Troubleshooting: Motor Overloading in Intermittent Duty

What does intermittent duty overload look like

A motor that trips only under certain conditions is a different problem from a motor that overheats every hour. In intermittent duty, the load may sit at a low percentage for most of the cycle, then spike to a high percentage for a few seconds. The motor may not be overloaded on average. It may be overloaded at the exact moment the breaker or thermal relay sees the high current.

The first sign is usually not a slow heat buildup. It is a sudden trip. The drive may show an overcurrent or overtorque fault. The operator may restart the motor and it may run fine for several cycles before tripping again. If the trip happens only when the load peaks, the issue is usually torque sizing or protection, not continuous power.

Consider a batch mixer that runs at low speed for ten minutes, then ramps up to high speed for ninety seconds. The average current over that eleven minute cycle might be well below the motor rating. Yet during the high speed phase, the draw can exceed the trip threshold. A thermal relay designed for continuous duty may not react to this short spike, but a magnetic breaker or a VFD with a fast overcurrent response will cut power immediately. The result is a nuisance trip that confuses maintenance staff because the motor temperature remains low at the time of the failure.

This pattern often appears in applications like hoists, cranes, and conveyor systems with sudden start stops. The electrical signature is distinct. Current spikes sharply, often exceeding the full load current by a significant margin, then drops back to normal. If the protection device is set for a longer time constant, it may ignore the spike. If it is set for a short time constant, it will trip. The key is to identify the duration and magnitude of the spike relative to the motor capability.

Is the torque sizing wrong for the actual load?

The most common cause of intermittent overload is a mismatch between the torque required by the load and the torque the motor can deliver. A motor selected for a steady load may look correct on paper. It may have the right full load current. It may have the right full load power. But if the load needs a burst of torque that exceeds the motor capability, the motor will stall or draw excessive current.

Torque sizing means matching the motor torque curve to the load torque curve. For a conveyor, the torque peak often happens during acceleration. For a pump, the torque peak often happens during start or during a surge. For a batch process, the torque peak may happen only during a specific phase. If the peak torque exceeds the motor breakaway torque, the motor slows down. As it slows, the current rises. The protection device trips.

A common mistake is to select a motor by full load current only. A 15 kW motor may have a full load current that matches the load average. If the peak load requires 25 percent more torque than the motor can supply during a short burst, the motor will overload. The selection error may be small. It may be a motor with the same power but a lower breakaway torque than needed.

Breakaway torque is the torque at zero speed. Rated torque is the torque at full speed. The difference between these two points matters for starting loads. A standard induction motor has a relatively flat torque curve, but it may not have enough breakaway torque for a heavy load start. If the load requires high torque at low speed, a standard motor may stall or draw excessive current.

Consider a screw compressor. It may require high torque to start the rotor, then settle into a lower torque profile at operating speed. If the motor is selected based on the running torque, it may lack the breakaway torque needed for startup. The motor may start under light load, but trip when the full load is engaged. The fix is not always a larger motor. It may be a motor with a higher torque capability or a different torque characteristic.

What to check in the load cycle

Before changing the motor, measure the actual load. The nameplate duty cycle is not the same as the operating duty cycle. The nameplate tells you how the motor was designed. The operating cycle tells you what the load actually does.

Record the load for a full cycle. Note the current, torque, speed, and duration. If the load has a fixed cycle, run it through several cycles. If the load is variable, capture a representative period. Look for the peak current and the peak torque. Compare those values to the motor capability.

The motor capability is not a single number. It has a torque curve. The curve shows torque versus speed. At zero speed, the motor has breakaway torque. At rated speed, it has rated torque. Between those points, the torque changes. The load must fit within the motor torque curve across the whole operating range.

Use a power analyzer or a VFD with load monitoring to capture the data. A multimeter gives you voltage and current, but it does not show the torque profile. A power analyzer records power, current, and phase balance. A VFD can log current and speed over time. You need to see the peak values and the duration of those peaks.

Compare the peak load torque to the motor torque at the corresponding speed. If the load torque exceeds the motor torque at any point in the cycle, the motor will slow down. As it slows, the current increases. This is where the overload happens. The goal is to ensure the load torque curve stays below the motor torque curve at all operating speeds.

