The efficiency value and IE code on an electric motor's nameplate are two critical pieces of information that tell how efficient the motor really is. However, reading these values correctly and comparing them with real field performance is an often-overlooked subject. In this article we cover how to read the percent efficiency value and the IE code on a motor label, the difference between nominal efficiency and efficiency at 75% and 50% load, and how to verify label efficiency with a field measurement (power analyzer). Our aim is to help you understand the difference between the value written on the label and the real performance in the field, and to make the correct purchasing decision.

Where Are the Efficiency and IE Code on the Nameplate?

On a motor label, alongside information such as power (kW), voltage, current, speed, frequency and power factor (cosφ), the efficiency (η, eta) and the efficiency class (IE code) are also given. Efficiency is usually stated as a percentage and the IE code as IE2, IE3, IE4 or IE5. We covered verifying all label fields before ordering in our nameplate and label reading article.

Specifically for IE3 motors, we detailed the kW, speed, cosφ and efficiency fields on the label in our reading the IE3 motor nameplate article. The label is the motor's identity; read correctly, it clarifies both the order and the performance expectation in the field.

What Does the IE Code Mean?

The IE (International Efficiency) code indicates the motor's international efficiency class. IE2 means high efficiency, IE3 premium, IE4 super premium and IE5 ultra premium. As the code increases, the motor's efficiency rises and its losses decrease. We compiled which power requires which IE class from which date in our IE3 and IE4 efficiency mandate article.

The IE code is also an indicator of regulatory compliance. Because a minimum IE class is mandatory at certain powers and speeds, the IE code on the label also determines whether the motor can be sold in the relevant market. That is why verifying the IE code in purchasing is important not only for energy but also for compliance. We covered the difference between IE2 and IE3 motors and when old-stock IE2 makes sense in our IE3 vs IE2 motor difference article. We discussed the question of staying with IE3 or moving to IE4 in our stay with IE3 or move to IE4 article.

How Is the Efficiency Value (η) Written?

On the label, efficiency is usually written as the full-load (nominal) value, as a percentage. On some motors the efficiency values at 75% and 50% load are also stated on the label. This multi-point efficiency information is very valuable for selection because it shows how the motor behaves at different load points.

Alongside the efficiency value, cosφ (power factor) is also an important label field and should be evaluated together with efficiency. At low load both efficiency and cosφ drop; their dropping together means that the total current and reactive power drawn from the grid increase. Therefore, when reading the label, you should look not only at the efficiency percentage but also at the cosφ value and the rated current. We covered the effect of rated current on cable, fuse and contactor selection in our rated current and cable-fuse selection article.

The Difference Between Nominal Efficiency and Part-Load Efficiency

The nominal efficiency on the label is the motor's efficiency at full load. However, in the field the motor often runs not at full load but in the 50-75% band. Therefore looking only at the nominal efficiency can be misleading; the efficiency at the load point where the motor will operate is more decisive.

Efficiency at 75% and 50% Load

In induction motors, efficiency is typically highest around 75% load; it drops slightly at full load and falls markedly below 50%. We detailed this behaviour in our efficiency-load curve article. Therefore a motor labelled with 95% nominal efficiency may perform well below that efficiency when run at 30% load.

Reading the efficiency value and IE code on a motor nameplate

The Effect of Oversizing on the Label-vs-Real Gap

When a motor is oversized, the high nominal efficiency on the label is not realised in the field, because the motor runs at a low load factor, in the lower region of the efficiency curve. We detailed how oversizing eats savings in our part-load efficiency curve and oversizing article. For this reason, the label efficiency is only close to reality when the motor runs at the correct load point.

Verifying Label Efficiency with a Field Measurement

The efficiency on the label is a value measured under standard test conditions. To see the real performance in the field, a measurement with a power analyzer is required. This verification is important for energy management, especially in high-running-hour and high-power motors.

Measurement with a Power Analyzer

A power analyzer measures the active power, current, voltage and power factor that the motor draws from the grid. By comparing the motor's shaft output power (mechanical power) with the electrical power it draws, the real efficiency can be estimated. This measurement also shows at which load point the motor actually runs, so that how much of the nominal label efficiency is realised in the field can be evaluated.

One point to note in field measurement is that directly measuring the mechanical output power is difficult. Measuring shaft torque and speed requires special equipment, so in practice the load factor is often estimated by using the electrical power drawn together with the motor's label efficiency. Even so, this approach gives a sufficient picture to see whether the motor is oversized and what its real operating point is. Making the measurement after the motor has reached thermal equilibrium, that is, after it has run for a while, makes the results more reliable.

