1. Electronic system to protect electric motors, wherein the motor (M, M′) is a three-phase electric motor, connected to a compressor of a refrigeration system, wherein the system comprises: a plurality of sensors (ST 1-n , SC 1-n ), being tension sensors (ST 1-n ) located before contactor (K, K′) and current sensors (SC 1-n ) are Hall-effect type sensors located between the contactor (K, K′) and feeding terminals (U, V, W, U′, V′, W′) of the motor (M, M′); a processor ( 2 ) to execute rules to evaluate currents and tensions measured in the motor (M, M′) from readings made by the plurality of sensors (ST 1-n , SC 1-n ); a communication interface ( 3 ) connected to the processor ( 2 ) to present the results of the application of the rules; and an actuation interface ( 4 ), connected to the processor ( 2 ) that acts on the contactor (K, K′); wherein the rules comprise one or more from the group consisting of: a1. detecting the feeding tension (U R , U S , U T ) during the procedure of motor (M, M′) start in each of the feeding phases (R, S, T), and the motor (M, M′) feeding currents in each feeding phase (R, S, T) and presenting the tensions and currents detected by the communication interface ( 3 ); A2. starting the motor (M, M′); A3. detecting the start currents (I R , I S , I T , I R′ , I S′ , I T′ ) and comparing said currents with the blocked rotor current (I max part ); if any start current is higher than the blocked rotor current, turning off the motor; B1. identifying an imbalance between the feeding phases (R, S, T) from the currents (I R , I S , I T , I R′ , I S′ , I T′ ); B2. identifying a current fluctuation (Δi/Δt) from the currents (I R , I S , I T , I R′ , I S′ , I T′ ) in relation to time (T); B3. identifying lack of phase in any of the phases (R, S, T) from the feeding, by determining a phase current (I R , I S , I T , I R′ , I S′ , I T′ ) equal to zero; B4. identifying an over current in any of the feeding phases (R, S, T) by comparing each of the phase currents (I R , I S , I T , I R′ , I S′ , I T′ ) with a limit value (l_opr_max) for the currents; B5. calculating the mechanical torque of the motor (M, M′) from the currents (I R , I S , I T , I R′ , I S′ , I T′ ) and the tensions (U R , U S , U T ) and presenting the value calculated for the torque; B6. identifying a harmonic distortion in feeding, from the currents (I R , I S , I T , I R′ , I S′ , I T′ ) and the tensions (U R , U S , U T ); B7. identifying a power factor out of operation standards by calculating cos φ and the power factor (FP) from the currents (I R , I S , I T , I R′ , I S′ , I T′ ) and the tensions (U R , U S , U T ); and B8. identifying possible damage to the motor from the determination of the escape current (l_earth_measured) in the line L Earth , if, in steps B, any at calculated and/or measured parameter is out of a corresponding range of operation and/or higher or lower than a given limit, the motor (M, M′) is turned off; C1. identifying a motor (M, M′) turnoff command; C2. turning off the motor relay/interface ( 4 ); C3. identifying an undue actuation of the motor by determining a phase current (I R , I S , I T , I R′ , I S′ , I T′ ) different from zero; if any of the phase currents (I R , I S , I T , I R′ , I S′ , I T′ ) is different from zero, to set off an alarm and register said failure.