Motor Startup
At system startup Electrical Submersible Pump motors draw more than five times greater instantaneous currents than their name plate current. Due to the increased current a high voltage drop develops across the cable and the voltage reaching the ESP motor may be insufficient to start the motor.
Normal starts require about 50% of the motor’s nameplate voltage to be available at the motor’s terminals.
The following simple formula, although being an estimate only, permits a rapid check on the cooperation of the cable and the motor at the start up conditions.
The ratio of the startup and nameplate voltage is found from:
U_start /U_np = (U_np – 4 I R)/U_np
where,
U_start: motor voltage at the startup, volts
U_np: motor nameplate voltage, volts
I: required motor current, amps, and
R: cable resistance, ohms
The selected cable size is satisfactory, if the this ratio is greater than 0.5, because in those conditions more than 50% of the nameplate voltage reaches the motor’s terminals and no startup problems will occur.
Switchboard and Transformer Selection
When specifying switchboards and transformers, the most important information is the maximum power of the ESP system required at the surface. This can be found from the necessary surface voltage and the motor current. Surface voltage is composed of the motor’s required terminal (nameplate) voltage plus the voltage drop across the power cable. Because at this phase of the installation design, the nameplate voltage has already been selected, and the cable voltage drop equals the product of the cable resistance and the motor current, the installation’s surface voltage requirement is found as:
U_surf = U_np + 1.732 R I
where
U_surf: Required Surface voltage, volts
U_np: motor nameplate voltage , volts
I : required motor current, amps, and
R: cable resistance, ohms
From the available models, a switchboard with a rated voltage above this value is selected. The power rating of the switchboard (in KVA units) is found from the formula used to find three-phase electric power.
P_surf = 1.732 U_surf I /1000 = 0.001732 U_surf I
where,
P_surf: required surface power, kVA
U_surf: required surface voltage, volts, and
I: required motor current, Amps
The necessary three phase step-up or step-down transformer should have the same kVA rating as the switchboard.
Reference:
Electrical Submersible Pump Manual; Design, Operations, and Maintenance; Second Edition; Gabor Takacs