When interpreting the measured values, the motor's duty type must also be considered. On a motor running continuously (S1) the measurement gives a stable picture; on intermittent or frequent start-stop applications, instantaneous measurements can be misleading. We covered the effect of duty type on selection and evaluation in our duty type (S1-S6) selection article.

How Is Efficiency Measured and Documented?

We covered how motor efficiency is measured under laboratory conditions and how label efficiency is documented in our efficiency measurement and IEC 60034-2-1 article. Although a field measurement is not as precise as a laboratory test, it is a practical and valuable tool for seeing the motor's behaviour at its real operating point. We examined CE and the declaration of conformity (DoC) with MEPS certification in our CE and declaration of conformity article.

Verifying nameplate efficiency with a power analyzer in the field

Reasons for the Label-vs-Real Gap

A difference between the label efficiency and the real field efficiency does not always mean a quality problem; it most often stems from operating conditions. Understanding the main reasons for this gap enables correct interpretation.

Load Factor and Operating Point

The most common reason is the motor running at part load instead of the full load on the label. A motor with 95% nominal efficiency shows lower efficiency at 40% load. This is not a defect but the natural behaviour of the efficiency curve. Therefore, in a field measurement, the motor's real load factor should be determined first, and then the efficiency interpreted.

Voltage, Frequency and Grid Conditions

Deviations in grid voltage, voltage unbalance and frequency difference affect the motor's efficiency. We covered voltage tolerance and grid fluctuation in our voltage tolerance and grid fluctuation article. We examined the effect of the 50/60 Hz difference on speed, power and efficiency in our rated voltage and 50/60 Hz difference article. For this reason, grid conditions should also be recorded when a field measurement is made.

Interpreting the Label Correctly in the Purchasing Decision

When buying a motor, the IE code and efficiency value on the label, interpreted correctly, are a strong indicator of quality and compliance. However, looking only at the high nominal efficiency is not enough; the motor's operating load point and the duty type of the application must also be evaluated.

Correct power selection is the prerequisite for the label efficiency to be realised in the field. Choosing the motor according to the real load demand both minimises the label-vs-real gap and secures the energy savings. We compiled the common mistakes made when buying an electric motor in our mistakes made when buying article; misinterpreting the label is one of the foremost of these mistakes. We covered label checking in the incoming and acceptance inspection of a stock motor in our incoming and acceptance inspection article.

Frequently Asked Questions

Is the efficiency value on the label realised exactly in the field?

Usually not, because the nominal efficiency on the label is the full-load value, while the motor often runs at part load in the field. When the motor is sized correctly and runs in the peak region of the efficiency curve (typically around 75%), it approaches the label value. At a low load factor, efficiency drops markedly. To see the real value, a field measurement with a power analyzer is recommended.

What is the relationship between the IE code and the percent efficiency?

The IE code indicates the efficiency class to which the motor belongs; each IE class defines minimum efficiency values for specific powers and speeds. So the IE code is a class and the percent efficiency is the concrete value within that class. At the same power, an IE4 motor carries a higher percent efficiency than an IE3 motor. The two appear together on the label.

Can I verify efficiency without a power analyzer?

A full efficiency measurement requires a power analyzer; however, as a simple check you can measure the current the motor draws with a clamp meter and compare it with the rated current on the label to get an idea of the load factor. This does not give the real efficiency but helps you understand whether the motor is oversized. For a precise evaluation, a measurement with a power analyzer is recommended.

Get a Quote

If you want to interpret the labels of your existing motors correctly, select a new motor with the correct IE class and power, or make an efficiency assessment, the HEM Motor expert team is at your side. Share your motor label data, application and load profile; let us determine the right solution for your needs together. To get a quote, reach us at +90 (532) 345 49 86 or write to us via our contact page. You can explore our product family on our IE4 electric motors and high-efficiency motors pages, and the full range on our homepage.

Checklist

  • Were the IE code (IE2/IE3/IE4/IE5) and the percent efficiency value on the label read?
  • Were the nominal efficiency and the 75%/50% load efficiency values distinguished?
  • Was the load factor at which the motor runs in the field determined?
  • Was the label efficiency compared with a field measurement using a power analyzer?
  • Were the grid voltage, frequency and unbalance conditions recorded?
  • Was the label-vs-real gap due to oversizing evaluated?