A study of the modified Kramer or asynchronous — Synchronous cascade variable-speed drive

LARGE wind tunnels require a wide range of speed and accurate speed control and if their use factor is high, a high efficiency over the working range is desirable. Also minimum disturbance to the power system is important in many cases. To meet these requirements a speed-control system has been adopted which, while not new in principle, involved the solution of a number of interesting problems. The system as shown in Figure 1 consists of a wound-rotor driving motor (A) whose secondary winding feeds a synchronous motor (S1) driving a variable-speed d-c generator (DC1) which in turn drives a d-c — a-c set (DC2 and S2) to return most of the secondary power back to the line. The name “modified Kramer set” is suggested by the authors since the scheme involves conversion of the secondary power to d-c and field control for the speed changes as in the well-known Kramer set. The term “modified” was used because the Kramer set used a rotary converter and the d-c power was usually fed into a d-c motor on the same shaft as the main motor. Another descriptive name would be asynchronous-synchronous cascade.