Method for operating an electronically commutated motor, and motor for carrying out one such method

The invention concerns a method for operating an electronically commutated motor (10) that is equipped with a current limiting arrangement which acts on a PWM controller (56). The latter, in operation, delivers PWM pulses (60) having a controllable pulse duty ratio (pwm) and a substantially constant frequency. In the event a predetermined upper limit value (Isoll; Iw) of the motor current (Iist) is exceeded, the current limiting arrangement effects a change in this pulse duty factor (pwm) in order to reduce the motor current (Iist). If the motor current (Iist) exceeds a predetermined upper limit value (Isoll; Iw) while the motor (10) is rotating, that limit value (Isoll; Iw) is raised by a timing member (260) during a predetermined time span (304) (FIG. 8: Iwmax), so that the available motor power is temporarily raised in the context of a load peak. If the rotor (24) is stalled, the limit value (Isoll) is not raised but instead is greatly lowered. The limit value (Isoll) is preferably raised as a function of the rotation speed of the motor (10) in order to utilize the improved motor cooling at high rotation speeds. At start-up of the motor (10), the limit value (Isoll) is briefly increased to a high value in order to enable dependable starting.

Electronically commutated motors are used for many driving tasks, e.g. in vacuum cleaners, equipment fans, medical devices, video recording devices, etc. Such motors are subject to many requirements, among which a low price is paramount. This means that a motor of this kind must be efficiently utilized for the particular driving task without being overloaded.

This is usually achieved by current limitation, i.e. the motor current is limited so that it cannot exceed a predetermined upper limit value. The power of such a motor is then, however, unnecessarily limited at startup, when a particularly high motor power is necessary. Many such motors could also be operated at higher power at high rotation speeds, since their cooling is then better and such a motor could absorb and deliver greater power. A motor of this kind could also, in many cases, temporarily deliver more power in the context of a load peak because it has a “thermal reserve,” i.e. the motor does not immediately overheat if an overload occurs briefly. There exist special circuits for this purpose with which a motor can be “simulated” by means of an electronic or mechanical model, but such solutions are too expensive for low-cost applications.

It is therefore an object of the invention to make available a novel method for operating an electronically commutated motor, and a novel electronically motor for carrying out such a method.

According to a first aspect of the invention, this object is achieved by means of the subject matter of Claim1. If a load peak occurs in such a motor so that the motor current exceeds its predetermined limit value, that limit value is increased by a timing member during a predetermined time span, provided the motor is rotating. The available power of the motor is thereby temporarily increased in the event of a load peak. Provision is preferably made, however, to prevent that increase from also occurring if the motor is stalled, since in that case the motor current should be as low as possible in order to avoid overheating and the resulting risk of fire. Preferred developments of such a method are the subject matter of Claims2and3. A motor for carrying out this method is the subject matter of Claim10.

Another approach to achieving the stated object is the subject matter of Claim4. Because at least one current pulse is generated and is conveyed to the voltage divider, the current flowing to the voltage divider becomes greater for the duration of that current pulse, and the upper current limit thus rises. This allows better utilization of the motor, especially when rotation-speed-dependent current pulses—which increase the upper current limit with increasing rotation speed—are generated. The reason for this is that a motor is usually cooled better as rotation speed rises, and can therefore deliver more power. This applies in particular to external-rotor motors. A motor for carrying out a method of this kind is the subject matter of Claim11.

A further approach to achieving the stated object is the subject matter of Claim7. The capacitor that is connected in parallel with a splitting resistor of the voltage divider is discharged before the motor is switched on. Immediately after switching on, it therefore acts as a short circuit for that splitting resistor and thus increases the upper limit value at startup temporarily, i.e. until that capacitor has charged. It is thereby possible, in a motor of this kind, to raise the starting torque without causing a long-term overload of the motor. A motor for carrying out a method of this kind is the subject matter of Claim12.

