📌 Introduction
Electrical motors are designed to operate within a specific temperature range. When the temperature exceeds that limit, motor winding, bearings, insulation, and lifetime are affected.
Overheating not only reduces efficiency but can also lead to breakdowns and unexpected shutdowns.
In industries, overheating is one of the most common reasons for motor failures. This post further covers :
🔍 10 Common Electrical Reasons for Motor Overheating
1. Overloading
Overloading is the situation where Motor draws more current, Higher than its rated capacity. Extra current produces inverse impact on winding’s insulation.
Motors are brave warrior. Even in a complete shaft lock situation, motor consume current much higher than its rated current.
At peak level, current get saturated at its peak level. In that mean time, winding get heated up & its insulation starts burning out.
Causes:
- Incorrect motor sizing
- Mechanical jams
- Extra load on shafts
- Damaged motor’s bearing
Effect: Higher current → more heat → insulation damage.
2. Low Voltage Supply
Electrical power= Torque* RPM
- Torque– Type of angular force
- RPM– Rotation per minute
Low voltage requires extra current to maintain the required torque. Voltage ↓ → Current ↑ → Heat ↑
A study published by ABB motors is shown below. Which is basically a graph representing the impact of variation of voltage & frequency ranges.
Graph contains :
- x axis represents frequency per unit
- Y axis represents voltage per unit
- 1 Zone A (represented in Dark) Where frequency is +-2% & voltage is +-5% . Temperature rise will be higher than at rating point.
- 2 Zone B ( outside the dark zone A area) Where frequency is +3 & -5% While voltage is +- 10% (short period run))
- 3 rating point point.
Motors are capable to deliver the rated torque in both the zones A & B. How ever, the temperature rise will be higher than at rated voltage & frequency. Motor can run only for short period of time in zone B.
3. High Voltage Supply
Higher voltage stresses winding insulation and increases magnetic losses, leading to overheating.
Often ignored in industries because “high voltage is good,” but it’s not. Clearly explained in a graph above.
4. Single Phasing
One phase drops during operation — motor continues on two phases and overheats very fast.
” When a 3-phase induction motor is supplied with only two phases, a proper rotating magnetic field is not produced. Due to insufficient and unbalanced magnetic flux, the rotor fails to develop adequate torque and slows down. As rotor speed reduces, the back EMF in the stator windings drops significantly, causing excessive current to flow. This high current leads to severe overheating and ultimately damages the motor windings . “
Common reasons:
Single phasing protection is crucial for safety.
5. Poor Power Factor
A low power factor increases current draw and generates more heat. Industries with inductive loads (motors) see this often.
Next, We are going to calculate the Ampere increase when power factor fall below.
Name plate 22 KW induction motor is shown below. Having a rated current is 41 Amps at supply of 400 Volts/ 50 Hz/P.f 0.83. Nominal efficiency is 93%.
What will the current at a power factor of 0.75 ?
Efficiency= (Output/Input)*100
- efficiency= {Output power (Mechanical power)/Input power (Electrical power)}*100
- 93={ 22000/(1.732*400*I*0.80}*100
- I={22000/(1.732*400*0.75*0.93)}
- I=45.52 Amps
Clearly seen, power factor decreases current in line increases. At power factor 0.83, current is 41 Amps. At power factor 0.75, current is 45.52 Amps.
6. Wrong Motor Starter Selection
There are multiple types of starters such as :
DOL Starter : DOL stands for “direct online starter“. In this starter, Motor get directly connected to power supply without any change in voltage & frequency. In this starter , There is no provision to control the high starting inrush current. Low rating induction motor up to 4 KW can easily run through this starter.
Star delta Starter : In this starter, Motor’s winding connection changes from star connection to delta connection. At starting, motor run in star winding connection While after some time connection changes to Delta winding. In this starter, starting current reduces to 1/3 times. High rating induction motors can run through this starter. This starter has only provision to limit the high starting current.
VFD : VFD stands for “Variable frequency drive“. VFD has a mechanism to control the speed through frequency control. Starting, running, loading & breaking widely control through this VFD mechanism. From low rating to high rating induction motors, All the motors can easily control through this starters.
Starters; not only starts the motor but it provides the protections from overload condition. Some starters have provision to control the high initial starting current. Such as Star-Delta starter & VFD.
But; DOL starter do not provides any protection from high initial starting current.
Wrong starter → high inrush current → heating.
7. Harmonics from VFD
Harmonics are higher frequency distortions which appeared on standard frequency line, Harmonics are integral multiple of standard power frequency .
Let suppose standard frequency is 50 Hz. Than Harmonics frequency will be 100,150,200,250,300,350,400 & so on. Harmonics distortions are created by non linear load which manipulates the overall waveform of standard power.
VFDs produce harmonic currents which heat the motor’s winding. Harmonics creates heating in the winding due to the following reasons.
Reasons behinds harmonics generation :
- High loss due to high RMS Current
- Skin effect
- hysteresis & eddy current loss
- Negative sequence Harmonics creates couter torque in industry.
- Reduced cooling effect
Solutions:
- VFD-rated motors
- Filters such as line reactors
- Better grounding
8. Wrong motor selection
Motor draws heavy current during startup. Frequent cycling does not allow cooling time. Typical in compressors, conveyors, cranes.
Motors are designed for particular duty type such as ‘continuous duty‘, ‘short duty‘ & ‘Intermittent duty‘.
- Continuous duty motors (S1) are specially designed for continuous run such as pumps & fans.
- Short duty motor(S2) runs for some time period & then there is a long rest period such as cranes motors.
- Intermittent duty(S3-S8) contains the type of motor which contains a period of starts, loads, break, rest. Such as motors of CNC & VMC.
Here motor selection is very important. A wrong selections of motor leads to improper behavior which leads to overheat.
9. Lack of Maintenance
Every motor has particular cooling type. Such as air cooled motor which includes an air fan behind the motor Which cools down the motor.
Air cooled Motors are designed with outer periphery fins & air from the fan sweep the heat away. These fins actually increases the overall heat rejection area. Higher the area, higher the heat rejection.
Over the time, due to hard environmental conditions, Fins around the motor get blocked due to dust & sand accumulation. Which results heat removal mechanism get inefficient.
That’s why preventive maintenance or servicing of the motor is very important. Servicing further includes Bearing, winding insulation improvement.
10. Voltage imbalance
Even 2–3% imbalance increases current drastically
🧰 Protection Devices to Prevent Overheating
- Thermal overload relay
- PTC thermistor
- PT100 RTD sensors
- Motor protection circuit breaker (MPCB)
- Bearing temperature sensors
🏭 Effects of Overheating on Motor
- Insulation damage
- Reduced winding life
- Bearing failure
- Reduced efficiency
- Unexpected shutdowns
📑 Real Industrial Example
A 7.5 kW induction motor in a packaging line overheated due to low voltage during peak factory operation. Installing a stabilizer and power factor correction solved the problem.
🧾 Conclusion
Overheating is preventable if monitored properly. Regular load checks, voltage measurements, and correct starter selection improve reliability and motor life.