1 Introduction
With the improvement of people's living standards, product quality, precision, performance, automation level, functionality, power consumption, and price have become major factors in choosing home appliances. Permanent magnet brushless DC motors combine the advantages of AC servo motors (simple structure, reliable operation, and convenient maintenance) with the excellent speed control characteristics of DC servo motors without a mechanical commutator, and are now widely used in various speed control drive applications. The emergence of Motorola's second-generation dedicated motor control chips has greatly facilitated the design of permanent magnet brushless DC motor speed control devices. These chips offer strong control functions, comprehensive protection functions, stable operation, and require simple peripheral circuits for the system, exhibiting strong anti-interference capabilities. They are particularly suitable for harsh working environments and applications with high requirements for controller size, price, and performance ratio.
2. Controller Structure and Principle
2.1 Controller Structure
The MC33035 is a second-generation dedicated integrated circuit for brushless DC motor control developed by Motorola. Combined with an MC3309 electronic tachometer to convert the rotor position signal of the brushless DC motor from F/V to generate a speed feedback signal, a closed-loop speed control system can be formed. By connecting six external power switching devices to form a three-phase inverter, a three-phase permanent magnet brushless DC motor can be driven. The controller circuit is shown in Figure 1. In the figure, S1 controls the motor direction, S2 controls the system start and stop, S3 selects whether the system operates in open or closed loop, S4 controls the system braking, and S5 selects the rotation speed.
The sub-position detection signal is in 60° or 120° mode, and S6 is used for resetting the control system. Potentiometer RP1 is used to set the desired motor speed, and LED L1 is used for fault diagnosis.
The system indicates that when an abnormal position detection signal, main circuit overcurrent, one of three undervoltage conditions (chip voltage below 9.1V , drive circuit voltage below 9.1V , reference voltage below 4.5V ), internal chip overheating, or low level at the start/stop terminal occurs, L1 will illuminate as an alarm, and the system will be automatically locked. Normal operation will only resume after the fault is cleared and the system is reset.
2.2 Control Principle
The three-phase position detection signals (SA , SB , SC) from the motor rotor position detector are fed into the MC33035. The chip's internal decoding circuit, combined with the control logic signals from the forward/reverse control, start/stop control, braking control, and current detection terminals, processes the signals to generate six raw control signals for the inverter's three-phase upper and lower bridge arm switching devices. The three-phase lower bridge switching signal undergoes pulse width modulation (PWM) processing according to the brushless DC motor speed control mechanism. The processed three-phase lower bridge PWM control signals (Ar , Br , Cr) are shaped and amplified by the drive circuit before being applied to the inverter's six switching transistors, generating the three-phase square wave AC current required for normal motor operation.
On the other hand, the rotor position detection signal is also fed into the MC33039 for F/V conversion, resulting in a pulse signal FB with a frequency proportional to the motor speed. FB is filtered by a simple RC network to form a speed feedback signal. Using the error amplifier in the MC33035, a simple P regulator can be constructed to achieve closed-loop control of the motor speed, thereby improving the mechanical stiffness of the motor. In practical applications, various external PI and PD regulator circuits can be connected to achieve more complex closed-loop regulation and control.
3-chip function
3.1 Structural composition and function of MC33035
Its main components include:
(1) Rotor position sensor decoding circuit;
(2) Internal reference power supply with temperature compensation;
(3) A sawtooth wave oscillator with a settable frequency;
(4) Error amplifier;
(5) Pulse Width Modulation (PWM) Comparator;
(6) Output drive circuit;
(7) Fault outputs such as undervoltage lockout protection chip overheat protection;
(8) Current limiting circuit.
Typical control functions of this integrated circuit include PWM open-loop speed control, enable control (start or stop), forward and reverse rotation control, and energy consumption braking control. With the addition of some external components, soft start can be achieved.
3.1.1 Rotor position sensor decoding circuit
This decoding circuit converts the rotor position sensor signal of the motor into six drive output signals: three upper drive outputs and three lower drive outputs. It is suitable for sensors such as Hall effect integrated circuits with open collectors or optocoupler circuits. Input pins 4, 5, and 6 are equipped with boost resistors. The input circuit is TTL level compatible with a threshold voltage of 2.2V . This integrated circuit is suitable for three-phase brushless motors with sensor phase differences of 60°, 120°, 240°, and 300°.
With three input logic signals, there are eight possible logic combinations. Six of these are normal states , which determine the different position states of the motor. The remaining two combinations correspond to abnormal position sensing states, i.e., open circuits or short circuits to ground on the three signal lines. In this case, pin 14 will output a fault signal (low level).
