Use of Magnetic Pickup with Windrock Analyzers

APPLICATION NOTES

When using the magnetic pickup for TDC (top dead center) triggering in reciprocating machine analysis, it is extremely important that the pickup triggers data acquisition exactly at TDC. Errors of only 2 degrees can produce measurement errors of up to 5% in horsepower and throughput calculations.

For the magnetic pickup to operate properly with Windrock analyzers, several issues must be addressed.

Pickup Distance from Target:

Often, the magnetic pickup is placed too close to the target. When this happens, the pickup can trigger data acquisition on scratches, nicks, or magnetic deformities in the flywheel or shaft. Usually, a gap of about 1/4 inch sufficient.

Target Type:

Some sort of target must be placed on the flywheel or shaft to trigger the magnetic pickup. The target must be of a ferrous material. It may be a hole drilled into the flywheel or shaft, or may be a protruding object such as a bolt head. Either way, the target must produce the largest magnetic interference of any other scratch, hole, or deformity in the flywheel or shaft. The center of the hold or bolt head must be placed at TDC of the machine. A hole is preferred because it is always round, and the center of the hole is the magnetic center of the target. A good choice for hole size is 1/4" diameter, 1/4" deep. Sometimes when using a bolt head, the flats of the bolt causes the magnetic center to be other than the geometric center of the bolt.

Sometimes, we have seen a sharp edge used as a target. This is OK, but due to the small magnetic interference, the gap must be smaller in order to get a sufficient signal.

Signal Polarity:

This is where many problems occur when using a magnetic pickup. The circuit in the Windrock analyzers arms itself on a POSITIVE going signal. The actual trigger occurs upon ZERO-CROSSING after the circuit is armed. Zero-crossing occurs at the magnetic center of the magnetic pickup or when the magnetic pickup is centered on the target.

When using a hole as a target, pin A of the magnetic pickup is the signal.

When using a protruding object as a target, pin B of the magnetic pickup is the signal.

If in doubt, use the analyzer in o-scope mode, or an oscilloscope to view the signal. The signal should look like the figure below. If the polarity is reversed from this, swap the signal lead and ground leads in the connector.

Size of Signal:

The magnetic pickup contains a permanent magnet with a coil. When the magnetic field changes due to the pickup going over a hole or object, a voltage is generated in the coil. The size of the signal is proportional to the magnetic field change. Several factors determine the field change : permanent magnet strength, number of turns in the coil, gap from pickup to target, material of target, and size of target.

The bottom line is that you want a signal of about 1V peak, and it must be at least 10 times the size of any other noise or spurious signals in the output of the pickup. If the polarity of the signal is reversed, the trigger point will be much later than actual TDC.