WO1999003193A1

WO1999003193A1 - Method for controlling an electric motor

Info

Publication number: WO1999003193A1
Application number: PCT/EP1998/004154
Authority: WO
Inventors: Hellmut Lorenz
Original assignee: Barmag Ag
Priority date: 1997-07-11
Filing date: 1998-07-06
Publication date: 1999-01-21

Abstract

The invention relates to a method for controlling an electric motor for small members, in particular in a texturing machine. A corresponding efficiency is determined for a preset voltage-frequency ratio. The efficiency is optimized through repeatedly varying the voltage-frequency ratio, at a constant frequency and for a given dynamic working point. The motor is driven, at the given dynamic working point, with the voltage-frequency ratio having reached the highest efficiency.

Description

Method for controlling an electric motor

The invention relates to a method for controlling an electric motor for small units, in particular in a texturing machine.

The small units, for example a texturing machine known from DE 33 24 243, are driven by electric motors. Small units are, for example, the friction false twister, the traversing device, the rewinder and the delivery units. The electric motors, which can be designed as asynchronous motors, for example, are each individually controlled in order to bring about corresponding speeds. Due to the different thread types, each of the drives must cover a relatively large speed range. When processing threads with large titers, the drives are set in the machine at correspondingly low speeds. In the case of large thread titers, however, high torque is also required from the electric motor, for example on the false twist unit. On the other hand, a relatively low torque is required when processing threads with a low titer, but this must be given off by the motor at higher speeds.

However, it is known that in the design of electric motors, optimal utilization occurs only in a narrow range around the nominal speed of the motor. Outside of this range, the engine works with relatively poor efficiencies.

Accordingly, it is an object of the invention to provide a method for controlling an electric motor, in particular in a texturing machine, in which the motor is optimally utilized at every operating point.

This object is achieved by the features of claim 1 solved.

The motors are usually designed for a design point with optimal utilization, which is characterized by the nominal speed. This design point of the motor is characterized by a voltage-frequency ratio that results from the mains voltage and the mains frequency. The motor characteristics such as the torque can be influenced by this voltage-frequency ratio. The invention now makes use of the knowledge that each voltage-frequency ratio has characteristic motor characteristics. At an operating point, the voltage-frequency ratio is therefore changed according to the invention at a constant frequency in such a way that the motor with optimal utilization, i.e. is operated with the most favorable efficiency for the operating point. The frequency that determines the speed of the motor remains unchanged during the change in the voltage frequency ratio. The change is made solely by changing the voltage, for example by changing the setting of a converter. The favorable voltage-frequency ratio determined by iteration is then maintained for the respective operating point. In this way, the power consumption of the motor can be minimized. In a machine with numerous individual drives in particular, this leads to considerable energy savings. This method is particularly advantageous in order to be able to operate small motors in the power range of <1 KW with optimal utilization, since the efficiency curve of a small motor has a particularly strong peak.

When using the method with drives of a texturing machine, the optimization of the operating point is carried out after a steady state of a new process setting has been reached. Since the thread parameters do not change significantly during the process, there is no constant need New hiring. Only after a change to another thread type has been carried out, a new optimization of the drive is carried out at the respective operating point when the process starts again.

By changing the voltage-frequency ratio to optimize efficiency, the slip of the electric motor will change. The method variant according to claim 2 is particularly advantageous here in order to adjust the frequency in a further step to maintain a target speed of the engine. This ensures that the electric motor maintains the desired speed at the operating point with optimum efficiency. Since the frequency / frequency ratio also changes as the frequency changes, the efficiency of the electric motor is preferably optimized again after the target speed has been adjusted. With this method variant, two iterative methods are thus carried out in succession and repeatedly.

Since a torque which is below the maximum torque to be emitted by the engine could be emitted by the engine both when the engine slips and when the engine slips very little, the method variant according to claim 3 is advantageous. The change in the voltage-frequency ratio is carried out in both directions by increasing and / or reducing the voltage. This allows the slip to be optimized in relation to the target engine speed.

The method variant according to claims 4 and 5 is particularly advantageous in order to quickly reach the optimal range with a strongly developed characteristic curve of the engine. This makes it possible to first find an optimal area in order to then determine the optimal efficiency in fine optimization with a small increment. In a particularly advantageous development of the method, a power consumed by the motor is measured at the respectively predetermined voltage-frequency ratio. As a result, the efficiency can be determined solely from the powers measured during the iteration. If the power has decreased due to a change in the voltage-frequency ratio, the voltage changes in the same direction until the minimum power is reached. If the power increases with a given change in voltage, the direction of change must be changed.

In a further advantageous method variant, when the voltage-frequency ratio changes, a current drawn by the motor is measured and compared with the respective output value. If the current decreases compared to the initial value, the voltage-frequency ratio changes further in the same direction until a minimum of the current is reached. This process variant can be carried out in a particularly simple manner.

The procedure for carrying out the method is described in more detail with the aid of the diagrams shown in FIGS. 1 to 3.

1 shows a diagram with the current and torque characteristics of an electric motor. This is an asynchronous motor. The speed is plotted on the abscissa. The speed n is given as a percentage of the synchronous speed. The torque M emitted by the motor and the current I absorbed by the motor are plotted on the ordinate. The torque characteristic M denotes the torque delivered by the engine. The current characteristic curve I characterizes the current drawn by the motor. Both characteristics apply to a specific voltage-frequency ratio of the motor. That at The nominal torque occurring is denoted by M _N and the current drawn by the motor at the nominal speed is denoted by I _N. The nominal torque is generated by the motor when S slips. The slip is the deviation between the nominal speed and the synchronous speed. The slip is greater the more the motor is loaded, ie the higher the torque required by the motor.

