DE4018146A1 - Electronically-regulated electric drive - has star-delta switching device in supply path of asynchronous motor - Google Patents

Abstract

The electric drive uses an arynchronous motor (M) supplied via a star/delta switching device (1) with measurement of the power supplied by the latter to the motor (M). The measured power is compared with a switching value (Pschalt) via a comparator (3) which activates a pulse source (4) supplying the switching pulses for the star/delta switching device (1). Pref. the power supplied to the motor (M) is measured at regular intervals by measuring the current. ADVANTAGE - Improved efficiency for energy saving.

Description

Three-phase asynchronous motors are enjoying increasing popularity Popularity as an electric drive source as it comes with Power electronics are capable of speed control and the cost of the components of this power electronics become less and less.

Although number of poles, slot shape, nominal power and others Parameters the operating behavior of each engine differently can determine, in principle, of a certain qualitative shape of the operating characteristics of such Engines are run out. Unfortunately they bring Operating lines of these engines also express that the normal operating range of the engine is only in a very narrow speed range between slip s = 0.1 to 0.03, or in other words, about an engine speed that between 90 and 97% of the synchronous speed of the rotating field of the Motors is. This is the only area Efficiency η good. (Within this speed range but also falls the available torque M des Motors strongly.) The load moment ML becomes significantly smaller than that Nominal torque MN, (which is usually the right edge of the  Operating range of the engine), so take the Efficiency η and also the power factor cos ϕ rapidly decrease; the current I, however, remains almost constant. This means, that motors should never be too big should, because of their low efficiency otherwise becomes very uneconomical.

However, there are use cases such as discontinuous Utilized funding, etc., where the load moment ML at full load and the required torque when idle diverge so much that they no longer both within the optimal operating range of the engine. Since that at Full load required torque is definitely available must, and thus the full load state within the normal Operating range of the engine characteristics must be Operating point for no load outside of normal Operating area, and thus already in the area of a very low efficiency η.

If one could achieve that such empty runs with a cheaper efficiency of the same three-phase Asynchronous motor could be coped with, so this would considerable energy savings over the sum of the Represent the motors in use.

It is also known the stator winding of such Three-phase asynchronous motor from star to delta connection switchable design. Instead of the star Triangle switch, a control unit is also possible, which is the one supplied by the power supply Continuously regulates the voltage to the optimal value. At Use of star- and delta-switchable motors Purposes where only a low starting torque is required is because the start-up has no or very little load (Easy start) that can go ahead Switchability of the stator winding from star to  Delta connection used to the high starting current To squeeze short-circuit runners. This is done by the The motor is started in the star connection, and automatically when a certain speed is reached or manually switched to a delta connection.

There is therefore the task of three-phase asynchronous motors, especially when using a star-delta switch are equipped, in smooth operation, i.e. during operation with little or no load, with less Loss of performance and thus a lower efficiency η to drive.

This task is carried out by the characteristic parts of the Claims 1 and 3 solved.

With three-phase asynchronous motors, a distinction must also be made between speed-controlled and non-speed-controlled motors. Since influencing the speed z. B. happens about the change in the supplied current, must be in the case of speed-controlled three-phase asynchronous motors that have a star-delta switch or said control unit between z. B. this switch and the motor, the electrical power P _e1 (z. B. by monitoring the current I) measured and controlled. Such a power electronics is missing in motors that are not speed-controlled.

By monitoring the electrical power P _{e1 as described} , it is constantly checked at which point within the normal operating range one is, since it is known how the operating characteristic of the electrical power P _e1 drops within the operating range in the direction of the synchronous speed.

At the same time, the monitored size is constantly increasing predetermined comparison variable compared. Sinks  monitored size for motors with star-delta switch below this switching threshold, it becomes an impulse triggered, which a switching back of the star Delta switch from the delta connection in the Star connection causes.

For motors with a stepless control unit, the voltage continuously adjusted to the required output.

As is known, there are three phases between one Three-phase line, that is, the so-called Delta connection, voltage differences of 380 volts each Alternating current. The voltage differences of these three Phases to the center leader, i.e. at so-called star connection, are each 220 volts Alternating current.

The operating voltage is thus changed by this switchover or regulation, so that the torque characteristic curve is influenced quantitatively while the zero crossing remains the same. However, this means that the required low load torque ML is delivered in a speed-controlled motor at a slightly lower speed than before, so that the motor runs in the range of a better, namely higher efficiency ν. This leads to a considerable saving in electrical power. In the case of speed-controlled motors, efficiency 2 also increases, but the speed is kept constant.

