Motor Control Board

[hakanokten]

Overview

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The motor controller is the AC driver board which is used to control the speed of the AC linear induction motor that the pod contains. With the AC driver board, we aim to enhance the process control, save the energy.

Behind the Scenes

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Required Control Components (Please check if we have the component)

• [ ] Starter
• [ ] Disconnect switch
• [ ] Contactor
• [ ] Circuit breaker
• [ ] Conducting wires
• [ ] Terminal blocks

Working Principles Motor speed is controlled by changing input line frequency. Motor torque and current are controlled by changing the input line voltage.

Single-phase AC motor starter system

• L1 and L2 are wired to hot and neutral of the power unit.
• Pushing the start pushbutton activates the switch M1.
• Contactors close, and the motor starts.
• Overload relays limit the current.
• Pressing the stop button disconnects L1 and L2. Then, the motor stops.

Non-reversing 3-phase motor starter systems

• L1, L2 and L3 wired to 3-phase AC voltage.
• Pushing the start button activates the switch M1.
• Contactors close, and the motor starts.
• Overload relays limit the current for all 3 lines.
• Pressing the stop button disconnects L1, L2, and L3. Then, the motor stops.

Reversing 3-phase motor starter systems

• The mechanical interlock prevents actuation of forward and reverse buttons at the same time.
• Contactors for the reverse system cannot energize until those of the forward system are open and vice-versa.
• Swapping any 2 of 3 phases in motor winds (on the diagram below, L1 and L3) makes the motor run in reverse.

The star-delta AC motor starter systems

• This reduces voltage disturbance on the power supply.
• The starting torque is proportional to the square of the supply voltage. It is also divided by 3.
• The motor stabilizes when the motor and resistive torques balance out around 75-85% of rated speed.
• The delta contactor is then closed and the star contactor is opened.
• The motor reverts back to normal.
• The contactors are electrically interlocked.
• The change from Star connection to Delta connection is controlled by a timer.
• In this way, short-circuiting between phases is prevented since star and delta contactors cannot close simultaneously.
• System requirements must be low to use this kind of AC motor since the motor will supply only 1/3 of its rated torque.
• A 30-50 milliseconds break in supply after the star contactor opens and the delta contactor closes.
• Six motor leads are required for delta connection.

Speed Control / Speed Drives

• Synchronous speed decreases as the equivalent impedance of the magnetic circuit and line frequency decrease. This increases the motor current and torque.
• If the voltage is not decreased, the magnetic flux may reach the saturation level. In order to keep the flux in the operation range, the voltage is reduced in a proportional way to frequency.
• The torque remains constant and the power increases linearly with speed as shown below.

-Speed Controller Block Diagram Explanation

• As can be seen clearly from the designed block diagram, the goal of AC speed control is to change the frequency input to the motor.
• The inverter is used to take the DC power from the rectifier and then convert it back to AC power (i.e. inverting the DC power).
  • The voltage and frequency controller is used to get the voltage and frequency inputs to the motor determined.

Motor Specs Needed for the Motor Control Board

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• Specific type of AC motor Please select one of the following options if you know:
• [ ] Single phase
• [ ] Non-reversing 3-phase
• [ ] Reversing 3-phase
• [ ] Open transition 3-phase
• Voltage needed for the motor (Please write in the brackets if you know)
• Input line frequency (Please write in the brackets if you know)
• Input line voltage (Please write in the brackets if you know)

Useful Links

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https://www.electricalengineeringtoolbox.com/2016/02/nema-three-phase-electric-motor-designs.html

[sni007]

Parts of a AC drive module :

1.SPWM signal generator ( either by using 2 555 timers or supplied by microcontroller(preferred) )

In PWM, Vdc is compared with a triangular wave . In SPWM, sinusoidal voltage signal is compared with a triangular wave and hence, the ac voltage output is obtained. ( 3 control signals for 3 phase each with 120 degree phase shift)

2.Switch mode power inverter circuit. Heatsinks needed to dissipate heat from the power MOSFETS.

Hovewer, since our motor will become a generator when breaking (decelerating), the power will flow from the ac motor to the dc link. Because of this, the voltage from the dc link will gradually increase. There 2 alternatives to solve this problem. First one is using a capacitor in paralel with a resistor so that the energy is dissipated. Other alternative is using a reversible current topology connected between the distrubution system ( battery) and the dc link capacitor. The second alternative is preferred when there is frequent braking in the system.

Overall, work in progress.