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Audel electrical course for apprentices and journeymen - Rosenberg P.

Rosenberg P. Audel electrical course for apprentices and journeymen - Wiley & sons , 2004. - 424 p.
ISBN: 0-764-54200-1
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Dual Voltages
Three-phase induction motors are usually wound for dual voltages, such as 240 volts and 480 volts, etc. The leads out of the motor are
Polyphase Induction Motors 321
numbered and connected for the voltage desired. This is accomplished by connecting internal windings in series or in parallel. The internal connections of the motor may be connected wye or delta, but wye is the most used.
A schematic of a three-phase squirrel cage induction motor is shown in Figure 30-9. This is delta-connected. The internal and external connections for a dual-voltage, delta-wound motor are illustrated in Figure 30-10.
o 1
Figure 30-9 Delta-connected induction motor.
HIGH VOLTAGE LINE 1-2-3
O "O o o o o
4 7 5 8 6 9
Figure 30-10 Connections for a dual-voltage delta-connected motor.
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Figure 30-11 illustrates the internal windings and external connections for a dual-voltage wye-wound motor.
Figure 30-11 Connections for a dual-voltage wye-connected motor.
An easy way to remember how to connect the leads of a dualvoltage motor, connected for wye, is shown in Figure 30-12. First draw the arrangements of wye windings. Then, starting at one point, draw a spiral so that it connects in order with all six coils. Begin at the starting point and number from 1 through 9 as you progress around the spiral. From this it is easy to see how to series or parallel coils as needed.
The following description is a method of determining the correct wiring sequence of a three-phase motor that for some reason doesn’t have the leads from the motor numbered. You don’t need to disassemble to find the proper numbering of the motor leads.
Step 1: Number the nine leads arbitrarily so that you have a starting point.
Step 2: With an ohmmeter or similar device, find the various coil sets and make a record of these coil sets, using the numbers that you originally assigned to the leads; assuming the motor is wye-connected, only three leads will show continuity to all of the thru leads. These three leads are the internally
Polyphase Induction Motors 323
3
Figure 30-12 Easy method for remembering the proper connections for a dual-voltage wye-connected motor.
connected wye. The remaining six leads belong to the three isolated sets of coils. See Figure 30-9.
Step 3: If the motor is Y connected, follow this procedure:
(a) Apply reduced voltage (40-100 volts) across one of the sets of coils that is in the internal, not in the wye. Use Figure 30-9A as an example, and put voltage across 6 and
7. Read the voltages across the internal wye that you have located (from 3 to 4, 4 to 5, and 3 to 5). Pick the lowest of these voltages. (Let us assume that this is between 3 and 4.)
(b) The terminal of the wye that the voltmeter is not attached to when the lowest voltage is read is the terminal that should be connected to the energized coil. In the example, since the voltmeter is connected across 3 and 4 when the lowest voltage is read, terminal 5 should be connected to coil 6 or 7.
(c) It now remains to find out whether 6 or 7 should be connected to 5 for correct operation of the motor. First
324 Chapter 30
connect 5 to 6 and apply reduced voltage to 3 and 7. Read voltage across 6 and 7. Now connect 7 to 5 and apply voltage to 3 and 6. Read the voltage across 6 and 7. The connection that resulted in the highest voltage across 6 and 7 is the correct connection.
Step 4: Repeat the above for the other coil sets. Remove the numbers you originally put on the leads and renumber them properly as shown in Figure 30-11.
Induction Motors on Start
Induction motors on start draw far in excess of full-load current. This current is termed locked-rotor current, and the motor nameplate has a code letter to indicate the locked-rotor current. The handiest method of interpreting the code letter is to use Table 430.7(b) of the NEC. This table is reproduced here as Table 30-1.
Table 30-1 [Table 430.7(b) of the NEC] Locked-Rotor Indicating Code Letters*
Code Letter Kilovolt-Amperes per Horsepower with Locked Rotor
A 0-3.14
B 3.15-3.54
C 3.55-3.99
D 4.0-4.49
E 4.5-4.99
F 5.0-5.59
G 5.6-6.29
H 6.3-7.09
J 7.1-7.99
K 8.0-8.99
L 9.0-9.99
M 10.0-11.19
N 11.2-12.49
P 12.5-13.99
R 14.0-15.99
S 16.0-17.99
T 18.0-19.99
U 20.0-22.39
V 22.4-and up
This table is an adopted standard of the National Electrical Manufacturers’ Association
Polyphase Induction Motors 325
For a motor with code letter G, the range is 5.6 to 6.29 kVA. Thus for a 20-hp motor, the actual kilovolt-ampere range would be 20 X 5.6 to 20 X 6.29 = 112 kVA to 125.8 kVA for locked rotor, which would also be the current drawn at standstill as the motor was first connected across the line for starting.
Assume this 20-hp motor to be three-phase, 230 volts, 54 amperes full-load current. Then the locked-rotor current can be calculated thus:
. . 112,000
Lower Limit = „ ----—— = 281 amperes
1.732 X 230 F
125,800
Upper Limit = „ ----—— = 316 amperes
1.732 X 230
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