Selecting Motor and Drive
From the basics of three-phase motors to making an informed selection: servo or asynchronous, IE efficiency classes, IP degrees of protection, and the cost-effective use of frequency inverters.
5 units · approx. 35 minutes total · Step 1 of 7
Getting Started
Modules 1 translates the requirements of an application into torque, power, and inertia. Module 2 builds on this and answers the next question: Which motor and which auxiliary electronics are suitable for this application? You will learn the basics of three-phase motors, a well-founded comparison between servo and induction motors, the significance of IE efficiency classes, how to interpret IP degrees of protection, and the economic considerations for a frequency inverter.
A decision tree, a matching exercise, flashcards, and a case study on a speed-controlled pump bring the theory to life; a self-test at the end shows whether the material has been mastered.
Learning Objectives – After completing this module, you will be able to:
- Identify the fundamentals and key specifications on the nameplate of a three-phase motor;
- Compare servo and induction motors based on positioning accuracy, dynamics, and cost, and select the appropriate one based on sound reasoning;
- Classify IE efficiency classes and their significance for operating costs;
- Identify degrees of protection and match them to appropriate operating environments;
- Assess the economic and technical benefits of a frequency inverter.
Learning units
Five short learning units (5–10 minutes) with learning objectives, examples, and a knowledge check. Progress is saved locally in this browser.
0 of 5 units
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- Unit 1 of 5
Three-Phase Motor Fundamentals
approx. 7 minutes
Open - Unit 2 of 5
Servo or Induction Motor?
approx. 8 minutes
Open - Unit 3 of 5
IE Efficiency Classes
approx. 7 minutes
Open - Unit 4 of 5
Reading IP Protection Ratings
approx. 6 minutes
Open - Unit 5 of 5
Variable Frequency Drives – When Do They Make Sense?
approx. 7 minutes
Open
Decision Tree: Servo or Asynchronous?
Three quick questions about positioning, dynamics, and speed control—you’ll receive a detailed recommendation with a link to the relevant guide.
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Questions and answers
- Does the axis need to position precisely or move exactly to a fixed position? (This refers to an accuracy better than approximately ±1°—typical for CNC machines, robotics, or packaging machines with fixed positions.)
- Yes, positioning is required – Fixed positions must be approached with precision
- No, not a positioning task – Pure rotational or conveying motion without fixed positions
- Are high dynamics (response times under 5 ms) or frequent start-stop cycles required? (For example, more than 10 positioning operations per minute in handling or packaging processes.)
- Yes, high dynamics are required – Very fast response times or many cycles per minute
- No, not critical – Normal accelerations are sufficient
- Does the speed need to be continuously adjustable, even though positioning is not required? (For example, speed-controlled pumps or fans without a fixed target position.)
- Yes, stepless speed control is required – Speed from 0 to maximum, but without positioning
- No, fixed or non-critical speed – Direct start or simple speed stages are sufficient
Recommendations
The first matching line applies.
- If Does the axis need to position precisely or move exactly to a fixed position?: Yes, positioning is required → Recommendation: Recommendation: servo motor – When positioning accuracy better than approximately ±1° is required, a servo motor with encoder feedback has the advantage: It achieves ±0.01° to ±0.001°, whereas an induction motor without feedback can only achieve ±5–10°. (Read more: Servo Motor vs. induction motor)
- If Are high dynamics (response times under 5 ms) or frequent start-stop cycles required?: Yes, high dynamics are required → Recommendation: Recommendation: servo motor – For response times under 5 ms or frequent start-stop cycles, the servo motor has the edge: a response time of under 5 ms and, for short periods, 3 to 5 times the rated torque, compared to 100–500 ms and 1.5—up to 2 times the rated torque for an induction motor. At the same time, check whether an induction motor with a high-quality vector drive would suffice—according to the learning unit, a servo motor is only technically necessary for positioning accuracies better than ±1° or an inertia ratio exceeding approximately 10. (Read more: Servo Motor vs. induction motor)
- If Does the speed need to be continuously adjustable, even though positioning is not required?: Yes, stepless speed control is required → Recommendation: Recommendation: Induction motor with a frequency inverter (vector control) – For stepless speed control without positioning requirements, a standard induction motor with a high-quality vector drive is sufficient—it offers many of the advantages of a servo system at significantly lower cost and with greater reliability. (Read more: Frequency inverters in use)
- In all other cases → Recommendation: Recommendation: Standard induction motor – There are no special requirements regarding positioning, dynamics, or stepless speed control. For continuous operation with a constant or adjustable load, the induction motor is the right choice: robust, low-maintenance, and with a mean time between failures (MTBF) of over 40,000 hours.
