Phase Academy · Motor Starting Simulator

Stopped

Configure parameters, then press Start Motor.

A peak

Peak Motor CurrentCALC

A peak

Peak Line Current (source)CALC

Rated FLC (NEC 430.250)

Peak PCC Voltage Dip

% FLT

Initial Start Torque

Accel Time (simulated)

Final Speed

pu²·s

I²t IndexCALC

% V

Min Motor Terminal V

Current vs Time

Speed vs Time

Motor / Load Torque vs Speed

Event Log

Mechanical Stress: --

Thermal Stress: --

VFD Output: Select VFD method to see notes.

Model Assumptions Info

FLC source: NEC Table 430.250 lookup (no nameplate entered)
Motor torque-slip curve: Piecewise 4-point approximation (start → pull-up → breakdown → rated). Typical values per NEMA design, not HP-specific NEMA MG 1 minimums or manufacturer test data.
Motor current-slip curve: Piecewise 3-segment approximation. LRC multiplier is a NEMA-design typical, not nameplate code letter.
Inertia (H): Base H × load JFactor × user multiplier × speed factor
Source impedance: Transformer Thevenin model (%Z on xfmr kVA base)
Voltage dip: Single time-step algebraic dip at each instant. No electromagnetic transient dynamics.
Impedance model: Source and cable impedances are combined as scalar magnitudes (|Z| = |Zsrc| + |Zcable|), not complex phasor addition. This slightly overestimates voltage drop vs. full R+jX phasor analysis at typical starting power factors.
Cable impedance: Lumped R+jX from NEC Chapter 9 Table 9 (AC resistance & reactance, uncoated Cu, 75°C, PVC conduit, 60 Hz, ohms-to-neutral per 1000 ft). These are 600 V-class values; at 4160 V they are indicative only.
Acceleration: dω/dt = T_net / (2H) with fixed dt. No rotor bar heating, no speed-dependent windage.
VFD model: Simplified V/Hz open-loop. No vector control, no detailed switching model.
Fault simulation: Conceptual only — simplified current multipliers and fixed trip times, not from relay coordination curves.

Side-by-Side Starting Method Comparison

Method A

Peak Motor I

Peak Source I

V Dip

Start Torque

Accel Time

Result

Method B

Peak Motor I

Peak Source I

V Dip

Start Torque

Accel Time

Result

Comparison Summary

Select two methods and press Compare for a multi-criteria comparison (current, dip, torque, success, accel time, I²t).

VFD Output Waveform Conceptual

PWM Output (Conceptual)

Freq: — Hz Volt: —% THD: N/A

A VFD converts AC→DC (rectifier), then DC→AC (inverter) via PWM. The motor sees a synthesized fundamental whose voltage and frequency are commanded by the V/Hz control. The waveform above is conceptual; switching frequency and dead-time effects are not modeled in detail.

Soft Starter SCR Firing Conceptual

SCR Phase-Angle Control (Conceptual)

Soft starters use back-to-back SCRs (thyristors) to chop the supply voltage during start. Motor torque is approximately proportional to V², so a low initial voltage may not produce enough torque to overcome breakaway. Many production units include bypass contactors that close once the motor is at speed, eliminating SCR conduction losses.

V/Hz Curve Conceptual

Below base speed: constant V/Hz keeps stator flux roughly constant, giving constant available torque. Above base speed: voltage is at limit, frequency rises further, flux weakens, available torque drops as 1/f (constant-power region).

Harmonic content depends on rectifier topology and any filter present. Compliance with IEEE 519 must be evaluated at the point of common coupling using the actual short-circuit ratio, not assumed from drive nameplate alone.

I²t Comparison Across Methods Calc

Cumulative I²t (in (I/I_FLC)²·s units) is computed by integrating the per-unit current squared over the simulated start. Lower curves indicate less thermal stress. Faults are applied to the active method only.

Practice Scenarios

Centrifugal Pump — 50 HP

HVAC chilled water pump. Variable torque load.

Easy

Reciprocating Compressor — 100 HP

High breakaway, constant torque. Evaluate soft starter limits.

Medium

Belt Conveyor — 25 HP

Constant torque with breakaway friction.

Easy

ID Fan / Blower — 200 HP

High inertia, variable torque. Repeated-start thermal risk.

Medium

Rock Crusher — 300 HP

Very high inertia. Weak bus. Keep dip below 15%.

Hard

Weak Generator Bus

50 HP on 500 kVA generator. Limit inrush to avoid collapse.

Hard

Design Challenges — configure settings, then click "Check Challenge"

Motor Starting Knowledge Quiz

Engineering Self-Test

Verifies that the calculation engine produces NEC-consistent FLC values and that starting-method formulas behave correctly. Tests are pass/fail with measurement tolerance.

