Most of the larger modern machines are electrically driven and are equipped with the Ward-Leonard system of control, which allows alternating current of fairly high voltage to be carried to the shovel over a very flexible electric cable. This cable is usually carried on a sled back of the shovel or on a reel on the shovel base. The current drives an alternating current motor, which is connected to, and drives, direct current generators, one for each of the operations of the shovel, and an exciter. Each direct current generator and the direct current motor which it drives are in a closed circuit. The field in each circuit is regulated by magnetic contactors or by rotating controls actuated by master controllers at the operator's position.