Rating of an Electric motor and thermal model

Rating of an Electric motor

Rating of an Electric motor

Electric motors are designed for different ratings. However, this rating can be found on the name frame of the motor. All Electric motors have there name frame fixed on its surface, these rating carries the important data of the electric motor. These data specify motor performance. They include Rated power, operating voltage, running speed, power factor, current, insulation class, and motor efficiency. 

Although Electric motors are rated in several ways and they all speak differently, as to the output produced by the motor. 

Type of electric motor rating 

These rating includes;

  1. Continuous rating 
  2. Continuous maximum rating 
  3. Short time rating 
  4. Intermittent rating 

Continuous rating: This rating of the electric motor, gives details as to the output that the electric motor can continuously produce without exceeding the specific temperature rise of the motor. In addition to this continuous output produced, it also shows that for every two hours, the motor can hold up to 25% overload even without insulation breakdown of the motor.

Continuous maximum rating: This rating is similar to the continuous rating, which shows the continuous output produced by the electric motor. However, the difference is that, this rating of the machine doesn’t give room for 25% overloading.

Short time rating: This rating of an electric motor shows the output that an electric motor can produce within a specific time frame. However, this is usually a short period of time and it might be 15mins, 30mins, 1hours or 2hours. This output is produced within a short period of time but without going above a specific temperature rise.

Intermittent rating: The intermittent rating of an electric motor, is more like the short-time rating. Here, it shows the output that an electric motor can produce within a short and specific period of time, just like that of the short-time rating. Although after this rating had been achieved, the motor is allowed to cool down for a specific period of time. Thereafter, the whole process continues. This period of time is calculated as the duty cycle of the motor. The duty cycle is the ratio between the period of time the motor was operational to the total time of the motor.

How to rate Electric motor

Here, I will be talking on the continuous rating of an electric motor. However, the continuous rating of a machine depends on the type of mechanical load acted upon. Therefore, the power rating of the motor can be calculated base on the type of impact on the load.

Rotational motion: For Electric drive system that impacts rotational motion on the load acted upon, the power rating is calculated as follows;

Power\quad rating=\frac { TN }{ 9.55\ast \eta }


{ \eta } represent the efficiency of the motor 

T shows the torque produced by the electric motor 

N stands for the running speed of the motor.

Linear motion: The mechanical energy produced by the electric motor, can also impact linear motion on the load. For linear motion, the power rating can be calculated as follows;

Power\quad rating=\frac { FV }{ 0.102\ast \eta }


F represents the force that is exerted on the load by the electric motor.

V shows the velocity of the motion of the motor.

{ \eta } represents the efficiency of the motor 

For an elevator: Electric motor is used on the elevator as one of the major device presents. However, the power rating for elevators is similar to linear motion.

Power\quad rating=\frac { FV }{ 2\ast0.102\ast \eta }

For pumps: The power rating of electric motor use for pumps can be calculated as follows:

Power\quad rating=\frac { HQ\ell }{ 0.102\ast \eta }

H= head + suction 

The head of a pump is the top height of the pump tank to the pump.

The suction of a pump is the distance between where the pump is located and the liquid.

Q represents the liquid flow rate.

{ \ell } is the Density of the liquid.

Electric motor rating with load variation 

The mechanical load of an electric motor can vary with time. However, an electric motor can handle load variation. Therefore of a continuous duty with load variation, the electric motor will neither be rating based on the maximum or minimum rating the motor, rather the rating is based on the average losses throughout the load cycle. 

Let’s say, an electric motor runs different loads. The minimum load was operated at a power rating of 10kW with respect to time. While the maximum load acted upon, operate at 100KW. However, during this variation, the average power with respect to the load was seen to be 75KW. Therefore, the electric motor will be rated base on the average power rating during the load cycle.

Temperature rise in electric motors 

During the operation of an electric motor, currents flow. However, there are power losses in an electric motor, these power losses are in the form of heat. Electrical heat, on the other hand, is a function of the current flowing. Therefore, the more current flows through, the higher the tendency of heat produced. This heat affects the most active part of the machine, usually the windings and core. 

Again, every electric motor has a rated temperature for which they can suitably work without experiencing any fault. When this temperature is exceeded above its rated, there’s also the tendency for heat to be produced in the machine. The effect of heat produced reduces the life span of the motor. It’s important to note that, for every 10-degree rise of the machine rated temperature, the insulation life span of the machine reduces by half. 

However, an excessive increase in the machine temperature or causing excessive current to flow through will lead to breakdown of the electric motor without warning. This increase in temperature of the motor can be a result of improper cooling or overloading which may lead to an increase in the current requirement of the motor.

Classes of insulation of the Electric motor 

There are various classes of electrical insulation. However, there are four main classes with their temperature range as shown below;

  • Insulation Class A with temperature ranging from 150-130 degrees Celsius.
  • The insulation Class B having a temperature ranging from 130-155 degrees Celsius.
  • Insulation Class F with temperature ranging from 155-180 degrees Celsius.
  • The Insulation Class H with temperature ranging from 180-200 degrees Celsius.

The class F insulation is the common type of electrical insulation used for an electric motor. Moreover, an electric motor is subjected to ambient temperature, which is the atmospheric temperature. For this reason, the electric motor is designed for a maximum ambient temperature of 40 degrees Celsius. Both the class of insulation and the ambient temperature rating of an electric motor is written on the motor’s nameplate.

Although when the ambient temperature is too high, there will be a tendency for the machine temperature to increase above the rated. So it’s best you take note of the surrounding temperature because it’s factored on the power rating of the motor/machine.

Thermal model of an Electric motor

Thermal model of an Electric motor
Thermal model of an Electric motor

The heat rise and cooling of an electric motor can be thermally modeled. This can be achieved from the general heat concept. This concept implies that the heat generated from a system is the summation of both heat absorption and heat dissipation. However, this can also be written as 

Heat absorbed = Heat generated – heat dissipated

It’s important to note that, some parameters are to be considered during the thermal modeling of an electric motor. These parameters include the following:

  1. The heat generated by the system, in joules per second.
  2. The heat transferred through the system, in joules per second per meter square per degree celsius.
  3. The weight of the active part of the electric motor, in kilograms.
  4. The cooling surface area of the machine, in meter square.
  5. The specific heat of the system, in joules per kilogram per degree Celsius.
  6. The temperature of the system in degree Celsius.

Heat Rise of the system

During the operation of an electric motor, at the point when the final temperature of the motor is reached. When this point is reached, the heat generated will be equal to the heat dissipated. Therefore, we can say that at this point, no heat is absorbed and it’s considered to be zero. 

0 = Heat generated – Heat dissipated 

The Heat generated = Heat dissipated 

However, the heating time constant of an electric motor is actually the set time for the electric motor to heat up to 63.2%(percentage) or 0.632 of its final temperature.

The cooling system of an Electric motor 

Every operating machine will get to a point when it will be switched off. At this point when the machine is turned off, no heat will be generated. In other words, the heat absorbed is equal the heat dissipated. Mathematically, the heat generated is equal to zero. This can be mathematically seen as to be;

Heat absorbed = 0 – Heat dissipated 

Heat absorbed = Heat dissipated 

Most importantly, the cooling time constant of an electric motor is the time required for the motor to cool, up to 36.8%(percentage) or 0.368 of the initial temperature rise of the motor, but above the ambient temperature.

You can learn more on Electrical/Electronic Engineering, by checking our Electrical/Electronic Engineering category, you can find this under Academic page. Thanks for coming around.

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