Test

Measured

Status

What This Simulator Calculates

Items tagged CALC are computed from inputs using simplified but defensible engineering formulas — NEC 430.250 FLC lookup (with 460/480 and 575/600 voltage scaling), transformer or generator Thevenin impedance, single-step voltage dip, V²-proportional reduced-voltage torque, 4-point piecewise torque-slip curve (start → pull-up → breakdown → rated), dynamic torque-balance acceleration (dω/dt = T_net / 2H), and trapezoidal I²t integration.

Items tagged Conceptual are illustrative visualizations (PWM/SCR waveforms, V/Hz curve, harmonic discussion, fault simulation) and are not numerically tied to a switching-device or protection relay model.

Motor Starting Fundamentals

An induction motor at rest has slip = 1 and presents a low impedance. The resulting locked-rotor current (LRC) is typically 5–8× full-load current (FLC) for NEMA Design B motors. As the rotor accelerates, slip decreases and current drops toward FLC along a curve set by the rotor and stator parameters.

NEMA B: typical LRC ~6.5× FLC, starting torque ~150% FLT, pull-up torque ~100% FLT, breakdown torque ~200% FLT.
NEMA C: higher starting torque (~220% FLT), pull-up torque ~140% FLT, for loaded starts.
NEMA D: very high slip (5–13%) and high starting torque, typical for punch presses and hoists.
NEMA A: similar torque to B but higher LRC (~7.5× FLC); less common in modern installations.

Note: Torque and LRC values used here are typical approximations, not HP-specific NEMA MG 1 minimums. Actual motor characteristics vary by manufacturer and frame size. The synchronous speed selector (2/4/6/8 pole) affects inertia scaling — lower-speed motors typically have higher rotor WR².

Starting Methods — Trade-offs

Across-the-Line (DOL): simplest, lowest cost. Use when supply is stiff and the load can tolerate the full torque transient.
Reduced Voltage (autotransformer): tap typically 50/65/80%. Motor current scales with tap; source current scales with tap². Torque scales with tap².
Soft Starter: SCR phase control ramps voltage from a starting value to full. Torque varies with V². Many production units include bypass contactors for steady-state operation.
VFD: commands both voltage and frequency, allowing a controlled current limit and a smooth acceleration. Typically limits inrush to ~100–200% FLC but introduces harmonics that must be evaluated.

Voltage Dip During Starting

The Thevenin voltage at the motor terminals during start equals source open-circuit voltage minus the IR/IX drop across the source impedance and feeder. This simulator uses:

For transformer source: Z_pu = %Z_xfmr/100 + (S_xfmr/S_utility) when a utility MVA is supplied.
For generator: Z_pu = Xd″/100 on generator base.
Cable: ohmic R+jX based on conductor size and length (60 Hz, typical PVC conduit).

A 15% dip is a common facility rule-of-thumb, not a universal NEC or IEEE code limit. Contactor drop-out, lighting flicker, and process equipment ride-through are the practical drivers.

VFD Principles

V/Hz control: below base speed, keeping V/f roughly constant maintains stator flux and available torque.
Torque boost: at very low frequencies stator IR drop dominates; adding extra voltage compensates and restores low-speed torque. Effective only at low frequencies.
Field weakening: above base speed, voltage is at its limit; further frequency rise reduces flux (constant-HP region, decreasing torque).
Harmonics: a 6-pulse VFD typically presents 5th, 7th, 11th, 13th input current harmonics. THD depends strongly on source impedance. Passive filters reduce distortion; 18-pulse or active front-end drives reduce it further but do not guarantee IEEE 519 compliance without a PCC study.

Common Mistakes

Soft starter on a high-breakaway load with low initial voltage: torque ∝ V² means 50% voltage gives 25% torque — the motor may not break free.
VFD ramp faster than the load can accept: the drive's current limit extends the actual ramp time, or the drive trips on overcurrent depending on configuration.
Long output cable to a VFD motor: can produce reflected-wave voltage stress at the motor terminals. Output reactor or dV/dt filter is often required for long runs.
Starting a large motor on a small generator: evaluate motor starting kVA versus generator capacity and Xd″. Voltage may collapse and the generator may trip.

Educational Disclaimer: This simulator is intended for engineering education. Calculations use simplified models (single-step voltage dip, lumped Thevenin source, piecewise torque-slip curve, no electromagnetic transients, no detailed thermal model). Fault simulation is conceptual — trip times are fixed, not from relay coordination curves. It does not replace utility coordination studies, ETAP/SKM/EasyPower modeling, motor manufacturer test curves, IEEE 519 PCC harmonic studies, or NEC, NEMA, IEEE, and OSHA compliance reviews. Do not use these results for actual engineering decisions.