Obtaining a DC supply controlled by rectifiers requires an AC power source. Consequently, the existing auxiliary generator was converted to a three-phase alternator with automatic voltage regulator. This arrangement gives improved control of traction current over conventional rheostatic systems and gives fine control when starting a train. Normally the driver has to maintain tractive effort well below the rail adhesion limit to give himself time to respond to wheel slip. The constant current systems of Class 74 were designed to allow the driver to apply power very near the limit of adhesion and the booster output is regulated in such a way that the maximum current of any traction motor group does not exceed the selected value. Thus, even when one motor (axle, wheel set) starts slipping, the voltage across the whole group of motors cannot rise because current in the non-slipping motors is fixed. As one motor begins to slip, its current consumption drops (electric motors use greatest current when stalled and least while free-running) which provides more current for the non-slipping motor (The current for the group is fixed and therefore shared across the motors in the group). The voltage drops proportionately and the slipping motor now partly deprived of power, begins to slow, the slip stops and everything returns to the state it was in before slip set in. Simply speaking the motors all work as a differentially balanced team so that as one motor begins to slip, the others act to balance everything which reduces the speed of the slipping motor almost 'by magic', but maintains torque at a point just below friction break-away. Recovery of wheel slip (providing the driver is not in-attentive or deliberately attempting to slip) is very rapid. It is this system that allowed the incredible acceleration for which Class 74 was known.