Motor Starting Voltage Dip Mitigation: Inrush, Acceleration, and Drive Selection

Published: June 2026
Technical Level: Advanced Category: Industrial Power Systems

Abstract

The mitigation of the voltage dip produced by starting a large motor requires the engineer to quantify the motor's inrush current and acceleration time, to assess the dip against the limits the system imposes, and to select among the available mitigation measures — reduced-voltage starting, reactive support, or a variable-frequency drive — the one that holds the dip within limits while delivering the required acceleration. This paper develops the calculation of inrush current and acceleration time, examines the role of reactive compensation in supporting the bus voltage during starting, and addresses the selection of the variable-frequency drive as the most complete mitigation. The emphasis is on the engineering decisions that follow once a motor-starting study has identified an unacceptable dip.

1. Inrush Current and Acceleration

The starting current drawn by an induction motor is determined by its locked-rotor characteristic, conventionally expressed through the motor's code letter or its locked-rotor-to-full-load current ratio, which for typical motors lies between five and seven. The locked-rotor apparent power that the motor demands at the instant of starting is the product of this ratio and the motor's rated demand, and it is this demand, drawn through the supply impedance, that produces the voltage dip. The inrush persists until the motor accelerates, and the acceleration time is governed by the inertia of the motor and its driven load and by the margin between the torque the motor develops and the torque the load demands at each speed:

tacc=2π J Δn60 (Tmotor−Tload)t_{acc} = \frac{2\pi \, J \, \Delta n}{60 \, (T_{motor} - T_{load})}

Where:

tacct_{acc} is the acceleration time in seconds.

JJ is the combined moment of inertia of the motor and driven load in kilogram-square-meters.

Δn\Delta n is the change in speed during acceleration in revolutions per minute.

TmotorT_{motor} is the torque developed by the motor in newton-meters.

TloadT_{load} is the torque demanded by the load in newton-meters.

The acceleration time matters because the voltage dip persists for its full duration, and a long acceleration — characteristic of a high-inertia load such as a large fan or centrifuge — prolongs the dip and the demand on the supply. The acceleration time also governs the thermal stress on the motor, because the high starting current heats the rotor throughout the acceleration, and an excessively long acceleration can overheat the motor. The starting study must therefore evaluate not only the depth of the dip but its duration, since both the disturbance to the system and the stress on the motor depend on how long the inrush persists.

As a worked example, consider a 200 hp induction motor driving a high-inertia centrifugal fan, with a combined motor-and-load inertia of J=85J = 85 kg·m², accelerating through Δn=1785\Delta n = 1785 rpm, with an average accelerating torque margin (Tmotor−Tload)=650(T_{motor} - T_{load}) = 650 N·m over the run-up. The acceleration time is:

tacc=2π⋅85⋅178560⋅650=953,31639,000=24.4 st_{acc} = \frac{2\pi \cdot 85 \cdot 1785}{60 \cdot 650} = \frac{953{,}316}{39{,}000} = 24.4 \ \text{s}

Where the symbols are as defined above. The dip therefore persists for roughly 24 seconds — long enough that the depth of the dip, not merely its occurrence, becomes the governing concern. If the locked-rotor demand of this motor is 1080 kVA and the bus has an available short-circuit capacity of 25 MVA, the approximate dip is 1080/(1080+25,000)=4.1%1080/(1080 + 25{,}000) = 4.1\%, holding the bus at about 95.9 percent of nominal. A 4 percent dip sustained for 24 seconds is acceptable for most equipment, but on a weaker bus — say 8 MVA short-circuit capacity — the same motor would produce a 1080/(1080+8000)=11.9%1080/(1080+8000) = 11.9\% dip held for the full 24-second acceleration, which would trip undervoltage relays and stall contactors, and it is precisely this combination of dip depth and acceleration duration that drives the choice between direct-on-line and reduced-voltage starting.

2. Reactive Support for Voltage Maintenance

One class of mitigation addresses the dip directly by supplying reactive power to support the bus voltage during starting, rather than by reducing the motor's starting current. The voltage dip is largely a reactive phenomenon, because the starting current of an induction motor is strongly lagging and the resulting drop across the predominantly inductive supply impedance is correspondingly large. A source of reactive power at the bus — a capacitor bank switched in during starting, or a dynamic reactive compensator — offsets a portion of the reactive demand and supports the voltage, reducing the dip without altering the motor's starting method.

The application of reactive support requires care. A fixed capacitor bank left connected during normal running can over-correct the power factor and raise the voltage excessively at light load, so reactive support for motor starting is generally switched, connected for the starting interval and disconnected afterward, or provided by a dynamic compensator that adjusts its output continuously. Reactive support is most attractive where the motor must be started direct-on-line for process reasons, where a reduced-voltage method would not provide adequate starting torque, or where the dip affects a sensitive bus that must be supported regardless of the starting method. Where the dip is severe, reactive support is often combined with a reduced-voltage starting method rather than relied upon alone.

