WO2012136212A2

WO2012136212A2 - Method for a safe operation of an electric motor

Info

Publication number: WO2012136212A2
Application number: PCT/DK2012/000034
Authority: WO
Inventors: Michael Laursen
Original assignee: Danfoss Drives A/S
Priority date: 2011-04-08
Filing date: 2012-04-03
Publication date: 2012-10-11
Also published as: WO2012136212A3

Abstract

A method for controlling an electric motor (2) is suggested, wherein at least one design limit of the electric motor (2) is complied with by controlling an increase and/or a decrease of the rotational speed of the electric motor (2).

Description

Method for a safe operation of an electric motor

The invention relates to a method for controlling an electric motor, wherein at least one design limit of the electric motor and/or at least one design limit of a device the electric motor is used for, is complied with by using an appropriate control of the electric motor. The invention further relates to a controller unit for controlling an electric motor. When designing an electric motor and/or a technical arrangement, comprising one or a plurality of electric motors, a variety of design limits have to be considered when constructing the motor and the technical arrangement, respectively. From a basic point of view the adherence to such limits is standard procedure.

For example, when constructing an electric motor a certain maximum rotational speed of the electric motor is automatically determined by the design of the electric motor. Of course, this design limit has to be complied with during operation of the electric motor. Otherwise damage or even destruction of the electric motor (and presumably other surrounding parts) is inevitable.

Such design limitations do not only exist for "stationary limits" like maximum rotor speed, maximum torque limit (as a fixed maximum, mechanical limit), electric current limits (as a fixed maximum value) or the like, but also for dynamic limits like ramp-up or ramp-down effects (for example acceleration and/or deceleration of the rotational speed of the shaft of an electric motor), dynamic electrical limits or the like.

While it is standard procedure to obey such limits (in particular by an

appropriate control of the electric motor itself), the way how to do it differs substantially. Such different approaches can differ in a variety of ways. In general, such controlling methods should be very robust (i.e. not vulnerable to small fluctuations of input and/or output parameters), easy to implement, and in particular the method employed should not give rise to the necessity of complex, heavy, sensitive, voluminous or costly additional components like sensors, drivers and so on.

Consequently, a plethora of different control methods (and appropriate devices for performing such methods) have already been proposed in the state of the art.

As an example, in the scientific paper "A Sensorless, Stable V/f Control Method for Permanent-Magnet Synchronous Motor Drives) by P.D. Chandana Perera, Frede Blaabjerg, John K. Pedersen and Paul Thogersen in IEEE Transactions on Industry Applications, Vol. 39, No. 3, May/June 2003, page 783 to 791 , a suggestion for a control method for a permanent magnet synchronous electric motor is proposed. The control method is described to provide a stable operation in a wide frequency range. However, experiments have shown that the limitations that are imposed by this method are too strict so that the operational capabilities of the electric motor are not fully used. This has the result that the electric motor, as well as the electric unit that controls the electric motor, have to be over-dimensioned, resulting in unnecessary cost, weight and so on. Furthermore, in European Patent EP 1 684 412 B1 a synchronous motor drive unit and a driving method for it is suggested. Here, a rotary sensor is used for gaining better information about the status of the electric motor. This adds additional cost and reduces the robustness of the system. It is therefore the object of the present invention to provide a method for controlling an electric motor that is improved over presently known methods for controlling an electric motor. Another object of the invention is to provide a controller unit for controlling an electric motor that is improved over controller units according to the state of the art.

