# Practical Work of Three-Phased Induction Motor Modeling in a Classical Spreadsheet Software
## 2 Three-Phased Induction Motor
### Starting of an Induction Motor  
The inrush current at the start of an induction motor can reach several times the rated current of the machine. When the application uses a drive or a starter, this device takes care of adapting the voltages applied to the machine in order to limit this current. The starting current is often an input variable of these devices. In the absence of a variable speed drive, several methods for limiting the starting current exist, which were developed before the advent of power electronics. They are still in use today in old installations, or as a cost-saving measure for applications that do not require a variable speed drive.  
Reduced voltage starting:  
The simplest way to limit the magnitude of the starting current is to reduce the voltage at the terminals of the stator windings during startup. The main disadvantage is that the motor torque is also reduced and this increases the time before the machine reaches steady state.  
Star-delta starting:  
During a star-delta start, the machine is first connected to the network with a star coupling, then, once started, it switches to delta coupling. Starting with a star coupling makes it possible to divide the applied voltage by \(\sqrt{3}\). Thus, the maximum absorbed  
Figure 3: Nameplate of a three-phased induction motor.  
current is divided by 3 compared to a direct start with a delta connection. The starting torque is also three times lower than with a delta start, but the over-current during the star-delta transition is lower than the inrush current of a start performed directly with a delta connection. When performed simply using contactors, this starting method is very economical, and thus very popular.  
Starting by auto-transformer:  
With this starting mode, the stator of the induction machine is directly connected to an auto-transformer, which allows for starting under variable voltage. The voltage is gradually increased, the and intensity of the current does not exceed the desired maximum value. This can be achieved by switching the windings of the auto-transformer. The drawback of this method is the need for an electrical specialist to manage both the auto-transformer and the induction machine.  
Resistive start:  
During a resistive start, resistors are inserted in series with the stator windings, which has the effect of limiting the voltage at their terminals. Once started, these resistors are short-circuited. This operation can be carried out gradually by an operator using starting rheostats.  
Starting at rated voltage: Rotor starting  
During a rotor starting, power resistors are inserted in series with the rotor windings, just as with the previously described resistive start, but with the rotor windings. This type of starting makes it possible to obtain a high starting torque with reduced stator starting currents, but it can only be implemented with wound rotor machines equipped with sliding contacts (rings and brushes) allowing the electrical connections of the rotor windings. These machines are more expensive than their \"squirrel cage\" counterparts.