Choosing a motor that is too small can cause it to stall or overheat, while a motor that is too large increases costs and reduces efficiency. Many engineers think this is just an experience issue, but the core reason is simple - the key parameters are not calculated correctly.
Motor selection is not about "guessing power"; it is a standardized calculation process. Engineers at Plutools have developed a universal method through years of industrial automation practice, applicable to conveyors, hoists, fans, pumps, screw conveyors, and other equipment.
1. Motor Selection Essentially Revolves Around 4 Key Parameters
All calculations ultimately come back to four quantities: load torque T, speed n, power P, and safety factor K.
Think of it as a main line: first determine how much "force" (torque) the equipment needs, then determine the operating speed. The combination of the two determines power, and finally multiplying by the safety factor gives the final motor specification. Plutools has found in practice that clarifying these four parameters can avoid 90% of common selection mistakes.
2. Core Formulas (Directly Applicable in Engineering)
The most commonly used formula for power is:
P = T × n ÷ (9550 × η)
This formula is straightforward: the motor output depends on "torque × speed," while accounting for transmission losses η. Typical efficiency references are: gear ≈ 0.9, belt ≈ 0.85, chain ≈ 0.9.
If the torque is unknown and you need to calculate it from power:
T = 9550 × P ÷ n
In practice, the calculated value cannot be used directly. You must apply a safety factor:
P_selected = P_calculated × K
The safety factor depends on whether the load has shocks or frequent starts/stops. The heavier the load or the more frequent the impact, the higher the factor should be.
3. Standard Selection Process (Engineering Workflow)
The process is important. First, identify the equipment type - whether it's a conveyor, lift, or mixer - because different equipment has different load characteristics. Then determine the operating speed, which is a commonly overlooked but critical step.
Next, calculate the load torque, derive the required power, and finally apply the safety factor to determine the selected motor power. After this, verify voltage rating, motor poles (which affect speed), mounting method, and protection level. Plutools strictly follows this workflow in industrial mobile robots and conveyor systems, ensuring stable and reliable operation.
4. Case Study: Conveyor Motor Selection (Step by Step)
Known conditions: conveying weight 500 kg, speed 0.5 m/s, drum diameter 0.2 m, friction coefficient 0.03, transmission efficiency 0.9, safety factor 1.2.
Step 1: Calculate Traction Force
F = G × g × μ = 500 × 9.8 × 0.03 = 147 N
Step 2: Calculate Drum Torque
T = F × D ÷ 2 = 147 × 0.1 = 14.7 N·m
Step 3: Calculate Drum Speed
n = 60 × v ÷ (π × D) ≈ 47.8 r/min
Step 4: Calculate Motor Power
P = T × n ÷ (9550 × η) = 14.7 × 47.8 ÷ (9550 × 0.9) ≈ 0.081 kW
Step 5: Apply Safety Factor
P_selected = 0.081 × 1.2 ≈ 0.097 kW
Conclusion: Selecting a 0.12 kW or 0.18 kW motor will meet the requirements. Plutools recommends slightly rounding up the calculated power in practice to ensure long-term stable operation.
5. Differences in Motor Selection for Various Equipment
Different equipment has completely different load characteristics. Using the same method for all can lead to correct calculations but wrong motor selection.
Fans and pumps: Power is proportional to flow rate × head, and changes cubically with speed, so small speed variations significantly affect required power.
Screw conveyors: High torque load with large starting resistance, recommended safety factor 1.5–2.0.
Lifts/hoists: Both static and dynamic torque must be considered; safety factor ≥1.5.
CNC machines/servo systems: Both acceleration torque and continuous torque must be verified.
Plutools engineers suggest evaluating load characteristics first, then combining with safety factor and transmission efficiency to calculate power, which minimizes waste and avoids selection errors.
6. Common Pitfalls (More Important Than Formulas)
Don't rely solely on power. If torque is insufficient, the motor will stall at low speed.
For low-speed, high-load scenarios, prioritize a "gear reducer + motor" combination instead of just increasing motor power.
For frequent forward/reverse or impact loads, increase the safety factor to avoid long-term overload operation.
Don't overlook environmental conditions. In humid or dusty environments, protection level should be at least IP54 or higher.
Plutools provides practical recommendations for protection levels, reducer options, and other solutions to ensure long-term stable operation in different working conditions.
7. Key Takeaways
Motor selection is not about "choosing higher power," but:
First calculate torque → then determine speed → derive power → finally apply the safety factor
Following this main line ensures correct motor selection for almost all scenarios, whether conveyors, hoists, fans, CNC machines, or servo systems. Experience from Plutools shows that this method works reliably in industrial automation and AGV systems.
