Join Date: Mar 2008
Location: Fernley, Nevada (near Reno)
how to build a single phase induction motor
What type of motor are you planning to build? Or do you just want to modify an existing one to be speed controlled?
Building an electric motor from scratch is bordering on impossible, unless you have access to a foundary, a machine shop, and sufficent knowledge of coil design to balance iron vs. copper vs. load vs. speed, etc.
If you want to control the speed of an existing motor, or buy a new one, there are basically 6 types of single phase motors that I'm familiar with;
1) Shaded pole. Usually small, (less than 1/4HP), commonly found in bathroom fans and the like. Very easy to speed control, in fact, most of these can be 'lugged down' even way down, and still not burn up. Starting torque is very low, running speed varies widely depending on load. Extremely inefficient, usually less than 30%.
2) Permanent split capacitor. Usually from 1/8HP to 1HP, mostly found on fans. Furnaces and outdoor A/C unit fans are common applications. Easy to speed control, can be 'lugged down' to some degree, very efficient. This design is very much like a 3 phase motor that's designed to run on single phase power. The capacitor functions as a sort of built-in phase converter.
3) Split-phase. Usually from 1/6HP to 3/4HP. This type has two windings, start and run, and a switch of some sort (frequently a centrifugal switch built into the frame), and thus, cannot be speed controlled. Some are two speed, with two run windings. Very little difference between no-load and full-load speed. Pretty good starting torque, very high starting current. Medium efficiency. Washers, dryers, and dishwashers usually have this type. Starting current is about 10 times the running current, that's why the lights dim when one of these motors starts.
4) Capacitor start. Usually from 1/4HP to 10HP. Very similar to the split phase, but with a capacitor in series with the start winding. This increases starting torque while reducing starting current. It also increases cost and complexity. Starting current is about 6 times the running current. Larger sizes (over 1HP) frequently have 2 capacitors, one for starting, the other for running. This type is called capacitor start-capacitor run. Efficiency is a bit higher with this type. A/C compressors, power tools, well pumps, etc. are usually one of these types. These cannot be speed controlled, but some are two speed, as with the split phase type.
5) Repulsion start-induction run. Usually 1/2HP to 3HP. Not many of these around, Baldor is the only manufacturer I know of that makes them. These have brushes that are used for starting, the above types have no brushes at all. Extremely high starting torque, moderate starting current. Due to the centrifugal switch that disconnects the brushes (like the start windings of split phase and capacitor start), they cannot be speed controlled. These do well on hard starting equipment that is at the end of a long, small wire.
6) Universal. This is probably the most common type of all. They have brushes, no start/run winding, high starting torque, moderate starting current, very high speed. The vast majority of them are less than 1HP, the largest I've worked with was 600HP though. . They're found in kitchen appliances, vacuums, power tools, etc. The starter, heater fan, etc in your car are this type. They're the only ones that will work on AC or DC power. Speed depends entirely on voltage and load. Easy to speed control using either armature feedback or a shaft mounted feedback device, and a speed controller.
Then there are 3 phase motors. The smallest one I've connected was 1/8HP, the largest was 15,000HP. Very simple design, no start winding, high start torque, high efficiency, easily speed controlled. Starting torque is about 6 times the running torque. Quite a few different starting systems exist for these, usually to reduce starting current. If asked, I'll describe them, but the post will be about as long as this one.
If you need precise speed regulation at varying loads, a 3 phase motor connected to a variable frequency drive that will accept single phase input is by far the best way to go. These are available from about 1/6HP up to about 3HP. The smaller ones will accept 120 volts, the larger ones are 240 volt. Simple, reliable, and not all that expensive.