Identify the correct electric motor

When you need to replace your electric motor, it is often necessary to find out which electric motor you need as a replacement. We know that it is not always easy to find the right replacement motor. Often it is an older model that is no longer manufactured or available.

In this blog, we consider as a starting point a 3-phase motor, as this is the most commonly used electric motor. In many cases, all information can be read from the nameplate on the motor.

"Find the right electric motor in 5 steps"

Five steps to choosing the right electric motor

  • Determining the nominal power and nominal voltage
  • Determining the speed (rpm)
  • Determination of the design
  • determination of the efficiency
  • Determining the size

Identify your need of a new electric motor in only 5 steps and optimize your equipment at the same time.

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Identify the correct electric motor

Which electric motor do I need?

STEP 1: Determination of rated power and rated voltage

The Rated power of an electric motor is expressed in kilowatts (kW). For older motors, the power can also be specified in horsepower (HP).
The Rule of thumb for conversion is: 1 HP = 0.75 kW.

The Performance of an electric motor is based on the maximum performance.
In our example, the electric motor has a rated power of 1.5 kW (or 2 hp).

Read the rated voltage according to the type plate (for example 230VD/400VY or 400VD/690VY).

If you select 1.5 kW and the brand Fenner in our ERIKS webshop, you will see all available Fenner ® motors with a rated power of 1.5 kW. Please be sure to select the other specifications of the motor.

In the following steps, we explain what you need to pay attention to.

Identify the correct electric motor

STEP 2: Determine the speed

The speed is usually indicated on the nameplate. This is the actual speed of the drive shaft, expressed as "revolutions per minute". This is the Rotation speed of the drive shaft and is often expressed in "revolutions per minute" resp. "rpm" expressed. Another way of representation is the notation of the number of "poles": a 2-pole motor has 3000 rpm. For a motor with 4 poles it is 1500 rpm, for a motor with 6 poles it is 1000 rpm, etc.

For technical reasons, the actual speed of the shaft always deviates slightly and is lower than the above figures. So always make sure to round up correctly. A good rule of thumb is: motor speed = 6000 divided by the number of poles. So a 4-pole motor runs with 6000/4 = 1500 rpm.

STEP 3: Design of the electric motor

An electric motor can be mounted in several ways. For example, this is only possible with a foot, a flange or a combination thereof. The design (or the mounting type) is determined by a Code of the European standard IEC 34-7 specified

There are five basic mounting methods for electric motors:

  • B3: Mounting by means of a foot
  • B5: Mounting by means of a flange (diameter of the flange is larger than the motor housing)
  • B14: Mounting by means of a flange (diameter of the flange is smaller than the motor housing)
  • B35: Mounting by means of a foot + flange (combination of B3 and B5)
  • B34: Mounting by means of a foot + flange (combination of B3 and B14)

The electric motor in the example has the design B14. This motor can be recognized by the fact that there is a thread in the screw holes of the flange. These are blind holes. A B5 flange is through holes without threads.

STEP 4: Selecting the right efficiency class

From the 16. June 2011 almost all motors on the market must be labeled and classified in an efficiency class according to the IE directive.

The efficiency classes for electric motors are:

  • IE1 = Standard efficiency (formerly EFF2)
  • IE2 = High efficiency (formerly EFF1)
  • IE3 = Premium efficiency
  • IE4 = super-premium efficiency

Since the 01. January 2017 all new electric motors with a power of 0.75 kW and more must comply with the IE3 standard. However, there are a few exceptions to this directive.

STEP 5: Determination of the efficiency class

The size of the electric motor is another important characteristic. As the dimensions for the European market are defined in the IEC standard Electric motors of different brands often interchangeable. Most manufacturers use the same dimensions for a given size.

The main dimensions according to the IEC standard are:

  • A (bore distance, front view)
  • B (Bore spacing, side view)
  • C (shaft shoulder to the first mounting hole)
  • D (diameter of the drive shaft at the shaft end D.E.)
  • E (wavelength)
  • H (axis height)

Identify the correct electric motor

For a standardized electric motor according to IEC standard, these dimensions refer to this. For example, when we talk about the size of a motor, we always mean the H dimension. This is the shaft height of a foot-mounted electric motor (B3).
For electric motors that comply with the IEC standard, most dimensions depend on the size specified or. Axis height of the motor from. This is the distance from the center of the drive shaft to the center base of a B3 motor. Denoted by the letter "H" in the following drawing.

Please also consider special designs such as increased protection classes, operating modes, special windings, roller bearings and other modifications.

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