Permanent Magnet Motor Sizing - How to Define Torque, Speed, Power and Duty Cycle

Permanent Magnet Motor Sizing - How to Define Torque, Speed, Power and Duty Cycle

Choosing the right permanent magnet motor starts with understanding what your equipment actually needs. A motor with the correct power rating may still struggle to start, overheat during continuous operation, or fail to deliver enough torque at certain speeds.

To size a permanent magnet motor correctly, you need to define four main requirements. These are continuous and peak torque, operating speed range, mechanical power, and duty cycle. You then compare these requirements with the motor's actual performance and operating limits.

For equipment manufacturers, engineers, and technical buyers, this approach helps prevent incorrect motor selection and makes discussions with suppliers more productive.

This guide explains what information to collect, how to estimate key specifications, and what to check before selecting a permanent magnet synchronous motor, or PMSM.

1. What Information Do You Need Before Sizing a Permanent Magnet Motor?

Before choosing a motor, identify the load it will drive, how fast the equipment needs to operate, and how operating conditions change over time. These details determine the motor's actual performance requirements.

1.1 Identify the Load Type and Operating Conditions

Not all equipment places the same demands on a motor.

A conveyor often requires relatively steady torque during normal operation, although starting with a full load may require additional torque. A centrifugal fan or pump generally requires more torque as speed increases. Equipment that repeatedly accelerates, stops, or changes load requires a motor capable of handling those variations.

The first question is whether the load remains stable or changes significantly during operation.

If the equipment runs at a nearly constant load, continuous torque and operating speed are the main starting points. If the load changes frequently, you must also identify the highest load, how long it lasts, and how often it occurs.

This distinction helps determine whether a motor's continuous rating is sufficient or whether additional peak torque capability is required.

1.2 Define the Load at the Motor Shaft

The torque required by the equipment is not always the torque required directly from the motor.

For example, a gearbox reduces speed while increasing the torque available at its output. Therefore, the motor may need to produce less torque than the driven equipment, but it must operate at a higher speed.

Consider an application requiring 100 Nm at the gearbox output. If the gearbox has a 5 to 1 reduction ratio and 90% efficiency, the motor needs approximately 22.2 Nm during steady operation.

However, this estimate does not include additional torque needed for acceleration.

When preparing specifications, confirm whether the torque and speed values refer to the motor shaft or the driven equipment. Also record any gearbox ratio and transmission efficiency.

For direct-drive systems, the motor must provide the required load torque without gearbox multiplication.

 Record the Required Operating Profile

A motor's operating profile describes what happens during a normal working cycle.

Required Information	What to Record
Load type	Constant, changing, or repeated load
Normal torque	Torque required during regular operation
Peak torque	Highest load and how long it lasts
Speed range	Minimum, normal, and maximum RPM
Starting conditions	Starting load and acceleration time
Working cycle	Running time, rest time, and starts per hour
Transmission	Direct drive or gearbox arrangement
Environment	Temperature, cooling, and installation conditions

For existing equipment, motor nameplates, operating records, and measured load data can provide useful information.

For new equipment, use the mechanical design requirements and clearly identify any estimated values.

The more accurately these conditions are defined, the easier it becomes to select a suitable motor without unnecessary oversizing.

2. How to Determine the Required Motor Torque

Torque describes the turning force a motor produces to move or drive equipment. It is especially important when the equipment starts under load, operates at low speed, or experiences sudden load increases.

Motor sizing should distinguish between the torque needed during normal operation and the additional torque required temporarily.

2.1 Calculate Continuous Load Torque

Continuous torque is the turning force a motor can sustain under specified operating conditions without exceeding its permitted temperature limits.

For practical selection, start by determining how much torque the equipment requires during normal operation.

If an existing motor delivers approximately 25 Nm while operating under its normal load, this provides a useful starting reference for a replacement motor. However, you should still confirm whether the existing motor is correctly sized and whether its operating conditions match the new installation.

