Motor selection by load profile is a core skill in engineering and maintenance. An undersized motor leads to overload and failures; an oversized motor leads to energy waste and high costs. IEC standard 60034-1 defines duty types and rules for thermal design. This article provides a practical guide to correctly selecting a motor based on its load profile.
Key Takeaway:
The load profile (S1–S10) determines the required motor power and size. A thermal analysis with duty factor, ambient temperature, and duty cycle (DC%) is essential. Use the Affinity Law (P ~ n³) for pumps and fans to quantify energy savings.
Understanding Load Profiles: S1–S10 per IEC 60034-1
IEC 60034-1 defines ten duty types (S1–S10) that describe different load scenarios. These directly affect the required motor power and size:
| S-No. | Duty Type | Description |
|---|---|---|
| S1 | Continuous Duty | Motor runs continuously under constant load. Typical applications: pumps, fans in district heating systems. |
| S2 | Short-Time Duty | Motor runs for a fixed operating time (seconds to minutes), then cools down to ambient temperature. The fixed duration defines this duty type. |
| S3 | Intermittent Duty | Repeated short-time operation with pauses (does not cool to ambient temperature). DC% 15–60%. E.g., hoists, crane systems. |
| S4 | Intermittent Duty with Starting | Like S3, but starting time is significant. Typical for presses, air compressors. |
| S5 | Intermittent Duty with Elec. Braking | Like S3, but with electromagnetic braking and frequent starts. High thermal stress. |
| S6 | Continuous Operation with Intermittent Load | Motor runs continuously, but load varies periodically. E.g., conveyor belt with variable load. |
| S7 | Continuous Duty with Elec. Braking | Like S1, but with frequent braking and starts. High thermal stress from braking energy. |
| S8 | Periodic Duty with Direction Reversal | Motor periodically reverses direction (forward/backward). High stress on drive and brakes. |
| S9 | Duty with Non-Periodic Load and Speed Variations | Irregular load and speed (e.g., rolling mill, wind turbine). Very high thermal demands. |
| S10 | Periodic Duty with Braking | Complex load profiles with multiple phases: acceleration, load, braking, pause. Crane systems, travel drives. |
Important: The higher the duty type (S3 vs. S1), the larger the motor can be sized. An S3 motor with DC=40% can have 50–100% higher power than an S1 motor of the same frame size, since it has cooling breaks.
Load Types: Constant, Linear, Quadratic
The way load changes with speed is a decisive factor in determining required motor power:
Constant Load (M = const.)
Torque remains constant regardless of speed. Power changes linearly: P = M × n.
Examples: Conveyor belt (friction), coil winders, escalators. Implication: With speed reduction (using a VFD), power drops linearly. A motor rated at 30 kW at 1500 rpm requires only 15 kW at 750 rpm. However, the motor must be sized for the highest required torque.
Linear Load (M ~ n)
Torque grows linearly with speed. Power grows quadratically: P ~ n².
Examples: Viscous friction (highly viscous fluids), bearings with damping. Implication: Rare in practice. Energy savings from speed reduction are moderate.
Quadratic Load (M ~ n²)
Torque grows with the square of speed. Power grows with the third power: P ~ n³. This is the Affinity Law.
M ~ n² ⟹ P ~ n³
At 80% speed: P = 0.8³ = 0.512 = 51% of rated power
Examples: Centrifugal pumps (without throttling), fans, blowers. Implication: With a variable frequency drive, massive energy savings (40–60%) are achievable. This is the basis for energy efficiency measures in pump and fan systems.
Practical Tip: Always verify the load profile in the machine drawing or with the machine manufacturer. Incorrect assumptions about load type lead to costly mistakes in motor selection.
Calculation Example: Chain Conveyor
Task: A horizontal chain conveyor transports boxes at a speed of 2 m/s. Total mass is 500 kg (boxes + chain). Friction coefficient is 0.05. What motor is required?
Step 1: Calculate resistance force
F = m × g × f = 500 kg × 9.81 m/s² × 0.05 = 245 N
Step 2: Determine speed and drum radius
Assumption: drum diameter 200 mm (r = 0.1 m). Peripheral speed is 2 m/s. Speed n = v / (2πr) = 2 / (2π × 0.1) = 3.18 rev/s = 191 rpm. With gearbox (ratio 1:7.9): n_motor = 191 × 7.9 = 1509 rpm ≈ 1500 rpm.
