DOL Motor Tripping on Start
A 15 kW, 380 V, three-phase induction motor drives a conveyor. The motor has a full-load current of 30 A, a power factor of 0.85 and an efficiency of 0.90. It is started direct-on-line and takes about six times full-load current for approximately 3 seconds. The circuit is protected by a 40 A Type C MCB and a 32 A thermal overload relay, fed by a 25 mm² four-core cable, 40 m long. The MCB trips intermittently during starting when the conveyor is fully loaded, but the motor runs normally once up to speed. The maintenance technician has already replaced the MCB once and checked the motor windings. The upstream board has a 100 A MCCB. The cable is clipped direct and has a tabulated current-carrying capacity of 110 A. The measured earth-fault loop impedance at the motor terminals is 0.42 Ω.
1. Explain why the 40 A Type C MCB trips during starting even though the motor runs normally afterwards.
Read the worked answer
At 30 A full load, a 40 A Type C MCB is only 1.33 times the full-load current. The starting current is about 6 × 30 = 180 A, which is 4.5 times the MCB rating, and it persists for about 3 seconds. A Type C magnetic trip is set at 5–10 times In, i.e. 200–400 A, so the magnetic element may not operate. However, the thermal element sees 4.5 times In for 3 seconds and can trip within that time, especially if the MCB is already warm. The 32 A overload relay is also set close to the motor full-load current and may begin to operate during a long DOL start. The cable is not the problem: its tabulated capacity of 110 A is well above the 40 A device rating. The measured Zs of 0.42 Ω gives a prospective fault current of about 220 / 0.42 = 524 A, which is adequate for a 40 A Type C but marginal for a Type D. The diagnosis is a starting-coordination problem, not a motor winding fault.
2. The technician proposes replacing the 40 A Type C MCB with a 40 A Type D MCB. Evaluate this proposal using the measured Zs.
Read the worked answer
A 40 A Type D MCB should not be fitted without checking the earth-fault loop impedance. Its magnetic trip is 10–20 times In, i.e. 400–800 A. The available fault current is about 524 A, so the device may or may not trip magnetically within the required time, depending on its actual tolerance. The maximum permitted Zs for a 40 A Type D MCB is lower than 0.42 Ω, typically around 0.38 Ω, so the measured value would not comply. A 63 A Type D would make the Zs requirement even stricter and would also reduce overload protection for the cable. Better solutions are: use a soft starter or star-delta starter to reduce starting current and start time; use a motor-rated MCCB or MPCB with an adjustable magnetic trip set above the starting current but within the fault level, after verifying Zs; or improve the earth-fault loop impedance if a Type D device is essential. The overload relay must remain set to the motor full-load current, and discrimination with the upstream 100 A MCCB should be checked.