Another approach to achieving the stated object is the subject matter of Claim8. In such a motor, the output signals of the rotor position sensors have a relatively low frequency that is proportional to the motor rotation speed. The invention allows this frequency to be increased, specifically in such a way that beginning at a rotation speed of zero (at which the increased frequency also has a value of zero), a signal is available whose frequency is increased by a factor of three, for example, in a three-phase motor; this makes possible, for example, a more accurate indication of rotation speed, more accurate rotation speed control, more accurate sensing of the rotor rotational position, and more exact adaptation of the upper limit value of the motor current to the instantaneous rotation speed. In many cases this allows expensive encoders to be dispensed with.

A preferred development of the invention according to Claim8is the subject matter of Claim9.

According to Claim13, the invention is suitable in particularly advantageous fashion for electronically commutated external-rotor motors.

In the description below, identical or identically functioning parts are labeled with the same reference characters and are usually described only once.

FIG. 1is an overview circuit diagram comprising an electronically commutated motor (ECM)10, here (as an example) a motor having three phases12,14,16in delta configuration with terminals18,20,22and a permanent-magnet rotor24. The latter is depicted, for example, as having four poles. It controls three Hall generators26,28,30that have angular spacings of 120 degrees el. and generate, during operation, signals H1, H2, H3that, as shown inFIGS. 4A through 4C, are each phase-shifted 120 degrees from one another. These signals serve for rotation speed measurement and to control the currents through phases12,14,16. ECM10is preferably an external-rotor motor.

Signals H1, H2, H3are conveyed to a signal processor36, which is depicted inFIG. 3and whose output signals are conveyed through a schematically depicted connection38to an output stage40that is depicted inFIG. 2and contains a control logic unit for controlling the currents in phases12,14,16. Terminals18,20,22are connected to output stage40. The latter is connected to an operating voltage +UB, e.g. to +24 V, +48 V, +60 V or the like, and is connected via a current measuring resistor42to ground44. All the current of motor10, which is labeled Iist(actual current value), flows through resistor42.

A voltage drop occurs during operation at resistor42, and is conveyed to an apparatus46which limits current Iistand serves as a dynamic motor protector. A rated speed nsollfor motor10can be set at this apparatus using a potentiometer48.

Apparatus46has conveyed to it via lead52a signal50that has three times the frequency of signals H1, H2, H3and is created in signal processor36.

An output signal from apparatus46is conveyed through a signal connection54to a PWM controller56which, as a function of the signal at connection54, supplies a PWM signal60. The latter has a frequency f of, for example, 25 kHz, corresponding to a period T=0.04 ms. This signal60has a pulse duty factor
pwm=t/T(1)
that is between 0 and 100% depending on the magnitude of the signal at input54. Signal60is conveyed through a connection62to output stage40.

Apparatus46preferably has the basic function, usual in such motors, of regulating the rotation speed to a desired value, e.g. to 10,000 rpm, and also limiting motor current Iistto a predetermined value that, as shown inFIG. 8, is e.g. approximately 4.2 A at 10,000 rpm.

At start-up, apparatus46is intended to limit the current briefly (e.g. for 0.5 s) to a higher value, for example to 7 A according toFIG. 7or12.

If greater loads occur briefly, apparatus46is intended to absorb those load increases by permitting a higher current of 5.5 A, for example, for one second, including at start-up; segments having a “normal” current of 3.5 A and lasting for example, 4 s are always intended to be present between these higher currents (5.5 A).

Lastly, when motor10is stalled (i.e. at a rotation speed of zero) the current is to be decreased to a low value, e.g. to 1.3 A, so that motor10does not overheat when at a standstill.

Furthermore, as shown inFIG. 8, in the case of an external-rotor motor10the permissible current Isoll(IwinFIG. 8) is intended to rise with rotation speed n, since an external-rotor motor in particular is cooled better with increasing rotation speed and therefore “tolerates” a higher current when running at high speed.

These functions naturally do not all need to be implemented in a specific motor10and can instead also be used only in part, and the numbers indicated are merely examples to facilitate comprehension.

