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

Technical Info