Which of the device is a single excited magnetic field system

Question

Question

This question was previously asked in

ISRO (VSSC) Technician Electrician 2019 Official Paper

Answer 1

A relay is an electromagnetic switch used to switch High Voltage or Current using low power circuits.
Relays often have two ratings: AC and DC. These ratings indicate how much power can be switched through the relays. This does not necessarily tell you what the limits of the relay are.
For instance, a 5 Amp relay rated at 125 V AC can also switch 2.5 Amps at 250 V AC. Similarly, a 5 Amp relay rated at 24 V DC can switch 2.5 Amps at 48 V DC, or even 10 Amps at 12 V DC.
Singly excited magnetic system is one which has only one source of excitation, examples are electromagnetic relays, moving-iron instruments, reluctance motors etc.
While, the doubly excited magnetic systems are those which has two independent sources of excitations, examples are synchronous machines, loudspeakers, DC shunt machines etc.