In Search of a Computerized SCR Triggering Circuit with Reference to

Traditional 3 Phases Induction Motor Driver

By : Wa Pong MAN Date : 3/5/2013

Background

• There is many aged lifts are still operating in buildings. SCR controlled 3 phases AC voltage controller was a common device to drive these lifts acceleration and deceleration in the past. It controls the terminal voltage of motor, so that the torque and slip of the motor is controlled.

• Since the Giant Transistor (GTR) appeared in the market in 1980’s, the lift’s manufactories make use of the GTR to build up the new induction motor driver by Vary Frequency Vary Voltage (VVVF) technique to improve their lifts’ performance and promote their band mark.

• That means there is no further new technical supporting for the aged lifts after the old analogue components were faded out in the market.

• However, most of the lifts owner just pay for repairing and is not willing to pay extra cost for replacement of whole new model driver.

• This situation has made the repairing and maintenance works become difficultly for the old model lifts. In order to continue the works, another way to replace the old components shall be found, a simple and flexible SCR triggering circuit board is a possible way to simplify the repairing work. The aim of this project is designing a new circuit board that can replace the old defective board to make the repairing works become more easily and efficiency.

Aged Lift Motor Control Circuit

Aged Lift Motor Control Circuit

Designed Triggering Circuit

• i) Zero Crossing Detector (ZCD) & Power Supply

• ii) Triggering Circuit, Input Signal & Analogue Processing

(Micro Controller Unit)

• iii) Driving Circuit

ZCD & Power Supply (Diagram)

• Each two photo coupler inputs connected in reverse parallel are used to detect one phase.

• The photo couplers also make isolation between AC supply and the DC logic circuit operation supply.

• The logic gates provide the voltage state signal and synchronization pulse every 60°.

Photo Coupler Logic Gate

Power Supply

ZCD & Power Supply (Waveform)

• When the phase voltage is

in positive cycle, its output

(R-, Y- and B-) is in logic

High (5V).

• A pulse will be generated to

MCU by a NAND gate when

the phase voltage is

crossing zero point and

both two photo couplers do

not conduct at same time.

ZCD & Power Supply (Actual Waveform Measured)

• Channel 1 was measuring R- as a reference

• & Channel 2 was connected to syn., Y- or B- respectively.

Micro Controller Unit (Diagram)

The minimum requirements of MCU in the circuit : 1. One interrupt input for synchronization pulse; 2. Four digital inputs for voltage state and GB3; 3. One analogue input for fire angle control signal (Analogue to Digital Converter); 4. Six digital outputs for triggering six SCRs; 5. Two timers/counters for delay fire angle and generating pulse chain.

Micro Controller Unit

• STC12C5A60S2 was used with development board.

• Its instruction set is fully compatible with INTEL 8051.

Micro Controller Unit (Analogue to Digital Convertor ADC)

• The ADC output value should be 0000H to 03FFH when 0 to 5V is input.

• The maximum ADC value when analogue input is 3.7V: (3.7V is the maximum external input for angle controlling)

ADCmax = 03FFh/5d x 3.7d

= 1023/5 x 3.7

= 757.02

= 757d

Micro Controller Unit (Variable Process)

0 – 3.7V

↓

0 – 757d

↓

757d – 0 (150。 – 0 °)

↓

755 – 604 (150。 – 120。)

603 – 302 (＜ 120。 – 60。)

301 – 0 (＜ 60。 – 0。)

↓

(150。 – 120。)

301 – 0 (＜ 120。 – 60。)

(＜ 60。 – 0。)

↓ ↓ (voltage state)

Fire Angle Delay Count TG_Word

← Analogue input voltage (F) ← ADC output value

← Changed to be descending order

as following the fire angle change

← Separated to 3 equal parts

← The value range is bounded in

301--0 with 3 flags to indicate the angle ranges

← By look-up table to find out the fire angle delay count and the triggering distribution word

Micro Controller Unit (Fire Angle Delay Count)

Fire Angle Delay Count = 65536 - (ADC value x (40000/301)) + 0.5

Where +0.5 is used to round up the results to

next upper integer when the resultant value greater than 0.5.

The maximum value after convertion.

The count value for 60° in 50Hz with

12MHz crystal.

The maximum count for 16-bit counter.

Micro Controller Unit (Fire Angle Delay Count Table)

• All the possible ADC Values were calculated and conversed to the Fire Angle Delay Count. It is used for the timer 0.

Micro Controller Unit (Fire Angle Delay Count Table)

• All the possible ADC Values were calculated and conversed to the Fire Angle Delay Count. It is used for the timer 0.

Micro Controller Unit (TG_Word :- AC Voltage Controller Diagram)

– The thyristors connect between ac supply and load.

– The voltage applied to the load can be controlled.

– The current flow into the load is controlled by thyristors T1, T3, T5; and thyristors T2, T4, T6 provide the return current path.

Micro Controller Unit (TG_Word :- AC Voltage Controller waveform)

• The waveform for the conduction angles of thyristors are shown on the left diagram for α= 60° and α= 120°.

• For α≧150°, there is no period for two conducting thyristors and the output voltage becomes zero at α=150°.

• Therefore, the range of delay angle is 0°≦α≦ 150°.

Micro Controller Unit (TG_Word :- waveform Details)

• Voltage State is the information regarding the status of the phases of the supply. It is necessary for the distribution of triggering pulses correctly.

