What is the RPM of a reel to reel recorder?
Audio tape speeds are measured in ips (inches per second) or in cm/s (centimeters per second), not RPM (i.e. there is no typical/common RPM).
The actual RPM of the reels changes with the amount of tape. The speed of the tape is not controlled by the reel motors them selves, but rather the capstan motor, which then ensures a constant speed (and typically the reels simply follow).
On the capstan a roller of varying diameter from model to model is mounted - typically around 1 - 2 inches, which means somewhere between 4-5 inches of tape is transported per round (1.5 * 3.14159 = 4.7). If the speed is e.g. 30 inches per second, then that gives 30 / 4.7 * 60 ~= 383 RPM for the capstan. The reels are harder because many different reel types exist, with varying starting inner diameter.
Some common speeds are
- 120 ips (304.8 cm/s): Loop bin duplication, analog instrumentation
- 60 ips (152.4 cm/s): Loop bin duplication, analog instrumentation
- 45 ips (114.3 cm/s): Digital audio recording
- 30 ips (76.2 cm/s): The highest standard professional speed.
- 15 ips (38.10 cm/s): Most common speed multi-track pro studio recording and otherwise professional speed for reel-to-reel including multitrack recorders.
- 7-1/2 ips (19.05 cm/s): Most common pre-recorded playback speed, highest domestic speed
- 3-3/4 ips (9.53 cm/s): Second most common pre-recorded playback speed
- 1-7/8 ips (4.76 cm/s): Lowest common reel-to-reel speed
7.5, 15, 30 are all common. Perhaps there are 3 3/4 ones also. There is a wide range of reel to reel type recorders. If your asking about a specific one you would need to provide some more info.
As the tape speed in front of the heads has to be constant (in ips as mentioned), there's no constant RPM for the reels as it is continuously changing as the reel empties or fills.