FIG. 2shows the essential elements of power section40. The latter contains a full bridge circuit comprising three upper transistors70,72,74that are implemented as p-channel MOSFETs, and three lower transistors76,78,80that are implemented as n-channel MOSFETs. A recovery diode70′,72′,74′,76′,78′,80′ is connected antiparallel with each of these transistors.

In upper transistors70,72,74, source S is connected via a lead82to +UB. In lower transistors76,78,80, source S is connected to a bus84that is connected via measuring resistor42to ground44, so that all of the motor current flows through resistor42.

Drain terminals D of transistors70and76are connected to winding terminal18of motor10, D terminals of transistors72and78to winding terminal20, and D terminals of transistors74and80to terminal22. When transistor70and transistor78, for example, are conductive, a current flows from left to right through phase12, and a smaller current flows through the series circuit of phases16and14. The magnitude of these currents depends substantially on the voltages that are induced in these phases by the rotating rotor24(FIG. 1).

The individual transistors70through80are switched on via AND elements. According toFIG. 4, for example, between 0 degrees el. and 60 degrees el.:

In this case transistors72through80are controlled as follows:Transistors70,74,76,78=0.Transistors72,80=1.

This is done by way of the AND elements depicted inFIG. 2. With the combination H1=1 and H3=0, transistor80is switched on; with the combination H2=1 and H3=0, transistor72is switched on. The other transistors remain blocked.

In this case transistors70and78are switched on and the remaining transistors72,74,76, and80are blocked.

The switching state of full bridge circuit70through80is thereby advanced after each 60 degrees el. so that winding phases12,14,16generate a rotating electromagnetic field in known fashion, as is common practice for such motors.

For this purpose, in transistor70gate G is connected via a resistor88to lead82and via a resistor90to the collector of an npn transistor92whose emitter is connected to ground and whose base is connected via a resistor94to the output of an AND element96that delivers a positive signal at its output (and thereby makes transistors92and70conductive) when a signal H1=1 is present at input98of AND element96, and a signal H2/=1 (corresponding to H2=0) is present at input100.

In the same fashion, transistor72is made conductive by an AND element102when the values H2=1 and H3=0 are present at that element102.

Transistor74is made conductive by an AND element104when signals H3=1 and H1=0 are present at that element.

Lower transistor76is made conductive, by an AND element106having three inputs, when H2=1, H1=0, and (at lead62) PWM signal60=1, i.e. PWM signal60switches on and off that particular lower transistor76,78,80that is presently being made conductive by the combination of signals H1, H2, H3.

An AND element108that is activated by H2=0, H3=1, and PWM signal60=1 serves to control transistor78; and an AND element110that is activated by H3=0, H1=1, and PWM signal60=1 serves to control transistor80.

The commutation system shown inFIG. 2is merely an example for better comprehension of the invention.

FIG. 3shows circuit36for creating signal50having three times the frequency of signals H1, H2, H3. A particular advantage of this circuit is that it is effective down to a rotation speed of zero, and that the frequency tripling permits a better measurement of the rotation speed of motor10and optimum adaptation of the current limitation operation to the instantaneous rotation speed of that motor.

AsFIG. 3shows, the current inputs of the three Hall generators26,28,30are connected in series. The upper current input of Hall generator26is connected via a resistor120to a lead122(e.g. +5 V), and the lower current terminal of lower Hall generator30is connected via a resistor124to ground44. Resistors120,124are preferably approximately the same size.

Associated with Hall generator26is a comparator126to whose two inputs128(+) and130(−) are connected the two outputs of Hall generator26. Output132of comparator126is connected via a pull-up resistor134to positive lead122, via a resistor136to input128, and via a resistor138to negative input140of a comparator142.

AsFIG. 3shows, Hall generator28has a comparator126′ and Hall generator30has a comparator126″. The circuit is the same in each case, and therefore the same reference characters are used (i.e. for example128,128′, and128″) and these parts will not be described again.

Output150of comparator142is connected to input140via a resistor152which effects a switching hysteresis; to lead122via a resistor154; and to ground44via a resistor156, a node158, and a resistor160.

