Journal of the

Home Metal Shop Club of Houston

Volume 6, Number 9 - September 2001 President - Dennis Cranston

Vice President - Tom Moore

Treasurer - John Hoff

Secretay - Ed Gladkowski

Web Master - Dick Kostelnicek

Editors - Keith Mitchell, Jan Rowland, David Whittaker

Membership Inforation

Membership is open to all those interested in machining metal and tinkering with machines. The purpose of the club is to provide a forum for the exchanging of ideas and information This includes, to a large degree, education in the art of machine tools and practices. There is a severe shortage of written information that a beginning hobbyist can use. This makes an organization such as this even more important.

President's Notes

Real estate agents say the three most important items are location, location, location. This seems to affect our organization as well. As long as we need to meet on Saturday afternoon and depend on community resources, the meeting location will continue to be problem. Maybe we need to dream a bit. What about renting a comer of a warehouse? (Air-conditioned of course) With the down turn in the economy, there should be some unused space available that the owner would lease to us for some token amount. Dues would have to go up, but maybe we could put some tools in the space that would make paying higher dues worthwhile. (Such as a tap disintegrator). Perhaps hold basic training classes in machining at night, for a fee of course. This current 'tempest in a teapot' might be a wake up call for for us to think about the possibilities in the future of the club.

This is also a good time to remind everyone that dues for the 2001-2002 year are due this month. Dues are $1 per month ($12 for the year) for existing members and $8 initiation fee for new members. Please see John Hoff to pay. Dennis Cranston

Regular Meeting

1:00 p.m. at the Collier Library, Houston, Texas, President Dennis Cranston presiding. There were 30 attendees, including 7 visitors.
A vote was taken on the choice of the second or the third Saturday of each month for club meetings. The vote went as follows:

The second Saturday carried as the new meeting day. Next month's meeting will be on Saturday, September 8.

Dick Kostelnicek asked the members present if they wanted to receive their newsletters electronically. 13 members agreed and 2 members preferred mailed copies. Dick will have a sigh-up sheet for electronic newsletters at the next meeting.

Joe Scott

Presentation

Joe Scott showed a video on the history of Springfield Armory and gave an interesting talk on chambering firearms and adjusting for proper head space. He also explained the legalities of gunsmithing.

Show and Tell

Dick Kostelnicek showed how to adjust a precision level using an inexpensive granite surface plate; also a homemade precision miter gauge for woodworking.

Richard Pichler showed suggested club business cards and cardholder; also a deluxe clothesline holder.

Jan Rowland showed a selection of very fine organ stops produced on his own-made CNC lathe.

Anthony Yon showed a piece of large diameter, heavy-wall aluminum tubing he successfully cut off using a carbide blade on his radial arm saw.

John Hoff showed his water-cooled heat exchanger for refrigerant liquid pre-cooling in his pursuit of energy conservation on his air conditioning system.

Visitors:

Reversing and Dynamic Braking of Single-Phase Induction Motors

By Dick Kostelnicek

August 17, 2001

Introduction

Single-phase induction motors drive many arbor-mounted cutting tools in the home workshop. Stationary grinders, table and radial arm circular saws frequently turn abrasive or cutting disks that are directly mounted on the motor's spindle. These disks have large inertial moments that allow them to free wheel for a long time after the power is turned off. A lengthy coast to complete stop often amounts to a major inconvenience.

Usually, motor rotation is not reversed. Furthermore, power may be reapplied prior to full stop. However, in the special case of a single face wheel grinder, it may be necessary to both stop and reverse the direction of rotation in order to grind on the opposite end of the table rest. For example, a lathe-threading bit requires that both the left and right sides be accurately ground. By reversing a face wheel's rotation, both sides of the bit can be symmetrically ground. This work can also be accomplished without reversal by using two face wheels, one on each end of the grinder's spindle together with complimentary tilted table rests. In the case of a grinder for carbide bits, the high cost of using two resin bounded diamond face wheels gives great incentive to reversing the rotation of a single wheel. However, waiting for a free wheeling grinder to coast to a complete stop prior to each reversal may not be worth the monitory savings.

