Franklin 2007 manual Transformer Capacity Single-Phase or Three-Phase, Effects of Torque

Application – All Motors

Transformer Capacity - Single-Phase or Three-Phase

Distribution transformers must be adequately sized to satisfy the kVA requirements of the submersible motor. When transformers are too small to supply the load, there is a reduction in voltage to the motor.

Table 4 references the motor horsepower rating, single- phase and three-phase, total effective kVA required, and

the smallest transformer required for open or closed three-phase systems. Open systems require larger transformers since only two transformers are used.

Other loads would add directly to the kVA sizing requirements of the transformer bank.

Table 4 Transformer Capacity

MOTOR RATING

HP KW

1.51.1

21.5

32.2

5 3.7

7.55.5

10	7.5
15	11
20	15
25	18.5
30	22
40	30
50	37
60	45
75	55
100	75
125	90
150	110

175130

200150

TOTAL	SMALLEST KVA RATING-EACH TRANSFORMER
EFFECTIVE	OPEN WYE	CLOSED
KVA	OR DELTA	WYE OR DELTA
REQUIRED	2- TRANSFORMERS	3- TRANSFORMERS
3		1
	2
4	2	1.5
5	3	2
7.5	5	3
10	7.5	5
15	10	5
20	15	7.5
25	15	10
30	20	10
40	25	15
50	30	20
60	35	20
75	40	25
90	50	30
120	65	40
150	85	50
175	100	60
200	115	70
230	130	75

NOTE: Standard kVA ratings are shown. If power company experience and practice allows transformer loading higher than standard, higher loading values may be used to meet total effective kVA required, provided correct voltage and balance is maintained.

Effects of Torque

During starting of a submersible pump, the torque developed by the motor must be supported through the pump, delivery pipe or other supports. Most pumps rotate in the direction which causes unscrewing torque on right-handed threaded pipe or pump stages. All threaded joints, pumps and other parts of the pump support system must be capable of withstanding the maximum torque repeatedly without loosening or breaking. Unscrewing joints will break electrical cable and may cause loss of the pump-motor unit.

To safely withstand maximum unscrewing torques with a minimum safety factor of 1.5, tightening all theaded joints to at least 10 lb-ft per motor horsepower is recommended (table 4A). It may be necessary to tack or strap weld pipe joints on high horsepower pumps, especially at shallower settings.

Table 4A Torque Required (Examples)

MOTOR RATING

HP	KW
1 hp & Less	0.75 kW & Less
20 hp	15 kW
75 hp	55 kW
200 hp	150 kW

MINIMUM SAFE TORQUE-LOAD

10lb-ft 200 lb-ft 750 lb-ft

2000 lb-ft

4