Connected to node158is the base of an npn transistor162whose emitter is connected to ground44and whose collector is connected to an output164at which a pulse train, having a frequency that is proportional to the instantaneous rotation speed nistof motor10, can be picked off.

The two resistors146,148, which are the same size, set input144of comparator142to approximately +2.5 V.

In the range 0 to 60 degrees el.,FIG. 4shows that H1=1, H2=1, H3=0. The output of comparator126″ is consequently connected to ground and the outputs of comparators126,126′ are not connected to ground, so that a current flows from lead122through resistors134,138, and134′,138′ to node140and from there through resistor138″ to ground44. As shown inFIG. 4D, this results in a potential at node140equal to approximately two-thirds of voltage U=5 V, and comparator142receives at its output150a high signal that is labeled 1 inFIG. 4E.

In the range 60 to 120 degrees el.,FIG. 4shows that H1=1, H2=0, H3=0, i.e. the outputs of comparators126′,126″ are connected to ground and the output of comparator126is high-resistance. A current then flows from lead122through resistors134,138to node140, and from there through resistor138′ to ground, likewise through resistor138″ to ground. As shown inFIG. 4D, this results in a potential at node140equal to approximately one-third of voltage U=5 V, and comparator142consequently receives at its output150a low signal that is labeled 0 inFIG. 4E.

In this fashion, after each 60 degrees el. the potential at output150jumps either from 0 to 1 or from 1 to 0, and signal50whose frequency is three times the frequency of signals H1, etc. is obtained there. That signal is also available at output164, e.g. for monitoring the rotation speed of motor10. Such monitoring is requested by many customers.

FIG. 5is an overview to explain the basic aspects of the invention. The voltage at measuring resistor42is conveyed through a resistor207and a smoothing capacitor208to negative input210of a comparator204whose output is labeled216. Positive input212of comparator204is connected to a node214whose potential determines the upper limit of the current in motor10, i.e. its available power. If that upper limit is exceeded, the pulse duty factor of pulses60, which are generated by a PWM generator56, is automatically reduced.

Node214is connected via a resistor240to ground44, via a resistor238to a node232, and via a resistor300to switch286of a timing member260that is connected via a capacitor262to output216of comparator204.

Node232is connected to lead122via a resistor234and a capacitor236parallel to the latter. It is also connected via a resistor230to the collector of a pnp transistor226whose emitter is connected to lead122and to whose base a rotation-speed-dependent signal f(n) is conveyed.

Output216is connected via a resistor202(having a value R2) to the input of PWM generator56, to which a rotation-speed-determining signal (“n signal”) is also conveyed, usually from a rotation speed controller or a manual rotation speed adjuster, via a resistor196having a value R1. Resistance value R1is substantially greater than R2. Typical values yielding a preferred ratio of R1to R2will be indicated below.

PWM generator56supplies, at an output190, PWM signal60that is conveyed through lead62(seeFIGS. 1 and 2) to commutation controller40.

Mode of Operation ofFIG. 5

As long as potential u210at input210of comparator204is lower than potential u212at its input212, output216of comparator204is high-resistance and has no influence on modules56and260connected to it. This is the case as long as motor current Iistis lower than an upper limit value that is defined by potential u212of node214.

That potential is in turn determined by the ratio among resistors234,238,240and by a rotation-speed-dependent current248that flows through transistor226and resistor230to node232, the potential at node232being smoothed by capacitor236. Potential u212at node214, and consequently also the upper limit of current Iist, thus rises with increasing rotation speed.

If current Iistbecomes too high, comparator204flips and its output216is connected to ground44. The potential change thereby occurring at output216is transferred through capacitor262to timing member260and switches on switch286, for example for one second, so that resistor300is connected in parallel with resistors234,238and potential u212of node214is raised for that one second so that output216of comparator204immediately becomes high-resistance again and current Iistcan once again rise. After that one second has elapsed, switch286opens and potential u212at node214drops back, causing current Iistonce again to be limited to a lower value. If output216is connected to ground in this context, a current flows from input194through resistor202and comparator204to ground44, thereby abruptly reducing the potential of input194. Pulse duty factor pwm (equation 1) of PWM signal60is thereby also immediately reduced in order to reduce motor current Iistand keep it below the desired upper limit. The frequency of signal60remains unchanged in this context, which is an important advantage.

