The coil of a current relay used with small single-phase compressors should have a resistance of approximately:

• A. 0.01 Ω
• B. 0.1 Ω
• C. 1 Ω
• D. 10 Ω

Answer: (B)

The coil in a current relay used with a small single‑phase compressor is designed to carry the motor’s current with as little voltage loss as possible while still producing a strong magnetic pull to operate the relay during startup. A value around 0.1 ohm provides a very small voltage drop at the high starting current, so most of the supply voltage stays across the compressor and the relay can respond quickly.

If the coil resistance were much higher (like 1 ohm or 10 ohms), the coil would drop more voltage and waste more power, potentially starving the compressor during startup and making the relay operate less reliably. If it were much lower (like 0.01 ohm), it would still be possible in principle, but it’s not practical given typical coil designs and tolerances; a practical, robust choice sits near 0.1 ohm.

So, a coil resistance of about 0.1 ohm is the best balance to ensure reliable relay operation without imposing a significant voltage drop on the running compressor.