Building Services Engineering l January/February 2022
Understanding the key elements is vital
Decoding a motor’s nameplate Whether you are looking to replace or repair a motor, it is important to give your supplier or service provider accurate details of the motor you currently have. The most relevant information should be recorded on the motor's nameplate. To help users understand the information on their motor nameplates, Karl Metcalfe (below), Technical Support at the Association of Electrical and Mechanical Trades, explains what key elements mean.
To specify a replacement motor or understand the requirements of a repair or rewind, several attributes need to be established. These include information such as the size and format of a motor, its power and speed ratings, how and where it has been designed to be used, its efficiency and a range of other factors. Most motors will have a nameplate that carries this information in a format that meets standards set out by one of two bodies, the International Electrotechnical Commission (IEC) and the National Electric Manufacturers Association (NEMA). Once you understand how the information is presented, it should be easy to read the nameplate. See Figure 1 and the related detailed explanations below. 1 – Frame size This indicates some of the motor's key dimensions and will usually be at the top of the nameplate as it is an important metric. Frame sizes conform to a standard that defines a motor's dimensions and outputs. The frame size is the height of the shaft's centre from the base of the motor's foot. A 315 frame, as in this example, will have a dimension from foot to shaft of 315mm. The number will be followed by a letter, S is for a short foot, M is for a medium foot, and L is for a large foot. This letter defines the dimension between the mounting holes on the front and back feet. 2 – Power The motor's power may be written in kilowatts (kW) or horsepower (hp). It will be written in kW on most motors, but older motors may be rated in hp.
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3 – Voltages Where there is more than one voltage figure listed, the delta (6) connection is the low-voltage connection, while the star ([) configuration is for a highvoltage connection. The wiring diagram for both delta and star configurations will often also be added to the nameplate. 4 – Speed/RPM This is the maximum speed of the motor in revolutions per minute without a load present, and this information can also inform how many poles the motor has. A motor requires a minimum of two poles per phase and a 2-pole motor rotates fully for each polarity change. Therefore, at 50Hz, the theoretical maximum speed a 3-phase motor can run at is 3000rpm. So, a motor labelled as having a speed of around 3,000rpm will be a 2-pole motor. The reality is slightly slower in induction motors due to losses from factors such as drag and windage – referred to as slip. As rpm halves, then the number of poles will have to double. Therefore, a motor rated at around 1500rpm (1490rpm in our example) will be a 4-pole motor. It follows then that a rating around 1000rpm will indicate a 6-pole motor, while a motor labelled near to 750rpm will have 8-poles, and a 10-pole motor will run close to 600rpm. 5 – Efficiency Efficiency is a significant factor where motors are concerned and is typically indicated by an IE number. While a lower efficiency motor can be repaired and reused, sometimes to an improved efficiency level, the Ecodesign Directive governs what IE rating a new motor must have based on the application and the wider system. IE1 is known as standard efficiency, IE2 is high efficiency, IE3 is premium efficiency and IE4 is super premium. There is a significant difference in efficiency between IE1 and IE4. At a power rating of around 4kW, an IE1 motor is around 80% efficient, while an IE4 motor is around 90% efficient. That represents halving the losses,
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