Induction motors with electrically separate start and run windings (four wire connections) can be easily reversed by flipping the connection of either winding but not of both. Ever since OSHA entered upon the scene, grinder and saw motors have been manufactured with three wire connections. In other words, one end of both the start and the run windings are connected internally and brought out as a single wire. Now, you can't just flip a winding to reverse these motors.

There are compelling reasons for not reversing the rotation. When a circular saw's rotation is reversed and the blade is flipped over, the work will cut normally but it may be violently kicked back or pulled away from the operator. Likewise, it is possible, but not safe, to grind with the wheel coming up from under the work rest. Additionally, reversing the saw or grinder rotation might cause poorly tightened arbor nuts to spin off, allowing wheel or blade to become detached from the arbor. Manufactures of intentionally reversible grinders secure their face wheels by bolting them onto a flange and keying the flange to the spindle.

Arbor nut spin off can also occur when rapid braking is applied to a motor spindle. Saws and grinders of recent manufacture often come without brakes. Rather than risk the loss of a retaining nut when breaking, the manufacture advises the tool user to wait till the motor comes to a complete stop. The operator is further cautioned not to use sticks or metal objects pressed against the wheel or blade in an effort to bring the motor to a more rapid stop. A typical six inch grinder takes 85 sec. while a ten inch table saw takes 100 sec. to free wheel to a complete stop.

The purpose of this article is threefold:

To show you how to electrically reverse the direction of rotation of a 3-wire single-phase induction motor.
To show you how to add dynamic electrically braking to a single-phase induction motor.
To warn you to "really tighten" your arbor nuts if you apply 1 or 2 to a grinder or circular saw.

Reversing the Motor

Figure 1 shows the original wiring diagram of a typical unidirectional stationary grinder. A high current surge flows through the motor run winding when the power is switched on. The initial surge is large enough to cause the current start relay to close, thereby applying line voltage across the start capacitor and start winding. The capacitor's effect is to cancel the inductance of the start winding and cause a 90 degree leading phase shift of the current compared to that flowing in the run winding. Since the start and run windings are physically perpendicular to one another and their winding currents have a 90 degree phase difference, a rotating magnetic field results in the motor's stator. This rotating field induces a current in the initially stationary rotor winding. The rotor magnet created by this induced current is dragged along with the rotating magnet within the stator, thereby spinning the motor in its initial direction. Electrically reversing either the run of start winding causes the rotating stator magnet field to turn in the opposite direction, thereby reversing the starting torque on the rotor. Fig 1

As the rotor speeds up, the current and resulting magnetic field that was induced into the rotor by the rotating stator field induces a reverse voltage back into the stator windings. This so-called back EMF (electromotive force) reduces the current flowing in the run winding. Eventually the reduced current will allow the start relay to open, thereby, disconnecting the starting winding. Current now flows only in the run winding. The stator magnetic field is no longer rotating but rather just reversing itself 60 times per second in a straight line across the spinning rotor.

This time changing unidirectional stator magnetic field can be thought of as being composed of two counter rotating magnetic fields. At 60 HZ, the rotation speeds are 3600 RPM right and left. Now put yourself on the turning rotor. The field that rotates in the same direction that you are turning appears to be rotating slower that the one going in the opposite direction. At full speed, 3450 RPM, the rotor sees two magnetic fields rotating in opposite directions, 150 RPM and 7050 RPM. The rotor voltages induced by these two fields are at 2.5 HZ and 117.5 HZ respectively. Note: Divide speed in RPM by 60 to obtain frequency in HZ. The self-inductance of the rotor inhibits the flow of current from the high frequency induced voltage. Only the current induced by the 2.5 HZ or 150 RPM relative (slip) speed supplies sufficient torque to keep the motor turning.

Even if the start and run windings are not separate (three wire connection), the initial rotor torque can still be reversed electrically. Rather than employing a start capacitor, an inductor is used in its place. This inductor must be large enough to shift the start winding current in the opposite or lagging phase direction. However, the additional inductance must not be so large as to impede current flow in the start winding, which already has its own large inductance. Hence, a trial and error method is appropriate. I found that the 24V secondary winding on a 110V - 24V Radio Shack transformer had just the right inductance for reversing a 1/2 HP 3450 RMP grinder. The starting torque is about 1/3 that produced when a start capacitor is used. Consequently, it takes longer to come up to final speed in the reverse direction.

