YR wound rotor motor working principle and selection: how to use the rotor circuit resistance to realize heavy load soft start?

YR-type wound-rotor asynchronous motors (typically including the YR, YRKK, YRKS series, etc.) are a specialized type of AC asynchronous motor. Their stators are identical to those of conventional squirrel-cage motors, but their rotors employWound structureand the three-phase windings are led out via slip rings and brushes.YR MotorThe core value lies in itsExcellent startup performanceBy connecting an external resistor in series with the rotor circuit, the starting torque can be artificially increased while significantly reducing the starting current, thereby enabling the control of high-inertia loads.Heavy-Load Soft StartIt serves as an ideal drive source for equipment such as ball mills, crushers, and hoists.

Core Technology Analysis: Why Can YR Motors Achieve “Soft Start”?

YR-wound rotor motors are irreplaceable in heavy-load starting applications due to their unique rotor circuit design and starting mechanism:

Activation Principle: The Magic of External Resistance

During the startup process, a series connection is introduced into the rotor winding circuit via the slip rings.Three-Phase Starting Resistor (Resistor Box)According to the principle of asynchronous motors, increasing the rotor circuit resistance shifts the motor's maximum torque point toward lower speeds.

• Professional Advantages: This design maximizes the motor's starting torque (up to 2.0 to 2.5 times the rated torque) while limiting the starting current to a low level (typically below 2.5 times the rated current). It effectively prevents significant impact on the power grid, making it particularly suitable for scenarios with insufficient grid capacity or extremely high load starting inertia.

Structural Distinction: Open vs. Closed

• YR Series (Open Type): Earlier models typically feature lower protection ratings (such as IP23) and are suitable for clean indoor environments.
• YRKK/YRKS Series (Enclosed Type): Featuring air-to-air or air-to-water cooling configurations with high protection ratings (e.g., IP44/IP54), these units are designed to withstand harsh environments such as mining and metallurgical operations characterized by dust or humidity.

Application Scenarios and Selection Pitfalls (Professional Selection Guide)

YRWire-wound motorsPrimarily used in applications with extremely high demands on starting performance. However, in actual selection and usage, the following misconceptions and key points must be clarified:

1. Load type determines everything: YR motors are designed forStartup conditionsDesign for fixed speed rather than continuous speed regulation (unless paired with a variable frequency drive). If your load consists of large fans, pumps, or similar equipment with relatively easy starting, prioritize YKK/YKS squirrel-cage motors. Only when drivingBall mills, large crushers, belt conveyors, or hoistsOnly when the time comes must YR motors be used.
2. Matching of the starting resistor: The motor body and starting resistor must be considered asA systemThe rating, resistance value, and cross-sectional area of the starting resistor must strictly match the motor parameters.
   • Avoid Pitfalls Guide: The technical parameters of commonly used liquid resistors (water resistors) or frequency-sensitive resistors on the market must be verified.electric motor factoryCalculate the motor's rated current to avoid excessive startup time due to improper matching, which could burn out the rotor windings.
3. Selection of Slip Ring Materials and Brushes: Slip rings are critical wear parts for YR motors. They should be selected with care.Silver-containing or high-copper alloysSlipper ring, and matched to the manufacturer's specifiedHigh-graphite-content carbon brushIncorrect brush grades can cause severe sparking, accelerating slip ring oxidation and wear.

Maintenance and Troubleshooting (Sleeve and Brush System Maintenance)

Maintenance work for YR wound motors is primarily concentrated on their slip ring and brush systems.

• Surface Inspection of Slip Rings:
  • Normal state: The slip ring surface should remain smooth, exhibiting a uniform bronze-like luster, free from pitting or burn marks.
  • Exception Handling: If black spots or arcing are observed on the slip ring surface, check whether the brush pressure is uniform. Use a dedicated grinding stone to polish the surface and remove the oxide film. Do not use ordinary sandpaper for polishing while the motor is running.
• Brushes andbrush holderMaintenance:
  • Brush Pressure: Pressure should be uniform and comply with manufacturer specifications (typically between 150-255 g/cm²). Excessive pressure accelerates wear; insufficient pressure causes poor contact and increased sparking.
  • Replacing the brushes: When the brushes wear down to one-third of their original height, they must be replaced as a set to ensure consistent length and contact area across all brushes.
• Initiate rheostat maintenance: Regularly inspect whether resistor connections are loose and check for signs of overheating or discoloration. For liquid resistor cabinets, periodically test the electrolyte concentration and temperature.

Frequently Asked Questions (FAQ)

Q: After the YR motor has completed its start-up, is it necessary to keep the resistor box connected? A: Not required. When the motor starts up to near its rated speed, the resistor will be disconnected, and the rotor windings will be directly short-circuited via the short-circuit device. If the starting resistor box remains in the circuit for an extended period, it will cause extremely low motor efficiency and sustained overheating.

Q: Can YR motors be used for speed regulation? A: Yes, but it is less efficient. Speed regulation achieved by series resistors in the rotor circuit results in significant energy loss as heat, making it suitable only for short-term applications or situations where efficiency is not a priority. For high-efficiency speed regulation, dedicated equipment should be employed.Cascade Speed Control System for Wound-Rotor Motorsor variable frequency drive.

Q: Is it normal to see slight sparks at the slip ring? A: Minor, uniform, and spotty sparks (level ≤ 1) may be considered normal. However, if the spark area increases, the color turns blue, or circumferential sliding sparks appear, this indicates an abnormality requiring immediate inspection of the brush neutral zone or slip ring surface condition.

Manufacturer's Closing Remarks

YR wound-rotor motors offer a proven solution for heavy-load starting challenges, but their stable operation critically depends on proper selection and professional slip ring/brush maintenance. When procuring or replacing starting resistors, never compromise matching precision for lower prices.

Need the latest technical specifications for the YR motor series and recommendations for selecting starting resistors? Please provide your load type and inertia data, and our engineers will calculate the optimal starting solution for you.