For new equipment, continuous torque can be estimated from mechanical load calculations or from known shaft power and speed.

The important decision is whether the candidate motor can deliver the required torque continuously at the intended speed.

Do not assume that a motor's advertised maximum torque represents its continuous capability.

2.2 Account for Starting and Acceleration Torque

Starting and acceleration can require more torque than normal operation.

A motor must overcome the existing mechanical load while also accelerating rotating components. Equipment with heavy rotating parts or short acceleration times may require substantial additional torque.

For example, a machine that takes 20 seconds to reach operating speed may place a different starting demand on the motor than the same machine accelerating in 3 seconds.

This difference depends on the combined inertia of the motor and driven equipment. Inertia describes how strongly rotating parts resist changes in speed.

When specifying a motor, provide the expected starting load, desired acceleration time, and information about large rotating components.

If these details are uncertain, request an acceleration torque assessment rather than assuming that normal running torque is sufficient.

2.3 Distinguish Continuous Torque from Peak Torque

Continuous torque and peak torque serve different purposes.

Requirement	Continuous Torque	Peak Torque
Purpose	Sustained operation	Starting or temporary overload
Duration	Continuous under rated conditions	Limited period
Main concern	Long-term temperature rise	Maximum torque and permitted overload time
Selection check	Meets sustained load	Meets temporary load at the required speed

Suppose an application normally requires 30 Nm but occasionally needs 50 Nm for a few seconds.

The motor must support the normal load continuously and provide the temporary 50 Nm without exceeding its permitted operating limits.

A motor with a 50 Nm peak rating is not automatically suitable. You also need to check how long it can maintain that output and whether the drive can supply the necessary current.

The practical rule is to match continuous torque to sustained demand and peak torque to the highest temporary demand. Both must be verified at the required operating speeds.

3. How to Specify Motor Speed and Operating Range

Motor speed is normally expressed in revolutions per minute, or RPM. It indicates how many complete rotations the shaft makes in one minute.

For accurate sizing, engineers should specify the entire required speed range rather than only the normal operating speed.

3.1 Define Rated Speed, Minimum Speed and Maximum Speed

Three speed values are particularly useful during motor selection.

Rated speed is the speed associated with the manufacturer's specified rated operating conditions. Normal operating speed is the speed the equipment uses most frequently.

Minimum and maximum speeds describe the full range the equipment must achieve.

For example, a machine may operate normally at 1500 RPM but require occasional operation between 500 and 2500 RPM.

A motor designed primarily for 1500 RPM does not automatically meet the full requirement.

The selection should confirm that the motor can operate within the required speed range while producing sufficient torque at each important operating point.

3.2 Check Torque Availability Across the Speed Range

A motor's available torque may change as its speed increases.

Some permanent magnet motor and drive combinations can maintain approximately constant torque across a defined speed range. At higher speeds, voltage and control limitations may reduce the available torque.

This means a motor may be capable of reaching the required maximum RPM but unable to deliver the required load torque at that speed.

The easiest way to check this is by reviewing the manufacturer's torque-speed curve.

This chart shows how much continuous and temporary torque the motor can provide at different speeds.

Compare your equipment's normal and maximum-load operating points with the chart. If a required point falls outside the permitted range, the motor or drive configuration needs reconsideration.

3.3 Consider Low-Speed Operation and Cooling

Low-speed operation deserves additional attention when the equipment requires high torque for extended periods.

Some motors rely on shaft-mounted fans for cooling. When motor speed decreases, the fan may move less air, reducing heat removal.

As a result, a motor that operates continuously at a certain torque near its rated speed may not maintain the same output at very low speed.

Motors using independent fan cooling or liquid cooling may have different operating capabilities.

If your equipment frequently operates at low speed under heavy load, ask the supplier to confirm the available continuous torque under those exact conditions.

Do not choose a cooling method solely from motor power. The operating speed, load duration, ambient conditions, and motor design must also be considered.