To ensure that the increase in the potential at node232and thus also at node214is as great as possible, resistor234is preferably selected to be substantially larger than the sum of resistors238and240. The voltage drop at current measuring resistor42is kept as low as possible. Potential value u212at node214for upper current limit Isollis thus also low, and because resistor300is connected in parallel it is easily possible to double upper current limit Isollif so desired.

At start-up, capacitor236is discharged and acts then as a short-circuit for resistor234, so that at startup, potential u212of node214is raised until capacitor236has charged. The starting current of motor10can thereby be greatly increased for a short time in order to ensure reliable starting, as depicted inFIG. 7at252. A longer-duration increase is possible with the variant shown inFIGS. 11 and 12.

One important aspect of the present invention is therefore voltage divider234,238,240, to which, as a function of motor parameters, signals of various kinds are conveyed from outside in order to limit or optimally utilize the available power of motor10. The various external influences on this voltage divider that are described represent, of course, only examples of the many possibilities offered by this principle.

FIG. 6shows details of a preferred embodiment ofFIG. 5. The same reference characters as inFIG. 5are used for parts identical, or identical in function, to parts in that Figure. PWM generator56contains a triangular signal generator having a comparator170whose positive input172is connected via a resistor174to lead122(+5 V), via a resistor176to output178, and via a resistor180to ground44. Output178is connected via a resistor181to lead122and via a resistor182to negative input184, which is also connected to the negative input of a comparator186and, via a capacitor188, to ground44. PWM signal60is generated at output190of comparator186. Output190is connected via a pull-up resistor192to lead122.

Comparator170with its various circuit elements generates a triangular voltage u184(seeFIG. 9) at, for example, 25 kHz at input184, and that voltage is conveyed to comparator186.

The output signal, for example, of a rotation speed controller200(indicated only schematically) is conveyed, as potential u194, to positive input194via resistor196, and input194is connected via resistor202to the output of comparator204, which is a constituent of an arrangement for current limiting.

The voltage at measuring resistor42, determined by motor current Iist, is conveyed through resistor207and filter capacitor208to negative input210of comparator204, as already described with reference toFIG. 5. The latter's positive input212is connected to node214, and potential u212there determines current Isollat which the current limiting arrangement is activated: the current is limited to a high value if the potential at node214is high, and to a low value if it is low.

Specifically, if current Iistbecomes sufficiently high that potential u210of input210becomes higher than potential u212of input212, comparator204flips and its output216goes to ground potential, so that a current flows from input194through resistor202to ground; as a result, potential u194at input194of comparator186abruptly decreases, pulse duty factor pwm of pulses60consequently becomes lower, and current Iist, is thus reduced because transistors76,78,80are switched on and off at that pulse duty factor, as described with reference toFIG. 2.

FIG. 9shows triangular voltage u184that is furnished by comparator170which serves as the triangular signal generator. This triangular voltage is compared in comparator186with potential u194at input194of that comparator.

If motor current Iistat time t10is higher than the predetermined value Isoll, comparator204flips, its output216becomes LOW, and a current flows through resistor202to ground44so that potential u194makes a downward jump195at time t10.

The result, as depicted inFIG. 9B, is that as of time t10the pulses of PWM signal60become shorter, and motor current Iistconsequently decreases until it is once again lower than Isoll. When that situation exists, comparator204flips back into its other state in which its output216is high-resistance, and no further current flows through resistor202.

A negative potential change at output216causes a transistor264to switch on and results in a temporary raising of upper current limit Isollas depicted inFIG. 7at304, and in this situation the length of pulses60temporarily increases again.

