Torque is vital to a successful motor replacement. The new motor must be able to accelerate and drive the existing equipment without exceeding the locked rotor time. In some cases, too-fast acceleration is detrimental to other equipment. The new motor must provide enough breakdown and locked rotor torque values, as well as the same revolutions per minute (rpm), power and current. New motors should provide equal or better efficiency when compared to legacy equipment.
To evaluate motor suitability, the end user should provide the speed-torque curve of the existing driven equipment, plus the total inertia on the motor shaft. In many of these cases, this information is not available from the driven equipment manufacturer.
The end user should provide records of the original motor’s datasheet, speed-torque curves and thermal limit curves for the motor application engineer to consult. Photography of the existing motor’s nameplate is of no use. On occasion, an end user may accept a motor with dissimilar dimensions and make the necessary site adaptations.
It is important to understand the deviations owners are willing to accept. It may not be required to provide a “drop-in replacement.” Even so, the electrical performance data is still a must, unless the driven equipment is changing. When the shaft height of the replacing motor is below the height of the existing unit, it may be possible to provide a transition base to match the foundation holes. The opposite makes a direct replacement impossible without foundation changes. Work closely with the application engineer for certainty.
Replacing Motors of the Same Manufacturer
Replacing motors of the same make and model could be perceived as being risk free and relatively straightforward. Many people assume the only necessary information is the corresponding motor model number. However, even in these cases there are potential risks.
Consider, for example, an original motor that has been modified. If the motor’s original nameplate is secured to the frame and provided as the motor identity and only frame of reference to the application engineer, the miscommunication could result in delays and extra cost. It is essential to understand if the motor to be replaced was ever modified and, if so, deviations the user is willing to accept.
In cases where a “drop-in replacement” is not required, the electrical performance data remains necessary unless the driven equipment has changed.