Vol. DIY Electronics Projects
Chapter 4 AC Circuit Projects

AC Lab - Large Induction Motor

In this hands-on AC electronics experiment, build a large AC permanent capacitor split-phase induction motor and learn the principles of AC induction motor function.

Project Overview

As shown in Figure 1, this is a larger version of the previous induction motor experiment


Large induction motor system.

Figure 1. Large induction motor system.


This is a permanent capacitor split-phase induction motor that provides an excellent visual example of the design and operation of AC induction motors.


Parts and Materials

  • AC power source: 120 VAC
  • Capacitor, 3.3 µF 120 VAC or 350 VDC, non-polarized
  • #33 AWG magnet wire, 2 pounds
  • Wooden board approx. 6 to 12 in. square.
  • AC line cord with plug
  • 5.1-inch dia. plastic 3-liter soda bottle
  • Discarded ballpoint pen
  • Misc. small wood blocks


Learning Objectives

  • Build a large, exhibit-size, AC permanent split-capacitor induction motor
  • Illustrate the simplicity of the AC induction motor



Below you'll find step-by-step instructions for this project.


Build the Stators

There are two different stator coils for this project, as illustrated in the illustration of Figure 1 and the schematic diagram of Figure 2(a). 


Induction motor schematic diagram (a) and stator winding cutaway (b).

Figure 2. Induction motor schematic diagram (a) and stator winding cutaway (b).


Step 1: First, create a form for building the 1.0-inch wide, 3200-turn L2 winding, as shown in Figure 2(b). Cut out a 1.5-inch wide section from a 5.1-inch diameter plastic 3-liter bottle to provide a 0.25-inch margin on each side of the 1.0-inch primary section. 

Step 2: Wind #33 AWG (American wire gauge) enameled magnet wire over the 1-inch center section of the form, as illustrated in Figure 2(b). The L2 winding of 3200 turns is approximately 744 Ω. To avoid counting the 3200 turns, scramble wind the wire over the one-inch width of the form until you reach a 1/8-inch thickness of magnet wire. 

You can also measure the resistance to determine how close you are to the 744 Ω target but do NOT cut the lead to the spool. Scrape the enamel from 1 inch on the free end, and scrape only a small section from the lead to the spool. Measure the resistance and estimate how much more wire you think you will need to hit the target. Apply enamel, nail polish, tape, or other insulation to the bare spot on the spool lead. Continue winding, and recheck the resistance.  

Step 3: Once the approximate 744 Ω is achieved, leave a few inches of magnet wire for the lead. Cut the lead from the spool. Secure the windings to the form with lacing twine or other means.

Step 4: Make cuts of 0.25 inch into the margins, spaced at 1-inch intervals around the circumference of both ends. Cut into the edge toward the wire.

Step 5: Bend up the cut margin regions at 90o to hold the wire on the form, as shown in Figure 2(b).

Step 6: Strip the enamel off 1-inch of the ends of magnet wire leads (if not already done).

Step 7: Splice the bare ends to heavier gauge insulated hook-up wire. Solder the splice.

Step 8: Insulate with tape or heat-shrink tubing.

Step 9: Secure the splice to the coil body.

Step 10: Repeat steps 1-9 for the second coil, L1. This will require a larger 1.75-inch wide section from the 3-liter bottle to allow 1.25 inches for winding and 0.25 inches on either side for the margin. Wind approximately 3800 turns of #33 AWG enameled magnet wire wound over the center 1.25 section of the form. The L1 winding of 3800 turns is approximately 894 Ω


Assemble the Induction Motor Coils

Step 11: The coils may be mounted in one corner of the wooden base. Alternatively, for more flexibility in use, they may be mounted to movable pallets. Refer to both the schematic diagram (Figure 2) and the illustration (Figure 1) for assembly. Note that the coils are mounted at right angles.

Step 12: As illustrated in Figure 2, inductor L2, the smaller coil, is wired to both sides of the 120 VAC line. The capacitor is wired in series with the wider coil L1. The capacitor provides a leading phase shift of the current with respect to voltage.