Controlling pulses60exclusively by way of pulse duty factor pwm, using a fixed frequency for PWM signal60, is very advantageous because, for example, it is possible always to work at 20 kHz or higher. That frequency lies beyond the range of human hearing, and motor10thus becomes quieter.

An arrangement220serves to increase the potential at node214(seeFIG. 8) in rotation-speed-dependent fashion. Pulses50(at tripled frequency) are conveyed through the series circuit of a capacitor222and a resistor224to the base of a pnp transistor226that is connected via a resistor228to lead122, to which the emitter of transistor226is also connected. The collector of this transistor226is connected via a resistor230to a node232, which is connected to lead122(+5 V) via a resistor234and a capacitor236parallel thereto. Node232is likewise connected via a resistor238to node214, and the latter is connected via a resistor240to ground44.

Resistors234(430 k),238(100 k) and 240 (8.2 k) constitute a voltage divider, and in the steady state, when no external influences are acting on the voltage divider, the potential of ground44is 0 V, node2140.076 V, node2321 V, and lead122+5 V.

Potential u212at node214determines the upper current limit to which motor current Iistis limited—for example, according toFIG. 8, to approx. 4.2 A at 10,000 rpm in continuous operation. This potential u212is conveyed to positive terminal212of comparator204; when it is low, comparator204already switches over at a low current Iistand reduces potential u194at input194of comparator186, thereby already reducing pulse duty factor pwm (equation 1) of pulses60at a low motor current Iist.

Raising the Current Limit as a Function of Rotation Speed

Arrangement220(FIG. 6) causes a current pulse248of constant pulse width to be generated at each pulse50(FIG. 4E). A particularly advantageous approach to achieving the constant pulse duration of current pulses248is to connect the base of transistor226to capacitor222, resistor224, and resistor228. The pulse duration is determined by the product of the capacitance of capacitor222and the sum of the values of resistors224and228, i.e. C222*(R224+R228). Current pulses248are conveyed to node232, so that an additional current248flows through resistors238,240and raises the potential of node214. This additional current248does not flow, however, when motor10is stalled, resulting in a low motor current when the motor is stalled.

Since more pulses50and248are generated per unit time as the rotation speed increases, this additional current through resistors238,240rises with increasing rotation speed so that the upper current limit rises with increasing rotation speed.

To ensure that the potential at node232and therefore also at node214is raised as much as possible, resistor234is preferably selected to be very much larger than the sum of resistors238and240.

Dynamic Current Raising in Response to Load Surges

An ECM10is designed so that it has a power reserve, i.e. its temperature is almost unaffected if increased power is demanded of it for only a brief time. If that same increased power were required from motor10on a continuous basis, however, it would overheat and be destroyed as a result.

It is thus very preferable to use dynamic current raising for load surges. This is accomplished with part260inFIG. 6, whose function has already been explained with reference toFIG. 5.

Output216of comparator204is connected via a capacitor262to the base of a pnp transistor264that in turn is connected via a resistor266to lead122. The collector of transistor264is connected to ground44. Its emitter is connected via a resistor268to lead122, via a resistor270to a node272, and directly to negative input274of a comparator276. Node272is connected via a resistor278to positive input280of comparator276, and via a resistor282to ground44.

Output284of comparator276is connected to the base of an npn transistor286, also via a resistor288to lead122and via a capacitor290to a node292, which in turn is connected to positive input280via a resistor294and to ground44via the series circuit of a resistor296and a diode298.

The collector of transistor286is connected to lead122, and its emitter via resistor300to node214.

When transistor286is conductive, resistor300(180 k) is connected in parallel with the series circuit of resistors234and238; the result is that potential u212at node214jumps to a higher value, and the upper current limit is raised as shown inFIG. 7, e.g. from 3.5 to 5.5 A.

If motor current Iistis too high, comparator204flips to LOW; this potential change is transferred through capacitor262to the base of pnp transistor264and makes it conductive, so that it bypasses resistors270,282and switches over comparator276, which is connected as a monoflop. Transistor264suppresses the positive pulses that are produced upon differentiation by capacitor262, so that only the negative pulses can cause comparator276to switch over.