An important advantage of this automatic star-delta changeover or voltage adjustment is that it can also be retrofitted to most electric motors. Retrofitting is of course particularly simple and economically inexpensive if the electrical power P _{e1 has} already been measured in this three-phase asynchronous motor, for example by monitoring the current I consumed. It would then have to _{be switched} only one comparator of the electric power P _e1 with a predetermined shift amount P or the controlled variable, and a pulse generator coupled thereto for the star-delta switch or the control unit be added. P _switch is the electrical power _consumed , during the course of which switching from star to delta connection makes economic sense.

Of course, with speed-controlled engines power electronics must first be added, which the electrical power mostly by monitoring the recorded current I and the input signal for the also provides the comparator needed.

If the engine to be retrofitted does not even have a star Have triangle switches for such a switch however, it would have to be retrofitted.

Using the following diagrams and schematic diagrams this can be explained concretely.

Show it

Fig. 1 unit is a circuit diagram of the drive according to the invention, with a star-delta change-over switch,

FIG. 2 shows typical starting characteristics with star-delta switching,

Fig. 3 shows the operating characteristics of a three-phase asynchronous motor between the speed 0 and the synchronous speed n _d and

Fig. 4 is an enlarged detail view of these operating characteristics, plotted against another reference variable.

Fig. 1 shows the basic circuit diagram for a drive unit in which the drive A is driven by a motor M. This motor is, for example, a squirrel-cage motor which has a star-delta changeover switch 1 , with the aid of which voltage can be removed either with respect to the phases with one another (380 volts) or with respect to the center conductor (220 volts) for the motor.

Instead of the star-delta switch, one can stepless voltage control unit can be used, the requires an economically higher effort, on the other hand, however, an even more precise control of the Operating point of the drive unit enables.

The star-delta changeover is used to start the motor, starting first in the star connection and later switching to the delta connection. To determine the changeover point, the electrical power P _{e1 is} measured by means of a measuring unit 2 between the changeover switch 1 and the motor M. This is compared in a downstream comparator 3 with a predetermined switching power P _switching and, depending on the result of this comparison of the star-delta change-over switch 1 is connected to the comparator 3 pulse generator 4 is operated.

Starting in the star connection serves to reduce the current I during the start-up phase. The effect of the star-delta connection is shown in FIG. 2, which shows the starting characteristics of a squirrel-cage motor with star-delta changeover of the stator winding, such as according to FIG. 1.

From Fig. 2 it can be seen that the torque characteristic in star connection is qualitatively the same, but quantitatively flatter than the torque characteristic in delta connection. The same applies to the corresponding current curves, but here the greatest difference in speed 0 can be seen. In order not to need the very high current peak for current I according to curve I, i.e. the current requirement in the delta connection, at this speed, the motor is started in star connection. Of course, this is only possible in cases in which the load torque M _L required by the drive A is lower than the torque M provided by the motor in the star connection.

In this case, the motor is started up in the star connection until a speed n _u , the so-called switchover speed, is reached, at which the torque M provided by the motor in the star connection with the load torque required by drive A. M _L covers.

Switching to the delta connection again results in a discrepancy between the torque M provided by the motor and the load torque M _{L still} required by drive A. This discrepancy is eliminated by a further increase in the speed up to the operating speed nb, at which these two torques are of the same size.

The switching process at the switching speed n _U initially increases the current intensity I required by the motor to the value according to the curve for the current I at the speed n _U. However, this is far from reaching the much higher initial value of this current curve. As a result of the further increase in the speed, in the case of FIG. 2 beyond the nominal speed n _N up to the operating characteristic number n _b , the current intensity I drops.

As can already be seen from FIG. 2, each motor is designed for a nominal speed n _N and thus also a nominal torque M _N. This point represents a theoretical operating point. In reality, however, such a motor is loaded over a certain range in terms of speed and torque, as shown in FIG. 3. Measured by the slip s or the synchronous speed nd, the normal operating range of an engine ranges between approximately 0.9 and 0.97 of the synchronous speed n _d . This normal operating area is shown hatched in FIG. 3. It is particularly noticeable that the power factor cosine ϕ decreases within this operating range with increasing speed and especially outside this operating range in the direction of the synchronous speed n _d faster and faster. This is due to the fact that the curve of the power factor cosine ϕ, like the curve of the torque M, approaches zero after passing through a maximum point of further increasing speed. The normal operating range is of course designed so that the point of the maximum power factor cosine ϕ is still within this operating range, namely usually given at the lower speed limit of the normal operating range.