The present invention provides a method for controlling an electric motor, wherein at least one design limit of the electric motor and/or at least one design limit of a device the electric motor is used for, is complied with by using an appropriate control of said electric motor and wherein the at least one design limit is complied with by controlling an increase and/or a decrease of the rotational speed of the electric motor. Thus, the rotational acceleration and/or the rotational deceleration of the shaft of the electric motor is controlled. This is often referred to as doo/dt where d/dt stands for the time derivative of ω and ω stands for the rotational speed of the shaft. The inventors have discovered to their own surprise that a surprisingly large number of design limitations of the electric motor itself and/or the machinery the electric motor is used for can be obeyed by simply controlling the electric motor in a way that this value du)/dt is controlled, in particular in that certain limits (wherein both upper and/or lower limits are possible) are imposed on this value dco/dt (i.e. the change of the rotational speed of the electric motor; typically the shaft of the electric motor). Of course, the control of this value need not be performed in a "direct" way. Instead, all types of influencing control possibilities are possible. As non-limiting examples, an appropriate supply with an electric current, "electric braking" (the use of the braking power when electric energy is generated by running the electric motor as a generator), "real braking" using mechanical brakes, hydraulic brakes, aero dynamical breaks or the like, adding an additional load on the respective device and so on is envisaged. Of course, apart from the suggested control, additional control is possible. In particular,

additional/alternative control methods can or even should be employed for certain design limits, in particular design limits that cannot be dealt with (or at least not be dealt with sufficiently) the suggested control. Those alternative methods can be employed additionally and/or alternatively.

In particular, the method of the present invention can be employed for at least one design limit, preferably for a plurality of design limits, wherein such design limits are taken from the group comprising induced voltage, maximum torque and maximum current. Typically, a maximum torque limit (in particular maximum mechanical limit) can be exceeded both during ramp-up and ramp- down of the electric motor (acceleration or deceleration of the rotating shaft). The maximum torque limit can be imposed by the electric motor itself and/or by the machinery the electric motor is connected to. The maximum current, however, usually occurs during ramp-up (acceleration of the rotational speed of the shaft) of the electric motor. The induced voltage is typically a problem during a ramp-down (deceleration of the shaft) of the electric motor. In particular, during a ramp-down of the electric motor, the mechanical energy stored in the angular momentum of the shaft (and presumably inertial effects of the machinery the electric motor is connected to) can be converted into electric energy (since the electric motor can act as a generator) so that a maximum voltage and a maximum current can be reached or exceeded (in particular, if said ramping down is intended to be performed too fast). This can lead to a damage or even destruction of various parts, like the electric insulation of cables, electronic components (in particular electronic components in the controller and/or power supply of the electric motor) or the like. In particular it is suggested to employ the method of the invention in a way that a shift of electric parameters due to motor load conditions is considered. It is a well-known effect that the "mechanical orientation" i.e. the reference frame of the rotating rotor that is connected to the shaft of the electric motor (the so- called d-q-system) is not identical to the rotating magnetic field system of the stator (the so-called stator reference system and/or x-y-system), if a load is imposed on the electric motor. Both reference frames are shifted by the load angle and the torque angle. The inventors have discovered that the presently suggested method performs much better if such a shift of parameters is considered. In particular, the limits can be better obeyed and/or it is possible to better exploit the respective limit. Therefore, the efficiency and preciseness of the suggested method can be improved. For further improving the suggested method it is suggested that an electric limit is imposed in the x-y-reference frame and/or calculated from the d-q-reference frame. By an electric limit in particular a limit for electric current, frequency, electric voltage, phase angles and the like can be used. The employed limits can comprise lower and/or upper limits and/or even "forbidden intervals in between".

A further improvement of the suggested method can be achieved if at least in part and/or at least at times at least one dampening function is used. This way, it is typically possible to reach a better stability of the control mechanism. In particular artificial effects (like oscillations) can typically be avoided. Also, if numerical controllers are used (at least in part and/or at least at times), usually numerical effects (some kind of Moire-effects) can usually be avoided as well. In particular, such dampening functions can be realised as filter functions, in particular as low-pass filters and/or high-pass filters.

