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Stepper Motors

The stepper motor is a device used to convert electrical pulses into discrete mechanical rotational movements.

Most common are 2-phase permanent magnet (PM) motors which provide discrete angular movement every time the polarity of a winding is changed.

CONSTRUCTION
In a typical motor, electrical power is applied to two coils. Two stator cups formed around each of these coils, with pole pairs mechanically displaced by 1/2 a pole pitch, become alternately energized North and South magnetic poles. Between the two stator-coil pairs, the displacement is 1/4 of a pole pitch. The permanent magnet rotor is magnetized with the same number of pole pairs as contained by the stator-coil section.

Interaction between the rotor and stator (opposite poles attracting and likes repelling) causes the rotor to move 1/4 of a pole pitch per winding polarity change. A 2-phase motor with 12 pole pairs per stator-coil section would thus move 48 steps per revolution or 7.5° per step.

STEP ANGLE
Step angles for steppers are available in a range from .72° to 90°.
Standard step angles are:

3.6º — 100 steps per revolution.

7.5° — 48 steps per revolution.

15° — 24 steps per revolution.

18° — 20 steps per revolution.

A movement of any multiple of these angles is possible. For example, six steps of a 15° stepper motor would give a movement of 90°.

ACCURACY
A 7.5° stepper motor, either under a load or no load condition, will have a step-to-step accuracy of 6.6% or 0.5º. This error is noncumulative so that even after making a full revolution, the position of the rotor shaft will be 360º ± 0.5º.

The step error is noncumulative. It averages out to zero within a 4-step sequence which corresponds to 360 electrical degrees. A particular step characteristic of the 4-step is to sequence repeatedly using the same coil, magnetic polarity and flux path. Thus, the most accurate movement would be to step in multiples of four, since electrical and magnetic imbalances are eliminated. Increased accuracy also results from movements which are multiples of two steps.

Keeping this in mind, positioning applications should use 2 or 4 steps (or multiples thereof) for each desired measured increment, wherever possible.

BIPOLAR AND UNIPOLAR OPERATION
Stepper motors are available with either 2-coil Bipolar, or 4-coil Unipolar windings. The stator flux with a Bipolar winding is reversed by reversing the current in the winding. It requires a push-pull Bipolar drive.

Care must be taken to design the circuit so that the drive transistors, which are in series, do not short the power supply by coming on at the same time. Properly operated, the Bipolar winding gives the optimum motor performance at low-to-medium step rates.

A Unipolar winding has two coils wound on the same bobbin (one bobbin resides in each stator half) per stator half. Flux is reversed in each coil bobbin assembly by sequentially grounding ends of each half of the coil winding. The use of a Unipolar winding, some times called a bifilar winding, allows the drive circuit to be simplified. Not only are half as many power switches required (4 vs. 8), but the timing is not as critical to prevent a current short through two transistors as is possible with a Bipolar drive.

For a Unipolar motor to have the same number of turns per winding as a Bipolar motor, the wire diameter must be decreased and the resistance increased. As a result, Unipolar motors have 30% less torque at low step rates. However, at higher rates the torque outputs are equivalent.

Information Courtesy: Thomson Industries, Inc.

 

 

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Decorative Sun Project

This is a decorative Sun made to hang on a door or wall. The center wood face is made from 0.5 inch Baltic Birch plywood and is 10 inches in diameter.

The center face part has several coats of white primer and the finish coat was several coats of Rust-Oleum bright copper.

The outer flame part is made from 0.050 inch white styrene plastic that has a faceted surface. This material was designed primarily for fluorescent light fixtures.

A flame pattern was created on cardboard and was traced onto a stack of four pieces of the styrene material that was then taped together and cut out at the same time. The flames were solvent welded together using Methyl Ethyl Ketone (MEK).

The outer flame part was also finished with several coats of Rust-Oleum metallic bright copper.

 

 

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Variable Transformer

A variable AC transformer (Variac) is a very handy device and should be part of any electrical experimenters lab.

The Variac allows adjusting the line voltage anywhere from 0 to 130 VAC. The voltage output is usually provided through a single or dual 120 VAC outlet located on the Variac housing.

A Variac is available in many different VA output ratings.

Some uses for a Variac:

1.) Adjusting the speed of small AC power tools

2.) Creating a variable DC power supply by adjusting the primary voltage of a power transformer with the secondary output connected to a bridge rectifier and capacitor filter.

