A motor that runs hot is telling you something about its load, its supply or its cooling. This guide lists each cause with its symptom and fix, then shows how to measure the real winding rise.
Heat is the main enemy of motor insulation, and every 10 °C above the rated temperature roughly halves winding life. Learn the common causes, how to confirm each one in the field and how to calculate winding temperature rise.
What Are the Main Motor Overheating Causes?
The main motor overheating causes are overload, supply problems such as voltage unbalance, low or high voltage and single phasing, too many starts, poor cooling, bearing faults, low speed running on a VFD, and high ambient temperature or altitude. Each one either raises the losses inside the motor or reduces its ability to shed heat.
- Overload and high current raise copper losses with the square of current
- Voltage unbalance of only a few percent causes a much larger current unbalance
- Blocked fans, dirt and low VFD speed cut cooling air
- Every 10 °C above rated temperature roughly halves insulation life
- Measure current, voltage and winding rise before replacing the motor
Heat inside an induction motor comes from copper losses in the windings, iron losses in the core and friction in bearings and the fan. If you need a refresher on where these losses arise, read our article on how an AC induction motor works or this short note on the induction motor working principle.
Fluke recommends thermal scans with the motor at no less than 40 % of design load, as suggested by NFPA 70B, because higher load makes problems easier to see. A baseline image taken under normal running conditions is what you compare every later scan against.
Motor Overheating Causes: Symptom, Cause and Fix Table
| Symptom | Likely Cause | Field Check | Fix |
|---|---|---|---|
| Current above nameplate FLC on all phases | Overload or wrong motor size | Clamp meter vs nameplate | Reduce load or resize motor |
| Phase currents differ widely | Voltage unbalance | Measure three line voltages | Fix supply, loose joints, PF capacitors |
| Motor hums, one phase near zero | Single phasing | Check fuses, contactor tips | Replace part, add phase loss relay |
| Hot at low and high voltage | Supply voltage out of range | Voltage at motor terminals | Tap change, larger cable |
| Hot after many starts per hour | Frequent starts or jogging | Count starts in DCS or PLC log | Soft starter or VFD, review logic |
| Hot frame, fan cover dusty | Blocked cooling | Look at fins and fan cowl | Clean, restore airflow |
| One end hotter, noise | Bearing fault or misalignment | Thermal scan, vibration | Grease, align, replace bearing |
| Hot only at low speed on VFD | Shaft fan too slow | Check speed and torque | Forced ventilation, resize |
| Hot in summer or on hills | High ambient or altitude | Ambient reading, site altitude | Derate or higher class motor |
This table is a practical start for field work, so print it for your maintenance team. Most motor overheating causes show up first as a change in current, voltage or frame temperature long before the protection trips.
The US Department of Energy motor tip sheet on voltage unbalance states that winding insulation life is cut in half for each 10 °C increase in operating temperature. Fluke adds that this applies even when the overheating is temporary.
Overload and Wrong Motor Size
Overload sits at the top of most lists of motor overheating causes.
Copper loss rises with the square of current, so 10 % extra current gives about 21 % more winding loss. Compare running current with the motor full load current on the nameplate, and read the service factor shown in motor nameplate details.
A worn pump impeller, a choked filter, a dragging conveyor or a higher density product can all push a motor into overload. A correctly set thermal overload relay should trip before damage, so a motor that runs hot without tripping often points to a wrong relay setting.
Voltage Unbalance and NEMA Derating
U = max deviation from average ÷ average × 100
DOE example: 462 V, 463 V, 455 V
Average = 460 V, deviation = 5 V
U = 5 ÷ 460 × 100 = 1.1 %
Extra rise ≈ 2 × U² = 2 × 1.1² ≈ 2.4 %
The DOE tip sheet recommends that voltage unbalance at the motor terminals should not exceed 1 %, and says that above 1 % the motor must be derated per NEMA MG 1. It also notes that current unbalance can be 6 to 10 times the voltage unbalance, which is why a small voltage problem makes one phase run much hotter.
Electrical Engineering Portal, quoting NEMA, adds that operating a motor above 5 % voltage unbalance is not recommended. Work through a full example in our guide on voltage unbalance calculation for motors.
When phase currents differ, rotate all three supply leads one position at the motor terminals and measure again. If the high current moves with the supply, the cause is the supply, and if it stays on the same motor lead, the cause is inside the motor.
Single Phasing, Low Voltage and High Voltage
Single phasing happens when one line opens, often from a blown fuse or a burnt contactor tip, and Fluke notes that it causes voltage imbalance, high current and heating. A three phase motor that loses one phase while running may keep turning but draws heavy current in the healthy phases, so fit motor protection relays with phase loss detection.
IEC 60034 1 expects motors to run normally within plus or minus 5 % of rated voltage and 2 % of frequency, which it calls zone A. Low voltage raises current for the same load, while high voltage raises magnetising current and core loss, so check voltage drop on long feeders.
Frequent Starts, Cooling Problems and Bearings
These mechanical and duty related motor overheating causes are easy to miss.
Grundfos lists frequent switching cycles, inadequate ventilation, high ambient temperature and a damaged or clogged pump among the main motor overheating causes on pump sets. For starts, compare your logic with the maker limit and consider the motor starting methods compared guide.
On VFD motors, bearing heat can also come from shaft currents that pit the races. Our article on VFD motor bearing currents explains the signs.
- Overload and high current
- Voltage unbalance
- Single phasing
- Low or high supply voltage
- Too many starts per hour
- Blocked fins or broken fan
- Bearing faults and misalignment
- Low speed on a VFD
- High ambient or altitude
VFD Low Speed Cooling, Ambient and Altitude
A standard TEFC motor uses a fan on its own shaft, so at low speed it gets much less cooling air while a constant torque load still demands full current. For long low speed running, fit a separately powered fan or pick a motor rated for the speed range, and read our page on VFD overload trip causes.
