Patent Application
20050135794
The invention relates generally to brushless DC motors and more specifically to methods and systems to control negative torque and provide variable speed control in a brushless DC motor.
A brushless DC motor typically has a stator with a plurality of teeth and a rotor with permanent magnets mounted on it. When wire-wound coils on the teeth are energized with current, the stator and rotor interact to produce positive or negative torque, depending on the direction of the current with respect to the polarity of the magnets. In motors of this type, an electronic inverter bridge controls energization of the stator winding for controlling the direction and amount of torque produced by the motor as well as for controlling the rotor shaft speed. The inverter bridge typically has a number of power switching devices for connecting the motor's winding or windings to a power supply.
The negative torque produced by the brushless DC motor is essentially due to the motor inductance. Once the rotor pole is past the magnetic neutral axis of the stator, the stator pole polarity has to be reversed. This reversal is done by reversing the current through the stator windings, by means of turning off or on a pair of power switching devices. Due to the motor inductance, it takes a certain amount of time for the current in the motor windings to reverse direction. Additionally, the amount of time for current reversal depends on the DC bus voltage and the magnitude of current. Due to the delay in current reversal (and hence delay in stator pole polarity reversal), the stator pole ‘pulls back’ the rotor pole instead of propelling it forward. This gives rise to production of negative torque, which will cause a deceleration of rotor as well as stress and vibrations in the mechanical assembly.
Some of the techniques being used for reducing negative torque include conduction angle control, current mode control (peak current control or average current control or hysterisis current control with low inductance motor), and advancement of position sensor along with a square wave control. These have some inherent limitations, for example, conduction angle control needs expensive microcontroller/DSP/ASIC for implementation. Current mode control works only with specially designed low inductance motors and to achieve the same torque level as a voltage mode motor, the current through the current mode motor has to be much higher. This results in higher losses in semiconductor devices and use of higher rating devices. Position sensor advancement usually works well only for a certain speed and is non-optimal for other speeds of operation
It would therefore be desirable to have a system and a method for controlling the negative torque in a brushless DC motor, which is simple, inexpensive and easy to implement.
Briefly, in accordance with one aspect, a method for reducing negative torque in a brushless single phase DC motor is provided. The method includes initiating (or starting) the motor in a normal mode of operation and activating an ON time control to cut off a voltage supply to the motor. The ON time control is applied after a predetermined time delay, the delay being defined by the motor parameters.
In accordance with another aspect, a system for controlling negative torque in a brushless DC motor is provided. The system includes a positional sensor for producing a sensor signal based on polarity of a rotor of the motor and a control circuitry for activating an ON-time control to cut-off a voltage supply to the motor, the activating of the ON-time control is synchronized with an edge of the sensor signal.
The foregoing and other advantages and features of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:
Referring now to the drawings,
A motor shaft 18 mechanically connects the rotor 16 to a particular device to be driven, such as a rotatable component 20. For example, the rotatable component 20 comprises a fan, blower, compressor or the like for use in a heating, ventilating and air conditioning system or refrigeration system. Although motor 12 is particularly useful for driving a fan, it is to be understood that motor 12 may be part of a number of different systems for driving other rotatable components. In addition, rotatable component 20 may include a connection mechanism for coupling it to the shaft 18.
A user interface, or system control, 22 provides system control signals to a control circuit 24. The system control signals take the form of motor commands representing, for example, turn on and turn off commands, desired fan speeds and the like. In response to the system control signals, the control circuit 24 then generates motor control signals. As represented by the block diagram of
A power supply 30 provides high voltage DC power to switches 28. Power switches 28 then provide power to motor 12 by selectively switching the power supply 30 in connection with the motor winding(s) (not shown) included in stator 14.
Referring further to
Power switches 28 energize the motor winding in a preselected sequence for commutating motor 12 in response to control circuit 24. The preselected sequence, as would be appreciated by those skilled in the art, depends on the positional sensor signal. In this instance, control circuit 24 selectively activates power switches 28 to control rotation in motor 12 as a function of the motor control signals. It is to be understood that power supply 30 may also provide power to operate control circuit 24.
The control circuit 24 generates its control signals as a function of the estimated zero crossings of the back electromagenteic field of motor windings. As is generally known in the art, the product of the current and the back electromagnetic field determines torque production in motor 12, and in conventional systems, the motor 12 develops considerable amount of negative torque when under normal operation. In one example, a square wave mode of operation is used for initiating the motor 12. Under square wave mode of operation, the position sensor signal is used directly, without any modifications, to control the motor voltage. For example, if the position sensor signal is ‘high’, motor gets+ve voltage and when the position sensor signal is ‘low’ the motor gets−ve voltage. To overcome the negative torque produced under normal square wave mode operation, the control circuit 24 includes an ON time control 34 which is turned on after a predetermined time delay after the motor is started under normal operation. ON time control is not activated right at start of the motor, because the ON time control employs fixed time duration pulses. When motor is stationary, these fixed time duration pulses will not be enough for the motor to start rotating. However once the motor speed has picked up, these pulses are sufficient to maintain the rotating motion. The predetermined time delay depends on the motor specification and also on load conditions. In an exemplary embodiment, a half HP (Horse Power) i.e approx. 340 Watts brushless DC motor was used and the ON time control was switched on after 10 seconds. Once the ON time control 34 is activated, the voltage (power) supply to the motor 12 is cut-off and the motor is allowed to run at the desired speed. The ON time control 34 is embodied in one example, by a low cost analog circuitry using for example a timer integrated circuit, a comparator and logic gates.
The positive torque is sustained by the use of ON time control 34 since the motor winding is now energized when the back electromagnetic field has crossed zero in the direction that will oppose the voltage energizing them. Since the voltage supply is cut-off, the current through motor 12 towards end of each cycle will reduce, and this aids the motor current to build rapidly in the reverse direction when a voltage of opposite polarity is applied during the next half cycle through the commutation circuit 24.
As would be appreciated by those skilled in the art, the negative torque reduction methods and systems as described in above embodiments will be advantageous for a variety of applications including but not limited to high ventilating air conditioning (HVAC), refrigeration equipments, home appliances like vacuum cleaners, washing machines and in air filtration systems.
While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.
