Method for controlling a brushless electric motor
The invention describes a method for controlling a sensorless, brushless DC motor that is controlled via a bridge circuit preferably using pulse width modulation (PWM) having precommutation. According to the invention, in the braking operation each commutation interval is divided into two phases. A drive phase, lasting from the start of the commutation interval up to zero crossing in which the motor is controlled with the normal PWM, and a braking phase, lasting from zero crossing up to the next commutation time in which at least two motor phases are short-circuited and/or in which the PWM is operated with recovered energy. The efficiency of the control circuit is as low as possible so as to ensure that the braking energy is consumed in the control circuit.
1. A method for controlling a brushless DC motor over a plurality of commutation intervals using a bridge circuit, the method comprising:
receiving, by a controller, an indication of a zero crossing of a back electromotive force (BEMF) voltage from a sensing circuit, the BEMF voltage being induced in phase windings of the brushless DC motor during each commutation interval of the plurality of commutation intervals;
for each commutation interval of the plurality of commutation intervals, dividing, by the controller, each commutation interval into a drive phase and a braking phase based on the received indication of the zero crossing of the BEMF voltage, wherein the drive phase lasts from a start of each commutation interval up to the zero crossing of the BEMF voltage, and wherein the braking phase lasts from the zero crossing up to a next commutation interval;
determining, by the controller, if an actual rotation per minute (RPM) of the brushless DC motor exceeds a nominal RPM; and
in response to determining that the actual RPM of the brushless DC motor exceeds the nominal RPM, short-circuiting at least two phase windings during at least one braking phase of the plurality of commutation intervals to dissipate mechanical energy in the brushless DC motor.
2. The method for controlling a brushless DC motor according to claim 1 , wherein the brushless DC motor is commutated without using sensors.
3. The method according to claim 1 , wherein the brushless DC motor is operated using pulse width modulation (PWM).
4. The method according to claim 1 , wherein in each braking phase at least two phase windings are charged with current via the BEMF voltage from mechanical energy in the brushless DC motor, the current being induced in part from energy made available from one phase windings by switching on at least two switches on a first side of the bridge circuit, followed by switching off a switch on the first side of the bridge, followed by switching on a switch on a second side of the bridge circuit, the second side being different from the first side.
5. The method according to claim 1 , wherein the bridge circuit has MOSFET switches.
6. The method according to claim 1 , wherein braking energy is stored in at least one capacitor.
7. The method according to claim 6 , wherein the braking energy stored in the capacitor in the braking phase is used in the drive phase for driving the motor.
8. The method according to claim 1 , wherein the brushless DC motor has a control circuit having only one single Hall sensor.
9. A brushless electric motor comprising:
a control circuit comprising a bridge circuit and a motor driver for controlling the brushless electric motor over a plurality of commutation intervals;
a controller electrically coupled to the control circuit and configured to:
receive an indication of a zero crossing of a back electromotive force (BEMF) voltage from a sensing circuit, the BEMF voltage being induced in phase windings of the brushless electric motor during each commutation interval of the plurality of commutation intervals;
for each commutation interval of the plurality of commutation intervals, divide each commutation interval into a drive phase and a braking phase based on the received indication of the zero crossing of the BEMF voltage, wherein the drive phase lasts from a start of each commutation interval up to the zero crossing of the BEMF voltage, and the braking phase lasts from the zero crossing up to a next commutation interval;
determine if an actual rotation per minute (RPM) of the brushless electric motor exceeds a nominal RPM; and
in response to determining that the actual RPM of the brushless electric motor exceeds the nominal RPM, send a signal to cause a short circuiting of at least two phase windings during at least one braking phase of the plurality of commutation intervals to dissipate mechanical energy in the brushless electric motor.
10. The brushless electric motor of claim 9 , wherein a diode is disposed at a supply voltage input of the control circuit to prevent braking energy from being fed back into a voltage supply, and wherein the control circuit has a capacitor for storing the braking energy dissipated due to short circuiting the at least two phase windings of the brushless electric motor during the braking phase.
11. The brushless electric motor according to claim 9 , wherein efficiency of the bridge circuit during each braking phase is kept low such that braking energy is consumed by the control circuit and is thermally distributed about the control circuit in an even manner.