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BLDC speed & direction control circuit construction

Discussion in 'Electronics Homework Help' started by usr:[email protected], Apr 27, 2019.

  1. usr:000@.com

    usr:[email protected]

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    Apr 27, 2019
    I am currently working on a final resubmission for an electronic engineering BTEC level 3 assignment. I have posted the original question, answer and tutor feedback below. Just wondering if anyone can make sense of the feedback and give guidance as to how to correct the answer.

    T6.
    As an alternative to the AC system in task 5, you have also been asked to produce a circuit design that can control the speed and direction of the conveyor using a DC motor drive system. With reference to circuit layout drawings and research sources, explain how it operates. (400 words)

    Answer:
    Modern conveyor systems use brushless DC motors with sensors, which offer sophisticated seed control. One method of speed control is trapezoidal commutation, which involves controlling the current in two phases at a time, with the third phase disconnected.

    PWM (pulse width modulation) is used in this speed control system. PWM involves varying the input voltage by generating a series of pulses and is the operating principle behind choppers (electronic static power device, a high speed switch that connects/disconnects load from voltage supply to get pulses). The chopper’s mark-space (Ton/Toff) ratio is controlled by thyristors (electronic switches) which vary the pulses’ average voltage (Vout = (Ton/(Ton/Toff)) x Vin). PWM varies the average voltage and current flow in the coils and so the motor’s speed and torque.

    The chopper circuit, uses a number of MOSFETs (metal-oxide-semiconductor field-effect transistor). A pair of MOSFETs controls one phase of the motor, so for a 3-phase BLDC motor 3 pairs of MOSFETs are required (4). These can be arranged in the format shown in the circuit diagram.

    The above diagram shows the layout of the speed drive circuit board. In this circuit the micro-controller PWM signals six inputs to the IC. Each of these inputs corresponds to the six MOSFETs (5). The three Hall effect sensors send signals to the MCU to control when it commutates (5).

    For every electrical revolution, a six-step commutation process is used (6). The rotor has two pairs of permanent magnets, meaning that two electrical revolutions spin the rotor once (6). The diagrams below show the rotor coil arrangements and corresponding Hall effect sensor outputs and voltages.

    Out of the two phases, which are energised during each step, one phase provides the current, and the other the current return path (6). The micro controller automatically controls which switches in the MOFSETs must be open/closed to provide negative/positive current to the two coils, the third coil will remain open (6). In more detail, the MOSFETs may be in the following states: High (top switch is closed, bottom switch is open), Low (top switch is open, bottom switch is closed), or OFF (both switches are open).

    In order to complete the drive system a regulated MOSFET power supply is required, which usually includes: step-down transformer, gate fault handling/driver control and timing/control logic (6).

    The direction of the motor can be reversed through interchanging the connection of any two power supply leads to the motor. The circuit is connected to a PLC, which can automatically control the direction through pre-programmed timing logic. The contactors interchange the wiring on the load side which allows a neater set-up. The circuit is shown below.

    One set of wires is fed through a contactor and another set through another contactor. These contactors require an interlocking device to prevent the contactors being operated simultaneously, this can either be a mechanical or electrical interlock. This circuit utilises an electrical interlock in the form of NC auxiliary contacts in each contactor. The PLC supplies the auxiliary contacts, each of which is wired in series with a coil, and controls the opening/closing of them and so the energisation of the coils. When one contactor is operating its auxiliary contact is opened, de-energising the other coil, preventing the other contactor from operating (7).

    Tutor feedback:
    T6 some interesting stuff here, but it is unclear from your diagrams how the 3-ph supply is converted to PWM dc using thyristors. Also electronic direction control is not shown.
    The only supply to a workshop will be 3-ph ac so you need to start with this and then convert to some form of pwm dc,( using thyristors is a common way to do this).
    To change motor direction there should be a simple electronic control circuit that controls the dc supply to the motor and not a PLC.

    Anyone have suggestions for a speed control circuit and a separate direction control circuit? It is for an automatic conveyor so has no push buttons/manual switches. Any help would be appreciated.
     

    Attached Files:

  2. Minder

    Minder

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    Apr 24, 2015
    The question asked included a request for a DC motor, why not go with a simple DC brushed motor, a DC supply is still needed but the controller is much simpler than a BLDC motor as shown.
    M.
     
  3. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    That would be my approach, too. The principle behind this is KISS.
    But feel free to use a BLDC motor, only be aware of the increased complexity and the higher risk for errors.

    Start with this part: convert 3 phase AC to DC. Note that you do not necessarily have to use all 3 phases, but it is possible and not difficult.
    Next generate the control signal for the motor (whatever type of DC motor you chose).
    You will then typically have to add a power stage to boost the control signal to a level that is suitable for driving the motor.
    If there are no manual controls, there has to be some kind of sensors to evaluate thr current speed of the conveyer (required for creating a feedback loop for regulating the speed of the conveyer). Also you will need at least one other sensor for control of the direction of the conveyor. Otherwise the whole matter of direction control is moot.
    You'll have to incorporate the sensors as input to the above mentioned control logic to create the signals for forward/backward motion and speed control/regulation.
     