Output284of comparator276is LOW in the idle state. When the monoflop is triggered, output284becomes HIGH for a period defined by components290,296,298and then flips back to LOW.

As long as output284is high, transistor286is switched on and an additional current flows through it and resistor300to node214, as already described. Transistor286acts in this context as an ideal switch, i.e. resistor300is decoupled from node214when transistor286is blocked.

The time during which output284is high is here approximately 1 second, and it is followed in each case by a period of at least 4 seconds during which output284is LOW; this results, as shown byFIG. 7, in short segments304of higher current separated from one another by long segments306of lower current. This prevents overloading of motor10, but allows adaptation to short-duration load surges that can occur in many drive systems.

When rotor24of motor10is stalled, the current limiting arrangement is continuously active, i.e. comparator204is continuously flipped, so that no pulses are transferred through capacitor262and circuit260is not activated.

No further pulses50are generated when rotor24is stalled, meaning also that no further current pulses248are generated. The current then drops as indicated by segment308ofFIG. 7, and at a standstill is limited to a low value310in order to prevent overheating of ECM10when it is stalled.

Upper current limit Isollis exceeded at time t20. As a result, comparator204switches to LOW and monoflop circuit260is activated (see description ofFIG. 5) so that upper current limit Isollis raised for the period T1(e.g. 1 second) determined by monoflop circuit260. Potential u216thus becomes high again at time t21.

At time t22motor current Iisthas returned to its normal level, for example because the brief additional load or interference is no longer present. Monoflop circuit260is deactivated after time period T1, and upper current limit Isollreturns to its original value. No further current excursions occur until time t24.

At time t24upper current limit Isollis once again exceeded, and output214is switched to LOW. Since monoflop circuit260, after completion of the upper current limit raising, does not permit a further raising for a period T2of, for example, 4 seconds (in order to protect the motor from overheating), the current excursion has no effect until time t26. Motor current Iistcannot rise further. Between t24and t26, as depicted, potential u216oscillates continuously between HIGH and LOW because here the current limiting function acts based on the present value of Isoll.

At time t26motor current Iistdrops back, in this example, below upper current limit Isolland output216goes back to HIGH.

At time t28upper current limit Isollis once again exceeded. Because time period T2has not yet elapsed, the upper current limit is not raised. That does not occur again until time t30, at which time period T2has elapsed. Starting at t30, upper current limit Isollis once again raised for time period T1. Motor current Iistcan thus briefly rise back to a higher value, as depicted at A.

At time t32motor current Iistdrops back into the normal range, and potential u216therefore becomes continuously high again. Time span T1ends at time t34, and the upper current limit is lowered back to the normal value.

The mode of operation of the arrangement ofFIGS. 5 and 6is based partly on the fact that potential u212at node214, which defines the upper current limit, is modified as a function of certain operating conditions so that it becomes either higher or lower; consequently the motor current is automatically limited, as a function of operating parameters of the motor, to various values in order to optimize utilization of the output capabilities of ECM10.

FIG. 8shows, as an example, the raising of the upper current limit in a motor that is designed for a rotation speed of approximately 10,000 rpm.

If the motor is stalled (rotation speed n=0), the motor current is limited to a value of approximately 1.4 A. Upper current limit Iw=f(n) rises to approximately 4.2 A at 10,000 rpm. The curve becomes flatter in the upper region and reaches a plateau; this flat region is placed, by the selection of electrical components222,224,228,230, in the vicinity of the motor's rated speed.

FIG. 8also shows a curve Iwmax corresponding to the raised upper current limit resulting from the activation of monoflop260. This causes the upper current limit at 10,000 rpm to be increased, for example, from approximately 4.2 to approximately 4.8 A; there is a corresponding increase in torque M, which is shown inFIG. 8on the left-hand scale and is proportional to the actual motor current. Since an external-rotor motor is effectively cooled at 10,000 rpm by the air turbulence that is generated, it can dissipate substantially more waste heat at that rotation speed than at a standstill, and the permissible motor current can therefore be substantially higher at 10,000 rpm than when the motor is stalled. This makes it possible to achieve higher rotation speeds, and thus greater power, with a motor of predetermined size.

