Compressor Motor: Nuisance Tripping and an Unsafe Upsize Proposal
A workshop compressor is driven by a single-phase 230 V motor rated at 2.2 kW output. The motor nameplate gives full-load current 13.5 A, power factor 0.82 and efficiency 0.78. It is connected by a 13 A plug and socket to a dedicated 16 A type B miniature circuit breaker (MCB) with a 30 mA RCD. The compressor starts against a receiver pressure and the motor draws about six times full-load current for 0.8 s. The MCB sometimes trips after the compressor has run for several minutes, and the 13 A fuse in the plug has blown twice. The motor terminal box is damp and there is a smell of hot insulation. The user wants to replace the 16 A MCB with a 32 A MCB and fit a 20 A fuse 'to stop the nuisance tripping'.
1. Calculate the input current from the nameplate data and compare it with the 13 A fuse and the 16 A type B MCB. Comment on the user's proposal.
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Output power is 2.2 kW, efficiency 0.78, so input power = 2200/0.78 = 2821 W. Input current = P/(V × pf) = 2821/(230 × 0.82) = 14.96 A. The nameplate says 13.5 A, which corresponds to a slightly different efficiency or test condition, but either way the current is at or above 13 A. A 13 A fuse is therefore at or beyond its rating in normal running; it will blow on a small voltage dip or a hot day. The 16 A type B MCB is also marginal: it is not designed for a continuous 15 A motor load with a high starting current. The type B magnetic trip is 3–5 times rated current, i.e. 48–80 A; the starting current is about 6 × 13.5 = 81 A, so it may trip on start as well as run thermally.
The user's proposal is unsafe. A 32 A MCB and a 20 A fuse do not remove the fault; they remove the protection. The cable and the motor are rated for a much lower current, and the motor's own overload protection would be bypassed. The correct response is to find out why the current is high, not to raise the trip.
2. Distinguish between the possible causes of the fuse blowing, the MCB tripping after several minutes, and the RCD tripping. State the tests you would carry out.
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Three different symptoms point to three different mechanisms:
- 13 A fuse blowing: sustained current above 13 A, or a starting surge. Calculate 15 A running; the fuse is undersized. Check running current with a clamp meter under load. If it is near 15 A, the fuse is the wrong device, not a fault.
- MCB tripping after several minutes: this is a thermal trip, not a magnetic start trip. Causes include an overloaded compressor (high receiver pressure, restricted discharge, worn bearings), low supply voltage, a failing motor, or a 16 A MCB that is marginal and derating in a hot enclosure. Measure current and voltage while it runs; compare with the nameplate. A clamp reading of, say, 20 A would show a mechanical or load fault, not a nuisance trip.
- RCD tripping: this is earth leakage, not overload. Damp in the terminal box, contaminated windings or a damaged cable are likely. The smell of hot insulation supports winding degradation. Test insulation resistance with a 500 V insulation tester; a motor in good condition should be well above 1 MΩ. Do not bypass the RCD.
Also inspect the plug and socket for heat damage; a 13 A plug running at 15 A will overheat.
3. Recommend a safe solution, including the protective device type and rating, and any cable or motor considerations.
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Recommended solution:
- Replace the 13 A plug/socket arrangement with a 20 A dedicated motor circuit: 2.5 mm² copper cable, a 20 A double-pole isolator adjacent to the compressor, and a 20 A type C MCB (or a motor-rated manual motor starter) to ride through the 81 A start. A type C magnetic trip is 5–10 × 20 A = 100–200 A, above the start current, while still giving short-circuit protection.
- Keep the 30 mA RCD; it is required for socket outlets and is protecting the user. Fix the damp terminal box, replace damaged seals and dry the windings. If insulation resistance is below 1 MΩ, the motor needs rewinding or replacement.
- Provide motor overload protection set to the nameplate full-load current (about 13.5–15 A), not to the MCB rating. The MCB is for short circuit; the overload relay is for the motor.
- Check the supply voltage and the compressor unloader; a compressor that starts against full receiver pressure will draw a long high current.
- Do not fit a 32 A MCB or a 20 A fuse on the existing 13 A plug circuit. The cable, plug and motor are not rated for it, and the fire risk increases.
Record the measured running current, insulation resistance and supply voltage before returning the compressor to service.