Construction and Working of Induction Cup Relay:
The working and construction of induction cup relays are similar to the induction motor or induction disc relay. The induction cup relay consists following parts 1. Electromagnets
2. Coils
3. Stationary iron core which is works the same as stator
4. Hollow cylindrical cup which is works the same as rotor
5. Damping magnets
6. Control Springs
7. Retraining springs
The stationary iron core is made from iron and works the same as the stator in an induction motor. the stationary iron core consists of two, four or more poles.
The electromagnet consists of two pairs of coils. When the fault occurs in a circuit, a current is flowing through these coils and a magnetic field is produced in these coils. these both fluxes have the same frequency but are displaced by phase shifts from each other. These two coils are energised the stationary iron core and produce a rotating magnetic flux in the air gap.
In a four-pole design, both pairs of poles are supplied from the same current transformer's secondary winding, but the phase difference between the two poles is 90 degrees. the phase difference between currents is done by inserting a inductor in series with the one pole Pairs and inserting a resistor in series with the another pairs of poles.
Due to these rotating fluxes, eddy currents are circulates in the hollow cylindrical cup causing it to rotate in the same direction as the rotating magnetic field. the rotation of the cylindrical cup depends on the direction of the rotation of the magnetic field and magnitude of applied voltage or currents and the phase angle between them.
The hollow cylindrical cup rotates in the direction of the rotating magnetic field between the magnets and stationary iron core. the moving contacts are attached to the cylindrical cup and fixed contacts are attached to the relay stationary body.
When the cup rotates, the moving contacts come near to fixed contacts and the trip circuit is completed. the relay sends a signal to the breaker and the breaker trip and faults current is interrupted.
The control springs are attached to the spindle of the cup to avoid rotation of the hollow cylindrical cup continuously. During the normal operating conditions, restoring torque of the spring is higher than the deflecting torque of the cup. so there is no movement of the aluminium cup. the restraining springs are used to provide restraining torque to the aluminium cup. When current does not flow through the electromagnetic coil restraining springs keep the aluminium cup in its reset position.
Advantages of Induction cup relay
1. The induction disc relays are less sensitive to the DC transients.
2. The induction disc relays produced steady non-vibrating. torques and parasitic torques due to current and voltage are small.
3. These relays have a very high torque to weight ratio.
4. Induction cup relays are very sensitive relays.
5. The induction cup relays have inverse time characteristics.
6. The induction cup relays are fast in operation and they have an operating time of fewer than 0.001 seconds.
7. These relays have lesser inertia of cup as compared to the inertia of disc in induction disc relays.
8. There is less magnetic leakage in the induction cup relay due to a more efficient magnetic system.
9. Induction cup relays are used for distance and directional relays.
10. The induction cup relays have high torque to inertia ratio.
11. due to an efficient magnetic system and light-weighted rotor, VA burden is greatly reduced in the induction cup relays.
the torque per VA is 3 times that induction disc relays.
12. The induction cup relays are more suitable for directional or phase comparison unit.