How to Size a Motor for Intermittent Duty Cycles

Why Peak Power Is Not Enough for Intermittent Loads

Standard electric motors carry a nameplate rating that assumes continuous operation at a specific load factor. The power figure printed on the nameplate represents the electrical input required to maintain steady-state temperature when the machine runs at that load indefinitely. This assumption fails for applications involving frequent cycling, such as batch mixers, hydraulic pump stations, or material handling equipment that pauses between operations.

The thermal behavior of a motor under intermittent duty differs fundamentally from continuous duty. During the heavy load phase, copper losses in the stator and rotor windings generate heat. The insulation surrounding these windings has a specific temperature limit. If the motor operates at full load, the temperature rises until heat generation equals heat dissipation. In an intermittent profile, this balance is disrupted. The motor heats up during the load phase and cools down during the rest phase.

If the rest period is too short relative to the thermal time constant of the machine, the temperature never returns to the baseline before the next load cycle begins. The winding temperature climbs incrementally with each cycle. After a sufficient number of cycles, the insulation reaches its maximum limit. At this point, the insulation begins to degrade chemically, leading to loss of dielectric strength. This degradation accelerates, resulting in short circuits or open circuits within the windings.

Sizing a motor for this scenario requires moving beyond simple torque checks or average power calculations. You need to calculate the heat balance over one full operating cycle. This process is known as intermittent duty motor sizing. It involves tracking the temperature rise and fall over time to ensure the peak temperature remains below the insulation limit after a large number of cycles.

Prerequisites for Accurate Sizing

Before selecting a frame, gather specific data from the application. You need the mechanical load profile, the operating environment, and the cooling conditions.

1. Load Profile Data. Record the torque or power required during the heavy load phase and the light load phase. Note the exact duration of each phase in seconds or minutes. For example, a hydraulic pump may draw 15 kW for 20 seconds and then draw 1 kW for 60 seconds. Precision here is critical because a small error in timing can significantly alter the predicted temperature rise.
2. Cycle Time. Determine the total time from the start of the heavy load to the start of the next heavy load. This includes the rest period. This value defines the frequency of the thermal cycling.
3. Ambient Temperature. Note the maximum ambient temperature the motor will see. Hot environments reduce the driving force for heat transfer from the motor surface to the air. If the ambient temperature exceeds the standard reference temperature, the allowable temperature rise decreases.
4. Enclosure Type. Determine if the motor is in an open room, a ventilated enclosure, or a sealed box. Enclosure type drastically affects heat removal. A sealed cabinet acts as a heat trap, raising the internal air temperature well above the room temperature.
5. Altitude. Check the installation altitude. Higher altitudes result in lower air density and reduced cooling. Air at high altitude carries less heat per cubic meter, which reduces the cooling capacity of the fan and natural convection.

Step-by-Step Sizing Process

The following steps outline the standard method for selecting a motor for intermittent duty. Each step includes the reason for its inclusion in the process.

1. Calculate the Heat Generation in the Load Phase.
   Measure the actual power consumption during the heavy load phase. Convert this into heat generated per second. You must assume the efficiency of the motor at this specific load point. If the load is high, efficiency may drop, generating more heat than the power input suggests. For instance, a motor operating at 110 percent of its rated current may have an efficiency of 90 percent, meaning 10 percent of the input power becomes heat. This step establishes the thermal input to the system.

2. Calculate the Heat Dissipation in the Rest Phase.
   Determine how much heat leaves the motor during the unloaded phase. This depends on the thermal time constant of the motor frame and windings. A general rule of thumb is that the heat loss rate is proportional to the temperature difference between the winding and the ambient air. You need the thermal time constant, which is provided in manufacturer datasheets. This value represents the time required for the temperature to change by a specific percentage. This step determines how much heat is removed before the next load burst.

3. Perform the Duty Cycle Calculation.
   Use the values from the previous two steps to find the steady-state temperature rise. If the heat generated in the load phase exceeds the heat removed in the rest phase, the temperature will rise indefinitely. The duty cycle calculation balances these two values. It tells you if the selected motor can reach a stable temperature. If the temperature rises every cycle, the selection is incorrect. You must ensure the temperature stabilizes at a level below the insulation limit after a large number of cycles.

