Back EMF
06/05/21: Page Origin

An electric motor can also be a generator, a coil of wire rotating in a magnetic field.   As the motor is turned by an AC wave applied to it, it also generates a voltage in opposition (reverse polarity) to the applied AC wave.   This is called the Back EMF.
Now when you apply a voltage to an inductor (like the winding in a motor or a transformer) a magnetic field springs up.   If you disconnect the applied voltage from the inductor while the applied voltage is high, the magnetic field collapses, which generates a self-induced or flyback voltage of reverse polarity to the original applied voltage.   If this inductor is a motor, it's rotation also generates a back emf which adds to the inductor's flyback voltage.  

Back EMF    (As I remember, EMF stands for Electro Motive Force)
This back EMF (the voltage generated by a rotating motor that counters the voltage being applied), is what keeps the motor from appearing as a very low impedance (like a short).   When you switch off the AC voltage that is being applied to and rotating inductor (like the motor armature windings) the generated back EMF, no longer having anything to absorb it, springs back in nearly equal voltage that was being applied.   If the AC voltage was high when the disconnect happened, the reverse induced-voltage adds to the back emf being generated by the rotating inductor.   Remember, a 60 cycle 120 volt AC voltage cycles from zero, to +169, to zero, then to -169, and back to zero again.
120 Volt RMS, 60 Hz sine wave
If you suddenly turn off the voltage (opening a switch), while it is at one of these peaks, nearly double the voltage (300 + volts) will appear on the winding connections of the motor.   A mechanical switch (relay) will tend to arc over as the high EMF voltage is reflected back while the contacts are close together (in an attempt to break the circuit).   If you turn off the motor while the voltage is near zero, very little back EMF will be seen.   This is why a mechanical relay will appear to work for a few times before the contacts are welded or vaporized.

SSRs, when de-energised, wait for the voltage to be near zero before actually breaking the circuit, hence the back EMF is very small.