Step 13 (Optional): The schematic and illustration show no power switch or fuse. Add these to your system is recommended but not required unless it will be used by non-technicians as an unsupervised exhibit.

Step 14 (Optional): If this device is intended for use by non-technicians as an unsupervised exhibit, all exposed bare terminations, like the capacitor, must be made finger safe by covering them with shields. The switch and fuse mentioned above are necessary. Finally, the enamel on the coils only provides a single layer of insulation. For safety, a second layer, such as an insulating wrapping, Plexiglas box, or other means, is called for. Replace all wooden components with Plexiglas for superior fire safety in an unsupervised exhibit.


Building the Rotor

The rotor must be made of a ferromagnetic material like a steel vegetable can, fruitcake can, etc. Figure 3 illustrates how to make the rotor.


Constructing the rotor.

Figure 3. Constructing the rotor.


It can be a flat lid, as shown in Figure 3, or a vegetable can that has been cut in half, which would look similar to Figure 1. A fairly long can, as shown in Figure 1, balances better than a flat rotor, as illustrated in Figure 3, due to the lower center of gravity.

The rotor may be smaller than the coil forms as in the case of a cut-down vegetable can. It can even be as small as the can lid rotor used with the previous small induction motor. It is also possible to drive a rotor larger than the coils, which is the case with the fruitcake can.

Step 15: Use geometry to locate and mark the center.

Step 16: Create a dimple in the rotor at its center.

Step 16 (a): Select an eighth-inch diameter (a few mm) nail—Figure 3(a)

Step 16 (b): File or grind the point round—Figure 3(b).

Step 16 (c): Place the rotor atop a piece of softwood—Figure 3(c)

Step 16 (d): Hammer the rounded point into the center—Figure 3(d). Practice on a piece of similar scrap metal. Take care not to pierce the rotor.

Step 16 (e): Shaping your rotor as a dished rotor or a lid, as shown in Figure 3(f) and (g), respectively, provides better than the flat rotor of Figure 3(e). The pivot point (e) may be a straight pin driven through a movable wooden pedestal or through the main board. The tip of a ball-point pen also works well for larger rotors. If the rotor does not balance atop the pivot, remove metal from the heavy side.

Step 17: Double-check the wiring. Check that any bare wire has been insulated.

Step 18: The circuit may be powered-up without the rotor. The lamp should light. Both coils will warm within a few minutes. Excessive heating in L2 indicates that more turns are required. Excessive heat in L1 calls for a reduction in the capacitance of C1. No heat at all indicates an open circuit to the affected coil.

Step 19: Mount the pivot to a movable wooden pedestal.

Step 20: Place the rotor atop the pivot and move it between both energized coils. It should spin. The closer it is, the faster it should spin. Both coils should be warm, indicating power. Try different sizes and styles of rotors. Try a small rotor on the opposite side of the coils compared to the illustration.


Try These Options:

  • It is possible to simultaneously spin more than one rotor. For example, in addition to the main rotor inside the right angle formed by the coils, place a second smaller rotor (can or bottle lid) near the pair of coils outside the right angle at the vertex.
  • It is possible to reverse the direction of rotation by reversing one of the coils. If the coils are mounted to movable pallets, rotate one coil 180o. Another method, especially useful with fixed coils, is to wire one of the coils to a DPDT polarity reversing switch. For example, disconnect L2 and wire it to the wipers (center contacts) of the DPDT switch. The top contacts go to the 120 VAC. The top contacts also go to the bottom contacts in an X-crossover pattern.


Author's Notes

Three models of this motor have been built using #33 AWG magnet wire because a large spool was on hand. AWG #32 magnet wire is probably easier to get and should work. Although the current will be higher due to the lower resistance of the larger diameter #32 wire. If a 3.3 µF capacitor is not available, use something close as long as it has an adequate voltage rating. A discarded AC motor run capacitor (bathtub shaped) was used by the author. Do NOT use a motor start capacitor (black cylinder). These are only usable for a few seconds of motor starting and may explode if used longer than that.


Related Content

Learn more about the fundamentals behind this project in the resources below.





Published under the terms and conditions of the Design Science License