VIl. (Total 8 points) The mofor controlling circuit is shown in Figure 9, in whi
ID: 2266839 • Letter: V
Question
VIl. (Total 8 points) The mofor controlling circuit is shown in Figure 9, in which the breaker K is an air switch. Analyze Pigure 9, then answer the following questions: (1) List protcctive elements and the corresponding protection functions; (2) Simply describe the operation process, (3) Describe the action of the timer relay TR in the system: (4) Explain why Normal-Close contacts M2, and M3 are designed in the last two laddet on the bottom right comer of the circuit. FU OL PB1 PB2 CR M1 MI M11 CR O- M3 OL M1 TR Motor M2 TR M3 TR M2TTT FRure 8Explanation / Answer
The motor controlling circuit is shown in figure. Given that the breaker K is an air switch.
From the given diagram we can say that The AC motor is connected to the three phase supply using an air switch K,
The air switch is designed to open a circuit under load. To quench the arc, special arcing horns are provided.
The neutral line of the three phase suplly is connected to the over load release through a fuse. Fuse is a over current protection device.
We know that for a three phase motor if one phase is open circuited then we can cut the circuit and hence the motor stop rotating. In the above circuit to use this concept , one of the phase is conecting to the circuit brekers. If the relay detects the faults then the circuit breaker PB1 will open and open circuit the entire process of the motor.
1. the protective elements in the above circuit are time relay, circuit breaker, fuses, air switches
The time relay here in the circuit is normally open and time closed relay . This is open in the normal condition and closes in the abnormal condition to protect the motor from the fault currents if the air switch is failed to do so.
Circuit breakers which are connected to one of the phases are protecting the motor from the over voltages.
fuses which are connected to neutral and one ohase are protecting the motor from the over currents. If there is a over current passing through the line then the fude will blow off and opens the circuit.
2. When the AC motor is running under normal condition no protection circuit will be in ON condition, but under faulty conditions i.e when the fault occurs in the supplu side or the motor side then the number of protection device will turn ON and protect the motor from the dangerous damages which will effect the performance of the entire power system. This is called Switch gear and protection.
3. The time relay circuit used in this process is normally open and time closed type as mentioned above.
The time realy here will be normally open which opens the entire branch. The time relay has the setting time to set by our need. If we set the time to 5 micro seconds then the relay will operate in 5 micro seconds after the fault had occured. The time relay cirvuit gives the signal to the circuit breakers to open when the fault has occured.
4. Yes the normally closed contacts M2 and M3 are connected in series with the time relays as shown, as already the relay is normally opened is present in the same branch then we have to design a normally closed contacts in the same branch otherwise when the relay is closed the contacts would have been opened. That is the reason we are using normally closed contacts in only last two ladders.
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