IEC 60034 1 bases its ratings on a site altitude up to 1000 m and a coolant temperature up to 40 °C. Above either limit, thinner or warmer air carries away less heat, so motors on hill sites or in hot plant rooms need derating or a higher insulation class.
Motor Insulation Class Temperature Limits
| Insulation Class | Max Temperature | Allowed Rise at 1.0 SF | Hot Spot Allowance |
|---|---|---|---|
| Class A | 105 °C | 60 °C | 5 °C |
| Class B | 130 °C | 80 °C | 10 °C |
| Class F | 155 °C | 105 °C | 10 °C |
| Class H | 180 °C | 125 °C | 15 °C |
These values from the Toshiba temperature rise application note assume a 40 °C ambient and the resistance method. Many modern motors use Class F insulation with a Class B rise, which Toshiba shows leaves a thermal margin, and our article on motor insulation classes explains this in detail.
How to Check Motor Temperature and Winding Rise
- Measure current on all three phases and compare with nameplate FLC
- Measure all three line voltages and work out unbalance
- Take a thermal image at 40 % load or more and compare with baseline
- Check fan, cowl and fins for dust and damage
- Listen to bearings and check vibration at both ends
- Count starts per hour from the control log
- Check VFD minimum speed and load type
- Test winding with a megger and a resistance test
A rising motor temperature often comes with falling insulation resistance, so add a megger insulation resistance test to the checks, as described in this guide to insulation resistance testing. Embedded sensors also help, so see our page on PTC thermistor protection.
The resistance method gives the average winding temperature from the change in copper resistance. Measure the cold resistance and temperature, run the motor to steady state, then measure the hot resistance quickly after stopping, using the same leads as in the motor winding resistance test.
R cold = 1.20 Ω at 25 °C
R hot = 1.53 Ω, ambient = 40 °C
T hot = (1.53 ÷ 1.20) × (234.5 + 25) minus 234.5
T hot = 1.275 × 259.5 minus 234.5 = 96.4 °C
Rise = 96.4 minus 40 = 56.4 K, below 80 K
The constant 234.5 applies to copper windings, so use 225 for aluminium windings. Because the motor cools while you measure, take the hot reading within the first minute after stopping or the true rise will be slightly higher than the result.
Before opening a terminal box on a hot motor, follow the isolation steps in our electrical safety audit checklist. Many motor overheating causes are found at the terminals themselves, where a loose lug runs far hotter than the rest of the circuit.
Write the running current, phase voltages and frame temperature on a tag at every motor once a month. A slow upward trend in these three readings warns you about motor overheating causes weeks before the motor trips.
The Toshiba application note below explains temperature rise, insulation classes, service factor and thermal margin in plain terms. It is a useful reference when you rank motor overheating causes on site.
The video below gives a quick recap of motor overheating causes.
Motor Overheating Causes FAQ
The most common motor overheating causes are overload, voltage unbalance, low or high voltage, single phasing and too many starts. Poor cooling, bearing faults and long low speed running on a VFD are also very frequent.
High ambient temperature and altitude reduce cooling as well. Measuring current, voltage and frame temperature usually points to the right cause within minutes.
The limit depends on the insulation class on the nameplate, which is 130 °C for a Class B winding and 155 °C for a Class F winding. These are winding limits, not frame temperatures.
Toshiba lists an allowed rise of 80 °C for Class B and 105 °C for Class F at a 40 °C ambient. Use the resistance method or embedded sensors to know the real winding temperature.
Yes, voltage unbalance is a major cause, because current unbalance can be 6 to 10 times the voltage unbalance. The DOE estimates extra temperature rise of about 2 times the square of percent unbalance.
It recommends keeping unbalance at the motor terminals within 1 %. Above that, NEMA MG 1 calls for derating the motor output.
A standard motor cools itself with a fan on its own shaft, so the airflow drops when the speed drops. A constant torque load still needs near full current at that low speed.
The result is high losses with little cooling air. Fit a separately powered cooling fan or choose a motor rated for the full speed range.
Start by measuring the current on all three phases and comparing it with the nameplate full load current. Then measure the three line voltages at the terminals and work out the unbalance.
A thermal image at normal load shows hot bearings, loose connections or blocked fins. For the real winding temperature, use the resistance method or embedded winding sensors.
Yes, insulation life roughly halves for every 10 °C rise above the rated operating temperature. The DOE, Fluke and EASA all quote this rule in their motor guidance.
Even short periods of overheating age the insulation, so the damage adds up over the years. Bearings also suffer, because grease breaks down much faster at high temperature and the bearing then adds its own friction heat.
Related Articles
- Motor Insulation Classes
- Voltage Unbalance Calculation for Motors
- Thermal Overload Relay Explained
- Motor Thermistor Protection With PTC Relay
- VFD Overload Trip Causes
External References
- Toshiba: Motor Temperature Rise Application Note
- Fluke: Using Thermal Imaging to Troubleshoot Motors and Drives
- Wikipedia: Induction Motor
What We Learn Today
- The main motor overheating causes are overload, voltage unbalance, single phasing, poor supply voltage, frequent starts, blocked cooling, bearing faults and low VFD speed.
- Current unbalance can be 6 to 10 times the voltage unbalance, so keep supply unbalance within 1 % at the motor terminals as the DOE recommends.
- Use the resistance method to find the real winding rise and compare it with the insulation class limit, since every 10 °C extra halves insulation life.