Understanding "Orders"

I'm hoping to initiate a series of articles dealing with various topics in the Machine Condition Monitoring space. The intent is to keep most of these pretty short, and as entertaining as possible.

When a sales guy thinks about 'orders' he is usually figuring out how much more he needs to sell to make quota and feed his family. When a Vibe guy mentions 'orders', it is a little different!

The first thing to get firmly centered in your frontal lobe is that most Vibe guys are looking at things that are rotating. Their entire view of the world is related to rotational movement. Most engineers are comfortable in thinking of frequency in terms of Hz or cycles per second. We understand that real-world signals are made up of many different frequency components. We know that we can use an FFT algorithm to 'break down' a complex time domain signal, and separate the individual frequencies from this waveform into the individual harmonic (single frequency) components. Some of us even have a good understanding of Nyquist!

This is great, but to the Vibe guy, the main information he needs is not the actual 'frequency value' in Hz. To the rotational equipment tech, it is an ingrained notion that the primary driving force in a machine is the actual operating speed of that machine. Machines will operate at differing speeds, and therefore chasing Hz all over the place makes their job... well let's just say it would be a lot more difficult!

Consider for example a 'fixed speed' induction motor, the single most common motor in industry; there are more induction motors on the planet than there are people! The name-plate on the motor may read "3600 RPM" or "1800 RPM" but what speed does the motor actually turn at? There is no accurate answer, because an induction motor must ALWAYS run slightly slower than the driving line frequency to function. If the shaft is spun at exactly the same speed as the rotating magnetic field generated in the stator (the outside, fixed windings of the motor) there would be no current 'induced' in the rotating 'rotor' coils, and no magnetic field generated trying to keep the rotor and stator fields aligned. It's really just magnets!

(If you are too young to recognize the little Scotty toys, drop me an email and I'll explain what you missed growing up!)

The 'lag' between the synchronous motor speed (nameplate RPM) and the actual rotational speed of the motor is known as "slip". The greater the slip, the higher the current induced in the rotor windings, and the greater the magnetic force created. This directly translated to torque, the more load applied to the motor, the greater the slip, and the higher the torque potential attempting to keep the rotor's magnetic field aligned with the stator. It's really pretty cool!

But I digress into motor design and theory, which is not what I started out to discuss, back on-task David!

The take-away from all this is that the speed of a motor is ALWAYS going to vary slightly, even on 'fixed speed' systems, depending on the load on the motor. Further, consider all of the variable speed systems, gearboxes, and other components in a mechanical system that will result in a 'change' in rotational speed of individual components... it get's pretty overwhelming in very short... 'order'. (OK, I know it was a bad pun, but I just couldn't stop myself)

What makes the Vibe guys happier, is instead of thinking of 'frequency' in Hz they view spectral data 'normalized' to rotational speed. This is their definition of Orders, further expressed in multiples of Orders, 1X, 2X, .5X, etc. 1X (one times operating speed) remains 1X if the motor is turning at 3600 RPM or 3582 RPM... or at 40,000 RPM. Frequency components that result from various mechanical features of the machine are also easily expressed in orders. A three vane pump will generate a 3X frequency, a 20 tooth gear will produce a 20X frequency component. These components will remain fixed in the order-normalized spectrum, regardless of the operating speed of the machine, greatly simplifying analysis and diagnosis.