Correct Gap Critical to Rotation Speed

Maintaining the correct gap between the sensor and the rotor is critical to correctly measuring turbine rotor speed. Setting the gap can be problematic if the correct gap is unknown or if it cannot be accessed with a feeler gauge.

First a little background on why the sensor gap is critical: The sensor, or “pickup”, is mounted perpendicular to the shaft, facing a toothed gear fixed to the rotor. Pickups can be either “active” or “passive” (see below). In either case, the pickup counts the teeth by sensing the difference in height between the tip of the tooth and the valley between the tooth. If the sensor is too close, it can’t reliably distinguish between the tip and the valley. If it is too far away, it can’t reliably register the tip. The correct gap will register a voltage differential which can be counted. The electronic circuits determine shaft rotation speed by dividing the number of teeth on the gear into how fast the voltage changes over a period of time.

Always use the manufacturer’s specification data to determine the correct gap spacing for the pickup. If the specification is not available, an initial setting of 0.025″ (0.64 mm) will work in most cases. Once the unit is running, the controls engineer can determine if the output voltage or signal level is sufficient for the type of control used.

Sometimes the pickup gap cannot be accessed with a feeler gauge. If so, an accurate setting can be obtained with a little math and a technique called “counting the flats”. The two most often found pickup sizes are the 5/8″ – 18 and the 3/4″ – 20 thread sizes. If you do the math, the 18 threads per inch (TPI) device will move 1 inch in the mounting hole if it is rotated 18 times. Looking at it the other way, it will move in the hole 0.055 inches if it is rotated one time. Breaking it down further, it will move 0.009″ if it is rotated one “flat” of the hexagonal shaped body or hex nut. In the case of the larger 3/4″ inch device, it will move 0.050″ per one rotation and 0.008″ per “flat”.

To “count the flats”, line up the tooth of the gear as close as possible to the center of the mounting hole until it looks like the picture above. Once the gear tooth is aligned with the center of the hole, screw the pickup down BY HAND until the face of the pickup gently contacts the tooth. Set the gap by unscrewing the pickup while counting the flats from a fixed reference point (can even be a line made by a Sharpie pen). For a 0.025″ gap unscrew it by 2 ¾ flats. The math would be 0.025″ (gap) / 0.009″ (movement per flat) = 2.77 flats or approximately 2 ¾ flats. For the larger pickup size that would be 0.025″ / 0.008″ = 3.1 flats or just tad over three full flats. Tighten up the locknut and you’re done!

There are many different types of pickups out in use today but the most common types used on steam and gas turbines are the “passive” type (sometimes called inactive pickups) and the “active” type. They look very similar but operate quite differently. Very simply, the typical magnetic or “passive” pickup is simply a coil of fine wire wrapped around a magnetized iron core that self generates a voltage. When the tooth of a gear passes in front of the iron core, a small voltage is generated and when the valley between the teeth passes the iron core, the voltage falls off. The “active” type pickup receives power from an outside source instead of self generating it. There is a small transmitter and receiver inside of the device that sends out a signal from the end of the pickup. When a gear tooth passes this signal, it changes the characteristic of the signal that is reflected back to the receiver. The internal electronics then interpret this and send out a voltage pulse. Although the passive sensor generates a sine wave and the active sensor generates a square wave, both sensors count cycles over time, which represents teeth rotation speed.

Please contact Mr. Turbine® for answers to any issue with Steam or Combustion Turbine Controls, or Generator and Exciter Controls for any motive power system.