Is the protection setting too low?

A motor can be correctly sized while the breaker or thermal relay is set too low. In intermittent duty, the breaker may be set for a short time limit. If the peak current is high but short, the breaker may trip even though the motor is fine.

Check the breaker type. A thermal magnetic breaker has a thermal element and a magnetic element. The magnetic element trips on high current. The thermal element trips on sustained current. For intermittent loads, the settings may need adjustment. A VFD also has settings for overload, overcurrent, and short circuit. These settings must be set for the load profile.

A common setting error is using a default setting that assumes a steady load. If the load has a short peak, the default setting may be too sensitive. If the setting is too high, the motor may be protected from short circuit faults but not from sustained overload. The correct setting depends on the load cycle.

For a motor with a thermal relay, the time constant matters. A standard relay may be set for a 60 second trip time. If the load peaks for only 5 seconds, the relay may not trip. But if the breaker is set for a shorter time, it may trip. You need to match the protection device time constant to the load cycle.

If the VFD is used, check the overload setting. A standard overload setting may be set to 110 percent of rated current for 60 seconds. If the load peaks at 130 percent for 10 seconds, the VFD may not trip. But if the setting is set to 110 percent for 10 seconds, it will trip. The setting must be adjusted to allow the peak current for the duration it occurs.

What to do when the motor is correctly sized

If the torque sizing is correct and the protection setting is correct, the problem may be elsewhere. Check the mechanical load. A misaligned coupling can add torque. A worn bearing can add drag. A belt that is too tight can add load. A pump that is overfilled or clogged can add load.

Check the electrical supply. A low voltage during start can increase current. A loose connection can add heat. A poor ground can cause imbalance. Check the motor windings. A shorted turn can cause high current at start. A damaged insulation can cause high current under load.

Check the VFD if present. A VFD can change the torque characteristics. If the VFD is set for a low torque limit, it may not allow the motor to reach the torque needed. If the VFD is set for a high torque limit, it may allow the motor to draw more current than the breaker expects. The VFD settings must match the motor and the breaker.

Mechanical issues often cause intermittent problems because they change with operating conditions. A misaligned coupling may be fine at low speed but cause vibration and extra torque at high speed. A worn bearing may have low friction when cold but high friction when hot. These changes in friction alter the torque required and can trigger overloads.

Electrical issues can also be intermittent. A loose connection may have low resistance when cold but high resistance when hot. As the resistance increases, the voltage drop increases, and the current required to deliver the same power increases. This can cause overloads that only happen under load.

Troubleshooting table for intermittent duty overload

Symptom	Likely cause	What to do
Motor trips only during start	Breakaway torque too low for load	Verify load breakaway torque and increase motor size or torque capability
Motor trips only during acceleration	Acceleration torque exceeds motor capability	Increase acceleration torque or reduce acceleration time
Motor trips only under peak load	Peak torque exceeds rated torque	Verify peak load torque and select a motor with higher torque capability
Motor trips during short bursts	Protection setting too low for peak current	Adjust breaker or VFD overload setting for intermittent duty
Motor trips after several cycles	Thermal protection set for continuous duty	Adjust thermal protection or duty cycle setting for intermittent duty

How to prevent intermittent duty overload

Prevention starts with the load data. Do not select a motor from the nameplate alone. Measure the load. Record the current, torque, speed, and duration. Build a torque curve for the load. Compare it to the motor torque curve.

Select a motor with enough torque margin. For intermittent duty, the margin should cover the peak torque. A motor with a higher breakaway torque may be needed. A motor with a higher rated torque may be needed. A VFD can help by controlling acceleration, but it cannot create torque that the motor does not have.

Set the protection for the actual load. The breaker and VFD settings must match the load cycle. A setting that works for a steady load may not work for an intermittent load. A setting that works for a short peak may not work for a long peak. The settings must be verified under the real load.

Keep the mechanical load healthy. Misalignment, wear, and drag increase torque. A well maintained drive system reduces the torque required. A clean, aligned, and properly tensioned drive system allows the motor to operate within its rated range.

Review the selection after changes. If the load changes, the torque sizing may no longer be correct. If the process changes, the peak torque may change. If the motor is replaced, the torque curve may change. Re verify the match after any change.