Figure 2 shows the reversing circuit diagram that employs a DPDT-Center-Off power switch. When up to speed, and the switch is flipped directly from forward to reverse without allowing the motor to come to a stop, the motor just continues to run in the same direction. Only when the motor has slowed sufficiently and will draw additional run current, will the start relay pull in and produce reverse motor torque. I use a special DPDT switch that can't be thrown from forward to reverse without first pausing at center off. The addition inductor (transformer secondary winding) and DPDT switch fit with plenty of room to spare in the pedestal base of my far-East-knock-off of a Baldor carbide grinder.

Recall my warning about tightening arbor nuts. Usually, a dual shaft grinder has a right hand threaded nut retaining the right wheel and left hand nut for the other. I use an impact wrench, set to a low torque, to tighten or loosen both arbor nuts. That way the rotor's own inertia acts as a backup wrench rather than holding on to the wheel on the other side of the grinder.

A note on current starting relays. These devices may be very sensitive to gravity. In order to sense a current change at high amperage, these relays have just a few turns of thick copper wire in their coil. Therefore, they need a very weak spring to balance the weak magnet force generated. With cheap designs, gravity provides just such a weak spring. In my grinder the relay was initially mounted at about a 75-degree slope. I had to remount the relay vertically in order to ensure relay dropout at a larger run winding current when the motor used the inductor for starting. Aligning the relay with gravity increases the effective restoring force. Alternatively, removing a few turns from the relay's coil will force it to drop out at a larger run winding current.

Dynamic Braking

Recall that a magnetic field in the motor's stator induces a current in the rotor as its windings cut through the stator field. If we connect DC (direct current) to the stator winding while the rotor is turning, a large DC will be induced in the rotor winding. There are no brushes to take this current from the rotor as would be done in a generator. Also, the rotor winding is short-circuited onto itself. This is ok when AC (alternating current) is flowing, since the rotor's self-inductance limits the rotor AC. However, the large DC goes into heating up the rotor resistance. Thus, the energy loss in the rotor winding caused the rotating wheels to slow. By simply applying DC to the run winding, we can provide dynamic braking to a free wheeling grinder.

How much DC is necessary to stop a 3450-RPM grinder in a just few seconds? Well, my solution uses a 12V-lawn tractor battery that can easily supply 12 amps for several seconds to the 1.0-ohm run winding resistance. Table 1 shows the stopping times for various 12V battery powered dynamic braking applications that I have constructed and used. You'll have to recharge the battery, so an inexpensive trickle charger is appropriate here.

Table 1. Free wheeling and Dynamic Braking Stop Times.
Application	Free Wheeling (sec.)	Dynamic Brake (sec.)

6" - 110V Grinder	85	6
10" - 110V Table Saw	100	3
12" - 220V Radial Arm Saw	115	4

Figure 3 shows the combined dynamic brake and reversing wiring diagram used on my 1/2 HP - 6" carbide grinder. The stop switch is a momentary DPDT toggle that disconnects both 120V power lines from the grinder while applying 12V DC to the motor's run winding. Fig3

Reversing Carbide Grinder with Braking

Figure 4 shows a photo of the finished grinder. On the left is an inexpensive silicon carbide vitrified face wheel, while on the right is a resin bounded diamond wheel. The left power switch has three positions; forward, off, and reverse. The right switch brakes the motor when held in the down position. Electrical clips, seen below the switches, connect to a 12V battery located beneath the grinder's stand.

Figure 5 shows component location in the grinder's pedestal base. The reversing inductor is at the left, and start relay is at the right. The start capacitor can't be seen as it is buried below the mass of wires in the center. The battery cable exits the pedestal at the bottom left through a rubber strain relief. The two operating switches are at the center top.

Figure 4. Finished carbide grinder.

Figure 5. Inside the grinder pedestal.

Originally, the work lamp was illuminated only when the grinder was under power, even though it has its own switch atop the reflector. This was a major inconvenience when setting up the tilting support table and miter protractor. I put a 60-minute crank-up timer on the lamp (not seen behind the grinder) so that the lamp eventually turns itself off.