MJB250LB4 - Marelli | 200 kW

Model. The MJB250LB4 is a Marelli MJB 4-pole alternator. Marelli reads the model as series MJB, frame 250, core LB and 4 poles. The MXB range uses an aluminium alloy 46100S T6 frame with cooling ribs; the MJB range steps up to a steel frame for the larger frame sizes. It is wound for three-phase service at 50 Hz. Electrical rating. At 0.8 power factor lagging this machine is rated 200 kW, which is 250 kVA, delivered as three-phase at 400 V 50 Hz. At 400 V line-to-line that is 360.8 A per line. This is the band where motor starting, not steady-state kW, usually sizes the machine: a direct-on-line motor of roughly 30% of the alternator kVA will pull the terminal voltage down by about 15% at start, which is normally acceptable, and anything larger needs a soft starter or a bigger frame. The 1500 rpm figure is synchronous speed, so there is no slip and no speed-dependent voltage droop. Frame and core. The 250 frame carries the LB core. Frame size fixes the stator bore and the shaft centre height; the core length fixes how much active iron sits in that bore, and therefore how much output the machine can make at a given temperature rise. The exact finished weight and outline drawing depend on the winding and the coupling, so they are confirmed on the certified datasheet issued with the order. The cooling circuit is IC01 open drip proof, and the frame is steel frame, cast iron end shields, steel shaft. Three-phase winding. This is a three-phase machine with a 12 re-connectable winding brought out to 12 re-connectable terminals, so it can be reconnected on site for the usual voltage pairs — 380/220 V, 400/230 V, 415/240 V, 440/254 V and 480/277 V on the 50 Hz build, and the 60 Hz pairs on the 60 Hz build. Reconnection is done on the link board inside the terminal box, not by rewinding, which is why one machine can serve a 380 V job and a 480 V job. The 2/3 pitch design is what keeps the fifth and seventh space harmonics down without skewing the stator, and it is the reason the line-to-line waveform stays within the distortion limits the standards ask for. Four-pole build. A four-pole machine turns at 1500 rpm on 50 Hz and 1800 rpm on 60 Hz. That is the standard choice for generator sets from a few tens of kVA up to the low megawatt range, because the lower rotor speed gives a longer bearing life, a quieter fan, a higher subtransient reactance that is easier to coordinate with downstream protection, and a rotor that can carry a substantial damper cage for parallel operation. The trade is weight: a four-pole machine of a given kVA is roughly twice the mass of a two-pole one. Excitation and voltage regulation. The excitation system is SHUNT standard; permanent magnet generator "PMG" optional, regulated by MARK V, holding steady-state voltage to +/-1.0%. Marelli uses the MARK VX and MARK V regulators with a brushless rotating rectifier built as a six-diode Graetz bridge. A PMG or an auxiliary winding is available as an option, and on the 225 frame the auxiliary winding is the standard arrangement. The regulator senses three-phase voltage and holds +/-1.0% from no load to full load at any power factor within the rating. Mechanical build. The frame is steel frame, cast iron end shields, steel shaft. Bearings are permanently lubricated, L10 life >= 20,000 h. Mounting is IM 2105 horizontal single bearing (two bearing optional). Maximum overspeed is 2250 rpm, and the machine is rated for 1000 m. The terminal board is Bakelite PF2736 with seven studs on the 160 and 180 frames and nine studs on the 180 to 225 frames, which is what allows the twelve leads to be reconnected for the full range of voltage pairs. The shaft is EN 10025 steel. Bearing L10 life is quoted at 20,000 hours or better under rated load. Where it fits. Industrial plant and institutional standby: a hospital theatre block, a small data room, a food processing line, or a water treatment works. These jobs usually ask for a PMG or AREP excitation system so that the downstream protection can discriminate on a fault. On a three-phase machine of 200 kW the practical constraint is usually the downstream protection and the cable, not the alternator itself. Core stack position. The LB core is stack position 8 of 9 in the MJB family, roughly 89% of the way along the family's lamination length range. It sits between the shorter LA and the longer LC. Long-stack builds carry the rating in the frame with the lowest flux density of the family. The extra iron costs length and weight, and buys a cooler-running core, lower magnetising current, better part-load efficiency and more margin for site ambient above 40 C or for a non-standard voltage. Reading the model code. A Marelli designation is read as the MXB or MJB series, a three-digit frame number, a two-letter core code, and the pole count. Here that gives MJB250LB4 for the model, 250 for the frame, LB for the core and three for the phase count, so the code on the nameplate and the code on this page match one for one. Quoting the full code rather than just the frame matters on this family, because the same frame number is built in several core lengths and the winding changes with the voltage. Frame position. The MJB range is built on 1 frame sizes — 250 — and the 250 frame is number 1 of them. No other frame in this range carries exactly this rating, so the 250 is the only build at 200 kW and there is no cross-frame decision to make. What the nameplate means once it is wired in. At 200 kW and 250 kVA this machine pushes 361 A down each line on a 400 V three-phase supply. That current, not the kVA figure, is what the switchgear and the cable have to survive, so the outgoing breaker should be a 630 A frame device set to trip above 451 A, and the run from the alternator terminals to the first panel should be 150 mm2 copper per phase for a normal 25 to 30 m span. Longer runs, aluminium conductors or tray-installed cable all move that number up, and the volt drop at full load is what decides it in the end. On the mechanical side the prime mover has to deliver about 224 kW at the coupling. The usual 1.10 to 1.15 margin over the alternator rating covers the fan, the excitation loss and the governor droop, and it is the reason a 200 kW alternator is normally quoted against a 224 kW engine rather than a 200 kW one. Fuel burn follows the electrical load almost linearly in this range. At a realistic 0.29 L per kWh for a modern common-rail set, running this alternator at full rating takes roughly 58 L per hour; at 75 per cent load it drops to about 45 L per hour and at half load to about 32 L per hour, because the fixed losses stay put while the variable ones fall away. Quote the breaker frame, the cable size and the engine rating together with the alternator, because a set that is right on paper and undersized on the cable is the most common way a good alternator gets blamed for a bad installation. Supply. Supplied as a genuine Marelli Motori unit — Marelli Motori S.p.A., built in Italy. MXB and MJB ranges are built in Italy and carry a European certificate of origin. Each alternator ships with the routine test certificate, the winding connection diagram, and the AVR commissioning sheet. Export packing is seaworthy plywood on a pallet sized to the frame, and the machine is strapped and shrink-wrapped inside it. Tell us the engine make and model, the coupling arrangement (single or two bearing), the voltage and frequency, and whether you need a PMG, and we will confirm the build before it leaves the works.