Introductory
Laboratory Category
Low Cost Category
2002 AAPT
Apparatus Competition, Boise State
University
Name: Judith L. Doyle (AAPT member) and Jerry Pepera
Address: Judith L. Doyle
14 Sunset Hill
Granville
Ohio
43023
Phone: 740-587-0019
Fax:
E-mail: doylej@denison.edu
Apparatus Title: A Different Kind of DC Motor
Abstract (40-50 words)
This DC motor can be used as an alternative assessment
when studying how DC motors work. The
students are given the motor and asked to come up with some explanation as to
why the armature continues to turn when there is no commutator to change the
direction of the current in the armature every half turn.
Judith L. Doyle, Ph.D Jerry Pepera
14 Sunset Hill, Senior Project Engineer
Granville, OH 43023 Rockwell Automation, Inc.
Doylej@denison.edu Mayfield Heights, OH 44124
I came home from the 2001 PTRA workshops in Rochester,N.Y., and was planning on doing a workshop on DC circuits for teachers. I began talking to my friend Jerry Pepera, a senior project engineer at Rockwell Automation, Inc., about making a motor with the teachers and trying to incorporate some different electronic components. We spent some time discussing it, and with my description of what I wanted to do and his ability in the area of electronics we came up with a DC motor that does not have a commutator. In this motor the transistor is used as a simple switch to turn on the field coil when the magnet is in the proximity of the reed switch at “just the right moment”. The transistor allows a small control current in the reed switch to control a much larger current in the field coil.
A teacher could use the motor in a couple of different ways. The students could build the motor and get first hand experience with DC circuits and adjusting components so the motor will run. Then have the students write an explanation in their own words of how the motor works and what each component does. (The motor is very inexpensive to build.)
The teacher could make enough motors so each lab group has one. Assuming the students have learned about DC motors using the traditional St. Louis motor, this one should present them with a challenge to explain why it works. Use this as an alternative assessment. Even if they do not know the names of the electronic components, they should be able to figure out what the components need to be doing so that the motor runs.
DIRECTIONS FOR MAKING A DIFFERENT KIND OF DC MOTOR
1. Glue magnets to wood armature with the same pole facing outwards. Motor will not spin if opposite poles face outward. Superglue works well for this and I have had better luck with the gel type. It might be necessary to smear a film of superglue on the wood armature and let it dry before proceeding.
2. Mount armature using screws and brass strips. Hold piece of wood containing field nail in place on base and check for adequate clearance. Armature should spin freely and just miss the nail by about 1/8”. Note: After you have wound the field coil, it won’t be possible to adjust the nail clearance.
Adjust the nail depth, if necessary, with a hammer and the provided piece of scrap wood.
3. Wind the field coil as shown leaving 6 inch tails on the finished coil. Carefully uncoil the magnet wire as you go being careful not to kink it. Try to fill the nail with as much wire as possible and dap a bit of superglue every few rows to keep it from unraveling as you wind. You will probably have several feet of magnet wire left over. It doesn’t matter if the coil is wound CW or CCW because you will polarity “phase” the field coil later with a 9V battery.
4. Mount field to base as shown.
5. Unfold paper clips and shape as shown. This is done in order to provide a convenient place to solder connection wires later.
6. Solder alligator clips to paper clips as shown.
7. Attach paper clips to base as shown with screws. Also, using a magic marker, label the polarity next to each screw. Note that this polarity is arbitrary until the coil is phased in the next step.
8. Using fine sandpaper or an Xacto knife, sand or scrape the enamel off the ends of the field wires.
9. Coil must be phased to determine which wire is positive and negative. Briefly connect the 9V battery across the field coil and observe armature behavior. If armature repels, polarity is correct. If armature attracts, polarity is backwards. Reverse polarity and try again. Solder the positive polarity end of magnet wire to the positive terminal on the wood block as shown.
10. Temporarily mount commutation circuit board with a rubber band and complete wiring as shown in diagram.
11. Apply 9V battery to finished unit (observe battery polarity). Motor should spin but, may not perform well. Move circuit board up and down to find commutation “sweet spot”. This is equivalent to advancing or retarding the timing (firing point) on the motor. Mark circuit board location with a pencil and remove rubber band. Super glue circuit board in place.
12. Add a drop of oil to bearings. Performance will improve noticeably.
Notes:
The transistor is used in this application as a simple switch to turn on the field coil when the magnet is in the proximity of the reed switch at “just the right moment”. The transistor allows a small control current in the reed switch (about 8 mA) to control a much larger current in the field coil (perhaps an amp or so).
It is possible to build this motor without the transistor using only the reed switch wired directly in series with the field coil. In this case, care must be taken that the field current does not exceed the contact rating of the reed switch. The reed switch can be replaced by a Hall Effect switch in this circuit and the motor should spin faster due to the much more precise switching action of the circuit.
