Page "Regenerative brake" Paragraph 23

The main disadvantage of regenerative brakes when compared with dynamic brakes is the need to closely match the generated current with the supply characteristics and increased maintenance cost of the lines.

With DC supplies, this requires that the voltage be closely controlled.

Only with the development of power electronics has this been possible with AC supplies, where the supply frequency must also be matched ( this mainly applies to locomotives where an AC supply is rectified for DC motors ).