Motor Temperature Rise (Causes and Limits)

Motor Temperature Rise (Causes and Limits)

My senior engineer, once told me, while we were checking an induction motor temperature, “To check if the motor is overheated, put your hand on its frame, and count to ten, If you bear the temperature then, the motor is OK, else, the motor is too hot.” 

This rule, is no longer applicable, because the motors are well designed to withstand higher temperature than your hands can do.

Besides, you can’t keep your palm on a surface that is hot for long enough to notice variations, and if you do, you’ll burn yourself.

So the right way to check a motor temperature rise is by using thermal imaging or temperature laser gun.

A constantly operated general purpose electric motor temperature can easily reach 80°C (176 F), and possibly even 100°C. (212 F).

Motor Temperature Effects

electrical short circuit damage
A burnt generator due to a short circuit – Photo Taken By Me

All electrical and rotating machines produce heat due to losses and frictions, and induction motors are not an exception.

Temperature rise is a serious enemy of motors, a motor with high temperature can get burnt because, the temperature decreases the winding insulation resistance.

In case of great decrease of the insulation resistance, it breaks down and a short circuit happens. And the motor burnout.

What Temperature Should an Electric Motor Run At?

The instruction manual and the motor nameplate clearly describe the temperature range of a motor.

The most frequent working ambient  temperature range for general purpose industrial motors is -20° C to 40° C.

This temperature range is suitable for a wide range of industrial applications.

However, the motor temperature can reach much higher than this range because, the modern motors are well designed to work with temperature rise more than 100° C, this is achieved by increasing the ability of the insulation of the winding to bear temperature rise.

Motor insulation class, is the key of motor temperature rise limits. A motor with class F insulation, has maximum permissible winding temperature of 155°C

I have written a detailed article about insulation class of motors, you can check it here

Allowable temperature rises at full load and 1.0 S.F.

A 60°C
B 80°C
F 105°C
H 125°C

Example Of Maximum Temperature of Motor

If we have a motor with insulation class F, running at 110°C.

Then the important question now is, Is this motor running too hot? The short and direct answer is No, This motor is not running too hot. Let’s find out why!

From NEMA insulation class table above, this motor has maximum Temperature Rise of 105°C, adding this temperature to the ambient temperature of 40°C gives us 145°C.

The surface of the motor is usually 30°C lower than the winding temperature, So this motor winding temperature is 110+30 = 140°C.

Which means that this motor is running at 5°C lower than its maximum Temperature Rise (Which is 145 for class F), and 15°C lower than its maximum permissible winding temperature (Which is 155). So the answer is No, This motor is not running too hot.

Overheating is one of the most typical problems with electric motors. Let’s discuss, What causes motor to overheat?

Causes Of Motor Temperature Rise

Electric motors, like any other complicated machine with several moving parts, are susceptible to problems like misalignment, bearing wear, and harmonic distortion.

Low voltage swich gear Power engineering system could be inspected using thermal image
Thermal image motors to check out its temperature rise

Unsuitable Motor

There is large number of sizes of motors. Choose a motor that can handle the load and with performance level that your project requires.

A motor that is higher in rating than requirements can waste a lot of energy, while a motor that is too little won’t be able to handle the load, causing more stress, heat and burnout.

 Wrong supply voltage 

A motor can be damaged by having over or under voltage issues.

Induction motors are constant power machines, when your motor lacks the proper voltage rating, it has to draw more current to provide the required torque, causing it to overheat.

This is why motors should have over voltage and under voltage protection relay. Motor voltage on the motor nameplate is the only reference for voltage supply.

Usually 5% up or down the rating voltage is the allowable limits.

A poor surrounding environment

Any motor needs space to breathe in order to work at its best. Your motor will struggle to cool down rapidly if it is operating in a hot environment.

The ambient temperature is set on the motor nameplate, usually ambient temperature is 40 °C.

