How are the motor load points defined?
Customer is often interested in defining the performance (e.g. losses and efficiency) of a motor drive for specific operating points. But how are the motor load points defined?
Over the years working with variable speed inverter fed motors I come across this topic. The customer is asking to fill in a performance data sheet. Part of that data sheet is a section asking certain inputs, typically efficiency or heat loss, for defined load points. These values may or may not be guaranteed values.
Unclear definition of motor load points
Although the question sounds very trivial, it is often not very clear how the operating points are defined. In most cases the customer defines those “load points” as percentage of the nominal load. Thus, you would see the request to provide losses e.g. at 100%, 75% and 50% load.
Interpretation of load points
While the above definition is very clear for a fix-speed motor, it leaves a room for interpretation when having in mind a motor supplied from a VFD.
What does “75% load” mean? Load is terms of mechanical (shaft) power is a product of speed and torque.
75% load may be a “100% speed and 75% torque” point. But it could also be “91% speed and 82% torque” assuming quadratic torque characteristic as example.
a) Assuming fix-speed
First option is to assume fix speed. This may not be realistic for a VFD driven motor but supposes that the definition of the load point is the same as for a DOL motor. Argument may be that the data sheet was originally developed for fix-speed motors and if not explicitly mentioned, the definition remains the same.
This first approach is quite often used as it is very straight-forward. For light load, such as 25%, this method is probably not realistic as the load point (100% speed and 25% torque) may be outside of the operating envelope.
b) Assuming specific load torque profile
Another approach is to assume a specific load torque characteristic. Most fans, pumps or compressor have a quadratic type of load torque (load torque increasing with second power of speed). Then the load point shall be on that curve. For a purely quadratic torque type of load the 75% load point in our example would correspond to approx. 91% of nominal speed and 82% of nominal torque.
At first look, this approach may sound more realistic for a variable speed motor. However, the torque-speed relationship may apply only between zero and minimum operating speed (i.e. during start-up). In the continuous operating speed range the load is depending on the actual process control. Instead of a curve there is an envelope with theoretically infinite number of points representing the same load but different combination of speed and torque.
c) Project-specific definition
Third option is to clarify the definition of the load point so that it is a realistic representation of a typical operation point. Especially if there are guaranteed values linked to such load points, the definition shall mirror the reality. Otherwise the derived calculations (e.g. energy savings over the life time of the equipment) will be erroneous.
Of course, to avoid misinterpretation, the best way is to determine each operating point by speed and torque or speed and power.
This simple definition of what xx% load actually means shall be properly clarified if the data associated with those load points has an importance.
Summary
Operating points expressed as percentage of nominal power are unambiguous for a fix-speed motor. However, for a variable speed motor there are many (theoretically infinite) combinations of operating speed and torque resulting in the same value of shaft power. Thus, the load point defined in % of nominal power or in kW (hp) is not a unique definition. One way is to assume a fix speed and the torque is then proportional to the shaft power. Another way is to find the load point on the torque-speed characteristic (if available).
In case the data sheet shall be filled in just to satisfy the process, it may not be worth to spend too many thoughts on the subject. However, if the corresponding values (e.g. losses) are guaranteed values stated in contract and maybe even used as part of the evaluation of the offer (in tender stage) then it is worth to clarify the definition.
The machine standards are continuously updated to reflect the operation under converter (inverter) type of supply. A good reference is the latest revision of IEC 60034-25 [1].
References
[1] IEC TS 60034-25 Rotating electrical machines – Part 25: AC electrical machines used in power drive systems – Application guide, ed. 4.0, 2022-06
[2] Load types and characteristics, https://mb-drive-services.com/load-types-and-characteristics/
[3] Motor start with VFD, https://mb-drive-services.com/vfd-motor-start/
