ESP Motor Voltage Calculation: A Comprehensive Field Guide for SUT Tap Selection

Post by **ALSEngineer** » Mon Apr 27, 2026 3:33 am

Step-by-step technical procedure for calculating ESP motor voltage, compensating for cable losses, and selecting SUT taps to optimize run life and efficiency.

In the oilfield, ensuring the longevity of an ESP system starts with a fundamental task: delivering the correct voltage to the motor terminals. This isn't just about reading a nameplate; it’s a dynamic process that accounts for power losses and grid fluctuations.

As a field engineer, you need a clear algorithm to determine the ideal secondary voltage of your Step-Up Transformer (SUT). This guide covers the essential logic used to calculate the required voltage, ensuring your motor runs cool and your system stays reliable. Whether you are dealing with a brand-new installation or troubleshooting an existing well, this calculation is your starting point.

Prior to the initial startup of an ESP system, a motor operating voltage calculation is required. The following data points are necessary to perform this calculation:

Motor Rated Voltage (from the Data Sheet).
Motor Rated Current.
Total Cable Length from the Step-Up Transformer (SUT) to the motor (including the downhole string, surface lead, and the cable connection to the junction box).
Measured Input Voltage (specifically for Across-the-Line / DOL Switchboards without a VFD).

Cable Conductor Cross-Section. Use the cross-section of the main cable string (the primary length). Minor differences in cross-section can be neglected. However, if there are significant lengths of different gauges, the voltage drop must be calculated for each section independently and then summed.

These parameters are located in the Equipment Records or the Documentation Package provided with the ESP unit.

For Across-the-Line (DOL) and Soft Start Switchboards:

Measure the actual input line voltage and divide it by the nominal service voltage of the surface equipment (480V for U.S. grids or 380V for GOST-standard equipment).
Calculate the voltage drop across the downhole cable (or retrieve it from a reference table).
Add the voltage drop value from Step 2 to the Motor Rated Voltage.
Multiply the result from Step 3 by the ratio obtained in Step 1.

For Variable Frequency Drive (VFD) Switchboards:

Input voltage measurement is not required, as the VFD provides an adjustable output voltage.
Calculate the voltage drop across the downhole cable using the same method as for Across-the-Line (DOL) systems.
Add the calculated cable voltage drop to the Motor Rated Voltage.
Apply the V/Hz frequency ratio. Multiply the result from Step 3 by a coefficient based on the programmed operating frequency. For example, if the drive is set to 66Hz (55Hz - GOST), the coefficient is 1.1; for 72Hz (60Hz - GOST), it is 1.2. This coefficient is calculated by dividing the operating frequency by the Motor Rated Frequency 60Hz (typically 50Hz for GOST-standard equipment).
Account for the Sine Wave Filter. If a motor flux/output filter is installed, multiply the resulting value from Step 4 by a factor of 1.1 to compensate for the filter's internal impedance.

Detailed instructions for motor voltage calculations are typically provided by either the operator (customer) or the ESP service provider, depending on the contractual agreement, and are formalized in the Standard Operating Procedures (SOP). This article is intended to provide a general overview of the Step-Up Transformer (SUT) tap selection procedure.