Next time you are in your workshop, count up the number of electric motors you own. Not only motors fitted to large equipment such as lathes, bandsaws, dust collectors etc but hand power tools as well. It will be quite a few.
I often get involved in discussions about failure of motors, using old washing machine motors for home brew disc sanders and modification of motor driven equipment. Usually, these discussions reveal a lack of knowledge about motors or appreciation of the problems that can occur if insufficient thought is given to their use. The following is a brief discussion of the most common topics to be considered when using electric motors.
Including the quality of the windings, the starting capacitor, the centrifugal switch and the cooling arrangements. A cheap motor is more likely to let you down due to failure of one of these components that a quality motor. You get what you pay for with motors and saving a few dollars can cause expense and frustration later.
A TEFC motor is "totally enclosed, fan cooled" which means that the windings and switch are fully encased in an enclosure with the shaft brought out through a sealed bearing. Heat generated internally is transferred to the case and is removed by means of a fan mounted externally blowing air over the case. Sometimes the case will have fins to increase the heat loss but more often than not in the motors we are talking about, the case is smooth. Irrespective, heat transfer can only take place effectively if the air path is open and the surface of the motor is clear of insulating material. In other words, the air inlet holes on the fan cowling must not be blocked and the surface of the motor being cooled must be clear of wood dust.
Use of a non-TEFC or open motor (such as an old washing machine motor) is asking for trouble. Dust can get into the switch and prevent it from operating, dust can pack around the windings and prevent them from being cooled, the bearings are often not capable of excluding dust and will fail or run hot. You might even get a fire.
If the motor is continually run overloaded or regularly stalled, it will get hot and windings will be more likely to fail, especially cheap motor windings. The motor should, ideally, be protected by a thermal overload which monitors the current being drawn, builds a 'thermal model' of the motor which trips when it decides that the motor has had enough. More often than not in a home workshop, either no thermal overload is present in the circuit or the overload device is a built-in thermal switch which is usually inadequate. The only protection for the motor is the common sense of the operator. If an unprotected motor is abused regularly or for long periods, it will fail.
The usual reaction to a hot or overloaded motor is that the motor is underpowered and should be replaced with one of a higher power rating. Unfortunately, this may not be the wisest move. Machinery is usually designed with a particular drive power in mind and increasing that power may overstress other components of the machine leading to more catastrophic (and expensive) failures and possible hazardous situations. Cheap equipment is often designed with no power headroom.
This is where I hear the horror stories. VS controllers are pretty cheap these days and not difficult to install. The single phase motor needs to be replaced with a three phase motor, but these are not expensive either. The problems come when a fixed speed motor is replaced with a variable speed motor of the same power. Remember that the motor is cooled by a fan running on the end of the motor shaft, if you reduce the speed of the shaft, the fan speed is reduced and cooling of the motor is dramatically reduced. Slow down to less than 25% of the normal speed and you will have little, if any, cooling available. A motor supplied from a VS drive controller will run hotter anyway due to reduced efficiency, making matters worse.
If you intend to change to VS drive, the motor power should be increased, but remember the comments about overstressing the machine. The step pulley drive should also be left in place and belts adjusted to give the required speed variation at the spindle with the motor running towards the top end of it's speed range.