    Last edited: Apr 27, 2019
  4. Minder

    Minder

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    One advantage with 3ph rectified DC is that the ripple is virtually nil, and 3x the supply frequency, requiring much less in the way of any smoothing capacitors compared to the same in 1ph rectification..
    M..
     
    hevans1944 and Harald Kapp like this.
  5. usr:000@.com

    usr:[email protected]

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    Apr 27, 2019
    Thanks for the replies.

    I am checking with the tutor if I am able to change the type of motor explained for the resubmission.

    If so I plan to go with a thyristor circuit, as this is what he suggested in the feedback and a description is given in the course revision notes (circuit shown in attached file 1). This circuit has a separate supply for the field winding (bottom right corner).

    If I have to incorporate two sensors can I connect them to the External Input side of the processor? Also, can Hall effect sensors be used as these are only mentioned in conjunction with BLDC motors?

    As for adding a power stage, can this be a simple step-up transformer?

    The course revision notes mention that thyristors can control the direction of the motor as well, but don’t explain how they actually do this. I understand that they will have to change the polarity of the supply, but if they are unidrectional thyristors as shown how does this work?
     

    Attached Files:

  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    The circuit you show is not suitable for control of direction. You show a controlled 3 phase bridge rectifier. Using suitable control of the thyristors' gates you can control the rms voltage available to the motor and thus speed only.
    You can modify your circuit by doubling the thyristor circuit with reverse polarity to allow control of direction, too. While this can be done, it is in my opinion overkill in terms of circuit complexity and number of components.

    A typical method to control speed and direction of a DC motor is a pwm controlled H-bridge. You would create the Dc olperating voltage from the 3 phase AC using a diode based bridge rectifier - no need for complicated gate drive circuits. The H-bridge can easily be controlled to set speed and direction.

    Of course. Otherwise how would you be able to read the sensor status and use it to control the motor?

    I doubt that Hall sensors are mentioned in conjunction with BLDC motors only. Hall sensors can be used to measure any kind of magnetic field.
    You should not ask "what can I use this or that sensor for?", ask "which sensor should I use to measure this or that effect (e.g. speed, direction)?".

    A transformer doesn't work with DC. An H-bridge as mentioned above can directly be operated from the high voltage rectified DC and using suitable transistors can also deliver the power required.

    This article might be interesting to you.
     
  7. usr:000@.com

    usr:[email protected]

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    Apr 27, 2019
    Tutor okayed changing the motor to a simple DC motor such as a cumulative compound motor. The tutor also wants 1 circuit for speed control and a separate one for direction control. I have chosen to use a thyristor circuit for speed control as per his advice and a MOSFET H-bridge with diode rectifier for the direction control circuit.

    I have read that PWM signals are sent from the processor to the thyristors to alter their operation, would it be correct to say that the voltage output of the thyristors is also PWM DC or refer to the firing angle delay of the thyristor being the factor changing the output?
    Just want to be clear with my explanation and don’t know whether to explain the thyristor gate input signal in terms of PWM or the thyristor output in terms of PWM? Don’t want it it sound like I am confusing PWM with firing angle delay.

    Also is PWM only used for speed control or also the switching of the MOSFETs in the direction control circuit, would a processor send PWM signals to the MOSFETs to switch on either the high-side or low-slide switches?
     
  8. Minder

    Minder

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    Apr 24, 2015
  9. usr:000@.com

    usr:[email protected]

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    Apr 27, 2019
    I am having a little trouble with the H-bridge direction control circuit since I can only find diagrams showing its connection to the micro-controller or rectified supply and not both. I am required to show both in the ciruit. I have included a diagram showing the H-bridge.

    I have read that in electronic circuits the ground symbol may mean the connection to the negative terminal of the supply. If this is the case, could the 12V supply to the bridge and lower ground symbol be connected to a 3-phase diode rectifier as shown in the second edited diagram I have included.
     

    Attached Files:

  10. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    1. You cannot drive a 12 V H-bridge from a 5V powered driver (at least not without additional measures).
      Why? Consider the Gate-Source voltages on the P-Channel MOSFETs and what these voltages are for low and high logic level on OUT_A and OUT_B. What will the state of the MOSFETs be in both cases? What will you have to do to correct the observed effect?
    2. The ground symbol is often (mostly incorrectl) used for the common supply rail which is in most cases the negative pole of the power supply. Therefore the connection to the negative side of the rectifier would be o.k.
      But note that ground in a system is "-" only in most cases and not by definition. It is equally possible to build systems where the common potential is the "+" of the power supply. You, the designer of the circuit, define where the common potential is.
    Why do you think that is the case? Live is so much easier when you split a task into smaller, manageable parts. Consider the power supply as one such part, the controller as another part and the H-bridge (the power section) as a third part. Look a the parts and their interfaces in terms of power (voltages, currents) and timing (frequencies, periods, dead-time etc.), then design each circuit, then connect the parts to create the whole circuit.

    Also: What is the voltage of the 3-phase input power? How are you going to create 12 V from that (using your rectifier)? Certainly not without additional measures. What could you use?
     
    Last edited: May 3, 2019
  11. WHONOES

    WHONOES

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    May 20, 2017
    Have you thought about a DC stepper motor? You can determine distance travelled by counting the pulses and is relatively straight forward to reverse.
     
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