DOL or star-delta for a 15 kW crusher conveyor that always starts loaded
A 15 kW, 415 V, 4-pole squirrel-cage motor drives the feed conveyor of a stone-crusher unit. Nameplate data: 27 A, 1440 rpm, efficiency 89 percent, power factor 0.86. The motor is 40 m from the MCC through a 4-core 16 mm2 copper armoured cable, and the plant is fed by a dedicated 250 kVA transformer with 5 percent impedance. Management has already bought a DOL starter and also has a used star-delta starter available free from another site. The conveyor is always started with 8 to 10 tonnes of stone on the belt and needs about 70 percent of full-load torque to break away. The adjacent CNC shop has complained that its drives reset whenever the crusher starts, and those drives are specified to ride through a 7 percent voltage dip. The electrical inspector has asked for a written justification of the starting method, with calculations, before the starter is energised.
1. Verify the full-load current from the nameplate data, estimate the DOL starting current and the voltage dip at the MCC busbar, and decide whether the DOL start alone explains the CNC resets.
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FLC check. I = 15,000 / (1.732 x 415 x 0.86 x 0.89) = 15,000 / 550 = 27.3 A, matching the 27 A nameplate. The motor is correctly rated for the duty.
Starting current. A crusher-duty cage motor takes about 6 times FLC: 6 x 27 = 162 A.
Starting kVA. 1.732 x 415 x 162 / 1000 = 116 kVA.
Transformer dip. Short-circuit capacity = 250 / 0.05 = 5,000 kVA. Dip at the transformer terminals = 116 / 5,000 x 100 = 2.3 percent. Add roughly 0.5 percent for the transformer-to-MCC cable, so about 2.8 percent at the MCC busbar.
Motor terminal dip. On the 16 mm2 feeder (about 2.4 mV/A/m), drop = 1.732 x 162 x 40 x 2.4 / 1000 = 27 V, that is 6.5 percent along the motor feeder only. The motor sees about 9 percent, which is acceptable for starting.
Judgement on the CNC complaint. A 2.8 percent busbar dip cannot reset drives rated to ride through 7 percent. The DOL start is therefore not the demonstrated cause; the CNC resets should be investigated separately (loose neutral, drive DC-bus setting, another large load, supply sag) with a power-quality logger before the crusher is blamed.
2. Evaluate the free star-delta starter against the load requirement and recommend a starting method with justification.
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Star-delta torque. In star, phase voltage is 1/1.732 of line voltage, so torque falls to one third of the DOL value. A DOL motor develops about 200 percent of full-load torque at start; in star that becomes about 67 percent.
Against the load. The conveyor needs 70 percent of full-load torque to break away with stone on the belt, and the belt is always loaded. 67 percent is below the requirement before any voltage dip is considered; with an 8 percent dip during starting, torque falls further in proportion to the square of voltage, to roughly 57 percent. The motor will sit at locked rotor, draw its starting current until the overload trips, and overheat the winding each time.
Mechanical effect. Even where star-delta does accelerate, the changeover from star to delta produces a current and torque transient that shocks the gearbox and belt.
Decision. Do not fit star-delta for this duty. Options: (a) fit the DOL starter, which meets the torque requirement and gives only about 2.8 percent busbar dip, with a class 20 overload relay because a loaded start may exceed 10 s; (b) if the mechanical shock or belt slip is unacceptable, fit a soft starter with a current limit of 3 to 3.5 times FLC and a kick-start of 0.3 s. The soft starter also gives phase-failure and under-voltage protection.
Interlock. Add a single-phasing preventer and set the magnetic short-circuit trip at about 13 times FLC (350 A).
3. State the protection settings and the commissioning records you would hand over, and explain how you would answer the inspector if he insists on star-delta.
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Settings.
- Thermal overload relay: set to nameplate FLC 27 A, class 20 for the loaded start; verify the actual start time with a clamp meter, since a class 10 relay will trip a 15 s start.
- Magnetic short-circuit setting (MPCB or MCCB): 13 x 27 = 350 A, below the 16 mm2 cable withstand.
- Under-voltage release: 70 to 80 percent of rated, so the motor drops out on a genuine sag and does not re-start into a loaded belt.
- Single-phasing preventer and phase-sequence relay fitted.
Commissioning records. Insulation resistance of motor and feeder (500 V megger, 1 Mohm minimum), earth continuity, contactor AC-3 rating at least 32 A, tightened lug torques, measured starting current and start time, measured dip at the MCC with a logger, overload relay type and setting, and the as-wired drawing signed by the electrician.
Answer to the inspector. Present the torque calculation: required 70 percent, star-delta available about 67 percent before dip. Offer the alternative of a soft starter or of a sequenced start, that is, start the empty belt and load it after acceleration, with the interlock documented. Ask for a witnessed trial showing the belt starting under load with the DOL; if the trial shows the belt cannot accelerate, the soft starter will be installed. This keeps the decision evidence-based rather than a matter of preference.