- Joined
- Feb 13, 2017
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- 2,138
RE motors:
An AC induction motor runs aligned to the power frequency. In the US, that is 60 CPS(cycles per second), known today as Hertz. Speed can be determined by the formula [(Poles X Speed) divided by 120] Using basic algebra, shuffle the variables around as needed. An induction motor runs slightly below the theoretical speed, usually ~150 RPM, called "slip". A synchronous motor runs at the theoretical speed. In the US, at 60 CPS, that will be 3600 (less any slip) for a 2 pole motor. Many (most) machine tools use 4 pole motors at 1800 RPM (less slip) and higher torque. The above applies to all induction motors, be they single, two, three, or six phase motors.
The end result is that 3600 RPM is as fast as an induction motor can run. To increase above that (synchronous) speed, one would need to increase the frequency of the supplied power. In England and some European countries, power line frequency is 50 CPS. This yields 3000 RPM for a 2 pole motor and 1500 for a 4 pole. There are positives and negatives to both. In the States, it was largely based on the economics of an installed system. To go further in that direction is a whole 'nuther subject.
Some time back, the US Navy experimented with 400 CPS on ships. That did increase speed significantly but had some "undesirable" side effects. It was not extensively pursued. The 400 CPS is also widely used in airplanes, primarily for weight and space savings. There are also VFDs (variable frequency drives) that can produce an output higher than 60 CPS. Motors are magnetic in nature, most 60 cycle motors can be run up to 120 CPS, maybe. Beyond that, there are significant losses of power.
"Series-Universal" motors, brushed and brushless, can run on AC or DC. Hence the term "universal". Newer "brushless" motors are based on magnetic principals that I am not familiar with. They likely work on DC, with a rectifier between the motor and the line. They can attain speeds well in excess of 10000 RPM. But they are newer technology. The few washing machines I have worked with over the years had induction motors. Newer machines may well have series-universal motors.
The bottom line here is that if your motor has a" nameplate" speed in excess of 5000 RPM, it is NOT an induction motor. There are probably brushes, somewhere. A "series-universal" is basically a DC motor that is magnetically adjusted to run on both types of power. There will be some minor sparking at the brushes. The final speed is determined by the type and number of windings of the armature. With 3 normal styles of windings and a wide number of them, speeds can vary widely. They are widely used on portable tools, with some sort of gear reduction as needed.
A lamp dimmer can be used as a speed controller for small series-universal motors. Such as my Dremel. They are handy for other uses, such as soldering irons. For larger motors, a router speed controller works well. Electrically it is identical to a lamp dimmer, with larger (higher current) components. In any case, such a speed controller should NEVER be used with an induction motor. A comprehensive book on motor theory would contain hundreds of pages, on motors alone. My explanation is merely an overview, at best.
RE grinders:
I have seen grinders with both right hand and left hand threads to prevent loosening under load. In most cases, a grinding tool has a rotation suited to the application. As an example, when one is grinding a shaft with a lathe, my bench grinder is not suitable. While the grinder should throw dust and sparks down toward the shears, it is turning the same rotation as the lathe, like gears meshing. I personally prefer to grind "up", with a good shield above. A personal choice there, but indicative of the application.
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