Method of Starting an Electronically Commutated Direct-Current Motor

The invention concerns a method of starting an electronically commutated direct-current motor with several cyclically activated coils by means of commutating signals emitted while the motor rotates.

There is always a problem involved in detecting and processing the actual direction and phase relation of rotation of motors of this type. Using several sensors to detect the direction and phase relation for example is known. The sensors, however, make the device more expensive. Systems with only one sensor that process the differently coded sensed scanned signals in order to detect direction and phase relation are also known. This solution, however, cannot be employed to correctly process the different codes below a prescribed minimum motor speed, at which point there is a substantial period during motor start-up when phase relation and direction of rotation do not conform to what has been set.

Getting a motor of the aforesaid type into the correct direction of rotation by initially stationing it in a prescribed position is also known. During this positioning period the commutation indexing is disengaged, and the commutation stage will only start the motor up from this prescribed initial state once the positioning phase is over.

It is important for a motor, especially one that drives the head drum in a videorecorder, to start moving in the correct direction as soon as possible, before the magnetic tape is threaded in.

The object of the invention is to obtain reliable start-up in the correct direction in the briefest possible time with only one sensor. This object is attained by the invention recited in the claim.

The invention will now be described with reference to the drawing.

FIG. 1 is a schematic diagram of a motor with a bipolar magnet as a rotor,

FIG. 2 is a graph illustrating the function of the system in FIG. 1, and

FIG. 3 shows the sequence of motions that occur during the motor's start-up phase.

The rotor consists of a bipolar magnet 4 with marks M1, M2, and M3, which operate in conjunction with a sensor 5, distributed around its circumference. The pulses m1 to m3 obtained by sensor 5 are supplied to a processing circuit 6 that cyclically switches a voltage U to outputs A1 to A3 every time it encounters a pulse. Outputs A1 to A3 are connected to stator windings 1, 2, and 3. The cyclic activation results in a rotating field for magnet 4.

Assume for the purpose of illustrating how the method works that magnet 4 is in an especially undesirable initial state and activating situation when the motor is started up. At time t1 the motor is placed into operation and it is assumed that output A3 applies voltage U to coil 3 of the stator. Magnet 4 is accordingly initially started up counterclockwise for a period T3.1. At time t2 mark M3 arrives at sensor 5 and generates a commutating pulse K1.1 due to the cyclic indexing of processing circuit 6. Since this supplies stator coil 1 with current, the motor is initially braked as indicated by the counterclockwise broken line. As soon as the north pole of magnet 4 rotates over winding 1' on coil 1, the coil will accelerate it will time t3. Subsequent to period T1.1 mark M2 arrives at sensor 5 and generates a pulse m2, which switches coil 2 on. Since magnet 4 is braked again because coil 2 of the stator attracts it, the magnet reverse direction clockwise at time t4. At time t5 mark M2 arrives at sensor 5 again and generates a pulse m2, which switches on the next coil, coil 3. Since coil 3 now extends along the new direction of rotation, the motor will run in the nominal direction. Subsequent to a period T3.2, coil 1 will be subjected to mark M3 at time t6. This terminates the motor start-up phase and the motor can be cyclically indexed by known commutation methods involving pulses K1 to K3.

The sequence of motions involved in the motor start-up phase is illustrated in FIG. 3. The individual phases described with reference to FIG. 1 are represented now by steps a through h. The just activated coil is indicated by the letter N and S. It will be evident that the direction of rotation changes at time t4 and that control in the correct sequence occurs at coils 1, 2, and 3 from the commencement of start-up.

Although the invention has been described with reference to a motor with three coils, employing the method to start a motor with more than three coils is within the scope of the invention.

The commutation marks M can also be coded to distinguish them from marks employed to provide other information.