How to size transformers install#
How to size transformers full#

If the voltage available at the motor is too low to achieve the required torque because the volt-drop due to is large, the motor will stall.

How to size transformers full#

The permissible volt-drop through the transformer impedance depends on what starting torque the load requires: if the load requires a certain torque at standstill or during acceleration to full speed, then there will be a minimum voltage at which this torque can be developed. Normal design practise would be to use motor starting gear to suit the motor rating, unless there was something unusual about the load starting characteristics. I'm not suggesting to size the transformer or switchgear to be able to continuously supply locked rotor current. RE: Transformer sizing for motor starting ScottyUK (Electrical) 9 Sep 03 12:09 If there is a good chance that this transformer will be used to supply future loads (like a second fan), I would take that into account.

How to size transformers install#

One other consideration is that it is extremely more expensive to have to engineer, design, purchase, install and checkout a transformer twice. You indicate that this is the only motor to be fed. My recommendation would be to install a transformer rated a minimum of 100 kVA. Thus the kVA required to meet the motor demand is (460)(95)(sqrt(3))/1000 = 75.7 kVA. By the way, concerning the fuses, you must use вЂњdual-element, time-delayвЂќ fuses that are designed to вЂњpass throughвЂќ the starting current without melting.ДЇor your specific application, a 3-phase, 460V, 75 hp motor requires approximately 95 amps (rated amps). In reality, the RMS (root-mean-squared) heating produced by the (very short duration) starting current will not harm the transformer (or the motor for that matter). The rated current on the nameplate is sufficient. Your application appears to be вЂњnormal dutyвЂќ (assuming this is a fan, the motor will be running continuously for long periods of time), thus locked rotor amps would NOT be necessary to size the transformer (or starters, fuses, overloads, cabling, conduit, etc.) to feed the motor. The only time that this may be a consideration is if the motor is вЂњpluggedвЂќ very frequently (start-stop-start-stop-start-stop-you-get-the-picture). The full-voltage starting current of a motor is approximately 6 times rated current (or approximately the locked rotor current). The following works from a practical standpoint:ДЇor virtually all industrial applications that I am aware of, Locked-Rotor current is never used to size the equipment feeding the motor. IвЂ™m not sure about all the theoretical impedance stuff mentioned, however, if you were to size all of your system components based on locked-rotor current, you would GROSSLY overestimate. I somewhat disagree with scottyuk and rbulsars.