I. Project Background

This project involves retrofitting an existing YQT630-6G wound-rotor inner-fed speed control three-phase asynchronous motor (900kW/10kV) with a Wolong WLKK630-6 900kW squirrel-cage high-voltage variable frequency drive (VFD) motor. The site is located at an altitude of 2,000m. The original inner-fed speed control system has a complex rotor circuit structure, limited speed range, and relatively low operating efficiency. The retrofitted solution adopts a "high-voltage VFD + high-altitude variable frequency motor" configuration, enabling highly efficient and energy-saving operation across a wide speed range while comprehensively improving equipment reliability and intelligent control capability.

• Original Motor: Inner-fed speed control three-phase asynchronous motor, Model YQT630-6G, 900kW, 10kV, commissioned in 2016.

• Associated Pump: Wilo centrifugal pump, Model SCPG20/24DV MED1-900/6.

• Years in Service: Both motor and pump have been in operation since 2016.
  
  

  

II. Comparison of Original and New Motor Parameters

III. Technical Advantages of the High-Voltage VFD Motor

3.1 Sound Speed Control Principle and Higher Efficiency

The inner-fed speed control system regulates rotor power by extracting a portion of rotor power and feeding it back to a feedback winding to reduce speed — essentially a slip power recovery type drive. At low speeds, system efficiency decreases linearly with speed. Variable frequency drive control directly changes the stator input frequency and controls the ideal no-load speed, maintaining high operating efficiency across the full speed range without incurring additional slip losses during speed reduction.

3.2 Wide Speed Range and Fast Dynamic Response

The WLKK VFD motor paired with a variable frequency drive operates under constant torque from 5–50 Hz and constant power from 50–60 Hz, achieving a speed ratio exceeding 10:1 — far beyond the original inner-fed system. It can precisely match operating conditions for fan and pump loads with quadratic torque characteristics.

3.3 Reliable Motor Structure and Easy Maintenance

• The squirrel-cage rotor construction completely eliminates slip rings, carbon brushes, brush holders, and rotor terminal boxes, removing the root cause of daily maintenance and replacement of these wear-prone components.

• Stator and rotor insulation is reliable, with shaft current insulation measures applied for long service life.

• The independent forced ventilation cooler has its own power supply, ensuring adequate heat dissipation even during prolonged low-speed operation.

3.4 High Energy Efficiency and Strong Energy-Saving Foundation

The full WLKK series supports optional GB IE2 or IE1 energy efficiency ratings. The new VFD motor's intrinsic efficiency is markedly higher than the aging inner-fed motor, achieving energy savings at the source.

3.5 Strong Compatibility and High Level of Intelligent Control

The high-voltage VFD supports seamless communication with DCS/PLC systems, enabling remote start/stop, stepless speed control, and online monitoring of operating data for refined energy management. A "one-drive-one auto-bypass" scheme automatically switches to power-frequency operation in the event of a VFD fault, ensuring production continuity.

3.6 Fundamental Improvement in Starting Method (Compared to Original Inner-Fed Motor)

This is one of the core upgrades in this retrofit with regard to system safety and grid impact.

Original Inner-Fed Motor Starting Method

The YQT2-630-6G is a wound-rotor asynchronous motor with rotor windings brought out through slip rings. Starting is typically achieved by inserting resistance or a frequency-sensitive resistor into the rotor circuit, using the inner-fed chopper circuit to control the equivalent rotor resistance for current-limiting starting.

• Starting Current: Typically 2–3 times rated current. Although lower than direct-on-line starting (5–7 times), it still causes notable impact on the grid and mechanical components.

• Starting Torque: A relatively high starting torque can be achieved by adjusting the rotor resistance, but torque pulsation occurs during starting.

• Starting Operation: Relies on contactors to short out and remove resistors or chopper adjustment; smoothness is limited and true stepless soft-starting from zero speed is not achievable.

• Equipment Limitations: The starting process is heavily dependent on reliable contact between slip rings and carbon brushes, which are prone to sparking and poor contact in high-altitude, humid, or dusty environments, and carbon brush wear requires regular maintenance.

High-Voltage VFD Motor Starting Method

The VFD motor is driven by the high-voltage variable frequency drive and uses variable voltage variable frequency (VVVF) control for soft starting.

• Starting Current: Starting from 0 Hz at low frequency, the starting current is strictly controlled within 1.0–1.5 times rated current, with minimal voltage drop on the grid and no impact.

• Starting Torque: Can deliver 150% or more of rated torque at low speed, suitable for heavy-load starting, with no torque shock throughout the process.

• Starting Process: Speed rises smoothly from zero with freely adjustable acceleration, significantly reducing mechanical stress and extending the service life of drive shafts, couplings, and driven equipment.

• Frequent Start/Stop Capability: VFD motors generate less heat and can handle frequent start/stop duty cycles; inner-fed motors are ill-suited for this due to rotor heating and carbon brush wear.

