Drives & Controls May 2022

37 www.drivesncontrols.com May 2022 MOTORS n How to decode your motors’ nameplates I f you are looking to replace or repair a motor, it is important to have accurate details of your current motor. The most important information is recorded on the motor’s nameplate. To specify a replacement motor or to 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 was designed to be used, its efficiency and 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 is easy to read the nameplate on most motors. The diagram on page 39 shows a typical motor nameplate that complies with the IEC standard. Here is what different sections represent: 1: Frame size This indicates some of the motor's key dimensions and will usually be at the top of the nameplate because 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 motor’s shaft centre from the base of its foot. A 315 frame, shown in the example, will measure 315mm from foot to shaft. The number is followed by a letter – S for a short foot, M for a medium foot, and L for a large foot. This letter defines the distance between the mounting holes on the motor’s front and back feet. 2: Power The motor's power may be expressed in kilowatts (kW) or horsepower (hp). On most modern motors it will be in kW, but on older motors it may be in hp. 3: Voltages Where more than one voltage figure is listed, the delta ( ) connection is the low-voltage connection, while the star ( Ｙ ) configuration is for high-voltage connections. The wiring diagram for both delta and star configurations is often added to the nameplate. 4: Speed/RPM This is the maximum speed of the motor in revolutions per minute without a load, and can also tell you how many poles the motor has. A motor needs at least two poles per phase and a two-pole motor rotates fully for each polarity change. Therefore, at 50Hz, the theoretical maximum speed a three-phase motor can run at is 3,000 rpm. So, a motor labelled as having a speed around 3,000 rpm will be a two-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, the number of poles will have to double. Therefore, a motor rated at around 1 500 rpm (1,490 rpm in our example) will be a four- pole motor. It follows then that a rating of around 1,000 rpm will indicate a six-pole motor, while a motor labelled near to 750 rpm will have eight poles, and a ten-pole motor will run close to 600 rpm. A great deal of useful information is contained on the nameplates attached to most motors. Karl Metcalfe, who provides technical support at the Association of Electrical and Mechanical Trades, explains what the various numbers, letters and symbols mean.