Soft starter controller

Retractable shade structures with motorized roll-up systems (Fig. 8) were controlled by light sensors to regulate the amount of sunlight that reaches the plants (Pass & Mahrer, 1997). The retractable shade used a black Raschel woven shade with 60% transmission. The mechanism implemented used wires split 60 cm as guides for curtain sustain. A long tube drived by a single gear motor rolled the wire over it and moved the curtain in any direction. The retractable roof was opened seven times between 9:45 AM and 14:14 PM, and a 1/2 HP motor started fourteen times, Fig. 9. The energy used to move the shade curtain during the day was 3.95 Ah at full voltage application.

The soft start motor controller uses an inverter drived by an ATM89C51 microcontroller, Fig. 10. The inverter is composed by a 12-120 V transformer switched on and off by a pair of MOSFETS. The microcontroller reads the battery voltage in order to predict maximum voltage that can be applied to the motor; for example a battery voltage of 10 VDC can provide 100V RMS (root mean square) to the motor. The PWM voltage presents many time delays (Fig. 10) stored as t1-t0, t2-t1, etc. in a look up table (LUT). At moment t0 the first MOSFET will be turned on for the period given by the first data in the Table. After counting this time it will arrive to t1 where it is turned off; the second data of the LUT will indicate the period that the MOSFET has to stay off, arriving to t2. At this moment the next value of the table is acquired and the same MOSFET is turned on until t3. The opposite MOSFET that operates as a switch will turn on and off during the negative part of the cycle.

The microprocessor program applies a starting voltage of 85 V RMS to the motor for a time period of 5 seconds, expecting it to turn. If it didn’t rotate the voltage is increased by 5 volts for another 5 seconds and so on until it turns on, Fig. 11. Once rotating the increments take place every 30 seconds until full voltage is supplied. With an uncharged battery the maximum voltage that can be supplied decreases changing the PWM timing constants. The rest of the waveform can be reconstructed from the first quarter and its RMS value can be obtained from Equation 1.

The method of charging lead-acid batteries is with a constant voltage, current-limited source. That method allows a high initial charge current until the battery reaches full charge. The battery charger is also managed by the microcontroller. The PV system presented one battery of 50Ah with a maximum discharge capacity of 50% and 3 storage days in case cloudy days were present. When the system was drived by the soft start controller the energy usage per day decreased from 28.4 Ah to 23.02 Ah, Table 5. The motor load is disconnected when the battery voltage drops below 11V and reconnected when it gets back to 12.5V. The number of panels of 30W each decreased from seven to six, depending on the voltage applied. If the curtains are opened 12 times instead of 7 the 50W solar panels installed decreased to 6 when started with 84 V, Table 6.

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