However, this causes that when the torque M decreases Motor at a higher speed and therefore automatically at operated a worse power factor cosine ϕ becomes.

In these cases, by switching the motor back from delta operation to star operation, that is to say by adapting the voltage, the speed is to be reduced by reversing the processes as described with reference to FIG. 2. This results in a shift to the left in the diagram in FIG. 3, that is, in the direction of the maximum point of the power factor cosine ϕ, and thus a reduction in the energy requirement by reducing the losses caused by the discrepancy between electrical power P _e1 and the mechanical power P. _mech .

This can also be explained on the basis of the diagram in FIG. 4, in which the load characteristics are plotted against the torque M.

For the sake of clarity, it should be pointed out that in the diagram of FIG. 4 the left edge of the diagram corresponds to the right edge of the motor area of the diagram in FIG. 3, that is to say the nominal speed nd. It is thus clear that the sharp drop in the power factor cosine ϕ and also the efficiency η at a torque which is below the nominal torque M _N , that is to say M / M _N less than 1, the range in FIG. 4 between the normal operating range and s = 0 corresponds.

The load characteristics plotted in FIG. 4 over the torque M, however, clearly show that a slight reduction in speed is achieved by changing from the torque characteristic in the delta circuit (M) to the torque characteristic in the star circuit (M) according to FIG. 2 , which depends on the course of the load torque M _L over the speed. In Fig. 4, however, it is easy to see that with a ratio of M / M _N of about 0.5 or even less, a speed reduction of only a few percent is sufficient to get into the normal operating range and thus values of the power factor cosine ϕ of more than 0.7 or even 0.8.

Fig. 4 also clearly shows the sharp increase in the electrical power P _e1 with only a slight decrease in the speed n, so that the only slightly increasing losses V in relation to the electrical power P _e1 or the mechanical power P _{mech drawn become} smaller and smaller.

One of the most important things about this invention is the fact that by measuring the required output to be delivered The voltage fed in is adjusted in this way - i.e. regulated - that the torque curve and the efficiency of the three-phase Asynchronous motor are in the optimal range. Hereby is the energy consumption of the respective load situation adapted and thereby reduced, and the range of use of a three-phase asynchronous motor expanded. Appropriate Three-phase asynchronous motors with variable load and discontinuous Operating behaviors are special for the application of these Suitable regulation.

Claims (6)

1. Electric drive unit with speed-uncontrolled three-phase asynchronous motor and a control unit for adjusting the voltage supplied by the power supply, characterized in that

• - The star-delta switch ( 1 ) is designed to be controllable,
• a measuring unit ( 2 ) for measuring the electrical power (P _e1 ) is arranged between the star-delta switch ( 1 ) and the motor,
• - a comparator (3) _(switching P) of the measuring unit (2) measured size with the switching amount and compares
• - A pulse generator ( 4 ), which can be activated by the comparator ( 3 ) to bring the star-delta switch into the other switching position.

2. Electric drive unit according to claim 1, characterized in that the control unit for adapting the voltage supplied by the power supply is a star-delta switch ( 1 ). 3. Electric drive unit with a speed-controlled three-phase asynchronous motor, characterized in that

• the drive unit has a control unit for adapting the voltage supplied by the power supply,
• - and a measuring unit ( 2 ) for measuring the electrical power (P _e1 ), which is arranged between the star-delta switch ( 1 ) and the motor,
• - The star-delta switch ( 1 ) is designed to be controllable,
• - a comparator (3) _(switching P) of the measuring unit (2) measured size with the switching amount and compares
• - A pulse generator ( 4 ), which can be activated by the comparator ( 3 ) to bring the star-delta switch into the other switching position.

4. Electric drive unit according to claim 3, characterized in that the control unit for adapting the voltage supplied by the power supply is a star-delta switch ( 1 ). 5. Drive unit according to claim 1, 2, 3 or 4, characterized in that the electrical power (P _e1 ) is measured at regular time intervals. 6. Drive unit according to claim 1, 2, 3, 4 or 5, characterized in that the electrical power (P _e1 ) is measured by measuring the current consumed (I).