4. How to Calculate Motor Power from Torque and Speed

Motor power represents how much mechanical work the motor delivers over time. It depends on both torque and rotational speed.

Calculating power helps establish the approximate motor rating, but it does not replace torque and duty cycle checks.

4.1 Use a Simple Torque, Speed and Power Calculation

When torque and speed are known, mechanical shaft power can be estimated using one basic relationship.

Power in kW = Torque in Nm × Speed in RPM ÷ 9550

For example, if a motor needs to deliver 20 Nm at 1500 RPM, the calculation is

20 × 1500 ÷ 9550 = approximately 3.14 kW.

This tells you the mechanical power required at that operating point.

However, a motor rated at 3.14 kW is not necessarily the correct selection. You still need to account for starting loads, continuous operating capability, and the manufacturer's performance limits.

Use the result to identify an appropriate range of candidate motors, not to finalize the selection.

4.2 Distinguish Shaft Output Power from Electrical Input Power

Motor catalogs generally identify rated mechanical output power, while the electrical system must supply enough input power to account for motor losses.

For example, if a motor delivers 3.14 kW of shaft power at an assumed efficiency of 94%, its electrical input would be approximately 3.34 kW.

The efficiency value in this example is illustrative, not a rating for a particular motor.

Actual efficiency depends on motor design, speed, load, and operating conditions. The drive also introduces additional electrical losses.

When evaluating electrical supply requirements, use the motor and drive manufacturer's specifications rather than assuming that shaft output power equals input power.

4.3 Why Equal Motor Power Does Not Guarantee Equal Application Fit

Two motors with identical rated power can produce different torque because they operate at different rated speeds.

For example, a hypothetical 5 kW motor rated at 1500 RPM produces approximately 31.8 Nm at its rated operating point.

Another 5 kW motor rated at 3000 RPM produces approximately 15.9 Nm at its rated operating point.

Both have the same rated power, but their rated torque values differ considerably.

For equipment requiring greater torque at lower speed, the first motor may be more suitable, provided its other specifications match the application.

This demonstrates why motor selection should consider torque, speed, and operating duration together, rather than relying only on kW.

5. How Does Duty Cycle Affect Permanent Magnet Motor Sizing?

Duty cycle describes how a motor operates over time, including how long it runs, how heavily it is loaded, and when it stops or rests.

It matters because motors generate heat during operation. Even when a motor can produce the required torque, it must also remain within acceptable temperature limits throughout the working cycle.

5.1 Understand S1, S2, S3 and Other Relevant Duty Types

Industrial motors may be assigned duty classifications under IEC 60034-1, which covers ratings and performance requirements for rotating electrical machines.

The most relevant duty types for preliminary sizing are summarized below.

Duty Type	Meaning	Typical Selection Situation
S1	Continuous operation at a constant load	Equipment operating for extended periods
S2	Short-time operation followed by sufficient rest	Equipment operating for a limited period before cooling
S3	Repeated operation and rest, with starting losses having negligible thermal influence	Intermittent equipment with repeated working cycles

For example, a motor operating continuously for an entire production shift is generally evaluated for continuous-duty requirements.

A motor running for several minutes and then resting may qualify for a different rating, depending on the actual thermal conditions.

If frequent starting contributes significantly to motor heating, S4 or another appropriate duty classification may be necessary.

These distinctions are also explained in the SEW-EURODRIVE operating duty guidance.

The correct duty classification depends on the real operating pattern, not simply whether the machine runs continuously or intermittently.

5.2 Evaluate Repeated Load Changes

Some equipment operates continuously but experiences repeated changes in torque.

For example, a machine may run under moderate load for most of each minute, then experience a much heavier load for several seconds.

Although the heavier load is brief, it produces additional heating that must be considered.

Engineers sometimes use an equivalent continuous torque estimate, commonly called RMS torque, to help evaluate this type of repeated loading.

