Induction motors are relatively simple compared to direct current motors, but there are certain characteristics that must be observed in an induction motor, such as the synchronous speed of a three-phase induction motor or slip in induction motors. We will answer these questions, in addition to teaching you step by step how to calculate the synchronous speed and slip of an induction motor, in addition to talking about slip and other characteristics of a three-phase induction motor, come on guys!
Undoubtedly , three-phase induction motors are the most used within industries due to their several advantages, such as, for example, having a simpler construction, low maintenance cost, power and control in relation to direct current motors and single-phase induction motors , but there are still many details to be known in an induction motor, such as features, applications and operation, such as the synchronous speed of the motor.
Induction motor: Synchronous speed.
Three-phase induction motors have a constant speed, because of the rotating magnetic field, so the speed of the rotating magnetic field is called the synchronous speed, so that the synchronous speed of the motor varies depending on the number of magnetic poles and the frequency of the electrical network in which the engine is installed
We can conclude that the synchronous speed of the induction motor is inversely proportional to its number of magnetic poles and directly proportional to the frequency, that is, the greater the number of poles in a motor, the lower its speed, and the higher the frequency on a bigger engine your speed will be.
It is only possible to change the synchronous speed of the three-phase induction motor by varying its number of poles, which will be at fixed speeds, or by changing the frequency, however it is important to note that the number of poles in a motor is a physical characteristic of the motor, which it is possible to change it during operation, the only exception being the Dahlander engine, that is, it is only possible to vary the speed of an engine by varying its frequency, such as using a frequency inverter.
We can see and understand better why the motor speed variation is a function of the number of poles and frequency according to the formula shown in the image below:
Synchronous speed: example 01
To make it clearer, we will find the synchronous speed in rpm in a three-phase induction motor, like four poles, connected to a 220 volt power supply and 60Hz frequency. To find the synchronous speed of the motor, just replace the values we have in the formula, as shown in the resolution in the image below:
Synchronous speed: example 02
Now we show in the image below the change in the synchronous speed of a six-pole motor, connected to a 220 volt power supply and 60Hz frequency. To do this, just replace the values, see how it is:
Induction motor slip:
We have to understand that the speed of the rotating field, or synchronous speed of the motor, does not always have the same speed in the rotor, as there is an exception for synchronous motors. The speed in the rotor can be called the nominal speed, and in the motors that have slip, this nominal speed will always be less than its synchronous speed of the induction motor.
The speed in the rotor is always less than the speed of the magnetic field due to several factors, among them we can mention the small friction that is generated between the bearings and the motor shaft and mainly the loads connected to the motor and among other losses.
The phenomenon that we define as slip on an engine, is nothing more than the loss of speed that we have in the rotor of an engine, that is, it is a difference between the speed of the magnetic rotating field of the engine and the speed measured on the engine shaft, the value we get from engine slip is provided in percent by most manufacturers, but can be calculated.
The slip concept consists of the difference between the speed on the motor shaft (the rotor) and the speed of the magnetic rotating field. It is important to mention that the motor is connected to a vacuum, that is, it does not have a load connected to the motor. In this condition, the motor rotation is practically the same as the synchronous speed, so we can say that the slip decreases according to how much the power increases.
The image below shows the formula used to calculate the slip on an induction motor:
We will calculate the slip for a three-phase induction motor fed with a voltage of 380V, 60Hz, synchronous speed of 1800 RPM, 4 poles and as a measured speed in the rotor of 1748 rpm. As in the examples we did earlier, it is only necessary to identify the data that will be needed to use in the formula and replace the values, as shown in the image below:
If you still have any questions regarding the concepts and how to calculate the slip on engines, the video below shows how to calculate and all the details about the slip.
We finish one more article here and we hope to have helped to remove all your doubts about how to calculate engine slip! If you still have any doubts or curiosity about this subject, leave it in the comments and we will respond.