Method for controlling an electronically commutated dc motor

#The present invention is directed at a method of controlling an electronically commutated DC motor.

#In an electronic control circuit for an electronically commutated DC motor (EC motor) having a three-phase stator winding and a permanent magnet-excited rotor as referred to in German Published Patent Application No. 43 10 260, three semiconductor switches configured as MOS FETs are each connected in series with a winding phase of the stator winding, and the three series circuits are connected in parallel. The control terminals of the semiconductor switches receive commutating signal-triggered control signals, which depend on the rotor position, so that the semiconductor switches may be triggered using a current flow angle, definable by the length of the control signals, within the commutating angle (block control). In order to avoid undesired features of block control in the lower rpm range (occurrence of high current peaks at low motor speeds and noisy operation), in a lower rpm segment at 100% triggering, the amplitude of the control signal is increased with increasing rpm up to a first rpm (linear control), then with increasing rpm up to a second rpm the amplitude of the control signals is increased to a maximum and at the same time the degree of triggering of the current flow angle (block length) is reduced from 100% to a lower value. Then, starting at the second rpm, with increasing rpm up to the maximum rpm, the degree of triggering is increased again from the lower value to 100% with maximum amplitude of the control signals. The pure block control in the upper rpm range may avoid the lower efficiency, which may be inherent in linear control.

#In variable-speed EC motors, also known as brushless DC motors, there are motor topologies where the power loss in the semiconductor switches does not increase with the motor output, but rather is higher in the partial load range than in the full load range. This may be undesirable in particular for drives whose intrinsic cooling increases with increased power output of the DC motor, such as for example in pump motors, which are cooled by the medium pumped. Such motor topologies may be encountered, for example, in EC motors having a single-strand or multistrand, even-numbered multiphase winding, for example, a double-strand four-phase winding or a three-strand six-phase winding. These EC motors may be controlled in cycled operation by pulse-width (PW) modulation. With increased pulse control factor of the cycle, i.e., switched-on time of the semiconductor switch in relation to the cycle length, the power losses in the semiconductor switches increase more than proportionally, so that such EC motors are not operated in the cyclic mode in the upper motor output range, and the motor output is changed by varying the block length via the block control. In this case, the time periods in which the individual winding phases are energized increasingly overlap. In this type of control, the maximum power loss in the semiconductor switches may occur shortly before the transition from cycled operation to block operation.

#FIG. 1 shows a block diagram of an EC motor having an electronic control.

#FIG. 2 shows a diagram of the power loss of the semiconductor switches in the EC motor as a function of the EC motor rpm.

#FIG. 3 shows a diagram of the control signals for the semiconductor switches in each winding phase for three different control modes.

#In the exemplary embodiment illustrated in FIG. 1 as a block diagram for elucidating the exemplary method according to the present invention, an electronically commutated DC motor, referred to hereinafter as EC motor 10, is set, i.e., regulated at a predefinable setpoint rpm nsetp. EC motor 10 has a two-strand, multiphase stator winding 11 having an even-numbered m, here m=4, winding phases 111-114 and a rotor 15 excited by a permanent magnet. Winding phases 111 and 112, as well as 113 and 114 of each winding strand are wound in opposite directions and inductively coupled. Each of winding phases 111-114 is connected in series with a semiconductor switch 12, which is configured in this case as a MOS-FET. The four series circuits each composed of one of winding phases 111-114 and one semiconductor switch 12, together with a capacitor 13, are connected into a parallel circuit, which is connected to a DC system 14, the point of common coupling of the four winding phases 111-114 being connected to the positive pole of DC system 14 and the point of common coupling of semiconductor switches 12 being connected to frame potential.

#Winding phases 111-114 are also connected to a commutating device 16, in which the voltages induced in winding phases 111-114 are further processed. Furthermore, in commutating device 16, an rpm signal is generated from the induced voltages which corresponds to actual rpm nact of EC motor 10 and is applied as an rpm-proportional DC signal to a comparator 17, configured for example as a differential amplifier and which also receives setpoint rpm nsetp. Comparator 17 compares setpoint rpm nsetp and actual rpm nact, and the difference is supplied to an rpm regulator 18. The regulator output signal is applied to the input of a pulse width modulator 19. Pulse width modulator 19 generates a separate control pulse train for each winding phase 111-114; these control pulse trains are gated with the commutating signals in commutation device 16. Semiconductor switches 12 of the individual winding phases 111-114 are triggered by the control signals obtained from these logic operations, so that each semiconductor switch 12 is cycled with an rpm-dependent pulse control factor when energized. The cycling of semiconductor switches 12 determines the magnitude of the direct voltage applied to EC motor 10, i.e., its stator winding 11, and the rpm is varied by varying this voltage; the nominal torque may be fully taken into account at all rpm levels.

#In an EC motor 10 thus controlled, the power loss of semiconductor switches 12 increases with increasing pulse control factor, i.e., pulse width divided by the pulse period, in other words, with increasing pulse width modulation. Therefore, the motor control via pulse width modulation is restricted to the lower half of the output power spectrum in cycled operation, and in the upper half the motor output is varied by block control, specifically by increasing the angle where each winding phase is energized beyond the commutating angle. In the exemplary embodiment of the four-phase EC motor according to FIG. 1, the commutating angle is 90 electrical degrees. The control mode is switched for semiconductor switches 12 at an rpm nb that is much less than the idling rpm, which is achieved using a 100% pulse width modulation when the winding is energized over an angle of 90 electrical degrees.