The direction of motor rotation is determined by the logic level of pin 3. When the logic state of pin 3 changes, the sensor signal is converted from its original logic state to NOT in the decoder. After further decoding, an inverted commutation output is obtained, causing the motor to reverse. The start and stop control of the motor is implemented by the enable pin 7. When pin 7 is floating, an internal current source keeps the drive output circuit working normally. If pin 7 is grounded, the three upper drive outputs are open (state 1), and the three lower drive outputs are forced to a low level (state 0), causing the motor to lose excitation and stop. At the same time, the fault signal output is zero.
When the braking signal applied to pin 23 is high, the motor performs braking operation. This opens the three upper drive outputs and makes the three lower drive outputs high, turning on the three lower power switches of the external inverter bridge, shorting the three winding terminals of the motor to ground, thus achieving energy-saving braking. The chip has a built-in quad AND gate circuit whose inputs are the braking signal from pin 23 and the three upper drive output signals. Its function is to wait until the three upper drive outputs have indeed turned high before allowing the three lower drive outputs to turn high, thereby avoiding the danger of the upper and lower switches of the inverter bridge turning on simultaneously. Its control truth table is shown in Table 1.
3.1.2 Error Amplifier
This chip incorporates a high-performance, fully compensated error amplifier. In closed-loop speed control, this amplifier offers a DC voltage gain of 80dB, a gain bandwidth of 0.6MHz , and an input common-mode voltage range from ground to VREF (typically 6.25V ), providing excellent performance. For open-loop speed control, this amplifier can be reconfigured as a voltage follower with a gain of 1, where the speed set voltage is input from its non-inverting input pin 11. Pins 12 and 13 are shorted.
3.1.3 Pulse Width Modulator
Unless overcurrent or a fault condition causes the six drive outputs to be locked out, under normal conditions, the error amplifier output is compared with the oscillator output sawtooth wave signal to generate a pulse width modulation (PWM) signal, which controls the three lower drive outputs. Changing the output pulse width is equivalent to changing the average voltage supplied to the motor windings, thereby controlling its speed and torque. The pulse width modulation timing diagram is shown in Figure 3.
3.1.4 Current Limitation
An external inverter bridge is grounded via a resistor Rs for current sampling. The sampling voltage is input to the current detection comparator via pins 9 and 15. A 100mV reference voltage is set at the inverting input of the comparator as the current limiting reference. If the current is too large during the rise time of the oscillator sawtooth wave, the comparator flips, resetting the lower Rs flip-flop and turning off the drive output to limit the current from increasing further. During the fall time of the sawtooth wave, the flip-flop is reset, turning on the drive output. Current limiting is achieved by using this cycle-by-cycle current comparison. If the maximum allowable current is Imax, the sampling resistor is selected according to the following formula:
To avoid malfunctions in current detection caused by commutation spike pulses, an RC low-pass filter can be set before the input at pin 9.
3. 2MC33039 Electronic Speed Meter
The MC33039 is an integrated circuit specifically designed for closed-loop speed control of brushless DC motors. The system can achieve precise speed control without the need for expensive electromagnetic or photoelectric tachometers. It directly utilizes the three output signals from the three-phase brushless DC motor rotor position sensor, converting them into voltages proportional to the motor speed via F/V conversion.
As shown in Figure 4 of the MC33039 block diagram, pins 1, 2, and 3 receive three signals from the position sensor. These signals are then processed by a buffer circuit with hysteresis to suppress input noise. An OR operation yields a signal equivalent to six pulses per pole pair of the motor. This signal then passes through a monostable circuit with external timing components CT and Rr, and the output signal fout from pin 5...
The duty cycle is related to the motor speed, and its DC component is proportional to the speed. After processing this signal with an external low-pass filter, a speed measuring voltage proportional to the speed can be obtained. In the waveform diagram of a three-phase motor application, fout is the output of pin 5, and Vout (AVG) represents its average value, i.e., the DC component.
4. Experiments and Conclusions
To better verify the feasibility and safety of the aforementioned theory, an experiment was conducted as designed.
4.1 Preparation
The main part of the experiment—the control circuit—was designed as a closed-loop system composed of MC33035 and MC33039. Due to limitations in the experimental conditions, we made some necessary adjustments to the experimental circuit, but these adjustments did not affect the system's functionality or the experimental results.