Yet another modification of the method can be achieved if at least one fast switch-off override function is provided. In particular, an emergency shutdown of the electric motor and/or the machinery, the electric motor is used for, can be realised by such an added functionality. For such an emergency shutdown, even a certain damage to (parts of) the arrangement is acceptable. As an example, the destruction of some kind of an electrical fuse device and/or a mechanical clutch device (at least one, preferably both can be preferably easily exchanged after an emergency shutdown) is usually perfectly acceptable in such a situation.

Furthermore, the method can be practised in a way that at least one design limit can be temporarily exceeded by a limited amount. This can be employed for one or a plurality of parameters. For example, a 10% excess electric current (or the like) can usually be tolerated for a short time span, for example for a time span of several seconds or minutes. This is, because a maximum current is usually at least partially induced by thermal effects. Thus, a comparatively small excess current for a comparatively small time span can usually be absorbed by the thermal capacity of the components involved. This way, the method can be easily employed for even a wider range of applications.

It is suggested to use the method for driving a variable frequency electric motor and/or a synchronous electric motor and/or a permanent magnet electric motor. First experiments performed by the inventors have shown that the suggested method is particularly suited for such purposes.

The present invention also provides a controller unit, such as a controller unit for driving an electric motor, wherein the controller unit is designed and arranged in a way to perform the methods described above. The controller unit will show the same effects and advantages as previously described, at least in analogy. Of course it is possible to modify the controller unit in the previously described sense, as well, at least in analogy.

In particular, it is possible to provide said controller unit with at least one inverter unit. Here, the necessary electronic components, in particular electronic components for controlling/limiting electric currents, electric voltages, frequencies and the like (wherein the control can consist in a control of respective time averaged values) are usually already present. Therefore, the controller unit can be typically easily adapted for performing the above described method.

It is further suggested to provide the controller unit with at least one electric current measuring device, preferably with an array of electric current measuring devices. Typically, such electric current measuring devices are comparatively cheap and precise. Quite often, said electric current measuring devices are already present with controller units according to the state-of-the-art for various reasons.

It is further suggested to provide the controller unit with at least one

programmable memory device. In this programmable memory device, different parameters, in particular different design limits of the electric motor and/or the machinery, the electric motor is used for, can be stored. Storage of these parameters can be done at the factory and/or by the customer, depending on the actual needs. The present invention and its advantages will become more apparent, when looking at the following description of possible embodiments of the invention, which will be described with reference to the accompanying figures, which are showing: Fig. 1 : a block diagram of a control loop for an electric motor;

Fig. 2: a more detailed block diagram of a part of the control arrangement shown in Fig. 1 ;

Fig. 3: a vector diagram showing the relation between the x-y and the d-q- systems.

In Fig. 1 a block diagram of a possible embodiment of an arrangement 1 , comprising an electric motor 2 and a control unit 3 is shown. The electric motor 2 is presently designed as a three-phase alternating current driven electric motor 2. The electric motor 2 is designed as a synchronous electric motor 2 with a permanent magnet rotor or salient pole rotor. Hence, the three-phase electric current is applied to the stator of the electric motor 2.

By varying the frequency of the applied three-phase electric current, the electric motor 2 can be driven at variable frequencies. For generating the variable frequency, three-phase electric current, an inverter unit 4 is used, that is presently supplied with electric energy by a DC (direct current) source 5. The inverter unit 4, more precisely the electronic switches of the inverter unit 4 (for example so-called IGBTs for isolated gate bipolar transistor), are commanded by the controller core 6 of the control unit 3. The controller core 6 can be designed as hardware components, as software algorithms or as a mixture between both. This statement can also be true for the other components of the controller unit 3. The output of the controller core 6 is mainly the active voltage, applied to the stator U_s,y, 7, the voltage vector angle Θ, 8 and the reference frequency for the stator u)_s,ref, 9 (these signals are outputted to the inverter unit 4), the load angle 36 (which is outputted to the current/torque limit control unit 10) and the current transformation angle 11 that transforms the three current phases l_s,u, ,v, ,w 13 from the external, stationary coordinate system into the x-y-system (the rotating stator reference system; see Fig. 3; signal is outputted to the current calculation unit 12). The current calculation unit 12 receives at its inputs the three current phases of the stator l_s,_u, ,v, ,w 13 that are sensed by a current sensor arrangement 14 that is placed between the inverter unit 4 and the electric motor 2.