In simple terms, this estimate represents the approximate steady torque that would produce a comparable current-related heating effect under suitable assumptions.

You do not need to perform a complex calculation to prepare an initial motor inquiry. Instead, record the torque during each operating stage and the time spent at that torque.

For example, provide 20 Nm for 58 seconds followed by 35 Nm for 2 seconds, repeated continuously.

The supplier can then evaluate the load profile using the actual motor's thermal characteristics.

5.3 Verify Overload Duration and Thermal Limits

Even when the average operating load appears acceptable, short periods of high torque may exceed the motor or drive's limits.

A motor capable of producing 50 Nm for two seconds may not be able to maintain the same torque for thirty seconds.

Likewise, repeated overloads can accumulate heat if the motor does not have sufficient time or cooling capacity to recover.

When reviewing a candidate motor, confirm the maximum overload torque, allowed overload duration, repetition frequency, and cooling conditions.

For continuous or demanding cyclic operation, the final selection may require a manufacturer thermal assessment.

6. How to Verify a Motor Against the Required Operating Profile

Once the initial torque, speed, power, and duty requirements are known, the next task is to verify whether a specific motor can meet them.

This step turns general sizing calculations into a practical product selection.

6.1 Compare the Load Profile with the Motor Torque-Speed Curve

Start by obtaining the candidate motor's torque-speed performance information.

Check whether the motor provides enough continuous torque at the equipment's normal operating speed.

Next, verify its available torque at the lowest and highest required speeds.

Finally, check whether the motor can handle temporary peak loads at the speeds where those loads occur.

For example, a motor may provide enough continuous torque at 1500 RPM but insufficient torque at 2500 RPM.

In that situation, selecting the motor based only on its rated power would overlook an important operating limitation.

Tools such as ABB DriveSize illustrate how motor and drive selection can account for different load types and working cycles.

6.2 Check Power Supply, Starting Method and Drive Compatibility

Permanent magnet synchronous motors must operate with a suitable electrical supply and control arrangement.

Many industrial variable-speed PMSMs use an electronic drive to control motor speed and torque. However, some specially designed line-start PMSMs can operate with different starting arrangements.

The required starting method should therefore be confirmed for the specific motor model.

When selecting a variable-speed PMSM, verify supply voltage, compatible drive type, continuous current, peak current, and required speed control functions.

A motor may have sufficient mechanical torque capability but still perform incorrectly if the selected drive cannot provide the necessary voltage or current.

The motor and drive should be evaluated together before final approval.

6.3 Confirm Cooling, Mounting and Environmental Constraints

Mechanical and environmental conditions can influence whether a motor is suitable.

High ambient temperatures, insufficient ventilation, and restricted installation spaces may affect continuous output capability.

Mounting dimensions, shaft diameter, mechanical connections, and enclosure protection must also match the equipment.

For applications where additional thermal management may be required, manufacturers may offer different cooling arrangements, including water-cooled permanent magnet motors.

However, cooling configuration alone does not establish suitability. The final decision must be based on the specific motor's rated performance and installation requirements.

7. Worked Example for a Variable-Load Conveyor

A simplified example demonstrates how the main sizing requirements fit together.

The same approach can be applied to other equipment by replacing the example values with actual operating data.

7.1 Define the Application Requirements

Consider a hypothetical conveyor driven by a permanent magnet motor.

The conveyor operates at 1450 RPM and normally requires 20 Nm of motor shaft torque.

Every 60 seconds, the torque increases to 35 Nm for 2 seconds before returning to 20 Nm for the remaining 58 seconds.

For this example, the values already represent the load at the motor shaft. Starting acceleration, braking, and additional transmission effects are excluded.

7.2 Estimate Power and Evaluate the Working Cycle

Using the basic power calculation, the normal mechanical power requirement is approximately 3.04 kW.

During the temporary 35 Nm load, mechanical power rises to approximately 5.31 kW at the same speed.