3.) Controlling the temperature of a heater or the speed of a fan motor.

4.) Re-forming power supply capacitors in old equipment by allowing the input voltage to the device to be increased slowly.

 

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Hot Wire Foam Cutter

Important Caution

The design of this foam cutter requires the use of an AC power transformer with a separate 120 VAC primary and a 24 VAC secondary winding in order to eliminate a potential shock hazard from the aluminum supports.

Construction Components

All measurements are approximate and non critical.

1.) The center support of the foam cutter is made from a scrap piece of 3in x 28in x 1in length of poplar wood.

2.) The side supports are made from a 18in x 1in x 0.125in length of aluminum angle.

3.) The rope used to tension the nichrome wire is a 60in length of nylon clothes line. A wood chopstick is used to apply tension to the cord. Note: The tension cord must be made of non-conductive material.

4.) The cutting wire is made from a 28in length of 27 awg nichrome wire.

5.) The black and white wire connections from the aluminum supports to the terminal block can be 16 or 14 awg TFFN stranded wire.

6.) Miscellaneous components include 16 awg #6 or #8 ring terminals, a two position terminal block, two small S hooks and some #6 and #8 machine screws and nuts.

7.) Power cable between transformer and foam cutter can be either separate 14 awg wires or a 14 awg two or three conductor cable.

8.) Power supply components consist of a 24 VAC transformer (Stancor P-8618) rated at 4 amps and a 0-130 VAC Variac transformer rated at 5A.

Note: Since most Variacs are NOT line isolated, a separate 24 VAC transformer is required to power the foam cutter.

Construction Notes

The aluminum support bars are spaced a nominal 24in apart and are attached to the poplar board with #8 machine screws.

Note: The aluminum bars also serve as electrical conductors to the nichrome wire.

The nichrome wire is looped several times through a nickel plated ring terminal and tightly crimped to the wire and attached to the aluminum bar with #6 machine screws and nuts.

Each aluminum bar is connected to the terminal block with a 16 awg wire using a 16 awg ring or spade terminal on each end.

The connection between the 24 VAC transformer and foam cutter is made with a length of 14 awg three conductor SJOOW power cable.

Finally, I added a two inch section of aluminum channel to the poplar wood support to allow me to clamp the foam cutter to my table saw in order to make long straight cuts using the table saw fence.

Using the table saw fence I have been able to cut strips of two inch styrofoam one tenth of an inch thick (0.1in).

Foam Cutter Voltage and Current Measurements
VARIAC OUTPUTXFRMR OUTPUTWIRE CURRENT
36 VAC7 VAC1.0 A
54 VAC11 VAC1.5 A
73 VAC15 VAC2.0 A
90 VAC18 VAC2.5 A
108 VAC22 VAC3.0 A

 

Construction Photographs

COMPLETED FOAM CUTTER.

 

TENSION CORD WITH S HOOK CONNECTION.

 

TENSION CORD WITH WOOD CHOPSTICK.

 

NICHROME WIRE CONNECTION TO ALUMINUM BAR.

 

VARIAC AND 24VAC POWER TRANSFORMER.

 

AIRFOIL SHAPE CUT FROM TWO INCH STYROFOAM.

 

SUPPORT CLEAT ON POPLAR SUPPORT.

 

REAR VIEW OF FOAM CUTTER CLAMPED TO TABLE SAW.

 

FRONT VIEW OF FOAM CUTTER CLAMPED TO TABLE SAW.

 

0.25 INCH AND 0.50 INCH FOAM STRIPS CUT WITH FOAM CUTTER.

 

Reference:

Variac and Cable Source

 

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Blade MCX2 RC Helicopter

Excellent beginners indoor helicopter. The coaxial main rotor design makes flying easier as it eliminates having to control a separate tail rotor while controlling the main rotor.

Ready to fly kit comes with preassembled helicopter, 4 channel 2.4 Ghz DSM2 transmitter, LiPo battery, LiPo charger, and 8 AA batteries for transmitter and charger. Recommended accessory: EFLC1005 6VDC (AC Input) power supply for charger. Eliminates having to use (4) AA batteries in the charger.

Comes with an excellent printed manual (also available online) with complete exploded view and replacement parts list.

Blade Helicopters

 

 

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