Voltage dip at motor starting
Estimate of the voltage dip on the substation busbars during direct-on-line starting of an induction motor, and of its reduced starting torque — to check whether a large pump or compressor can be started from a given transformer.
S_st = √3 · U · K_i · I_nS_k = S_t / u_kΔU = S_st / (S_st + S_k)M_st / M_st,n = (1 − ΔU)²Only the transformer impedance is included: the supply network and the cable increase the dip, while adding impedance magnitudes without their angles gives a small margin. The substation load before starting is not included. The starting torque falls with the square of the voltage: pumps started against a closed valve usually have a margin, pumping units and reciprocating compressors must be checked. If the dip is large, use a soft starter or a variable frequency drive.
Source: Estimate from the transformer short-circuit power (impedance addition method)
Inputs
You can change a field's unit: the value is converted to the formula's units automatically.
Line-to-line voltage on the motor side: 380 V, 6 or 10 kV.
From the motor catalogue: usually 5–8 direct-on-line.
From the nameplate; oil-immersed 6–10/0.4 kV transformers: up to 400 kVA about 4.5 %, 630–1 000 kVA 5.5 %, 1 600–2 500 kVA 6 %.
Unit converter for this formulaVoltage · Current · Apparent power · Fraction and percent
- V1
- kV0.001
- U_st — Busbar voltage during starting–
- M_st — Starting torque, % of the rated value–
- S_st — Motor starting kVA–
More in Motors and transformers
Reactive power of a capacitor bank that raises the power factor from cos φ_1 to cos φ_2, and the resulting reduction of the supply current and line losses.
Design apparent power from the installed load, the demand factor and cos φ, and the transformer rating from the standard series for the allowed loading and the number of transformers in the substation.
Synchronous speed from the supply frequency and the number of poles, slip from the rated speed, and the speed on a variable frequency drive — for example, of an ESP motor or a pumping unit drive.
Results are engineering estimates from standard formulas; for design decisions check them against the codes, project documents and specialists' calculations. The formulas carried over from the original set are unchanged, and their errors are described in the notes.