The following is a general summary of all the stages involved
Transformer Sizing Considerations
The transformer‘s size should allow it to start a motor with a higher current without experiencing an excessive voltage drop. It is typical practice to size the transformer to withstand a voltage drop of no more than 5 to 10 percent while starting a motor.
Following are the steps to calculate the transformer rating:
Full Load Current of a Transformer = KVA/(√3 X Secondary Voltage)
Short Circuit Current of a Transformer = Transformer Full Load Current / Impedance
Analysing Motor Parameters
- Identify the motor’s rated power (in kW or HP).
- Find the rated voltage (in V) for the motor.
- Find the motor’s power factor (typically listed on the nameplate or datasheet of the motor).
- Take note of the starting method used by the motor (Star-Delta, Direct-on-Line, Soft Starter, VFD, etc.).
Motor Full Load Current = (Motor Size x 1000)/( √3 x Motor Voltage x Power Factor)
ILM=(KW x 1000)/(1.732 x VM x P.F)
Voltage Drop of a Transformer (in Secondary)
The following formula can be used to estimate the voltage drop of a transformer produced by the inrush current of a motor & the maximum KVA rating of the transformer:
VD = Voltage Drop
IRSM = Inrush Current
Q1= Maximum KVA of Transformer at Short Circuit
An electric motor’s type and features are identified using motor codes. They are often made up of a series of letters & numbers, each representing a distinct aspect of information.
Motor codes are a useful tool in identifying & selecting the appropriate electric motor for a given application.