• No Slip Rings or Carbon Brushes: No sliding electrical contact during normal operation or starting — intrinsically safe and maintenance-free.

Comparison Summary

IV. Energy Savings Analysis

4.1 Speed Control Energy-Saving Mechanism

Fan and pump loads with quadratic torque characteristics follow the affinity laws: flow rate is proportional to speed, and shaft power is proportional to the cube of speed (P ∝ n³). When the required flow drops to 80%, speed reduces to 80%, and theoretical shaft power is only 51.2%. Variable frequency speed control can directly reduce to the corresponding speed, offering enormous energy-saving potential.

4.2 Expected Overall Energy Savings

For fan and pump loads operating on average at 70–85% of rated speed or below:

• Theoretical energy savings: 25–35%

• After accounting for VFD efficiency (96–98%) and motor efficiency improvement (approximately 1–3%), estimated actual overall energy savings: 10–20%

• Based on measured data from similar retrofit projects, operating current typically decreases by 15–20% after high-voltage VFD retrofits, demonstrating significant energy-saving results.

4.3 Energy-Saving Advantage Over Original Inner-Fed System

When the inner-fed system reduces speed, losses occur in the slip power recovery circuit, and the speed control device itself also consumes power. The retrofit completely eliminates these losses, and the higher efficiency of the VFD motor body means energy savings at low-load conditions are far superior to the inner-fed system.

Note: Final energy savings must be verified through on-site measurements. It is recommended that at least 7 days of continuous operation monitoring be conducted both before and after the retrofit.

V. High-Altitude (2,000m) Considerations and Countermeasures

5.1 Motor Insulation Requirements

At 2,000m altitude, air dielectric strength decreases by approximately 15%. The motor must:

• Have insulation class no lower than Class F, with temperature rise evaluated against Class B criteria, providing sufficient margin.

• Apply anti-corona treatment to the end turns of the 10kV high-voltage windings (corona-resistant varnish coating and corona-resistant tape wrapping).

• Increase electrical clearances and creepage distances to comply with GB/T 21707-2025 insulation specifications for variable frequency motors, with special attention to corona resistance and partial discharge resistance.

5.2 Motor Heat Dissipation and Derating

At high altitude, air density decreases and heat dissipation deteriorates. Motor temperature rise increases by approximately 5–10°C, and the power correction factor is approximately 0.92. The motor for this project must be custom-designed as a high-altitude type with independent forced ventilation cooling (IC416/IC616), with the cooling fan independently powered to ensure adequate heat dissipation at all speeds. Temperature rise testing must be completed at the factory.

5.3 VFD Derating

Above 1,000m altitude, the VFD must be derated by 1% for every 100m of additional altitude. At 2,000m, approximately 10% derating is required.

VFD selection capacity: ≥ 900 ÷ (1 − 10%) = 1,000 kW. A rated power of ≥ 1,200 kW is recommended to ensure adequate margin.

5.4 Other Environmental Adaptations

• The VFD cabinet must have enhanced ventilation and heat dissipation with increased airflow.

• Outdoor installation must account for protection against extreme temperature differentials, windblown sand, and UV aging.

• Anti-condensation heaters must be installed for cold-weather conditions, and low-temperature bearing grease must be used.

• The motor overall must be custom designed for high-altitude and cold-climate service conditions.

VI. Key Points of the System Retrofit Technical Scheme

6.1 Scope of Retrofit

6.2 Starting and Bypass Scheme (Optional Based on Actual Requirements)

A "one-drive-one automatic bypass" scheme is adopted. Under normal conditions, starting is performed by the VFD via soft start from 0 to 50 Hz. In the event of a serious VFD fault, the automatic bypass cabinet closes the power-frequency circuit breaker and switches the motor to direct power-frequency grid operation. (This causes significant inrush current and is intended only as an emergency short-term operating mode; the grid capacity at the site must be confirmed as adequate.) Manual bypass to power-frequency starting is also available during routine maintenance.

6.3 Installation and Commissioning

• Strict shaft alignment to avoid additional vibration.

• Cooling air circuit installed independently to meet high-altitude heat dissipation requirements.

• Complete power-frequency/VFD switching tests, low-speed and high-speed temperature rise evaluation, and protection interlock testing.

VII. Applicable Standards

VIII. Economic Benefit Estimate

IX. Summary

This retrofit upgrades the original inner-fed speed control motor system to a high-voltage variable frequency motor system. It not only achieves significant energy savings of 10–15%, but also fundamentally eliminates the maintenance challenges associated with slip rings and carbon brushes, reduces starting current from 2–3 times rated to within 1.5 times rated, and realizes truly smooth soft starting and efficient wide-range speed control. Through high-altitude motor customization, VFD capacity derating selection, output filtering, and shaft current protection measures, safe and reliable operation at 2,000m altitude is assured. The investment payback period is approximately 3–4 years, with significant economic and social benefits, making this the ideal pathway to achieving energy conservation, carbon reduction, and intelligent operation and maintenance.