We know that the rated rotational speed of an industrial motor is based on two fixed factors: the number of poles and the supply frequency. So if you install a 4-pole, 480V motor in a 480V, 60Hz system, you already know how fast that motor is going to rotate. Right?
In theory, yes. But the real world has a bad habit of throwing curve balls at us.
Many factors may affect the rotational speed of a motor system at the final output shaft (e.g., gearbox shaft). Using a tachometer to test a motor’s speed when disconnected from its load and again when connected to its load can make the difference between accurately calling the job done and leaving a motor failure time bomb behind you.
Never assume the motor will rotate at its rated speed or the motor system will produce the intended rotational speed to the final load. Always use a tachometer to verify that it does. It’s important to remember that this caution applies for the whole motor system, including electronic and mechanical drives, not just the motor.
Just in case you don’t know how poles and speed relate in a 60 Hz system, a 4-pole motor runs at 1800 RPM (minus slip). An 8-pole motor runs half that fast, and a 2-pole motor runs twice that fast.
Let’s examine a few things that could go wrong, and you’ll see why tachometer testing is essential to properly completing a motor installation and troubleshooting some types of motor problems.
- Shaft and/or bearing problems. Although these may permit a motor to turn at the rated speed while simply drawing more current (or shaking violently), they could be severe enough to impede rotation.
- Single phasing. This problem really means one of the three phases is missing. The cause is often a blown fuse in one phase of the supply. The other two phases must work harder to turn the motor. They might be able to turn it, but draw excess current in doing so. Using a tachometer can help you see what’s going on, especially during starting. Under these conditions, the motor should take much longer to get up to rated speed.
- Mechanical drive is wrong. The gearbox driven by the motor’s output shaft is really part of the motor system. You can use a tachometer to see if the gearbox is turning at the intended speed. If not, maybe it needs an oil change (this is a common problem, as these boxes too often get treated as if they don’t need maintenance). Or maybe the box has the wrong gear ratio. You can also uncouple the motor and see if the gearbox is getting the right RPM input.
- Electronic drive is wrong. Suppose you replace a variable frequency drive (VFD). You set it up per the settings used for the one it replaced. But how can you take As Left data to verify it’s turning that motor at the correct speed at each operational point? You already know the answer to that question: Use a tachometer.
- Load is mismatched. Consider a scrap grinder system. If the motor cannot develop enough torque for the conditions imposed (e.g., unregulated feed rate or irregular size per items fed), you’ll see big swings in motor speed. In what situations might you find a mismatch in your plant?
- Investigation is needed. When checking a motor that has a presumably steady load (or no load at all), the tachometer reading should be steady within a very narrow range. If it varies by much, something is wrong. The tachometer won’t tell you what is wrong, but at least you will know to look. It could be that load is not steady as presumed.
These are just a few ways in which a tachometer can be helpful. In many cases, the tachometer won’t give you a definitive answer by itself, but will provide a puzzle piece that lets you more efficiently solve a problem.
When installing or changing any part of a motor system, always take As Found rotational measurements (if the motor can safely operate) and As Left rotational measurements. Only the tachometer can provide these data.
