Analog Three Phase Self Excited Brushless Direct Current Motor

A brushless direct current three-phase motor that is self driven and therefore does not require externally generated waveforms for its operation. The circuit connected to the motor is analog and reduces the complexity and present cost of the driver circuitry. There is no electronic commutation of the currents in the stator coils as is the case with other brushless motors.

DETAILED DESCRIPTION OF THE INVENTION

The motor in this embodiment of this invention has its rotor magnets rotating axially, but the motor can be constructed to make the magnets rotate radially as well.

InFIG. 1, a circular end plate10of the motor, has six stator cores8made of soft iron laminates permanently attached to it. The stator cores have copper wire coils9wound around them and each coil has its diametrically opposite coil connected to one of its ends, so that both coils form a series or parallel circuit consisting of two coils. The coils are connected in such a way that their stator cores will have the same electromagnetic polarity when an electric current flows through them. This is shown inFIG. 2where there are six coils designated. L1L2, L3L4, L5L6connected in series. When the motor components are assembled, the end plates fit snugly into the motor casing and therefore will not move freely.

InFIG. 1, a circular plate7is rigidly fixed to the motor shaft5and four permanent magnets6are affixed to this plate. The magnets are placed onto the plate with alternate polarities facing the stator cores, to which they are in close proximity. The motor shaft fits into the bearing3at the center of end plate4and also fits into the bearing in the center of end plate10. The plate7is designated as the rotor, but the entire combination of this plate, the magnets and the shaft will rotate when the motor operates, so from here on, the term rotor will include these three components. The rotor is free to turn in the bearings.

The end plate4inFIG. 1has three Hall sensors2permanently affixed to its upper surface and they are affixed so that they are one hundred and twenty degrees apart. The Hall sensors are reactive to one magnetic polarity and not the other, depending on which side of the sensor is facing the magnets and which magnetic pole is facing the sensor. Whenever the magnetic pole to which the sensor is reactive is in the immediate vicinity of the sensor, the sensor will output a signal. The signal from each sensor is fed into the drive circuit ofFIG. 2and they are used to synchronize the switching on and off of the current flowing through each stator coil pair.

A cylindrical container1holds the motor components in place and also serves as a cover to keep dust and other contaminants out of the motor.FIG. 1is an expanded view of the motor and so the components might appear further apart than they are in an actual motor. In an actual motor, the distance between the top of the stator cores and the lower end of the rotor plate7is in the order of 0.05 inch (1.26 MM). The upper side of the magnets is around 0.1 inch (2.5 MM) from the lower side of end plate4.

The rotor of the motor is made to rotate by switching the stator coils on at the appropriate time. This is achieved by placing Hall sensors one hundred and twenty degrees apart and adjacent to the circular path of the magnets on the rotor. InFIG. 1, any pair of stator coils can be designated as L1L2by the reader and the other stator coils follow in sequence. The same principle is true for the Hall sensors. InFIG. 2, when HS1has a high output (it is not active), Q1is turned on, and current will flow through L1L2, causing an electromagnetic field to form around the stator cores surrounded by these two coils. This is assuming that PNP transistor Q5is on and is supplying power to the circuit. If the stator poles have become north poles, both diametrically opposite poles will repel the magnets that are north poles and facing the stators. At the same time, they attract the magnets that have south poles and are facing the stators. This action eliminates the need for electronic commutation and also, all four magnets are interacting with at least two stator poles at all times. This is useful for reducing torque ripple.

At the point where magnetic south poles are positioned directly above L1L2there is zero torque, due to the attractive forces between the magnets and north poles of the stator cores. Hall sensor HS2is switched off at this point and so L3L4are energized by Q2. Magnetic north poles are again repelled and magnetic south poles are attracted to the cores of L3L4. The action continues and soon L5and L6are energized. When L5and L6have interacted, the rotor has gone through one complete cycle and Q1is again turned on to start a new cycle

The motor needs to run as efficiently as possible and have a means for controlling its speed. The circuit ofFIG. 2provides for smooth speed control and efficient power conversion. Its operation is as follows: Electrical power is supplied as voltages between UNREG+V, REG+V and GND− (seeFIG. 2). The three Hall sensors have their outputs connected to REG+V via three resistors R1, R2, R3. This makes the outputs normally high, but if a magnet is activating a sensor, that sensor's output will be low. When power is turned on, one, or two, sensors might be activated (low output). If two sensors are activated, then the third sensor will be high. Let us say that HS1is left high when power is turned on. This high will cause NPN transistor Q1to go low at its collector and current will flow through L1L2when a voltage is supplied by the collector of PNP power transistor Q5. This action causes the network of R4, R5, and R6to go low at their point of common connection. This common point is directly connected to the inverting input of voltage comparator U1. The Non-inverting input of U1is connected to the speed control potentiometer VR1.VR1sets the voltage on the non-inverting input of U1. The motor will have a minimum speed at which it will operate, so VR1can set+input of U1to the voltage representing the lowest speed at which the motor will operate. The lowest speed setting will be such that it is always higher than the short circuit voltage across Q1, when Q1has been turned on. This allows the motor to easily start up when power is turned on at first and a low speed is required.

Let us assume that at this instance, the speed is set to minimum. Because Q1has been turned on by HS1, its collector goes low and because the+input of U1is higher than the—input, the output electrode of U1which is connected to R7goes high. This causes current to flow through R7into the base of NPN transistor Q4. Q4has a capacitor C1connected to its collector to reduce noise in the circuit. Q4's collector goes low and draws current through the base-emitter junction of PNP power transistor Q5. Q5therefore conducts and applies voltage to the common junction of the stator coil combinations L1L2, L3L4, L5L6. During this time, Q1has been on, so current will flow through Q5, L1L2and Q1to GND−. The rotor will turn because the electromagnetic fields created by the current flowing through L1L2cause their stator poles to interact with the magnets in their immediate vicinity. The rotor is turning slowly because the rotor will need several rotations before the motor is up to speed. The first time that Q5supplies current, it will be the largest current it supplies. It is the largest because the reactance of the coils will be at their lowest, when the rotor is at its lowest speed. After the rotor starts turning, its inertia keeps it going and the voltage at the common point of R4R5R6eventually increases. This increase in voltage occurs because as speed increases, the reactance of the coils increases, thereby reducing the current through Q1. Q1's collector voltage rises as a consequence. As Q1collector voltage increases, U1−input goes higher than U1+input and Q5is turned off. Because Q5is off, there is no power to any of the three transistors Q1, Q2, Q3, so the motor slows down. It slows to a speed where the voltage at U1−input is again lower than the voltage at U1+input. Q5is turned on again. The motor speeds up again. The cycle continues, but the voltage on U1−input can only rise to the level that will be enough to surpass the set voltage on U1+input, before the power to the output transistors Q1, Q2, Q3will be cut off by Q5. Therefore, the speed at which the rotor will turn is limited by the voltage set by VR1. If VR1sets a higher voltage, then U1−input needs a higher voltage from the R4, R5, R6junction before the power to the output transistors can be shut off. This means that the motor will have to be running at a higher speed and is therefore on for a longer time than it is off. Speed control of the motor has therefore been achieved. The fact that the motor has to be on for a longer time in order to go faster, and on for a shorter time to go slower, means that power to the motor is pulse width modulated. Pulse Width Modulation is a more efficient way to power a motor.