Current induction
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The company’s fire pumps have been upgraded to include remote control; a control panel was installed about 300 meters away from the original distribution cabinet. Now, the pumps start and stop properly, but while the ammeter on the on-site distribution cabinet shows normal readings, the current reading at the remote control panel is almost zero. Upon inspection, it was found that the current transformer on site is of the 100/5 type, whereas the one used for remote control is of the 150/5 type. Moreover, these transformers are installed at the lower end, i.e., at the motor input side. Could it be that the larger size of these transformers along with overly long wiring are causing this issue? What should be done in this situation?
Calculation of the load on the CT secondary side: S = I² * Z. As an example, let’s use a current transformer with a rated secondary current of 5 A. According to the information provided, the length of the secondary circuit is 600 m, and the cross-sectional area of the wire core is assumed to be 2.5 mm². The resistance at 20°C is given by R = 0.0172 * 10^-6 * 600 / 2.5 * 10^-6 = 4.128 Ω. The internal resistance of the ammeter and contact resistance are not taken into account; only the resistance of the wiring is considered. S=I2Z= I2R= I2ρl/s=52×4.128=103.2VA ； Typically, current transformers of the 150/5A class have rated output capacities of 5 VA and 10 VA. It is much smaller than the calculated output capacity. Therefore, the current meter shows no reading. Assuming the rated current of the current transformer is 1 A: S = 4.128 VA. It is recommended that the owner replace it with a current transformer of type 150/1 A with a rated output capacity of 10 VA (it is also possible to increase the cross-sectional area of the wires accordingly).
Thank you, your explanation is easy to understand; I’ll give it a try later
Hello! If the original poster manages to solve this problem, please share the specific steps taken to do so if it’s not too much trouble. Thank you! ! !
My electrician says that using a 50/5 transformer should work, but according to your calculations the current will still be 5A, and the load on the output will remain the same; so it probably still won’t work. It’s a bit unclear to me.:dizzy:
Hello, OP, what is the power rating of your motor? Based on your description of the issue, it appears that the current transformers installed in the distribution cabinet on site are of the 100/5A type. Could the rated operating current of your motor exceed 50A? Based on the calculation formula, 50/5A also doesn’t work. We can do the calculations: assuming the operating current of the motor is 50A, then with a current transformer of 50/5A, the output current will be 5A. Using the formula S=I2Z=I2R=I2ρl/s, we get S=52×4.128=103.2 VA. On the other hand, the current transformer you are using, which is 150/5A, results in an output current of 5/3 A, or approximately 1.67A. Applying the same formula, we get S=1.672×4.128=11.47 VA. The difference in output power between the two is roughly 9 times. It will work or not; just give it a try and you’ll find out. This won’t cost much either. I suggest the original poster try 150/1A or 100/1A. Please note that the ammeter specifications of your remote control cabinet must also match those of the newly purchased current transformer.
:handshake thank you