The first thing to adjust is the power supply. The motor used in the experiment is a three-phase six-pole motor, n0=1500r/min , I0=10A , U0=50V. An LM317 voltage regulator is connected between the power supply and pin 17 of the MC33035 to ensure that the Vcc of the MC33035 is within the permissible range. The LM317 is a 50V input, 15V output voltage regulator. Its basic circuit structure is shown in Figure 5.
Secondly, in this closed-loop speed control system, three Hall effect integrated circuits are used as rotor position sensors. The reference voltage ( 6.24V ) at pin 8 of the MC33035 is used as their power supply. The output signals of the Hall effect integrated circuits are sent to the MC33039 and MC33035. The motor in the experiment is six-pole, and the signal is input from pin 5 of the MC33039.
The number of pulses output is 3 × 6 = 18 pulses per revolution of the motor. The timing element is selected based on the maximum speed of the motor. In the experiment, the maximum speed of the motor is 1500 r/min, which is 1500/60 = 25 r/s. At this time, the number of pulses output per second is 25 × 18 = 450. That is, its frequency is 450 Hz, and the period is about 2.2 ms. According to the MC33039 manual, the timing element parameters are R1 = 1 MΩ, C1 = 750 pF, and the pulse width generated by the monostable circuit is 95 µs . Pin 8 is connected to the reference voltage of the MC33035. The output of pin 5 is connected to pin 12 of the MC33035 through resistor R3, which is the error amplifier inverting input terminal. The amplifier gain is 10 at this time, and capacitor C3 is used for filtering and smoothing. The MC33035 oscillator parameters are: R2 = 5.1 kΩ, C2 = 0.01 µF , and the PWM frequency is about 24 kHz.
Furthermore, since it was impossible to construct the NPN-PNP inverter bridge shown in Figure 1, N-channel VMOS transistors were used. This allows for the construction of a six-channel inverter bridge circuit. Because the upper drive signal can only directly drive p-channel VMOS transistors while the lower signal can directly drive N-channel VMOS transistors, an inverter is added to the circuit between the upper bridge arm and the inverter bridge to invert the drive signal. The resulting circuit diagram is shown in Figure 6.
4.2 Experimental Conclusions
Before actual operation of the motor, rotate the motor manually and use a multimeter to measure whether the three output signals of the Hall sensor installed on the motor are consistent with the truth table of the output signal of the MC33035. The experimental results are shown in Table 2.
The results of manual testing are consistent with the theoretical truth table.
The experimental waveforms of the motor under power supply are shown in Figures 7 and 8.
In both figures, the upper curve represents the sensor output SB, the lower curve in Figure 7 represents Sc, and the lower curve in Figure 8 represents SA. The comparison shows that the experimental output matches the theoretical output.
When measuring the current waveform, first, lead out the main circuit of a motor inverter, install a current sensor on the wire, and then connect a 5Ω measuring resistor to ground. Then, use an oscilloscope to measure the current of the current sensor, i.e., the waveform flowing through the resistor, which is the motor current waveform. See Figure 9.
However, the waveform in Figure 9 does not match the theory. While the distribution within the cycle is somewhat similar, the waveforms are significantly different. This is because the motor is running under no-load conditions. In the experiment, the brushless motor operates under no-load conditions, and each commutation of the inverter generates a greater inrush current than under full load conditions, resulting in fluctuations in the smooth current without the load-consumption effect.
Next, waveform tests were performed to test the startup acceleration. The experiment used the upper drive output of a certain MC33035 and the fout of the MC33035 as the experimental objects. The measured waveforms are shown in Figure 10 .
Theoretically, this waveform should be the output waveform from the upper side, which should not change due to the speed controller. However, the fout waveform should change the number of pulses in one cycle with the speed controller, thereby changing the average voltage across the motor and thus changing the motor speed.
However, due to various objective reasons during the experiment, the displayed waveform exhibited a phase loss phenomenon. Nevertheless, it can still be seen in the figure that the number of pulses in each cycle of the lower drive wave gradually increases, indicating that the motor is accelerating.
In the fault test, a potentiometer was connected to the power input terminal of the control circuit to change the power supply voltage Vcc of the control circuit and observe the circuit's response to the fault signal. During the test, the power supply voltage Vcc was continuously reduced from 15V . When it reached approximately 10.5V, the alarm circuit drove the LED to light up, triggering the fault alarm.
5 Conclusion
Although some phenomena that did not conform to the theory occurred during the experiment, the overall results basically met the expectations, proving the practical feasibility of using small brushless DC motors as household transmission devices.
