Overview: Voltage Dip vs. Voltage Drop During Motor Starting
A voltage dip (sag) during motor starting is a transient reduction in bus voltage caused by high inrush current flowing through source impedance. This is distinct from a steady-state voltage drop caused by conductor resistance. Benshaw reduced-voltage soft starters (RVSS) — including the RSI, RediStart MICRO, RediStart MX³, and iStart series — control SCR firing angle to ramp voltage from a programmable initial level up to full voltage, directly limiting peak inrush current and the resulting voltage dip.
Prerequisites
- Benshaw soft starter model and firmware version confirmed (see nameplate or parameter
P0.01on MX³/iStart) - Motor nameplate data: FLA, LRA, service factor, NEMA design letter
- Source impedance or available fault current at MCC bus (from utility or transformer data)
- Acceptable voltage dip limit — typically ≤10% of nominal for sensitive loads, ≤15–20% general industrial per IEEE 519 / NEMA MG1-12.45
- Parameter programming access (keypad, Benshaw PC software BenStart Suite, or Modbus RTU via RS-485)
Key Parameters Controlling Voltage Dip
| Parameter | MX³ / iStart Label | Typical Range | Effect on Voltage Dip |
|---|---|---|---|
| Initial Voltage (Pedestal) |
P1.01 – Starting Voltage |
15–70% Vnominal | Lower initial voltage → lower inrush at t=0; reduces initial dip spike |
| Voltage Ramp Time |
P1.02 – Accel Ramp Time |
1–30 s (up to 120 s some models) | Longer ramp → lower peak current → smaller dip; increases motor heating |
| Current Limit |
P1.03 – Current Limit |
100–500% FLA | Hard ceiling on SCR current; most direct dip control; overrides ramp |
| Kick Start (Boost) |
P1.05 – Kick Voltage / Time |
50–100% V, 0–2 s | Brief voltage boost to break static friction; increases initial dip — disable if dip is the concern |
| Soft Stop Ramp |
P1.06 – Decel Ramp Time |
0–30 s | No effect on starting dip; controls stop profile |
The Current Limit parameter (P1.03) is the most effective single control for minimizing voltage dip because it applies a closed-loop current feedback regardless of ramp position. Setting current limit to 250–300% FLA is a common starting point for loads with moderate inertia (fans, pumps). High-inertia loads (flywheels, large conveyors) may require 350–400% FLA to complete acceleration.
Step-by-Step Configuration Procedure
- Record baseline data. Start motor with factory defaults. Measure peak starting current with a clamp meter or power analyzer. Note bus voltage dip with a power quality meter (e.g., Fluke 435) or historian tag. This is your benchmark.
-
Access parameter mode. On MX³/iStart keypad: press
PROG→ enter access code (default0or225depending on firmware) → navigate to Start Parameters group. -
Set Current Limit first. Set
P1.03to 300% FLA. This is the primary lever. Formula for expected bus dip:ΔV% ≈ (I_limit × Z_source%) / 100whereZ_source%is transformer impedance + cable impedance expressed as percent of rated kVA base. -
Adjust Starting Voltage. Set
P1.01to 30–40% Vnom for centrifugal loads (pump/fan), 50–60% for high-breakaway torque loads (conveyors, compressors). Too low → motor fails to develop breakaway torque → extended acceleration → more heating. -
Set Ramp Time. Begin at
P1.02= 10 s. Increase in 2 s increments if dip target still not met. Do not exceed the motor's locked rotor time rating (from motor datasheet or NEMA MG1 Table 12-6). -
Disable Kick Start if enabled. Set
P1.05kick time =0unless breakaway torque is confirmed insufficient. -
Test start and monitor. Perform 3 consecutive cold starts spaced ≥5 min apart. Record: peak current, ramp duration, bus voltage dip, motor winding temperature (if thermistor connected to
T1/T2input). All three starts must achieve full speed and dip must be within target. -
Verify thermal margin. Motor thermal model trips in Benshaw starters via
E07(Motor Overload) orE09(Motor Thermistor Over-Temperature). If either fault appears during test, the ramp time is too long or current limit too high — reduce ramp time or accept a slightly larger dip.
