotational speed of the motor reaches its rated value.

Reactors limit the starting current in the final section of the start value, which is acceptable for the engine, and for the mains. Unlike conventional reactor start-up, when the voltage drop across the reactor must be at least 70% of the mains voltage, in the present method with the specified advance, i.e. when connecting reactors when, for example, the engine has already been "promoted" to a speed less than 80% of the nominal value, the desired voltage drop across the reactor is reduced several times, which increases the voltage applied to the motor. And as overclocking this voltage approaches the supply voltage and at the end of the acceleration is almost equal to this voltage, because a relatively small reactive current of the motor, released to the nominal frequency of rotation, creates the small inductance of the reactor is negligible voltage drop. Therefore, the acceleration in this case is increased, compared with the acceleration in the forward start, the very naznacite is Ino, for example, 10-20%, and therefore the amount of heat generated in the windings of the motor at this stage of acceleration, is also increasing slightly.

In the end, because of the repeated cuts of the heat dissipation in the first stage of acceleration in General, the engine start is not only limited to the required values of current consumption of the network and the current flowing through the motor, but with a significant decrease in heating windings compared with the direct start.

In an improved embodiment of the invention, for the purpose of protection of frequency Converter and motor from sudden surge when disconnected from the inverter, before disconnection of the frequency Converter, with it, the currents through the windings of the motor is reduced to zero, that is, the frequency Converter is "locked". When the motor some, very little time is spinning with slow, which in some cases need to be considered when setting the set frequency of the frequency Converter trips.

In one of specific embodiments of the invention measure the frequency of the output voltage of the frequency Converter and conduct switching of the motor when the measured value of the output voltage reaches the specified pre-determined value.

In al the previous alternative embodiment, measuring the rotational speed of the motor and unplug it from the frequency Converter, when the measured value of the rotational speed reaches a value corresponding to the specified predetermined frequency value of the output voltage.

Both options allow a simple way to determine the time of switching the electric motor.

Next, the case study method in accordance with invention

Figure 1 shows the dependence of the envelope of the current I through the motor and the frequency n of rotation from the time t for the case of direct connection of the motor to the mains supply after it is disconnected from the frequency Converter.

Figure 2 shows the dependence of the envelope of current through the motor I and the frequency of its rotation n from time t when connecting the motor after it is disconnected from the frequency Converter to mains through the reactor.

The method in accordance with the invention was tested to run is loaded on the fan induction motor with a rated voltage of 6.3 kV with a capacity of 800 kW. While the frequency Converter was performed on the voltage 0,66 kV and fed from a step-down transformer, and the engine was connected through a step-up transformer (the total capacity of both transformers accounted for approximately 10% of the nominal power of the motor).

The frequency Converter costituiscono from zero to half the supply frequency (n=0,5n _Mr.), so that the current through the motor does not exceed 2I_nom.Further, the inverter current has reduced to zero, the motor is disconnected from the frequency Converter and connected to the mains directly (figure 1) or through the reactors (figure 2).

As can be seen from figure 1, 2, on the first segment starting when the motor is fed from the frequency Converter, the current through the motor is maintained at the level 2I_nom.The frequency of rotation of the motor is increased almost linearly, and acceleration to n=0,5n_Mr.was 10 C.

In the second section of the start, of course, there was a momentary current surge to a value of approximately 4I_Mr.and then a rapid decrease power and increase speed to nominal values. With the reactors (figure 2) inrush current was smaller, and the acceleration to n_Mr.more than without reactors.

The obtained characteristics of start very close to the frequency characteristics of start, when the power to the motor from the frequency Converter, is designed for full voltage motor nominal frequency and power close to the power of the electric motor.

Due to a significantly lower voltage of the frequency Converter and the relatively small time transformers cost used in the way that device is and in General, even if reactors and switching elements, was about 4 times lower than the value of the frequency Converter at full supply voltage.

Literature

1. Kostenko BTW, Piotrovsky L.M. Electric machine. Volume 2 of the AC Machine. HP, Energy, 1973.

2. Petrov, L.P. Thyristor power converters for asynchronous electric drive. M, Energoatomizdat, 1986.

1. Method soft start AC motor in which the motor is connected to the frequency Converter, to produce frequency start the motor to the predetermined frequency of the output voltage of the frequency Converter, after which the motor is disconnected from a specified frequency Converter and connect directly to the outlet, characterized in that the said predetermined frequency value of the output voltage is less than the nominal frequency of the motor power.

2. The method according to claim 1, characterized in that the predetermined frequency value of the output voltage is less than 80% of the nominal frequency of the motor power.

3. The method according to claim 1 or 2, characterized in that over a direct connection to the power grid, the electric motor is connected to the mains through the reactor for a time, during which the rotational speed of the motor reaches nominally is about values.

4. The method according to claim 1 or 2, characterized in that before disconnection of the frequency Converter, with it, the currents through the windings of the motor is reduced to zero.

5. The method according to claim 3, characterized in that before disconnection of the frequency Converter, with it, the currents through the windings of the motor is reduced to zero.

6. The method according to claim 1 or 2, or 5, characterized in that the measured frequency of the output voltage of the frequency Converter and disconnect the motor from the frequency Converter when the measured value of the output voltage reaches the specified pre-determined value.

7. The method according to claim 4, characterized in that the measured frequency of the output voltage of the frequency Converter and disconnect the motor from the frequency Converter when the measured value of the output voltage reaches the specified pre-determined value.

8. The method according to any one of claims 1, 2, 5, characterized in that the measured speed of the motor and unplug it from the frequency Converter when the measured value of the rotational speed reaches a value corresponding to the specified predetermined frequency value of the output voltage of the frequency Converter.

9. The method according to claim 4, characterized in that the measured speed of the motor and disconnect it from the inverter frequently what you when the measured value of the rotational speed reaches a value corresponding to the specified predetermined frequency value of the output voltage of the frequency Converter.