Course Overview
The previous units provided the individual building blocks—torque, power, and speed; duty type and overload factor for intermittent duty; and inertia. This final unit combines them into a sequence, as suggested by the checklist in the guide:
- Determine the load profile and load type: Which duty type (S1–S10)? Constant, linear, or quadratic load?
- Calculating torque and speed: based on force and lever arm, or based on the application.
- Calculate power: from torque and speed (M = 9550 · P / n, see Unit 1).
- Apply (multiply by) the safety factor: 1.1–1.25 to the calculated torque or power.
- Check the overload factor for intermittent duty: For intermittent duty (S3–S5), a smaller size may be possible (see Unit 2).
- Evaluate ambient temperature: At > 40 °C, reduce motor power or select a larger motor.
- Specify energy efficiency: at least IE3, preferably IE4.
- Specify the mounting configuration and degree of protection: B3/B5/B14, IP54/IP55/IP65, depending on the installation situation.
- Check the motor-gearbox heat balance: Losses in the gearbox must be able to be dissipated through cooling.
- Compare data sheets and contact the manufacturer; if you have any uncertainties or complex requirements, consult an application engineer.
Two Common Mistakes
Mistake 1 – Incorrect assumption about the load type: Whether a load increases constantly, linearly, or quadratically with speed is a key factor in determining the required motor power when the speed changes. The guide expressly warns: Incorrect assumptions about the load type lead to costly errors in motor selection—the load type must be verified in the machine drawing or directly with the machine manufacturer, not estimated.
Error 2 – Incorrect application of the overload factor for intermittent duty with long cycles: IEC 60034-1 assumes a duty cycle of ten minutes for S3, unless otherwise specified. For significantly longer cycles (e.g., 30 minutes on, 30 minutes off), the winding heats up considerably more during the on-phase than during the 10-minute cycle, despite the same duty cycle (ED %) – the table factors from Unit 2 do not automatically apply in such cases. Therefore, when submitting an inquiry, the cycle time and load duration should always be specified in addition to the duty cycle.
Mnemonic
According to the learning unit, a complete motor inquiry should always include the following: duty type and duty cycle, load type, torque/speed/power, ambient temperature and safety factor, design, degree of protection, and energy efficiency class, as well as the efficiency of any gearbox used. Modules 2 of this Academy provides an in-depth look at how to select motors and inverters in detail.
Knowledge check
Answer all three questions, then click "Check". From 2 of 3 correct answers, the unit counts as completed. You can retry at any time.
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