Benshaw Fault Codes Related to Starting Configuration
| Fault Code | Description | Likely Cause in This Context | Corrective Action |
|---|---|---|---|
E01 |
Shorted SCR / Phase Loss | SCR firing fault during ramp | Check SCR gate signals; verify input voltage symmetry |
E05 |
Stall / Accel Timeout | Motor did not reach full speed within P1.04 (Accel Timeout) |
Increase current limit or reduce ramp time; check load |
E07 |
Motor Overload (I²t) | Ramp time too long → excessive heat in motor windings | Shorten ramp time or raise current limit to accelerate faster |
E09 |
Motor Thermistor Over-Temp | NTC/PTC thermistor tripped during extended start | Allow cool-down; reduce ramp time; check motor ventilation |
E13 |
Undervoltage | Bus dip exceeds UV trip threshold (~80% Vnom) | Reduce current limit; increase ramp time; check source capacity |
When Soft Starter Parameter Adjustment Is Not Enough
If bus dip remains unacceptable after optimizing all starting parameters, the source impedance is too high for the motor size. Consider these solutions:
-
Larger transformer or dedicated feeder: Reduces
Z_source%directly. - Power factor correction capacitors switched in during start (use motor-rated capacitors; switch out after acceleration to avoid self-excitation).
- Variable Frequency Drive (VFD): Controls current even more precisely than an RVSS; eliminates voltage dip almost entirely but adds harmonic distortion and cost.
- Wye-delta transition starter: Legacy method; produces a transition transient spike — generally inferior to modern RVSS for dip control.
- Contact Benshaw Technical Support (+1 412-968-0100) with transformer kVA, %Z, cable impedance, and motor FLA/LRA data for an application-specific starting current calculation.
01 hex) for remote parameter write during commissioning. Register 40101 = Current Limit %, 40103 = Accel Ramp Time (×0.1 s). Verify register map against your firmware revision in the Benshaw MX³ User Manual (P/N 890038).FAQ
What is the best Benshaw soft starter parameter to reduce voltage dip?
Set the Current Limit parameter (P1.03) first — it provides closed-loop control of SCR current regardless of ramp position. Start at 300% FLA and reduce in 25% increments until dip is within target, verifying the motor still accelerates to full speed without triggering fault E05 (Accel Timeout).
How long should I set the soft starter ramp time to minimize voltage sag?
Longer ramp times reduce peak current and voltage dip, but must not exceed the motor's locked-rotor time rating. For NEMA Design B motors, a 10–15 s ramp is a practical starting point; never exceed the motor manufacturer's locked-rotor withstand time (typically 8–20 s depending on frame size and NEMA design letter).
Why does my Benshaw soft starter throw an E07 fault when I extend the ramp time?
Fault E07 is a motor overload (I²t) trip. A longer ramp forces the motor to carry near-locked-rotor current for more seconds, accumulating thermal energy faster than the I²t model allows. Shorten the ramp time, raise the current limit so acceleration completes faster, or verify the motor's service factor and locked-rotor time rating.
Can I set the initial starting voltage too low on a Benshaw soft starter?
Yes. If starting voltage (P1.01) is set below the motor's minimum breakaway torque requirement, the motor will not accelerate and will stall, triggering fault E05 (Accel Timeout) and causing prolonged high current draw. For high-breakaway loads (conveyors, positive-displacement pumps), set initial voltage to at least 50–60% Vnominal.
When should I use a VFD instead of a soft starter to control voltage dip?
Use a VFD when the available source impedance is high (transformer %Z > 5% and motor LRA > 6× FLA), when dip must be kept below 5%, or when the process also requires variable-speed operation. A VFD limits starting current to approximately 150% FLA versus 250–400% FLA for a soft starter at minimum current limit.