As the heart of industrial equipment, motor selection for different operating conditions impacts both efficiency and cost. This article focuses on three typical operating conditions: frequent starts and stops, heavy-load starting, and constant torque/constant power. It breaks down the selection logic to help you avoid common pitfalls and achieve efficient matching of your power system.
01 Frequent Start-Stop Conditions: Heat dissipation and mechanical shock are core challenges.
Typical scenarios: Automated sorting lines, stamping machines, elevator door operators, and injection molding machine mold opening and closing.
Key Pain Points:
Fatal Heat Accumulation: Instantaneous currents during start-stop operations can reach 6-8 times the rated current, and insufficient heat dissipation can directly burn the insulation layer.
Mechanical Shock: Repeated starts and stops accelerate bearing wear and cause vibration and noise.
Braking Energy Management: Braking feedback energy can damage the drive.
Selection Principle: “Three Highs”:
High heat dissipation capacity: Forced air-cooled motors, insulation grade ≥ H (180°C).
High mechanical strength: Cast aluminum housing + reinforced bearings, bearing model suffix “C3” for clearance.
High Start-Stop Frequency: Variable frequency motors/servo motors, with an allowable start-stop frequency of ≥ 300 times/hour.
02 Heavy-Load Starting Conditions: Overcoming “Stationary Friction” is Key to Success
Typical Applications: Ball Mills, Crusher, Compressor, Large Fan
Physical Essentials: Starting torque must overcome the static inertia of the equipment + load resistance, typically requiring 150%-200% of the rated torque.
Key Selection Formula: Starting Torque Requirement = Load Static Friction Torque × Safety Factor (≥1.5)
Motor Solution Comparison:
High-Slip Motor: Small and Medium-Sized Equipment (≤75kW), High Starting Current, Low Energy Efficiency
Wound Rotor Motor: Large, Heavy-Duty Equipment, Requires Slip Ring and Carbon Brush Maintenance
Variable Frequency Soft Start: For Precise Control, Requires Additional Investment in a Frequency Converter
Practical Tips:
Torque Verification: Check the motor’s T-n curve to ensure that starting torque exceeds load requirements.
Use Reduced Voltage Starting with Caution: Y-Δ starting torque is reduced to 33%, making it suitable only for light loads.
03 Performance Game of Constant Torque vs. Constant Power Conditions
(1) Constant Torque Conditions – Powerful at Low Speeds
Typical Scenarios: Conveyor Belts, Hoists, Extruders
Characteristics: Load torque is independent of speed (e.g., lifting heavy objects)
Key Points for Motor Selection:
Ordinary asynchronous motors are preferred: their natural characteristics are close to constant torque.
Variable frequency motors are required for speed regulation: ordinary motors have poor heat dissipation at low speeds
Power Calculation Formula: P = T·n/9550 (T is torque Nm, n is speed r/min)
(2) Constant Power Conditions – No “Power Loss” at High Speeds
Typical Scenarios: Machine Tool Spindle, Centrifuge, Electric Vehicle Drive
Characteristics: Constant output power, the higher the speed, the smaller the torque (e.g., machine tool cutting)
Wide Speed Permanent Magnet Synchronous Motor: Constant power range can reach 1:4
Magnetic Weakness Control Capability: Maintaining power through magnetic weakening in high-speed areas
04 Overlooked System Matching Issues
Overlooked system matching issues can lead to failures even with the correct motor selection:
Inertia mismatch: When the load inertia is greater than 5 times the motor rotor inertia, the servo system is prone to oscillation.
Voltage sag: A voltage drop greater than 15% during heavy-load startup can cause contactor tripping.
Harmonic interference: The lack of an inductor at the inverter output can lead to motor insulation breakdown.
Selection is both technical and economic.
The motor purchase cost accounts for only 10% of the total lifecycle cost. The right motor selection solution can improve capacity utilization by reducing downtime, optimize energy efficiency, and reduce operating costs. Extending lifespan reduces spare parts expenses, creating returns for your business that far exceed the value of the hardware.
