Electrical Transformers and Rotating Machines
Electrical Transformers and Rotating Machines
4th Edition
ISBN: 9781305494817
Author: Stephen L. Herman
Publisher: Cengage Learning
Question
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Chapter 21, Problem 9RQ
To determine

The conductor sizes, overload heater size, circuit breaker size, fuse size, minimum NEMA and IEC starter sizes, and the upper and lower range of starting current for this motor.

Expert Solution & Answer
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Explanation of Solution

Given:

Horsepower of the motor (P) is 75hp.

Line voltage (V) is 230V.

Power factor (F) is 80%.

Full-load current (I) is 185A.

A temperature rise (ΔT) is 40°C.

Three phase synchronous motor.

Code letter is A.

Conductor is copper wire with THHN insulation.

Transformer ratio (T) is 300:5.

Calculate the maximum ampacity (Ii) using the relation.

  Ii=IF

  Ii=185A80%×(1100%)=1.25×185A=231.25A

Refer to Table 310.15(B)(16), “Conductor sizes” to obtain the size of conductor as 4/0AWG corresponding to copper conductor with THHN insulation and 260A nearest value to 231.25A.

Refer to Table 430.52, “Percentage of full load current” to obtain the percentage full-load current as 300A corresponding to inverse time breaker and synchronous type motor.

Refer to Figure 21-19, “Fuse size” to obtain the fuse size as 300A nearest to percentage of full-load current 300A.

Refer to Figure 21-21, “IEC motor starter by size, horsepower” to obtain the size of IEC starter as S corresponding to 75hp and 230V.

Refer to Figure 21-20, “NEMA motor starter sizes” to obtain the size of IEC starter as 4 corresponding to 75hp and 230V.

Refer to Table 430.7(B) “Locked-Rotor indicating code” to obtain the upper and lower range of starting current as 0kVAhp3.14kVAhp corresponding to code letter A.

Calculate the upper limit (I1) using the relation.

  I1=P×C1V(3)

  I1=75hp×0kVAhp230V(3)=0A

Calculate the lower limit (I2) using the relation.

  I2=P×C2V(3)

  I2=75hp×3.14kVAhp230V(3)=0.5911kA×(103A1kA)=591.1A

Thus, the conductor size is 4/0AWG_, fuse size is 300A_, minimum NEMA motor starter size is 4_ and IEC starter size is S_, and the upper range of starting current is 0A_ and lower range of starting current is 591.1A_.

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