A DC motor is extensively used for industrial applications where a precise speed control and a constant torque are desired. It is inversely proportional to its field current. In case of field current failure, the motor speed will rise to dangerously high level. A field failure protection is therefore necessary to cut off the armature supply in case of field current failure.
|R1||Pick-up Resistor 5 Ohm, 25 W|
|All R||20K , 25W|
|LED||Light Emitting Diode|
|RE||Field Failure Relay|
The basic circuit of the field failure protection uses an ordinary 6V electromagnetic relay of the open type with I0 amps rated sturdy contacts. This relay can be used on a manual autotransformer-controlled DC drive and motorised or thyristor controlled drives. This motor has a shunt field current of 1.13 amps at 220V DC. A 5-ohm (25~watt) wire-wound resistor (R1) connected in series with the motor field produces a 5.6-volt drop across resistor (R1) as long as the field current exists, thus energising the 6V DC relay connected across the resistor as shown in the diagram below
In case the motor field current fails due to any fault, the voltage drop across resistor (R1) will be zero which denergizes the relay (FFR) and cuts off the armature supply.
The circuit diagram of a manual aut0transfotmercontrolled DC drive of a 230V, 5HP DC motor with a separately excited shunt field of 230 volts ( l.l3 amps) and the FFR (field failure relay) circuit are shown in circuits diagram respectively. When the start pushbutton S2 is pressed, the contactor C is energised through S2 (N/O contact), limit switch S3 and stop pushbutton S1 contact).
The limit switch S3 is actually a part of the autotransformer, and it is so mounted that its contacts remain closed only when the autotransformer setting is at zero position. At all other settings of the autotransformer, the limit switch contacts remain open. This is a safety device introduced, so that the motor can be started only from the minimum position of the autotransformer setting, thereby starting at reduced voltage and current. If the motor is started on a high armature voltage, the starting current will be very high, especially if started on load, as is usually the case.
Project Submitted By :- E. AUGUSTINE
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