It is also possible in the context of the invention to temporarily switch off the motor current completely when the motor is stalled, and to attempt a restart at regular time intervals.

Capacitor236at node232effects a smoothing of the potential at that node, resulting in a stable target value at comparator204.

Arrangement220is thus particularly advantageous for external-rotor motors, but can of course be used in all motors in which cooling improves with increasing rotation speed.

Current Raising at Start-Up

Capacitor236(1.5 [mu]F) has the additional function of being discharged at start-up and functioning briefly at that time as a short circuit for resistor234. The potential at node232is thereby briefly raised to +5 V, and the potential at node214rises to 0.38 V so that current Iistis limited to a high value. This is shown inFIG. 7at252, where after start-up the current limit drops within 0.5 second from 7 A to 5.5 A, so that motor10can start at a very high torque that is nevertheless quickly reduced.

The duration tSTARTof the starting pulse is defined approximately as:
tSTART=C236*R234*(R238+R240)/(R234+R238+R240)  (2)
Preferred Values of Components inFIG. 6

FIG. 11shows a preferred variant of the manner in which the rotation-speed-dependent signal50(FIG. 4E) created by circuit36(FIG. 3) at its output150is conveyed to node232for a circuit according toFIG. 5orFIG. 6. This variant differs fromFIG. 6by having the two components231,233. The remaining components are largely identical toFIG. 6and are therefore not described again. InFIG. 11, the base of pnp transistor226is connected to ground44via the series circuit of a resistor231and a capacitor233.

At start-up, the previously discharged capacitor233is charged through resistors228and231. The voltage drop at resistor228during this charging operation makes transistor226conductive temporarily, thereby connecting resistor230in parallel with resistor234so that the potential at node232is greatly raised during this period. The duration Tstart233of this raising is determined approximately by
Tstart233=(R228+R231)*C233(3).

Pulses248(FIG. 6) at a rotation-speed-dependent frequency are then conveyed through resistor230to node232in order to raise the upper current limit with increasing rotation speed, as described with reference toFIGS. 5 and 6and depicted inFIG. 8. During Tstart233, the value Isoll(which is defined by the potential at node214) exhibits an elevation in the form of a plateau239(FIG. 12) which is overlaid on the elevation due to capacitor236and allows a longer start-up raising of upper current limit Isollto be achieved. Greater inert masses can thus be accelerated, and the raised current allows a high dynamic starting torque. At the same time, the motor is protected in the event of stalling, since in such a case transistor226is blocked and motor current Iistis limited to a low value (seeFIG. 7).

Preferred Values of Components inFIG. 11

The invention thus concerns a method for operating an ECM10that is equipped with a current limiting arrangement. The latter acts on a PWM controller that, during operation, delivers PWM pulses having a controllable pulse duty factor pwm and a substantially constant frequency. If a predetermined upper limit Isollfor the motor current is exceeded, the current limiting arrangement causes a modification of pulse duty factor pwm for pulses60delivered by PWM controller56in order to reduce the motor current. If the motor current exceeds a predetermined upper limit value Isollwhile the ECM is rotating, that limit value is raised for a predetermined time period304(FIG. 7), and as a result the maximum available motor power is temporarily raised—usually for a few seconds—in the event of a load peak. If rotor24is stalled, the limit value is not raised but is instead lowered further. Upper limit value Iwis preferably also raised to a plateau as a function of rotation speed n of the motor, as depicted inFIG. 8. The aforesaid features can be applied individually or in any combination.

The invention makes it possible, in very simple fashion, to utilize the power of an ECM10in better fashion than before, without requiring a special (thermal) simulation of the motor for that purpose. The definition of upper current limit Isoll(IwinFIG. 8) in the motor's rotation speed range is variable within wide limits. Many other variants and modifications are of course also possible in the context of the present invention.