Load Profile, Torque, and Inertia
5 units · approx. 33 minutes total · Step 1 of 7
Getting Started
This module focuses on the first step in any drive design: deriving the correct parameters from an application for subsequent motor selection. You will learn how torque, power, and speed are related; how load profiles and duty types according to IEC 60034-1 influence the permissible motor size; and how the inertia of a load can be reduced to the motor shaft.
Calculation exercises, flashcards, and a case study on a conveyor belt starting up bring the theory to life; a self-test at the end shows whether the material has been mastered. Module 2 builds directly on this and covers the selection of motors and frequency inverters.
Learning Objectives – After completing this module, you will be able to:
- Convert torque, power, and speed using the formula M = 9550 · P / n;
- classify a load profile based on duty types S1–S10 and the duty cycle (ED %);
- Calculate the moment of inertia for standard geometries and reduce it to the motor shaft;
- Evaluate the inertia ratio J_red/J_Motor using the traffic light rating system;
- Understand the entire process from the initial request to motor sizing and avoid common mistakes.
Learning units
Five short learning units (5–10 minutes) with learning objectives, examples, and knowledge checks—standalone learning content for this module, maintained independently of the website’s guides. Progress is saved locally in this browser.
0 of 5 units
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- Unit 1 of 5
Torque, Power, and Speed
approx. 6 minutes
Open - Unit 2 of 5
Load Profile and Duty Cycles
approx. 7 minutes
Open - Unit 3 of 5
Calculating Mass Moment of Inertia
approx. 7 minutes
Open - Unit 4 of 5
Reducing Inertia to the Motor Shaft
approx. 7 minutes
Open - Unit 5 of 5
From Load Profile to Motor Size
approx. 6 minutes
Open
Calculation Problem 1: Torque
A motor delivers a power output of P = 5.5 kW at a speed of n = 1,450 min⁻¹.
Calculate the torque M.
Computer Exercise 2: Inertia and Reduction
A full-bore cylinder has a bore diameter of m = 20 kg and a stroke of r = 0.1 m. A downstream gearbox has a gear ratio of i = 10.
Calculate the moment of inertia J and the inertia reduced to the motor shaft (J_red).
Flashcards: Torque and Inertia
Eight technical terms from the glossary—click on the card (or press Enter or the spacebar) to see the definition.
Flashcards
Tap to see the definition.
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- Torque
- Rotational torque that causes or counteracts a rotational motion—expressed in newton-meters (Nm), defined as the product of force and lever arm (M = F · r). Source: Glossary “Torque”
- Rated torque
- The maximum continuous torque that a machine or drive component can transmit under rated conditions—the most important selection parameter for motors, gearboxes, and couplings. Source: Glossary “Rated torque”
- Rated speed
- The speed of a motor or gearbox specified for rated operation—expressed in min⁻¹—and the reference point for all other performance data of the drive. Source: Glossary “Rated speed”
- Moment of inertia
- Describes a body’s resistance to a change in its rotational motion (unit: kg·m²) — analogous to mass in linear motion; depends on the distribution of mass relative to the axis. Source: Glossary “Moment of inertia”
- Overload factor for intermittent duty (f_Ü)
- A factor indicating the extent to which a motor can be overloaded during intermittent operation (S3) compared to continuous operation—depending on the duty cycle ED: the shorter the ED, the higher the permissible overload factor. Not to be confused with the application factor K_A from Module 3 (often referred to as the “service factor” in the catalog). Source: Glossary “Overload factor for intermittent duty (f_Ü)”
- Efficiency
- Ratio of output power to input power (η = P_ab / P_zu) – the measure of a drive system’s energy efficiency, expressed as a value between 0 and 1 or 0–100%. Source: Glossary “Efficiency”
- Gear ratio
- Ratio of input speed to output speed (i = n₁/n₂)—the key parameter of a gearbox for speed reduction and torque amplification. Source: Glossary “Gear ratio”
- Reduced moment of inertia
- J_red = J_Last / i² – a gearbox reduces the load inertia, calculated back to the motor shaft, by the square of the gear ratio i; for example, when i = 5, it is reduced to one twenty-fifth. Source: Glossary “Reduced moment of inertia”
All cards in this unit have been shown. The unknown ones are now in the review stack.
Case Study: Starting a Conveyor Belt
A horizontal chain conveyor (often referred to simply as a “conveyor belt” in operation) transports cardboard boxes at a speed of 2 m/s. The total mass of the boxes and the chain is 500 kg, and the coefficient of friction is 0.05. The conveyor operates continuously under a constant load. The designer has already calculated: resistive force F = 245 N; motor torque (after the gearbox and accounting for efficiency) approximately 3.4 Nm at about 1,500 min⁻¹; required power approximately 0.53 kW. With a safety factor of 1.15, the required power is approximately 0.61 kW—a catalog motor with 0.75 kW (the next IEC rating level above 0.61 kW), 1,500 min⁻¹, and IE3 efficiency class is selected.
Key Questions
- What type of load does a friction-laden conveyor correspond to, and what does that mean for motor power when the speed changes?
- Which duty type according to IEC 60034-1 is appropriate for this conveyor belt, which operates continuously with a constant load?
- The purchasing department suggests ordering a significantly larger motor than calculated, just to be on the safe side. According to the learning unit, what are the arguments against this?
View Worked Solution
1. A friction-laden conveyor is a constant load (M = const., independent of speed). The power changes linearly with speed (P = M · n): A motor rated at 30 kW at 1,500 min⁻¹ requires only 15 kW at 750 min⁻¹. Nevertheless, the motor must be sized for the highest required torque.
2. S1 (continuous duty) with constant load—the motor runs continuously under the same load, without cooling breaks as in intermittent duty (S3).
3. According to the learning unit, oversizing not only increases the purchase price but also permanently reduces the power factor (cos φ) and results in additional energy costs. Instead of simply increasing the size across the board, the learning unit recommends correctly applying the existing safety factor (1.1–1.25) and selecting a suitable IEC standard motor from a standard power rating.
Technical basis exclusively: Motor selection based on load profile (opens in a new tab)
Self-test
10 questions about this module—immediate feedback with explanations and source links. The module is considered complete if you score 7 out of 10 points or higher.
Self-test
10 questions for this module. You see right after each answer whether it was correct.
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Before each answer, you state how confident you feel. This helps distinguish knowledge gaps from uncertainty. It has no effect on the score.
Question 1 of 10
Advanced Topics (optional)
The sources for this module, in case you’d like to delve deeper. Reading them is not required for the learning units and the self-test.
7 min. reading time
Torque Calculator (opens in a new tab)Online Calculator
Moment of Inertia Calculator (opens in a new tab)Online Calculator
Curriculum v0.1 (Beta) · As of 17.09.2026 · content carefully prepared and reviewed – final sign-off to follow