3. Variable-Frequency Drive Selection

The variable-frequency drive is the most complete mitigation of the starting dip, because it controls both the voltage and the frequency supplied to the motor and can therefore accelerate the motor at full torque while drawing little more than rated current. By raising the frequency and voltage together from a low starting value, the drive maintains the motor near its most efficient operating point throughout the acceleration, essentially eliminating the inrush and the dip it would otherwise cause. The drive also provides speed control during normal running, which for many loads — particularly the fans and pumps whose power demand falls steeply with speed — delivers energy savings that can justify the drive on operating cost alone, independent of its starting benefit.

The selection of a drive for starting mitigation considers the load's torque-speed characteristic, because the drive must be rated to supply the torque the load demands throughout the acceleration and during running; the harmonic distortion the drive injects into the supply, which may require input filtering or a multi-pulse rectifier to satisfy the harmonic limits of IEEE 519; and the cost, which is the highest among the starting methods. For a load that benefits from speed control during running, the drive is frequently the natural choice regardless of the starting consideration, and its elimination of the starting dip is then an additional benefit. For a constant-speed load whose only difficulty is the starting dip, the drive competes against the less costly reduced-voltage methods and reactive support, and the selection turns on the severity of the dip, the starting torque required, and whether the additional capabilities of the drive justify its cost.

4. Conclusion

The most consequential finding is that the choice of mitigation is dictated by the load's torque requirement as much as by the dip itself: reduced-voltage methods that limit inrush also reduce starting torque by the square of the applied voltage, so a high-breakaway load may be unable to accelerate under the very method that fixes the dip. The mitigation that controls the disturbance and the torque the load demands are coupled constraints, not independent choices.

The most common implementation failure is selecting a reduced-voltage starter from the dip calculation alone, without verifying that the reduced starting torque still exceeds the load's breakaway and accelerating torque across the speed range — producing a motor that satisfies the bus-voltage limit but stalls on its load, drawing locked-rotor current indefinitely.

The engineer should next formalize the selection as a decision sequence — direct-on-line, then autotransformer, then soft starter, then variable-frequency drive — with the explicit torque-margin and harmonic criterion that promotes a design from one step to the next, because the choice among mitigation methods is the next problem the starting study sets up and the one most often made by habit rather than by the load's actual torque-speed demand.

Related Work

The analysis in this paper connects to several companion studies in this library. Readers concerned with the upstream and downstream engineering will find Motor Starting Analysis develops a closely related aspect of the same problem, while Variable Frequency Drive Harmonic Mitigation in Manufacturing extends the treatment into an adjacent domain. For the broader methodological context, Industrial Facility Power Distribution provides complementary depth.

The dependence of the starting voltage dip on source strength is shown in Figure 1, which plots the retained bus voltage against motor locked-rotor MVA for grids ranging from a weak 2 MVA source to a stiff 20 MVA source.

Bus voltage remaining during motor starting as a function of motor locked-rotor MVA, for four source-strength scenarios. The shaded band marks the region below the 0.85 pu contactor dropout threshold.

Figure 1. Bus voltage remaining during motor starting as a function of motor locked-rotor MVA, for four source-strength scenarios. The shaded band marks the region below the 0.85 pu contactor dropout threshold.

The figure makes the design margin explicit: on the 2 MVA source the bus voltage approaches the 0.85 pu contactor dropout limit as the motor size grows, whereas the 20 MVA source holds the bus above 0.98 pu across the full range. In practice this is why across-the-line starting of a large motor is acceptable on a stiff bus but requires a reduced-voltage starter, soft starter, or variable-frequency drive when the available short-circuit MVA at the motor terminals is low.

References

[1] IEEE Standard 3002.7-2018, IEEE Recommended Practice for Conducting Motor-Starting Studies and Analysis of Industrial and Commercial Power Systems, IEEE, 2018.

[2] IEEE Standard 399-1997, IEEE Recommended Practice for Industrial and Commercial Power Systems Analysis (Brown Book), IEEE, 1997.

[3] IEEE Standard 519-2022, IEEE Standard for Harmonic Control in Electric Power Systems, IEEE, 2022.

[4] NEMA MG 1-2021, Motors and Generators, National Electrical Manufacturers Association, 2021.

[5] IEEE Standard 141-1993, IEEE Recommended Practice for Electric Power Distribution for Industrial Plants (Red Book), IEEE, 1993.

[6] B. K. Bose, Modern Power Electronics and AC Drives, Prentice Hall, 2002.