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 (S
1
) driving a variable-speed d-c generator (DC
1
) which in turn drives a d-c — a-c set (DC
2
and S
2
) 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.