Matching Exercise: IP Codes
Assign the appropriate IP code to each description using the drop-down list, then click “Check.”
Dust-protected, protected against water splashes from any direction—standard industrial motor for use in dry to slightly damp industrial environments.
Dust-protected, protected against water jets from any direction—for outdoor installation and covered outdoor areas.
Completely dustproof, protection against water jets—minimum requirement for food processing and wash-down applications.
Completely dustproof; can be briefly submerged to a depth of 1 m for 30 minutes—typical for deep-freeze environments, mining, and maritime shipping.
Technical basis: degrees of protection Reference (opens in a new tab)
Flashcards: Motor and degree of protection
Seven technical terms related to motors and degrees of protection—click on the card (or press Enter or the spacebar) to see the definition.
Flashcards
Tap to see the definition.
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- IP-Schutzklassen
- Classification system according to IEC 60529 for the protection of electrical equipment against the ingress of foreign objects (first digit) and water (second digit)—e.g., IP54 or IP65. Source: Glossary “IP-Schutzklassen”
- Design
- Refers to the mounting type of a motor according to IEC 60034-7, e.g., B3 (foot mounting), B5 (flange), or B14 (small flange)—which is crucial for the mechanical connection. Source: Glossary “Design”
- IE-Effizienzklasse
- International efficiency class for electric motors, ranging from IE1 to IE5; the higher the class, the lower the losses for a given power output. Source: Glossary “IE-Effizienzklasse”
- Servo motor
- Motor with encoder feedback for high-precision positioning (±0.01° to ±0.001°)—in contrast to an induction motor without feedback (±5–10°). Source: Glossary “Servo motor”
- Synchronous reluctance motor (SyRM)
- A motor without permanent magnets that achieves IE4/IE5 efficiency levels when driven by an inverter, with a lower rotor moment of inertia than an induction motor—an intermediate solution between a standard motor and a servo motor. Source: Glossary “Synchronous reluctance motor (SyRM)”
- Inertia ratio
- J_red(J_Motor, load inertia reduced to the motor shaft): up to 3 is ideal, 3–10 is acceptable, and values above about 10 may lead to resonance and stability problems. A gearbox reduces the load inertia at the motor by a factor of 1/i². Source: Glossary “Inertia ratio”
- Frequency inverter
- An electronic device for continuously controlling the speed and torque of three-phase motors by varying the frequency and voltage. Source: Glossary “Frequency inverter”
All cards in this unit have been shown. The unknown ones are now in the review stack.
Case Study: Pump with a Square Performance Curve—Inverter: Yes or No?
A centrifugal pump has been operating at a fixed speed for years and is adjusted to the required flow rate via a throttle valve—a classic quadratic load profile. The pump runs for about 7,000 hours per year. The maintenance department is inquiring whether it is worth converting to a speed-controlled drive with a frequency inverter.
Key Questions
- What potential savings does the guide cite for a throttling centrifugal pump, and what is the physical basis for this?
- What operating life and payback period does the guide specify for this exact scenario?
- What additional information should the maintenance department have on hand before submitting a request?
View Worked Solution
1. For a throttling centrifugal pump, the decision table indicates a potential savings of 40–60%. This is based on the law of proportionality: With a quadratic load profile, power decreases with the cube of the speed (P ~ n³)—at 80% speed, only about 51% of the rated power is required.
2. For a “centrifugal pump (throttled)” with 6,000–8,000 operating hours per year, the guide recommends using a variable-speed drive with a payback period of 1.5–2.5 years. With approximately 7,000 hours per year, this pump falls within this range.
3. To ensure an accurate quote, please specify the motor power (kW), annual operating hours, the load profile (quadratic), the cable length between the motor and the inverter, the ambient temperature at the installation site, and the required EMC category according to EN 61800-3.
Technical basis exclusively: Frequency inverters: When is it worth using them? (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
Servo motor vs. Induction motor (opens in a new tab)7 min. reading time
IE efficiency classes IE1–IE5 (opens in a new tab)8 min. reading time
Degrees of Protection: Complete Reference (opens in a new tab)7 min. reading time
Frequency Inverters: When Is It Worth Using Them? (opens in a new tab)6 min. reading time
Curriculum v0.1 (Beta) · As of 17.09.2026 · content carefully prepared and reviewed – final sign-off to follow