Step 3: Calculate torque
M_drum = F × r = 245 N × 0.1 m = 24.5 N·m
With gearbox and losses: M_motor = M_drum / ratio / η_gearbox ≈ 24.5 / 7.9 / 0.90 ≈ 3.4 N·m
Step 4: Calculate motor power
P = M × ω = 3.4 N·m × 2π × 1500 / 60 = 3.4 × 157.08 = 534 W ≈ 0.75 kW
Step 5: Add safety factor
With safety factor 1.15: P_required = 0.75 × 1.15 = 0.86 kW. Motor selection: 1.1 kW three-phase motor, 1500 rpm, IE3 class, 4-pole.
This is a typical scenario for continuous duty (S1) with constant load. The motor would deliver 100% rated power in duty type S1.
Thermal Design: Duty Factor & Ambient Temperature
Every motor has a maximum permissible temperature (e.g., 130 °C for Class B per IEC 60034-1). This temperature is reached when the motor runs at its rated power at 40 °C ambient temperature. Deviations must be accounted for:
Duty Factor for Intermittent Operation
A motor in intermittent duty (S3 with DC=40%) can be operated at higher loads since it has cooling breaks. The duty factor accounts for this:
| Duty Cycle (DC%) | 15% | 25% | 40% | 60% |
|---|---|---|---|---|
| Duty Factor (typical) | 1.50–1.60 | 1.25–1.35 | 1.10–1.20 | 1.00–1.10 |
Meaning: For an application with DC=40% and a required torque of 10 N·m, you can select a motor with rated torque 10 / 1.15 ≈ 8.7 N·m (approx. 5% savings). However, caution: duty factors specified by the manufacturer can vary depending on motor type.
Ambient Temperature Correction
Motors are typically designed for 40 °C ambient temperature. Deviations require power adjustments:
- At 50 °C: Reduce motor power by ~10% (or choose a larger motor)
- At 60 °C: Reduce motor power by ~20%
- At 20 °C: Increasing motor power by ~10% is possible (better cooling)
Rule of thumb: For every 10 °C deviation from 40 °C ambient temperature, the permissible motor power changes by approximately 10%. This is a rough approximation; precise values can be found in the motor datasheet.
Motor-Gearbox Combination: Design & Heat Losses
When motor and gearbox are combined, heat losses from the gearbox must be taken into account:
Efficiency of typical gearboxes:
- Spur gear: 96–98% per stage
- Worm gear: 50–90% (depending on ratio)
- Planetary gear: 94–97% per stage
- Bevel gear: 95–97% per stage
The losses result in heat generation. A poorly matched motor and an undersized gearbox can lead to overheating and failures.
Example: A motor with 10 kW drives through a spur gear with 98% efficiency. The losses are 10 kW × (1 - 0.98) = 0.2 kW = 200 W. This must be compensated by gearbox cooling (ventilation, heat dissipation). If the gearbox is too small or insufficiently cooled, the temperature quickly exceeds the permissible range (typically <80 °C oil temperature).
Motor Selection Checklist by Load Profile
- Determine load profile: S1–S10? Ask the machine manufacturer or refer to the operating manual.
- Analyze load type: Constant, linear, or quadratic? This determines power at variable speeds.
- Calculate torque and speed: M [N·m] = F [N] × r [m], then P [W] = M × 2πn / 60.
- Add safety factor: Multiply by 1.1–1.25 depending on application safety requirements.
- Check duty factor: For intermittent duty (S3–S5), account for the duty factor.
- Evaluate ambient temperature: At >40 °C ambient temperature, reduce motor power or choose a larger motor.
- Energy efficiency: Choose at minimum IE3, preferably IE4.
- Define mounting style and protection rating: B3, B5, B14? IP54, IP55, IP65?
- Motor-gearbox heat losses: Verify that combined heat losses are absorbed by cooling.
- Contact the manufacturer: For uncertainties or complex requirements, consult an Application Engineer.
TEA Recommendation: Checklist for Motor Selection by Load Profile
Use this structured decision guide:
Required for every motor selection:
- Duty type (S1–S10), duty cycle (DC%)
- Load type (constant, linear, quadratic)
- Torque [N·m], speed [rpm], power [W]
- Ambient temperature, safety factor
- Mounting style, protection rating, energy efficiency class
- Gearbox efficiency, total system heat balance
Optional for optimization:
- Speed control (variable frequency drive) for energy savings?
- Servo motor for high precision or dynamic performance?
- Noise requirements, EMC requirements?
Our Application Engineers are happy to assist you with complete system design. Send us a request with the operating parameters – we will calculate the optimal motor selection with a cost-benefit analysis.
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