Detailed Circuit Operation
During idle, the reed switch is in the open posistion and no current flows through the field coil. If one of the armature magnets is brought in proximity to the reed switch, it will close and a small control current I1 will flow through the reed switch. Some of this current will split and flow through R1 but the majority (I3) will flow through the B-E junction of the transistor. The base-emitter junction is said to be forward biased with a forward voltage voltage drop of approximately 0.6 VDC. This will enable a much larger current flow (I4) from the collector to the emitter and the transistor will enter saturation. The voltage drop between collector and emitter (Vce) will be near 0 V. As long as the voltage drop between the collector and emitter stays low, power dissipation in the transistor will also stay low and it will behave efficiently as a switch.
In practice, the amount of collector current that the transistor can sink and still stay in saturation is related to the dc gain of the transistor (Hfe). The gain in a transistor is usually very high (300-400) until it enters saturation and then drops dramatically to about 10 or so.
In our circuit, what is the largest value of collector current that we can support and still guarantee saturation ?
First, we must calculate the base current:
I1 = I2 + I3 (Ref: Kirchoff’s current law – the sum of all currents entering and leaving a node must equal zero.)
(9V –0.6V)/1000 ohms = 0.6V/1000 ohms + I3
8.4 mA = 0.6 mA + I3
I3 = 7.8 mA
DC Gain = I4/I3
10 = I4/7.8 mA
I4 = 78 Ma
If we wanted to support a collector current of 1 Amp, what value of resistor would we
Need for R2 ?
DC GAIN = I4/I3
10 = 1 Amp/ I3
I3 = 100 mA
I1 = I2 + I3
I1 = ( 0.6V / 1000 ohms ) + 100 mA = 100.6 mA
R2 = (9V – 0.6V) / 100.6 mA = 79 ohms
What is the tradeoff with this large of a base current ?
Ans. The power dissipation in R2 can become quite high.
Pd (r2) = I^2 * R2 = (100.6 mA)^2 * 79 ohms = 0.89 Watts !
A rule of thumb in circuit design is that the resistor should be sized for twice the expected power dissipation. The 79 ohm resistor would need to be rated at 2 W and would be larger in size. If the circuit were designed to not quite enter saturation, power dissipation in the transistor would rise and dissipation in the base resistor R2 would drop.
The transistor in the provided circuit board probably doesn’t quite reach saturation but, all I had on hand were 1/8 watt resistors. Assuming a collector current (I4) of 1 Amp, what is the actual DC gain of our circuit ?
In our circuit, it’s hard to calculate the true value of I4 since it’s largely based on the field coil characteristics and the counter EMF of the motor. Every time the motor magnets pass by the field coil, they induce a current in the field coil which opposes the direction of current flow in the coil. This is known as “Counter EMF” and it will limit the final value of collector current. This also implies that if you lock the rotor by grabbing the shaft, the current will rise dramatically in the transistor and will get noticeably hotter !
Other useful things to do in the lab:
- Using an oscilloscope, measure the collector and base waveforms. During the on time of the transistor, what is the voltage between collector and emitter ? How close to saturation is the transistor ?
- Based on the frequency of the waveform, how fast is our motor spinning ? Do you think it would be possible to design a circuit that serves as a tachometer ?
3. Notice the bounce in the waveform edges. Why do you think the reed switch causes this to happen ? Replace the reed switch with a hall effect sensor and repeat the experiment. Are the waveform edges cleaner and more precise ? Did the motor speed up when you did that ? Why?
Equipment required to construct apparatus:
Parts List
------------
All Electronics
-----------------
QTY Description Catalog # Price each
1 Reed Switch 1 AMP @ 50 VDC RSW-14 $0.50
1 Bipolar Hall Effect Switch UGN-3030T $0.60
1 NPN Bipolar Power Transistor TIP3055 $0.65
2 1k ohm 1/4W resistor 1K-1/4 $0.05
Radio Shack
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15 ft 28 gauge magnet wire #278-1345 $3.99 (See note 1 below-this much
wire will make several motors.)
2 Ceramic disc magnet #64-1883 $1.59 ( Pkg. of 5. See note 2 below)
1 perforated PC Board #276-1396 $3.69 (Dimensions are 6x8 inches –
enough to do a bunch)
2 alligator clip #27-1540 $2.49 pkg of 8
1 9 volt battery $2.69
Total Cost for one motor approximately $6.74
Misc.
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sheet brass Any hobby shop
scrap pine Any Home center such as Home Depot/ Lowes etc...
Notes:
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1. Magnet wire can be another gauge but, 28 gauge is easy enough to work with and a reasonable number of turns can be wrapped on the nail. Radio Shack doesn't carry 28
gauge but, they have a 3 pack of magnet wire spools (22/26/30 gauge) which would work ok. (#278-1345/ $3.99)
2. Be sure ceramic magnets are polarized on the ends. The Radio Shack magnets listed will work. I actually ran into some round ceramic magnets at a craft store that were polarized on the radius and the motors would not spin. It wasn't until I tried attracting a Radio Shack magnet with a craft store magnet did I realize what had happened !