Read my detailed article about motor specs and nameplate understanding

Improper use

Motors can run continuously or intermittently depending on their design. Don’t forget to follow the manufacturer’s instructions when using your motor!

An intermittent duty motor won’t be able to cool down between cycles if you run it too long.

Motor duty cycle is a letter on the motor nameplate, I have written a detailed section about it in my article here.

Altitude

Your company’s location may have an impact on your motor performance. Your machine may not cool as effectively.

It’s critical to select a motor that’s rated for the location of your workshop.

A lack of ventilation

If something is obstructing the ventilation holes for your electric motor,temperature rise will not dissipate, and will accumulate within the motor body, causing damage. Broken cooling fan can cause a temperature rise as well.

Regular maintenance on your motor can help reduce this risk.

Many times, I found plastic bags on the ventilation of motors at my workplace, In this case the motor gets hotter.

Fortunately the preventive maintenance program allows us to discover any issues like this.

Motor Bearing Temperature

Most electric motors use ball bearings that have been pre-greased and sealed. Motor bearing operating temperature is typically in the range of 140-160 degrees Fahrenheit.

This measure should be taken at the bearing outer ring for all bearing applications.

To estimate the proper bearing temperature if the outer ring cannot be accessed, take the measurement at the housing and add 15° to 20°F.

Special attention should be paid to the bearing lubricant for electric motors operating in high-temperature conditions.

  • Heating in electric motor bearings is mostly a result of the lubricant used in the motors. A low-temperature grease may be used unknowingly to lubricate open bearings, for example, when the conventional operating temperature is not reached.
  • It is also possible that the user has over-lubricated the bearing, causing the bearing balls to eject grease as they revolve, as well.
  • The churning effect causes a sharp temperature rise. The presence of incompatible greases within the combination can also lead to a reduction in the consistency of the grease and possibly the overall viscosity.

Follow the bearing manufacturer’s guidelines when regressing to avoid overheating.

Motor Temperature Monitoring

Temperature dependent resistors are frequently built into motor windings to monitor the temperature.

As temperatures rise, these PTC resistors become more resistant and serve as sensors. They detect a temperature increase at the motor and activate the relay to continue processing the alarm or the motor stopping.

A number of temperature sensors can be installed in each motor, which are monitored by the station operator.

The temperature of motor bearings is measured by Pt 100 RTDs while the temperature of motor winding is measured by Cu10 RTDs.

Temperature Monitoring That Is Flexible And Stable

In such cases, the 4116 Universal transmitter is a wonderful alternative. A wide range of AC and DC voltages can power the PR 4116, making it easy to incorporate into existing control cabinets.

It can be used to measure Pt100 or Cu10 RTDs without any modifications to it. The transmitter is also exceptionally stable over the long-term, reducing the need for regular zero and span calibration.

Motor Temperature Sensor

In order to provide direct thermal contact between the motor windings and the sensor, motor protection sensors are PTC limit temperature sensors.

The PTC sensors should be connected in series to detect the temperature of all phases’ windings.

A drive is supposed to shut off power to the motor when a winding’s temperature reaches its maximum permitted value as a result of these temperature sensors.

Due to their tiny size, PTC assemblies can be installed in tight spaces where protection is required, such as inside motor windings, and can endure typical varnish dip and bake processes, as well as other handling activities.

To cover a broader region, a number of sensors can be connected in series. There will be an increase in resistance and a fault state if any of the assemblies are exposed to excessive heat.

Conclusion

  • Temperature decrease insulation resistance and can damage a motor and make it burn out.
  • To judge a motor temperature, use temperature tools like laser temperature detector or thermal imaging.
  • Do not use your hands to detect motor over heat. This is not safety action and can burn your hand.
  • Normal temperature of a motor depends on its design, and is listed on its nameplate as insulation class.
  • If a motor is overheated, you should check it and solve the issue.
  • Large electrical motors are usually thermally protected by means of temperature sensors.
  • Some motors can work properly at a temperature of 155° C

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