2 shows a diagram with several torque characteristics of a motor, each with changed voltage-frequency ratios. In the diagram, the speed n is drawn on the abscissa and the torque M emitted by the engine on the ordinate. Several torque characteristics are entered in the diagram with the designation U, U ₂ and U ₃ . The torque characteristic U _j corresponds to the torque delivered by the motor at a voltage-frequency ratio with the voltage U _j and a predetermined constant frequency. The speed selected at the operating point is determined by the frequency. This speed is registered with n _set in the diagram. The torque characteristic curve U denotes the torque delivered by the motor in a voltage-frequency ratio with the voltage U and the U ISIN line _3, the torque output from the motor at a voltage to frequency characteristic with the voltage U. ₃ The voltage U ₂ > U _j and U ₃ > U ₂ . The frequency is constant.

At an operating point, a torque M is now demanded from the engine. With an output-voltage-frequency ratio with the voltage U _{j, it} follows that the torque is output by the motor at the speed n _j . The slip then results from the difference between

and n,.

FIG. 3 is a diagram with the efficiency of the engine from FIG. 2 shown. Depending on the changed voltage-frequency relationships, there are different efficiency curves Ui, U ₂ and U ₃ . With reference to FIG. 2 it is shown in FIG. 3 that the torque Mi, which is delivered at the speed n, leads to an efficiency ιy, the motor.

According to the invention, the voltage-frequency ratio is now changed by changing the voltage at a constant frequency. The voltage Ui is increased to the voltage U ₂ . It follows that the torque Mi output by the engine is output at the operating point at a speed n ₂ , as shown in FIG. 2. The speed n ₂ in conjunction with the efficiency characteristic U ₂ leads to an efficiency η ₂ . The comparison between the efficiencies rji to η ₂ leads to the result that η _{2 is} greater than r /,. τ? ι corresponds to the efficiency rj _before , and the efficiency η ₂ corresponds to the efficiency rj _n ac -

Since the change in voltage has led to an increase in efficiency, the next step is to further increase the voltage. This results in the torque characteristic U ₃ , which is obtained with a voltage-frequency ratio with a voltage U ₃ . As a result, the torque Mi is output by the engine at the operating point at a speed n ₃ . However, the rotational speed n ₃ results in the underlying efficiency curve U ₃ to an efficiency rj. ₃ However, the efficiency r ₃ is less than η ₂ . In this case, η _{2 is} the efficiency i7 _before , and η ₃ corresponds to the efficiency η _after . The change in the voltage-frequency ratio has led to a deterioration in the efficiency. Thus, the output value of the voltage-frequency ratio is set at the operating point that existed before the change. The motor is operated with the voltage U ₂ at the operating point identified by the torque Mi. The slip results from (v ^^ - n ₂ ).

In this example, a lowering of the voltage and thus a reversal of the change direction to optimize the efficiency is no longer necessary, since the increase in the voltage has already led to an optimal value. In the event that U ₃ had been used as the output voltage, a further increase in the voltage would have resulted in a deterioration in the efficiency. Only a lowering of the voltage from U ₃ to U ₂ in this case leads to an improvement in efficiency.

At an operating point, the power output by the engine is characterized by the required torque. The torque is essentially constant at the operating point, so that the power output is also constant. The efficiency can thus be determined by directly measuring the power consumed by the motor or the current consumed by the motor. To optimize efficiency, the power or current consumed must therefore be minimized. The motor is then controlled by an iterative power measurement or current measurement for the purpose of power or current minimization.

Claims

PATENT CLAIMS

1. A method for controlling an electric motor for small units, in particular in a texturing machine, in which the motor is operated with a predetermined voltage-frequency ratio, which is formed from a voltage applied to the motor and a frequency determining a speed of the motor, and at Which are carried out at an operating point for delivering a torque: a) Determination of an efficiency τ belonging to the current voltage-frequency ratio? _before ; b) changing the voltage-frequency ratio at constant frequency; c) Determination of an efficiency η _according to the changed voltage-frequency ratio; d) comparison of the efficiencies and in the case τ? _after > r / _before : repetition of steps a) to d) and in the case of rj _after <_ τ / _before : _{recording the} voltage-frequency _ratio belonging to the efficiency η _voτ .

2. The method according to claim 1, characterized in that a frequency correction to maintain a target speed of the engine in the operating point is carried out in the operating point.

3. The method according to claim 1 or 2, characterized in that the change in the voltage-frequency ratio by increasing / decreasing the voltage by a predetermined step size takes place, the direction of change being reversed at least once.

4. The method according to claim 3, characterized in that the step size for changing the voltage-frequency ratio is variable.

5. The method according to claim 3 and 4, characterized in that the step size before the reversal of the change direction is larger than the step size after the reversal of the change direction.

6. The method according to any one of claims 3 to 5, characterized in that the increase / decrease in the voltage by a step size in

Range less than or equal to one volt is performed.

7. The method according to any one of claims 1 to 6, characterized in that a power consumed by the motor is measured at the predetermined voltage-frequency ratio.

8. The method according to any one of claims 1 to 6, characterized in that a current drawn by the motor is measured at the predetermined voltage-frequency ratio.