Motor Starting

Sizing of Generators for Motor Starting (Direct on Line)

Assumptions

For Motors up to 200kW

6-7 times running current on start up of motor

Efficiency 90% (0.9) PF 0.85

For motors above 200kW

5-6 times running current on start up of motor

Efficiency 95% (0.9) PF 0.85

Allowance

Average transient voltage trip will be around 15% of rated kW of alternator. Allow max. 35% voltage dip on main input terminals on capital load.

25% for voltage dip 10% for VHz engine dip

Note: contactors may start to "drop out" at around 75% rated voltage.

Check voltage dip and locked rotor kVA curve on spec sheet for relevant alternator and multiply running kVA by assumed starting current multiplier to give starting (locked rotor) kVA.

Approximate current per line or phase taken at full rated HP by motors of average efficiency and power factor.

Motor Rating Direct Current Alternating Current
  110V 220V 550V 240V 380V 415V 550V
HP Amp Amp Amp Amp Amp Amp Amp
0.5 5.7 2.8 1.1 3      
1 10 5 2 6 1.9 1.7 1.3
2 18 9 3.6 10 3.6 3.3 2.5
3 26 13 5.2 15 5.1 4.6 3.5
5 42 21 8.4 24 8 7.3 5.5
7.5 60 30 12 35 11.6 10.6 8
10 80 40 16 46 15.1 13.8 10.4
15 117 59 23 67 22 20 16
20 154 77 31 88 29 27 21
25 190 95 38 110 37 34 26
30 227 114 46 130 43 40 30
40 300 150 60 180 59 54 41
50 375 187 75 210 73 67 50
50 445 223 89 253 87 80 60
60 520 260 104 291 102 94 70
80 600 300 120 332 117 107 81
100 740 270 148 412 145 133 100
125   460 184 545 181 166 125
150     220   217 199 150
175     256   253 232 175
200     292   288 264 199
250         353 323 244
300         421 385 291

For more information contact us now on 0800 146 763 or click here to send an enquiry

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