• TG_WORD is firing

command. It is sent to the gate of SCRs through the output port of MCU according to the analogue input voltage and Voltage State.

Micro Controller Unit (TG_Word Table)

b5 b4 b3 b2 b1 b0 Hex.

Fire Angle Voltage_State (B-,Y-,R-) Tg6 Tg5 Tg4 Tg3 Tg2 Tg1

0°≦ α < 60° 101 1 1 1 0 0 1 39

001 1 1 0 0 1 1 33

011 1 0 0 1 1 1 27

010 0 0 1 1 1 1 0F

110 0 1 1 1 1 0 1E

100 1 1 1 1 0 0 3C

60°≦ α < 120° 101 1 1 1 0 0 0 38

001 1 1 0 0 0 1 31

011 1 0 0 0 1 1 23

010 0 0 0 1 1 1 07

110 0 0 1 1 1 0 0E

100 0 1 1 1 0 0 1C

120°≦ α < 150° 101 0 1 1 0 0 0 18

001 1 1 0 0 0 0 30

011 1 0 0 0 0 1 21

010 0 0 0 0 1 1 03

110 0 0 0 1 1 0 06

100 0 0 1 1 0 0 0C

Micro Controller Unit (TG_Word Table)

Micro Controller Unit (Program )

• 2 subroutines were added in the main program.

• The aim - converting the internal variables into ASCII format,

• & then sends it to the computer through the UART.

Micro Controller Unit (Triggering Waveform)

The channel 1 connected to the input reference signal B- and the channel 2 was monitoring the triggering signal for SCR T1.

• After entered the program into MCU and did debugging of the software, the output waveforms could be recorded as right diagram.

• The output waveforms were matching with the expected waveforms.

Fire angle = 0° Fire angle = 150°

Fire angle = 60° Fire angle = 120°

Driving Circuit

Overall Test with Resistance Load

• The circuit was connected on site with 3-phase power supply with resistance load.

• The basic functional test was carried out to confirm the performance. The light bulb is a resistance load and it is easy to observe its operation.

• The output voltage can be adjusted with turning the variable resister on the development board.

• From the site result, it was quite match to the expected output performance.

Overall Test with Real Motor

• The circuit board was connected into a real lift control panel.

• The inspection mode (manual control and low speed operation mode) was selected.

• Both Up and Down low speed running were confirmed and the lift was in proper operation.

Improvement and Fine Turn (ULN2003A Overheat)

• Though the operation of the lift was found normal while using the new designed board, the operation temperature of the driver IC ULN2003A was hot during the trial run.

• According to the Hong Kong Regulation and the EN81, the highest ambient temperature in lift machine room can be 40°C. In fact, during the summer time, the temperature inside the enclosed lift control panel will be over 50°C during the lift running. Therefore, if the initial temperature of the IC is 50°C or above, the operating temperature of the IC will be exceeded 85°C and over the maximum allowed temperature of the IC.

• That means it will affect the reliable of the lift operation. Therefore, to increase the reliability of the lift, to lower down the operating temperature of the driver IC is most important by the following methods.

Improvement and Fine Turn (ULN2003A Overheat --- Software)

Software Fine Turn:

Referring to the waveform of the SCRs triggering sequences mentioned before, the triggering pulse need to extend 30° after the phase voltage reached zero if the fire angle is greater then 120 °.

For simplify the program writing, the designed triggering pulse was extended 60° at any fire angle. Therefore, the extension was modified and reduced to 30°.

This modification can help to reduce the power dissipation and operation temperature of the driver IC ULN2003A by shortening the triggering period.

Improvement and Fine Turn (ULN2003A Overheat --- Hardware)

Hardware Improvement:

• Adding a heat-sink, it can help to release the heat energy from the IC to the surrounding.

• However, there is no suitable heat-sink can be obtained from the market.

• Therefore, another 2 driver ICs were proposed & used in the circuit.

• It can help to share the power dissipation of the existing driver.

• This suggestion is similar function as increasing the releasing heat surface area three times.

Final Test

• The operating speed of the lift, voltage and current of the hoisting motor were measured and recorded.

• Both values of the new board and the existing board were recorded with the inspection and normal mode.

• It was found normal as the existing board.

• The acceleration, constant speed, deceleration and leveling operation of the lift were same as the existing board.

Result

• The advantages are that it can carry out the replacement work in an easily way and efficiency to replace the existing old analogue components with a new heart - Micro Controller Unit (MCU) on the existing circuit.

• No need to have additional adjustment work while installation of the new design circuit, it is because the three phases balance adjustment work is performed by the MCU with crystal clock generator.

• An accuracy clock can make the fire angle delay for each phase same as each other.

• As a result, this new circuit board is match to the objectives as following.

– Low initial cost,

– Low installation cost,

– Low maintenance fee, &

– Extend the old model lift life.

Recommendation • Alarm Signal:

• Noise:

• Hardware Coordination:

An alarm signal from the MCU should be added for stopping the lift operation when the MCU detected any abnormal. (ex. input phase loss or phase reversal)

Add Schmitt-trigger gates or other circuits between the photo couplers and the logic gates in the existing zero crossing detection circuit to filter away unwanted noise from power line, for stabilize the synchronization signal and voltage state signal.

Because the triggering output will become high when program downloading to MCU or MCU power up. It should be inserted inverters beyond the driver ICs and modify software to change the polarity of the triggering signal to avoid the pulse transformer over current.

THANK YOU