Find the right electric motor in five simple steps

By Marnix Veraart - In Products - 27 Jan 2020

Find the right electric motor in five simple steps

Which electric motor do I need? Find the right electric motor in 5 steps.

You need to replace your electric motor and want to know which electric motor you need. It is not always easy to find the correct replacement motor. You might need to replace an older model that is obsolete or discontinued. To help you, we will explain in 5 steps how to choose the right electric motor in the ABF webshop.

In this blog, we assume a 3-phase motor, as this is the most commonly used type of electric motor. In many cases, all the information can be obtained from the nameplate on the motor.

Below, you will find an example of a nameplate for an older generation of a Siemens electric motor. You can use this information to check for yourself whether a similar product is available in our webshop.

Old-style Siemens nameplate.

Which electric motor do I need?

STAP 1: Determine nominal power

The nominal power of an electric motor is normally expressed in kilowatts (kW). On older motors, the power may also be expressed in horsepower (hp). The conversion is: 1HP = 0,75kW.

The power of an electric motor is based on the maximum power for a constant load. In this example, the electric motor has a nominal power of 1.5kW (or 2HP).

Select nominal power.

When you select 1.5 kW and the brand Siemens in our electric motors webshop, you will be shown all of the Siemens motors with a nominal power of 1.5kW (view in webshop). However, you will still need to select the motor’s other specifications. We will explain how to do this in the following steps.

STEP 2: Determine the design of the electric motor

An electric motor can be mounted in several ways, such as with a foot only, an inner flange, an outer flange or a combination of these. The design (or mounting method) is defined by a code from the European IEC 34-7 Standard.

The five most commonly used mounting methods are:

The design can often be read from the nameplate. However, it can also be determined using the drawings below.

Mounting options B3, B5, B14, B34 and B35.

The electric motor in our example has mounting ‘’B14’’; this is an inner flange. This design can be identified by the threaded holes in the face of the motor. For a “B5” design, the diameter of the flange exceeds the diameter of the motor, and the boltholes are unthreaded.

Select mounting.

TIP: To increase the chance of finding the right motor, if you need a motor with mounting “B14” select both “B14” and “B34” (inner flange with foot-mounting), as most motors supplied by ABF are fitted with removable feet. By detaching the feet of a B34 motor, you’ll end up with its B14 equivalent (view in webshop).

B34 motor, frontal view. Combination of inner frame with threaded holes in the face (B14) and foot mounted (B3) motor.
Example of electric motor with removable feet.

STEP 3: Determine nominal speed

The nominal speed is usually stated on the nameplate. This is the actual rotational speed of the output shaft, and is expressed in “revolutions per minute”, or “rpm”. The rpm on the nameplate below is 2860 revolutions per minute. An alternative way of expressing this speed is by listing the number of poles; a 2-pole motor has an “rpm” equivalent to 3000. For 4 poles this is 1500 rpm, 1000 rpm for 6 poles, and so on.

Due to technical reasons the actual speed of the shaft always deviates slightly and is lower than the figures given above. So, make sure you always round up correctly. The common rule of thumb is: Speed of the motor = 6000 divided by the number of poles. So, a 4-pole motor has 6000 / 4 = 1500 rpm.

Below you will find an overview of the theoretical revolutions per minute and the corresponding number of poles.

Relation between no. of poles and theoretical revolutions.

In the webshop, we select the nearest speed (rounded up). For the example used here we have a 2-pole motor with a nominal speed of 3000rpm. This 3000 rpm is the theoretical speed.

So, select the correct speed in the webshop by rounding up the required speed. In our example, a 2-pole motor with 3000 revolutions per minute (view in webshop).

Select the required rpm.

STEP 4: Determine the frame size

The frame size of the electric motor is another important characteristic. As the dimensions for the European market have been laid down in the IEC standard, electric motors of different brands, but with matching characteristics, are often interchangeable. Most manufacturers use the same dimensions for a particular size.

The most important dimensions according to the IEC standard are:

For electric motors adhering to IEC Standards, most dimensions are directly related to the Shaft Height, or Frame Size of the motor. This is the distance between the centre of the output shaft and the centre and bottom of the feet of a B3-style motor, as indicated in the diagram below by the letter “H”.

Example dimensions of B3-style electric motor.

In our example, the electric motor has a frame size of 90. In the webshop, select ‘90’ as the IEC frame size (view in webshop).

Selecting IEC frame size.

STEP 5: Determine the correct efficiency class

From 16 June 2011, most motors on the market must be marked in accordance with the IE directive and classified with an efficiency class.

The efficiency classes for electric motors are:

Since 1 January 2017, all new electric motors with a power rating of 0.75kW or above must meet the IE3 standard (however, there are exceptions to this rule).

The electric motor in our example has an energy efficiency of IE1. In order to meet current regulations, select IE3 in our webshop (view in webshop).

Selecting energy efficiency.

The right electric motor?

In our webshop, we have now selected the following filters:

You should now have found the correct electric motor. The selection of motors shown should meet the specifications of the electric motor you are trying to replace (view in webshop).

When comparing the nameplates of the old, and the replacement motor, it is clear that the specifications match. Please note, this particular motor is of mounting “B34” but is fitted with detachable feet and inner flange (an option Siemens provides).

Comparison of old and new Siemens nameplate.

Siemens nameplate

The following image shows an explanation of a Siemens nameplate. Please note that this image refers to a “standard” Siemens motor. Non-standard Siemens motors, or motors with certain extra options are denoted with the addition of “-Z” at the end of the Article Number.

In the examples used above, the “-Z” stands for the option of detachable feet on a B34 motor.

Explanation of Siemens nameplate.

Hopefully, by following these 5 easy steps, you will find the right electric motor in our webshop. However, if you have further questions, please do not hesitate to contact our electric motors specialist for more advice on the matter. We are here, for you!

To electric motor webshop E-mail our electric motor specialist
Marnix Veraart Product Content Specialist +31 (0)165 788 208

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