Using the current transformation angle 11 , the measured motor phase currents l_s,u, ls,v, .w 13 are transformed from the (fixed) coordinate system into the rotating coordinate system that is rotating with is the same frequency as the voltage vector, and that represents the magnetic field of the rotating stator system (x-y-system). This rotating speed is equivalent to the inverter output speed ω₅,_Γβί, 9. The overall current vector I is (vectorially) split up into an active component l_s,_y 16 that is parallel to the voltage vector and a reactive component l_s,x 5 that is perpendicular to the voltage vector. The relations are depicted in Fig. 3.

These two current vector parts l_s,_x, l_s,y 15, 16 are fed into the l_s,_y monitor unit 17 and the current/torque limit control unit 10 (only active component l_{s y} 16 is directly used).

The functionality of the l_s,_y monitor unit 17 and the current/torque limit control unit 10 is shown in more detail in Fig. 2.

In the l_s,_y monitor unit 17, the reactive component of the electric current l_SiX 15 is passed through a low-pass filter 18 to avoid unwanted oscillations. Also, filtering is advantageous, if the current limit should react on peak current values and/or average current values. Then, the maximum allowed active component of the current l_s,_y,iimit 20 is calculated in box 19 in a way that the overall current I is not exceeding the allowed maximum. Parallel to this, from various input parameters 15, 16, 36, the actual torque 23 (without correction/clipping) is calculated in box 22. This actual torque 23 is compared in box 24 with the maximum allowed torque (which might be stored in a writable memory) and, in case the actual torque 23 exceeds this limit, cut down to the maximum allowed value 25. The maximum allowed active stator current corresponding to this torque limit l_s,_y,torque limit 25 is passed over to logic unit 26, where the more restrictive current 20, 25 is selected (or if no cutting applies the actual commanded current l_s,_y is selected). Therefore, the main output of the logic circuit 26 and hence of the l_s,_y monitor unit 17 is the reference active current of the stator l_s,_y,ref 27. Another output of the l_s,_y monitor unit 17 is the signal flag 21 that indicates whether a clipping takes place presently, or not.

The reference current l_s,_yiref 27 is first of all passed through another low-pass filter 18 in current/torque limit control unit 10. Then in current limit PI controller 29 the corresponding value for the ramp frequency ω_Γ3Γ1ρ 30 (that also obeys the limit(s) for dco/dt) is calculated and passed to the controller core 6.

Parallel to this part of the signal processing of controller unit 3 in Fig. 1 , another control path particularly pertinent for a ramping down of the electric unit 2 is present. If a ramp-down of the electric unit 2 is commanded, the mechanical inertia in the electric motor 2 (and other components (not shown) the electric motor 2 is connected to) will be converted back to electrical energy because the electric motor 2 is now operating as a generator. The thus produced electrical energy has to be "absorbed" by the inverter unit 4 and/or the DC source 5. However, the inverter unit 4 and/or the DC source 5 are limited with respect to their electrical parameters, so that only a certain, somewhat limited ramp down speed can be achieved without overloading the inverter 4 and/or the DC source 5. Therefore, the current voltage at the DC source 5 is measured by an appropriate voltage sensor 31 and compared with the maximum allowed voltage in comparator 28. If the critical limit is reached, a correction signal 32 is generated. This correction signal 32 is converted in voltage limit control unit 33 into a second ramp frequency u>_ramp 34 (that also obeys the limit(s) for dco/dt) that is passed over to the controller core 6. In general, the first ramp frequency idramp 30 will yield a limit for a ramp up, while the second ramp frequency ω_Γ3Γηρ 34 will yield a limit for ramp down sequences of the electric motor 2. The controller core 6 will use this information (together with other sensor information) to actuate the inverter 4 in a way that neither the maximum torque, the maximum current, nor the maximum voltage at the DC source 5 and/or within the inverter unit 4 is exceeded. This way, damages to the arrangement 1 can be at least largely avoided.

