Speed Control Motor/Brushless Motor Structure and Operating Principle of Brushless Motors
Structure of Brushless Motor
Brushless motors have built-in magnetic component or optical encoder for detecting the rotor position. The position detectors send signals to the drive circuit. The brushless motor winding uses three-phase wiring in a "star" connection. The rotor uses permanent magnets.
The detection magnetic component uses a Hall effect IC. 3 components are provided on the inner side of the stator and as the rotor turns, each Hall effect IC outputs a digital signal.
Drive Method of Brushless Motors
The motor windings are connected to switching transistors, where 6 transistors constitute the inverter. The top and bottom transistors turn ON and OFF according to a predetermined sequence to change the direction of current flow in the windings. The mechanism of brushless motor rotation can be described as follows:
In step [1] of the transistor's switching sequence, as shown in the following figure, transistors Tr1 and Tr6 are in the ON state. At this time the winding current flows from phase U to phase W, and excites U and W to N and S poles, respectively. This causes the rotor to rotate 30˚. When this operation is repeated 12 times, the rotor turns by 1 rotation.
Switching Sequences of Individual Transistors
| [1] | [2] | [3] | [4] | [5] | [6] | [7] | [8] | [9] | [10] | [11] | [12] | [13] | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Tr1 | ON | ON | ON | ON | ON | ||||||||
| Tr2 | ON | ON | ON | ON | |||||||||
| Tr3 | ON | ON | ON | ON | |||||||||
| Tr4 | ON | ON | ON | ON | |||||||||
| Tr5 | ON | ON | ON | ON | |||||||||
| Tr6 | ON | ON | ON | ON | ON | ||||||||
| Phase U | N | − | S | S | − | N | N | − | S | S | − | N | N |
| Phase V | − | N | N | − | S | S | − | N | N | − | S | S | − |
| Phase W | S | S | − | N | N | − | S | S | − | N | N | − | S |
Control Method of Brushless Motors
The drive circuit of a brushless motor is connected to the motor and mainly consists of 6 blocks according to the structure shown in the figure.
Power Supply Circuit (Converter)
The power supply circuit converts AC voltage from a commercial AC power supply into DC voltage by rectifying it with a diode bridge and smoothing it with a capacitor. The DC voltage is supplied to the output circuit and is used to drive the motor.
DC input brushless motors do not have a power supply circuit. The DC voltage is supplied directly from the DC power supply to the output circuit.
Speed Controller
The speed controller compares the speed command with the motor speed (Velocity actual value) output from the speed converter and outputs a voltage command so that the motor speed follows the speed command.
A control system using information from the speed command and Velocity actual value is called a speed loop.
Speed Converter
The speed converter converts the frequency of the hall effect IC output into a Velocity actual value and outputs it to the speed controller.
A three-phase brushless motor has 3 hall effect ICs, which output 3 pulse signals for each pair of N- and S-poles of the rotor magnetic poles. By detecting the up and down edges of the pulse signals, 6 signals are obtained.
Excitation Signal Generator
The excitation signal generator instructs the power supply controller which of the 6 switching elements in the output circuit will carry the current.
As shown in the figure, the rotor magnetic pole angle is detected from the combination of output signals from the hall effect ICs (Ha, Hb, and Hc) located inside the motor. The switching elements connected to the torque-generating windings in response to the direction of the rotation direction are directed to the power controller.
Voltage Controller
The voltage controller outputs a signal to apply the voltage commanded by the speed controller to the switching elements of the output circuit as indicated by the excitation signal generator.
Output Circuit (Inverter)
The output circuit applies PWM control to the motor windings according to commands from the voltage controller.