Matthew Conville, P.E.
EASA Technical Support Specialist
When we consider putting a machine into service, we must consider the duty rating of the machine. If we do not, there is a good chance that the machine being placed into service will have thermal degradation of the windings. Not every application is created equal. For example, a crane motor doesn’t need the same duty rating as a punch press motor that runs continuously, even though they may have the same horsepower ratings. Likewise, a chop saw motor wouldn’t need to have the same duty rating as a pump motor where the pump is operated continuously.
NEMA MG-1-1.40 and IEC 60034-1, 4 provide detail on what duty classifications there are.
NEMA MG-1-1.40.1 is for a machine with a continuous rating (essentially, a motor capable of sustaining a given load for an indefinitely long period of time). NEMA MG-1-1.40.2 is for a machine that has a short-time rating. Effectively, this is the amount of time the machine can operate within its temperature rise from an ambient temperature start.
This clause indicates the motor shall remain de-energized long enough for the machine to get back within 9°F (5°C) of ambient temperature before starting again. We know this isn’t always what happens in application, so it’s important to educate the end-user and select an appropriate machine. Typically, on NEMA machines, we will see duty ratings like those in Figure 1. The machine manufacturer defines which rating is applicable for the machine given its design and performance testing results. When you look on a NEMA motor nameplate, you will see the designation next to “duty” (see Figure 2).
Consider Turndown Ratio
If the AC motor needs to be operated at variable speeds other than what is listed on the nameplate, it should have a turndown ratio listed to meet that criterion.
These turndown ratios are listed on the nameplate for Constant Torque (CT), Variable Torque (VT) or both. It is important to consider a turndown ratio in variable speed applications for machines with an integral fan on the shaft for cooling purposes. As the shaft turns slower than rated speed, we can expect less cooling air to be circulated within the machine. As the volume of cooling air is reduced, we will see an increase in temperature rise for the electric machine depending on machine loading. This could impact cooling for the stator, rotor and bearings, leading to an increased risk of component failure. For example, if we need to have constant torque and speed control at a speed range of 100 rpm to 1780 rpm, on a 1780 rpm, 4-pole motor, we need to have a turndown ratio of at least 17.8:1 (1780 rpm/100 rpm). In this instance, a turndown ratio of at least 20:1 CT is recommended. This ratio is calculated by using this equation:
See Figure 3 for examples of common turndown ratios for motors.
Duty Types with “S” Ratings
Machines that conform to the IEC 60034-1, 4 have a different duty system. These duty types have “S” ratings. These “S” numbers go from S1 to S10. The most common will be S1. This is also typically the default rating if there is no rating specified when ordering equipment. This means the motor is rated for continuous operation at nameplate load and speed.
The S2 classification is the equivalent of the short-time ratings of NEMA MG-1-1.40.2. If we needed a motor that needs to operate at varying loads and speeds like what we might see on a conveyor, we should consider an S9 duty motor instead. See Figure 4 for all the IEC 60034-1, 4 duty ratings. See Figure 5 to see where the duty rating is located on an IEC nameplate.
Questions to Consider
Now that we have this information, how can we apply it so we purchase the motor with the right duty rating? Ask plenty of questions about the application. Here are a few examples:
- Will this motor be operated continuously? If not continuously, how long will the motor be operating under load?
- Will the motor be de-energized between the periodic operation? If de-energized, how long before the next start?
- Will this motor be started and stopped a lot?
- Will the speed need to be varied in service from the nameplate? If so, by how much?
- Will it be operated by a drive system?
These are all great questions to help you pick the right duty rating for the machine.
The most important thing to remember is this: Any time we operate the motor outside of its nameplated duty parameters, the winding temperature can be higher than the insulation system will allow.
As a result, the winding life may be shortened due to thermal degradation. As a reminder, for every 18°F (10°C) increase of the winding's temperature, we decrease its life by half. Any time we plan to operate a motor at less than its rated speed, we should partner with the motor manufacturer to determine what turndown ratio is required or which “S” rating we should choose (i.e., S8-S10).
AVAILABLE IN SPANISH
Print