In this case, we are going to look at the behaviour of the single-phase induction motor of a fan coil for air conditioning a hotel room, and what happens when the motor is connected to a voltage and electrical frequency different from that of its manufacture.
As we can see on the motor’s nameplate, it has been manufactured to work at a voltage of 115v and 60Hz. The reality is very different since this motor has been connected to a voltage of 127v and 50Hz.
Relationship between Voltage and Frequency
The relationship between voltage and frequency is a constant, U / F = Cte
Induction motors (IM) can operate with supply voltage values higher than the nominal one (which is the one shown on the rating plate, (115v) of up to approximately 10%. But so as not to produce higher heating to those allowed by the materials with which the motor has been built (magnetic plate), (Fe iron), conductors (Cu copper), casing, shaft, bearings, etc.)
From here on, the values corresponding to those on the nameplate will be indicated with a subscript (1)
E1 ≅ U1 = 115V, f1 = 60Hz
That means that we get:
E1 = K x Φ1 x f1 ; E1 = K x Φ1 x f1
If it is divided it remains.
In other words, the magnetic flux (Φ2 ) in the actual operating conditions of the motor is approximately 30% higher than the magnetic flux ( Φ1 ) corresponding to the operating conditions for which the motor has been designed; indicated on its nameplate.
The consequences of this 30% increase in magnetic flux are as follows:
- On the one hand, excessive heating of the magnetic plates of the motor, both in the moving part (rotor) and in the fixed part (stator).
- At the same time, as the magnetic flux is generated by a part of the current absorbed by the motor (called magnetising current, (Iµ). Increasing the flux of 30% implies increasing the magnetising current (Iµ) by a value much higher than 30% as there is no proportionality between the increase in flux with that of the current, due to the saturation of certain parts of the motor’s magnetic circuit (see Figure).
- The grease in the bearings is deteriorated due to overheating, which means that it no longer performs its lubricating function correctly, affecting the proper functioning of the bearings, which can also be damaged. The figure clearly shows the deteriorated appearance of the fat.
- In the figure below, you can see parts of the stator windings darker in color than the rest of the winding. This darker color is due to the current greater than the nominal one that has passed through the conductors.