In Fig. 3, finally, the connection between the x-y-system and the d-q-system is shown in form of a vector diagram. The d-q-coordinates are representing the reference frame of the rotating rotor. The d-q-transformation is also known as the Park-transformation. The x-y-system is connected to the (also rotating) magnetic system of the stator part of the electric motor 2. Depending on the mechanical load on the electric motor 2, a shift between the x-y-coordinate system and the d-q-coordinate system will usually occur, namely in form of the load angle 36 and the torque angle 35. Once again, both the x-y-coordinate system and the d-q-coordinate system are rotating together with the rotor at the same frequency ω. Of course, small and typically short fluctuations between the two rotating frequencies ω may occur if one or several of the different shifting angles 35, 36 varies. The embodiments of the invention described above are provided by way of example only. The skilled person will be aware of many modifications, changes and substitutions that could be made without departing from the scope of the present invention. The claims of the present invention are intended to cover all such modifications, changes and substitutions as fall within the spirit and scope of the invention. Reference list

1 arrangement 19 box

2 electric motor 20 .y.limit

3 control unit 21 signal flag

4 inverter unit 25 22 box

5 DC source 23 actual torque

6 controller core 24 box

7 U_S,y 25 Is v .torque limit

8 voltage vector angle Θ 26 logic unit

9 (A)_s,ref 3 0 27 l_s,y,ref

10 current/torque limit control unit 28 comparator

1 1 current transformation angle 29 current limit PI controller

12 current calculation unit 30 tOramp

13 Is.Ui ,Vi ls,w 31 voltage sensor

14 current sensor arrangement 35 32 correction signal 15 ls,x 33 voltage limit control unit

16 Is y 34

17 l_s,y monitor unit 35 torque angle

18 lowpass filter 36 load angle

Claims

I a i m s

1. Method for controlling an electric motor (2), wherein at least one design limit of the electric motor (2) and/or at least one design limit of a device the electric motor (2) is used for, is complied with by using an

appropriate control of said electric motor (2), wherein the at least one design limit is complied with by controlling an increase and/or a decrease of a rotational speed of the electric motor (2) and wherein the at least one design limit is taken from the group, comprising induced voltage, maximum torque and maximum current.

2. Method according to claim 1 , characterised in that at least at times

and/or at least in part a shift (35, 36) of electric parameters due to motor load conditions is considered.

3. Method according to claim 1 or claim 2, characterised in that at least in part and/or at least at times at least one electric limit is imposed in the x- y-reference frame and/or calculated from the d-q-reference frame.

4. Method according to any one of claims 1 to 3, characterised in that at least in part and/or at least at times at least one dampening function (18) is used.

5. Method according to any of the preceding claims, wherein at least one fast switch-off override function is provided.

6. Method according to any of the preceding claims, characterised in that at least one design limit can be temporarily exceeded by a limited amount.

Method according to any of the preceding claims, wherein the method is used for driving a variable frequency electric motor (2) and/or a synchronous electric motor (2) and/or a permanent magnet electric motor (2).

8. Controller unit (3) for controlling an electric motor (2), characterised in that said controller unit (3) is designed and arranged in a way to perform a method according to any of the preceding claims.

9. Controller unit (3) according to claim 8, further comprising at least one inverter unit (4).

10. Controller unit (3) according to claim 8 or 9, further comprising at least one electric current measuring device (14).

11. Controller unit (3) according to any of claims 8 to 10, further comprising at least one programmable memory device.