Inrush current · Voltage dip · Speed ramp · DOL / Star-Delta / Soft Starter / VFD
This simulator models the inrush current, voltage dip, and motor speed ramp-up transient for four starting methods — Direct On-Line (DOL), Star-Delta, Soft Starter, and VFD. Electrical engineers use it when evaluating motor starting impacts on system voltage quality and selecting the appropriate starting method to limit disturbances.
Induction motors draw locked-rotor current (LRC) at starting, typically 6–8× the full-load amperes (FLA). The FLA is determined from NEC Table 430.250 for 460V three-phase motors — a 50 HP motor draws 65 A FLA, so locked-rotor current is approximately 6.5 × 65 = 423 A peak inrush. This large current spike through the system source impedance (transformer and feeder impedance) causes a momentary voltage dip at the motor terminals and all connected buses.
The voltage dip calculation follows: ΔV% ≈ (I_start × Z_system) / V_rated × 100. For a system with 5% source impedance, a motor drawing 6× FLA causes approximately a 23% voltage dip at its terminals during the first few cycles. IEEE 519 guidelines limit voltage dip to ≤5% for frequently started motors and ≤10% for occasional starts to prevent flicker, contactor dropout (contactor hold-in voltage typically 85% of rated), and motor stall.
Starting method selection dramatically changes the transient: DOL produces 6–8× FLA inrush; Star-Delta reduces voltage to 1/√3, limiting current to 1/3 of DOL (≈2.2× FLA in star) but causes a transition transient when switching to delta; Soft Starters use SCR voltage ramping to limit inrush to 2–4× FLA; VFDs ramp voltage and frequency together, limiting inrush to approximately 1.0–1.1× FLA with essentially no voltage dip.
NEC Article 430 governs motor branch circuit conductors, overcurrent protection, controllers, and disconnects. NEC 430.22 requires branch circuit conductors rated at 125% of motor FLA. NEC 430.52 governs overcurrent protective device ratings (up to 250% of FLA for inverse-time breakers to accommodate starting current without nuisance tripping). NEMA MG-1 defines motor starting torque, locked-rotor current codes (letter A through V, indicating LRC/FLA ratio), and service factor. IEEE 519 limits voltage disturbances and harmonics. IEEE 1159 governs power quality monitoring.
The voltage dip during motor starting is the primary coordination concern. The worst case is when the motor is the largest load on a weak source (high source impedance). Loads sensitive to voltage dip — computers, PLCs, contactors, adjustable-speed drives — must be evaluated. Contactor dropout occurs when voltage falls below approximately 85% of rated voltage during a start.
Star-delta starters save cost but introduce a transition transient when switching from star to delta at partial speed — current briefly spikes to 3–4× FLA during this transition, which can be worse than the original DOL start if the motor is loaded. Soft starters eliminate this issue but cannot provide variable speed control. VFDs are the most controllable option and also improve running efficiency through speed control, making them the preferred choice for variable-torque loads (fans, pumps) despite higher initial cost.
Select the starting method, then set the motor size (HP), system voltage, and source impedance percentage. The key metrics panel shows FLA from NEC 430.250, peak inrush current and multiplier, calculated voltage dip, and motor kVA. Click "Animate Start" to see the current transient and speed ramp play out in real time on the chart. The NEC 430 branch circuit panel shows the required conductor ampacity (125% FLA), maximum OCPD rating (250% FLA for inverse-time breaker), overload relay setting, and disconnect sizing.
At the moment of starting, the rotor is stationary and the motor is essentially a short-circuit transformer. The back-EMF (counter-electromotive force) that normally limits current in a running motor has not yet developed. As the motor accelerates, back-EMF builds and current decreases. For a squirrel-cage induction motor, locked-rotor current is typically 6–8× FLA until the motor reaches about 80% of synchronous speed.
NEMA Design B is the most common general-purpose squirrel-cage induction motor design. It has locked-rotor current of 600–650% of FLA (code letter F or G) and starting torque of 150–175% of full-load torque. NEMA Design C has higher starting torque (200–250%) for constant-torque loads. NEMA Design D (high resistance rotor) has very high starting torque but poor running efficiency.
VFDs are preferred for: variable-torque loads (centrifugal fans, pumps) where energy savings during part-load operation justify the cost; systems where voltage dip limits prevent DOL or star-delta starting; processes requiring precise speed control; and systems with frequent starts where motor thermal protection is a concern. VFDs also eliminate across-the-line starting torque shock to mechanical systems.
IEEE 519 and utility service agreements typically limit voltage dip to 3–5% for frequently started motors (more than once per hour) and 5–10% for occasional starts. A 10% voltage dip causes lights to flicker noticeably and can drop out sensitive electronic equipment. The acceptable limit at the point of common coupling with other utility customers is typically 3%.
Per NEC 430.32, the overload relay must be set at or below 115% of motor FLA for motors with a service factor of 1.15 or higher, or 115% of FLA if the motor has a temperature rise of 40°C or less. For all other motors, the setting must be at or below 115% of FLA. In practice, set the overload relay at the highest allowable value that still protects the motor from sustained overload.
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