An actual-size, single-sided PCB for the Fig. 3: Component layout for the PCB
motor controller circuit of Fig. 1 is shown in Fig. 2, with its component layout shown in Fig. 3. It is recommended to use bases for ICs.
Before connecting the circuit to starter assembly, a bench test is required for the adjustment of timer. Apply 3-phase power to the circuit. Observe pin 3 of IC5 (NE555), which should go ‘high’, provided the sequence is correct. Else, interchange any two phase wires. As ‘sequence OK’
signal at pin 3 of IC5 goes ‘high’, relay RL1 energises and IC6 (IC555) is activated. As a result, relay RL2 energises after a delay of 15 seconds for one second.
Now adjust preset VR2 such that 30-minute-du-ration pulse train (time period 60 minutes) is available at pin 14 of IC8 (CD4060). Flip switch S2 to 30-minute position. Se-lect the required run time using rotary switch S3. On completion of the selected run time, ‘time over’ LED should glow and the timer should stop. Relay RL1 should de-energise.
After resetting the timer with the help of switch S1, relay RL1 should energise once again. Then after a delay of 15 seconds, relay RL2 should again energise for one second. Now short momentarily pin 14 of counter CD4017 (IC10) to ground thrice. On the third touching, Q3 of IC10 will go ‘high’ and LED3 will glow, followed by de-energi-sation of relay RL1. The mains should be interrupted completely to reset IC10.
Current transformers X1 through X3, step-down transformer X4, and relays RL1 and RL2 may be mounted side by side in a compact box as shown in Fig.
4. The PCB may be mounted over the transformers and relays using insulated spacers. Current transformers are to be connected before the starter relay contacts.
Over-current adjustment can be done only after connecting the load. Connect Fig. 4: Layout of cabinet for mounting
trans-former relays and the PCB
Fig. 5: Creation of virtual neutral from 3-phaes 3-wires system
all the wires to the starter point and the load. Keep wiper contact of VR1 towards ground side and switch on the 3-phase supply. Relay RL1 activates. After 5 sec-onds, relay RL2 also activates and the mo-tor starts running. Now slide the wiper of VR1 and mark the position just before the motor trips. (Remember that such trips will be counted by latching counter.)
Caution. Some parts of this circuit contain live 3-phase voltages. So avoid touching the circuit with bare hands.
Note. In the case of non-availability of neutral terminal, assembler a circuit as shown in Fig. 5. Connect ‘N’ marked wire (shown in Fig. 1) to two more transformers X5 and X6 that are identical to X4. The secondaries of these transformers (X5 and X6) are kept open, while the secondary of X4 is connected to the power-supply circuit as shown in Fig. 1.
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Readers’ comments:
Q1. Please clarify the following:
1. Which starter in the circuit starts the motor?
2. Is the starter manually operated or automatically?
Ramaswamy Iyer Through e-mail Q2. When a 3-phase motor is started, it takes six times the rated current. So the current sense circuit will trip the motor during start-up. If we adjust the overload current setting for starting current, this will not trip the motor during normal running current through the load. Is there any initial bypass provided for over-current trip?
G. Saravana Mohan Salem Q3. The contactor-type starter can be used for starting motors up to 10 HP. As I need to control motors of 15 to 20 HP, please clarify the following:
1. Can I use star-delta starter (which can reduce the starting current and can be used for motors up to 25 HP) instead of the contactor-type starter? If no, suggest a proper alternative as the starting current of up to 40A may affect other components.
2. Are there any current-reducing circuits used to withstand the high start-ing current while usstart-ing the contactor-type assembly?
3. At the time of testing, what HP mo-tor was used with contacmo-tor-type starter assembly?
4. The 12V, 300-ohm, 1 C/O relays (RL1 and RL2) specified in the circuit are not available. The available relays are 12V, 200-ohm, 1 C/O and 12V, 150-ohm, 1 C/O.
So which relay should I use? What is the purpose of using VR2?
Ramaswamy Iyer Through e-mail Q4. I am facing the following problems in the project:
1. In Table II, the turns ratio of cur-rent transformers (CTs) is 12 for both 6HP and 20HP motors. If the ratio is same, the secondary currents of CTs work out to be different, i.e. 1.8A for 20HP motor and 0.8A for 6HP motor.
2. In the phase-sequence indicator cir-cuit, you have connected an RC (1-mega-ohm-0.1µF) combination to the reset pin of IC5 (NE555). In such a case, can the reset pin get a high input.
Abhijeet S. Bhosle Through e-mail EFY: a1. 1. The starter comprises a con-tactor, an ‘on’ button (N/O), and an ‘off’
button (N/C). The contactor in Fig. 1 of the project uses three main N/O contacts connected to R, Y, and B phases and one auxiliary N/O contact, which is wired as shown in Fig. 1. The contactor coil is rated at 415V AC. At EFY, we used ML1 contactor from L&T to make the starter assembly.
2. You can manually operate the start-er by making use of ‘on’ and ‘off’ buttons.
In automatic operation you don’t have to use these switches. The circuit does it through relays RL1 and RL2 as per the logic explained in the project.
The author, D. Dinesh replies:
a2. A heavy current flows through the motor winding for a moment only. A certain delay is provided by capacitors C12 (47 µF) and C17 (100 µF) to account for this.
a3. 1. One can use star-delta starter
for a higher HP motor, provided that star-to-delta changeover is done either on releasing the ‘on’ pushbutton or after a fixed time delay. A typical semi-automatic star-delta starter made by L&T is Mark1 type bearing catalogue No. SS96255.
2. A delay is provided by capacitor C12 to bypass the power-on surge current.
3. The circuit was tested using 6HP borewell pump.
4. If the specified relay is not avail-able, one can also use 12V, 200-ohm, 1C/O relay.
a4. 1. Current transformers are used to sense the load current and these draw only a few milliamperes of current to bias the transistor. The bigger core is used to cater to the wire gauge. One can use a smaller core to hold this gauge by main-taining the specified turn ratio.
2. Pulses at pin 14 of IC08 (CD4060) should be adjusted to obtain 30-minute delay by varying preset VR2. Pin 4 of IC5 is wired through RC network for delayed reset at power-on (starting).