Selecting a Three-Phase Motor: Fundamentals for Practice

A three-phase motor (asynchronous or synchronous) is selected on four key criteria: required power (P = M × 2πn/60), protection rating (IP per IEC 60034-5), mounting configuration (IEC 60034-7), and energy efficiency class (IE per IEC 60034-30-1). An incorrectly sized motor leads to energy waste, shortened service life, or production downtime. This guide provides a structured decision-making framework based on solid technical principles and proven practice.

Motor Types: Asynchronous vs. Synchronous

Asynchronous Motor (Induction Motor)

The asynchronous motor is the workhorse of industry. The rotating magnetic field of the stator induces currents in the rotor – hence the name "asynchronous": the rotor runs slightly slower than the stator field (slip). It is robust, low-maintenance, and affordable, making it the standard choice for most drive tasks.

Synchronous Motor (Permanent Magnet Synchronous Motor)

Synchronous motors rotate in sync with the stator field – without slip. They use permanent magnets in the rotor and require a VFD/inverter for starting and speed control. In return, they achieve higher efficiency (IE4/IE5), more precise speed control, and a more compact form factor – at higher cost.

For the majority of applications in mechanical engineering and conveying technology, the asynchronous motor offers the best price-performance ratio. Synchronous and servo motors pay off at high operating hours (>7,000 h/yr), large power ratings (>30 kW), or controlled dynamics with a VFD. The full comparison of both concepts — with advantages, disadvantages, MTBF, costs, and concrete selection criteria — can be found in the guide Servo Motor vs. Asynchronous Motor in detail. Whether a frequency inverter is economically worthwhile is covered in the guide of the same name; IEC-frame asynchronous motors in standard mounting configurations are available in the TEA asynchronous motors range.

Power Calculation

Mechanical power is calculated from torque and speed. This is the core formula for any motor selection:

Practical Example: You need a torque of 150 N·m at 1,500 rpm for a conveyor system. The required motor power is:

P = 150 × 2π × 1500 / 60 = 150 × 157.08 = 23,562 W ≈ 23.6 kW

In practice, you must add a safety factor (1.1–1.25) to compensate for wear, contamination, and temperature variations. You would select a motor rated 27.5 kW (standard size 30 kW).

Common grid frequencies are 50 Hz (Europe) with synchronous speeds of 750, 1,000, 1,500, or 3,000 rpm. At 60 Hz (Americas/Asia): 900, 1,200, 1,800, 3,600 rpm respectively. How to systematically analyze the load profile of your application is explained in the guide Motor Selection by Load Profile.

Understanding Characteristic Curves: Torque-Speed Behavior

The torque-speed characteristic curve is the heart of every motor. It shows what torque the motor delivers at each speed. For asynchronous motors:

• Starting Torque (M_A): The torque at startup (n=0). Typically 1.3–1.8× rated torque. Too low a starting torque causes starting difficulties; too high causes a strong inrush current to the grid.
• Breakdown Torque (M_K): The maximum torque at partial load, usually at 70–80% of rated speed. If exceeded, the motor stalls (breakdown point).
• Rated Torque (M_N): The continuous torque at rated speed and rated power.
• Overload Capability: Asynchronous motors can sustain a short-term (10–20 s) overload of 50–100% of rated torque.

The characteristic curve of an asynchronous motor typically shows a steep rise to the breakdown point, then a drop at higher loads. This is normal and demonstrates the stability of the motor. For quadratic load profiles (such as pumps or fans), starting torque is less critical; for constant loads (conveyors), adequate breakdown torque is essential.

Starting Methods: from Direct-on-Line to Frequency Inverter

Direct mains starting draws a multiple of rated current. The starting method determines starting current, available starting torque, and grid loading:

Star-delta reduces starting current to roughly one third, but equally reduces starting torque — suitable only for starting against low load. Where full torque at low current is required, or where speed control is needed anyway, the frequency inverter is the best choice. The values are indicative; the motor datasheet is binding.

Protection Ratings per IEC 60034-5

The protection rating defines how well the motor is protected against foreign objects, dust, and moisture. The IP designation follows the code "IPxy", where the first digit indicates solid particle protection and the second indicates moisture protection. For the complete reference of all combinations from IP00 to IP69K plus a comparison with NEMA enclosure types, see our IP protection ratings reference.

The industry standard is IP55. This is the right balance between cost-efficiency and protection for manufacturing environments, wet areas, and near-outdoor installations. IP65 is required for wash-down applications or areas with direct water jets. IP54 is sufficient for dry indoor environments (storage, clean offices).

Mounting Configurations per IEC 60034-7

The mounting configuration defines how the motor is mechanically attached. The IEC 60034-7 standard distinguishes several main configurations:

B3 – Foot Mounting (Horizontal)

The motor is mounted horizontally on feet and bolted to the machine bed through holes in the base. B3 is the most common configuration and is ideal for coupling to gearboxes, pumps, and fans. The motor has no mounting flange on the drive end.

B5 – Flange Mounting with Large Flange

The motor is flange-mounted directly to the machine via a large flange with a centering spigot on the drive end. It has no feet. B5 is particularly suitable for vertical or angled mounting and wherever an axially centered connection is required.

B14 – Flange Mounting with Small Flange

Similar to B5 but with a smaller flange and through-holes instead of tapped holes. B14 motors have no feet and are flange-mounted directly to gearbox or machine housings. Commonly used on planetary gearboxes, small pumps, and compact drives.

Additional configurations (B6, B7, B8, B9) exist for special applications. When selecting a motor, you must clarify: How will the motor be mounted? Are there designated mounting holes or feet? Consult machine drawings and interface specifications.

Energy Efficiency and IE Classes per IEC 60034-30-1

The IE classes (efficiency classes per IEC 60034-30-1) range from IE1 (being phased out) to IE5 and indicate how efficiently a motor converts electrical into mechanical power. Under EU Regulation 2019/1781, IE3 currently applies as the minimum class for motors from 0.75 to 1,000 kW; in the 75–200 kW power range, IE4 has even been mandatory since July 2023. For motor selection, this means in practice: choose at least IE3 — for long operating hours or high electricity prices, the surcharge for IE4 usually pays back within a few years of operation.

The complete IE1–IE5 overview with efficiency threshold values, EU mandatory classes, permitted exceptions, and a detailed energy savings calculation example can be found in our guide on the IE efficiency classes for motors. The full TCO calculation method — including energy costs, maintenance, and downtime risk — is described in the guide TCO Calculation in Drive Trains.

TEA Recommendation: Motor Selection Checklist

Follow this systematic checklist for reliable motor selection:

For questions about motor selection or the sizing of complex drive systems, contact our Application Engineers. We help you find the optimal solution for your specific application.

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