4. Adjust for Ambient and Enclosure Conditions.
   The standard thermal rating assumes a specific ambient temperature, usually 40 or 45 degrees Celsius. If your site is hotter, the allowable temperature rise decreases. If the motor is in a closed cabinet, the effective ambient temperature inside the cabinet will be higher than the room temperature. Apply derating factors for these conditions. This step prevents failure due to environmental mismatches. For example, a motor rated for 40 degrees Celsius ambient may need to be derated by 10 percent for every 10 degrees Celsius above that limit.

5. Check the Torque Margin.
   Verify that the selected motor can deliver the peak torque required during the heavy load phase. Intermittent motors often have higher torque capability than their continuous rating suggests. However, you must ensure the torque curve covers the entire load profile. This step confirms mechanical capability. The motor must not stall during the load phase, and the torque must be sufficient to overcome any mechanical friction or load inertia.

6. Select the Frame and Insulation Class.
   Choose a motor frame that matches the calculated thermal limits. Look at the thermal rating motors in the manufacturer catalog. Higher insulation classes, such as Class H, allow for higher temperature rises. This may allow you to use a smaller frame or handle a harsher environment. The frame size also affects the thermal mass, which influences the thermal time constant.

7. Verify the Stopping Frequency.
   Check the number of starts per hour. Frequent starting creates additional heat in the windings due to high inrush currents. If the starting frequency is high, you may need a higher frame or a different starting method. This step addresses the thermal stress of the transition phase. Starting a motor from rest draws several times the rated current, generating significant heat in a short period.

Common Mistakes in Selection

Engineers often make errors that lead to unexpected motor burnout.

Ignoring the Rest Period. Many buyers assume that a short rest period is sufficient. If the rest time is less than the thermal time constant, the motor never fully cools. The temperature accumulates over dozens of cycles. This is a frequent cause of early failure in batch processing applications. For example, a mixer that runs for 30 seconds and rests for 10 seconds will heat up significantly more than one that runs for 30 seconds and rests for 60 seconds, even if the average power consumption is similar.

Using Continuous Ratings for Intermittent Loads. Selecting a motor based only on the nameplate power ignores the thermal dynamics. A motor may be small enough for the average power but too small for the peak heat. You must use the duty cycle calculation to find the true limit. The nameplate rating is a snapshot of continuous performance, not a predictor of intermittent thermal behavior.

Underestimating Enclosure Heat. Placing a motor in a non-ventilated enclosure raises the internal temperature significantly. Without derating, the winding insulation degrades faster. Always check the ventilation requirements for the specific enclosure. A sealed cabinet can raise the internal temperature by 20 or 30 degrees Celsius above the room temperature, effectively reducing the thermal margin to zero.

Forgetting the Starting Current. Starting a motor draws high current. This creates heat in the windings. If the duty cycle includes many starts, this heat must be accounted for in the total thermal budget. The starting phase is often short but intense, and the heat generated during this phase can be substantial relative to the running phase.

Comparison of Thermal Rating Motors

The table below shows how different insulation classes handle temperature rise. These values are general references for selection.

Insulation Class	Max Winding Temp (C)	Typical Ambient Limit (C)	Selection Note
Class B	130	40	Standard for general industrial use.
Class F	155	40	Preferred for higher ambient temperatures.
Class H	180	40	Used for compact motors or hot environments.

Class F and H motors offer more thermal margin. This margin allows for higher peak loads or shorter rest periods without exceeding the insulation limit. For intermittent applications, the extra margin can be the difference between a reliable motor and one that fails within the first year.

Final Verification Step

Do not order the motor immediately after the calculation. Perform a final check using the manufacturer’s specific thermal data.

1. Input your calculated load profile into the manufacturer’s sizing software or thermal calculator.
2. Request a thermal profile report. This document shows the predicted winding temperature over time.
3. Compare the peak predicted temperature to the insulation class limit.
4. Ensure the peak temperature does not exceed the limit, even after multiple cycles.
5. Confirm the torque curve matches the application requirements.

This verification step catches errors in the manual calculation. It provides a documented proof of selection for your maintenance records. The thermal profile report is a valuable document for troubleshooting if the motor fails in the future. It allows maintenance teams to verify that the operating conditions match the design assumptions.

Reference to Related Topics

For applications where the speed changes during the load phase, refer to the guide on calculating motor torque for conveyor drives. This topic covers how torque changes with speed.

If you are using a variable speed drive to control the intermittent load, refer to the buyer guide on selecting VFDs for variable speed motors. The drive settings affect the thermal load on the motor. Proper VFD configuration can help manage the heat generation during the duty cycle.