motor start synchronous voltage drop

Motor Start Synchronous Voltage Drop: An Overview

The starting of large electric motors presents a significant challenge to electrical power systems, primarily due to the phenomenon of synchronous voltage drop. This article details the causes, impacts, and mitigation strategies for voltage dips that occur when synchronous motors are started directly online. We will analyze the technical principles, compare common mitigation techniques, and present a real-world case study.

1. The Mechanism of Voltage Drop During Motor Start

When a synchronous motor is started across-the-line (direct-on-line), it initially behaves like an induction motor with its field winding shorted. The inrush current drawn during this period is typically 6 to 8 times the motor's full-load current. This high-current demand creates a substantial voltage drop across the system impedance (transformers, cables, etc.), governed by Ohm's Law (ΔV = I_start Z_system*). This voltage dip is synchronous because it occurs immediately and simultaneously with the switching event.

The severity of the dip depends on:

• Motor Starting kVA: The product of starting current and rated voltage.
• System Short-Circuit Capacity (SCC): A stronger grid (higher SCC at the point of common coupling - PCC) has lower impedance and thus experiences less voltage drop.
• Source Transformer Impedance: Often the largest contributor to system impedance in industrial settings.

A severe voltage drop can cause:

• Nuisance tripping of sensitive equipment.
• Contactor dropout in other motors.
• Flickering of lighting systems.
• Instability or stalling of the motor itself if terminal voltage falls too low.

2. Mitigation Techniques: A Comparative Analysis

Several engineering solutions exist to limit the inrush current and mitigate voltage drop. The choice depends on cost, system requirements, and motor design.

Technique	Principle	Pros	Cons	Typical Voltage Dip Reduction
Direct-On-Line (DOL)	Full voltage applied directly.	Simple, low cost, high starting torque.	Highest inrush current & voltage dip.	Baseline (0% reduction)
Autotransformer Starter	Applies reduced voltage via taps (e.g., 50%, 65%, 80%) during start.	Significant inrush reduction; adjustable tap settings.	Bulky; torque drops with square of voltage; complex switching.	~40-60% reduction vs. DOL
Soft Starter (SSR)	Uses thyristors to ramp up voltage gradually.	Smooth acceleration, adjustable start profile, compact.	Generates harmonics; provides no energy savings at run state.	~50-70% reduction vs. DOL
Variable Frequency Drive (VFD)	Controls both voltage and frequency from near zero upwards.	Lowest inrush (<1.2x FLC), full control of torque/speed, energy savings.	Highest cost; requires more space; may require output filters.	>90% reduction vs. DOL

3真实案例: Cement Plant Raw Mill Drive Upgrade

A cement plant in Southeast Asia operated a 2.5 MW, 6 kV synchronous motor for its raw mill using an autotransformer starter。 The existing system caused a consistent 15% voltage dip at the 11 kV PCC， disrupting other process lines whenever the mill started。

Solution: The plant replaced the autotransformer starter with a modern medium-voltage VFD。 The VFD allowed the synchronous motor to be started at a controlled low frequency， limiting the starting current to 110% of full-load current。

Results:

• Voltage Dip: Reduced from 15% to less than 2% at the PCC。
• Grid Compliance: Met utility strict flicker standards (IEEE 519)。
• Operational Benefits: Enabled smooth， controlled starts， reducing mechanical stress on the mill gearbox。
• Energy Efficiency: The VFD provided optimized speed control during operation， yielding approximately 8% energy savings。

This案例 demonstrates that while VFDs represent a higher capital investment， they provide a comprehensive solution for severe starting scenarios with significant operational benefits。

4 Frequently Asked Questions (FAQ)

Q1: How is acceptable voltage dip level determined?
It is defined by standards and facility requirements。 For example， IEEE Std 141 recommends that for "frequent starts，" dips should generally be limited to 3-4% at the PCC， and up to 10-12% for "infrequent starts。" Sensitive equipment like PLCs may require dips below 5-7%。

Q2: Can synchronous motors be started like induction motors?
Yes， but only using specific methods。 Most modern synchronous motors are started as induction motors using an amortisseur winding（damper winding） embedded in the rotor pole faces。 This winding provides the initial torque during acceleration before DC field excitation is applied for pull-in synchronization。

Q3: When is a soft starter preferable to a VFD for large motors?
A soft starter may be sufficient if:

• The primary goal is only to reduce starting current/voltage dip。
• Constant speed operation after start is acceptable。
• The load has high inertia but does not require speed control during operation。
• Budget constraints are significant。 It serves as a cost-effective middle ground between DOL and VFD。

Q4: What role does utility grid strength play?
It is arguably the most critical external factor。 A "weak" grid（low short-circuit capacity） will experience much larger电压 dips for the same motor start compared to a "stiff" grid（high SCC）。 A preliminary utility fault level study at the PCC is essential before selecting large motor starting methods。

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References & Basis
The technical explanations are based on established electrical engineering principles found in texts such as IEEE Std 3004。8-2016 （Recommended Practice for Motor Starting Studies） and 《Electrical Machines， Drives， and Power Systems》 by Theodore Wildi。
The case study is synthesized from multiple documented industry application notes from drive manufacturers（e.g., ABB， Siemens） applied to cement plant auxiliary drives， with specific performance figures generalized for illustration。