The rated power written on an electric motor's nameplate is not, as everyone assumes, a fixed value valid under all conditions. This power is set against a specific reference condition: 40C ambient temperature and 1000 metres altitude. When the ambient temperature rises above this reference, the power the motor can safely deliver falls; this phenomenon is called derating. It frequently occurs in hot environments such as around furnaces, in foundries and in rolling mills. In this guide we cover why derating occurs, how cast iron motors behave at high temperature, and how the correct selection is made.

Against Which Condition Is the Rated Power Set?

According to the standards, a motor's nameplate rated power is given against a reference of 40C ambient temperature and 1000 metres altitude. This is the power the motor can safely deliver continuously under these conditions. This reference is important because the real factor limiting the power a motor produces is the temperature its winding insulation can withstand.

While a motor runs, it both produces heat from its own losses and absorbs heat from the environment. The winding temperature is the sum of the ambient temperature and the motor's own heating. The motor is designed so that the winding stays within the safe limit when the ambient is 40C. But when the ambient temperature rises, cooling the winding becomes harder, and while delivering the same power the insulation can reach dangerous temperatures. That is precisely why, as the ambient heats up, the power the motor can deliver must be reduced.

Cast iron motor derating graph at high ambient temperature

Derating: As Temperature Rises, the Deliverable Power Falls

Derating rests on simple logic: as the ambient temperature exceeds the reference value of 40C, the power the motor can deliver without overheating decreases. So a motor running in a 50C ambient cannot safely deliver the full power written on its nameplate; it must "sacrifice" part of that power.

The practical meaning of this is: in a hot environment, a motor selected by looking at the nameplate power is actually undersized for what it should be. As a result, the motor is constantly stressed, overheats, and its insulation wears out prematurely, failing early. That is why, when selecting a motor for hot environments, the basis must be not the nameplate power but the reduced power deliverable at that temperature. For the right selection, you can get an application-specific quote from the electric motor prices page.

In Which Environments Does the Temperature Exceed 40C?

In many industrial environments the ambient temperature easily rises above 40C. In these environments, motor selection must be made taking derating into account:

  • Around furnaces: Near heat-treatment, ceramic and glass furnaces, the ambient temperature is continuously high.
  • Foundries: Ambient heat runs high on melting and casting lines.
  • Rolling mills: On hot rolling lines, motors are exposed to high temperature.
  • Boiler rooms and enclosed hot spaces: Hot environments with limited ventilation.

Motors that will run in these environments must withstand both high temperature and, often, high dust. That is why cast iron motors are preferred in these applications; because the cast iron housing dissipates heat well and is mechanically durable.

Cast iron electric motor in furnace and rolling mill environment

Three Ways to Make the Correct Selection

There are three basic approaches to the correct motor selection at high ambient temperature:

  • Reducing power (applying derating): Selecting the motor to run with a load below the nameplate power. That is, in a 50C ambient, using a motor with a higher nameplate power at the reduced power suited to that environment.
  • Increasing frame size: Selecting a motor one frame class larger to deliver the same power. A larger frame provides better cooling and thermal reserve.
  • Providing thermal reserve with a higher insulation class: Selecting a motor with insulation of a higher temperature class, increasing the margin of temperature the winding can withstand.

These three methods can be applied individually or together. The right approach is determined by the ambient temperature, the motor's operating profile and the dust/protection requirements. What matters is ensuring the motor runs at a safe temperature in the hot environment without endangering its insulation.

A Hot-Environment Motor Solution With Manufacturer Assurance

High ambient temperature is a frequently overlooked but critical parameter in motor selection. When not calculated correctly, the motor is constantly stressed and fails early. HEM Motor offers cast iron motors with a suitable insulation class for hot environments, from the viewpoint of manufacturer assurance and strong stock; it provides the solution suited to your application with derating calculation, frame-increase and thermal-reserve options. Selecting the right motor for the hot environment secures both production continuity and motor life. For a solution and quote specific to your application, review the electric motor prices page.

Frequently Asked Questions

Against which condition is the motor's rated power set?

The rated power is set against a reference of 40C ambient temperature and 1000 metres altitude. This is the power the motor can safely deliver continuously under these conditions. When the ambient temperature exceeds this value, the deliverable power falls.

What exactly is derating?

Derating is the reduction of the power a motor can safely deliver as the ambient temperature exceeds 40C. Because cooling the winding becomes harder in a hot environment, the insulation can reach a dangerous temperature when the motor delivers the same power; that is why the power is reduced.

How is a motor correctly selected in a hot environment?

There are three ways: reducing power (applying derating), selecting a larger frame class to improve cooling, or providing thermal reserve with a higher insulation class. These methods are applied individually or together, according to the ambient temperature and the application.