This does not necessarily mean the application requires a motor with a continuous rating of 5.31 kW.

Instead, the motor must provide sufficient continuous capability for the repeating operating cycle and enough temporary torque for the two-second peak.

The exact continuous rating depends on the candidate motor's current-related heating, cooling arrangement, and other losses.

7.3 Identify the Remaining Selection Checks

Before approving a motor for this example, confirm that it can support the repeating load profile at 1450 RPM.

The manufacturer should also verify that 35 Nm is available for two seconds at the specified repetition frequency.

If the conveyor starts under load or accelerates quickly, those conditions must be added to the sizing requirements.

The key lesson is that normal power, peak power, and operating duration describe different parts of the same selection problem.

A suitable motor must satisfy all three.

8. Common Permanent Magnet Motor Sizing Mistakes

Most sizing problems can be avoided by checking the complete operating requirement rather than focusing on a single specification.

Three mistakes deserve particular attention.

8.1 Selecting a Motor by Rated kW Alone

Rated power does not tell you how much torque a motor can deliver at every speed.

A motor with sufficient rated power may still be unsuitable for sustained low-speed operation or high starting loads.

Before comparing motor prices or dimensions, confirm that the required torque is available across the equipment's operating speed range.

8.2 Ignoring Peak Loads or Using Peak Torque as a Continuous Rating

Temporary torque capability should never be treated as unlimited continuous output.

If the equipment frequently starts under load, accelerates rapidly, or experiences periodic overloads, those conditions must be included in the specification.

Otherwise, the motor may experience excessive heating, protective shutdowns, or inadequate acceleration.

Check both continuous and peak torque ratings, including the conditions under which they apply.

8.3 Applying an Arbitrary Safety Factor Instead of Checking the Actual Duty

Choosing a motor substantially larger than the calculated requirement may appear safer, but oversizing is not always the best solution.

A larger motor can increase cost, dimensions, and rotating inertia without resolving problems related to drive compatibility, cooling, or operating speed.

Instead of adding an arbitrary percentage to the power rating, first identify which operating conditions are uncertain.

Where necessary, provide an appropriate margin based on load variation, measurement accuracy, manufacturer recommendations, and expected operating conditions.

9. Permanent Magnet Motor Sizing Checklist Before Requesting a Quote

The final step is to turn your calculations and operating requirements into a clear specification that a motor supplier can review.

A well-prepared specification reduces unnecessary clarification and helps ensure that proposed motors are evaluated against the same operating conditions.

9.1 Complete the Motor Sizing Specification Sheet

Before submitting a technical inquiry, check that the following information is available.

Equipment type and mechanical load characteristics

Normal continuous torque in Nm

Maximum required torque and its duration

Minimum, normal, and maximum operating speed in RPM

Estimated mechanical shaft power in kW

Starting conditions and acceleration time

Running time, rest time, and starts per hour

Gearbox ratio and transmission arrangement, if applicable

Available power supply and intended drive configuration

Cooling requirements and ambient operating temperature

Motor mounting dimensions and shaft requirements

If all values are not yet known, identify which are confirmed and which are preliminary estimates.

This allows the supplier to focus on the missing information that could affect selection.

9.2 Confirm the Final Specification with the Motor Supplier

After preparing the specifications, ask the supplier to compare your requirements with actual motor performance data.

The review should confirm continuous torque, peak torque, speed range, duty cycle capability, drive compatibility, and cooling requirements.

For challenging operating conditions, request additional calculations or testing where appropriate.

If you are evaluating a motor for a new equipment design or an existing installation, you can submit your operating requirements to Hiest for project-specific technical review.

The most reliable permanent magnet motor sizing decision is based on actual load requirements, not simply the closest available power rating. By defining torque, speed, power, and duty cycle together, engineers can identify suitable motor options, avoid common sizing errors, and move forward with clearer technical specifications.