power tune up

7/31/2019 Power Tune Up

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From ShopNotes MagazineAll rights reserved page 2 of 5 2003 August Home PublishinOne copy for personal use. Other copies prohibit

Squeeze the trigger on a drill andthe chuck spins to life. Flip theswitch on a router and the motor firesup in a heartbeat. Its easy to takethese things for granted.

But with extended use, you maynotice that a power tool isnt quite upto speed. Maybe it runs rough, ortheres excessive sparking inside thecase. Worse yet, you turn on the tooland nothing happens. Thats usually

when it gets stuck on a shelf gath-eringdust instead of making it.

Of course, you could take the tool

to a repair shop, but that can be expen-sive. Plus its not always necessary.Sometimes a simple fix is all it takesto get a portable power tool runningas good as new.

Power Problems If there is aproblem, quite often it can be tracedback to one thing an interruptionin the electrical currentthat flows intothe tool, through the switch, and on tothe motor. Now dont worry. You donthave to be an electrical engineer totroubleshoot problems with a powertool. But itdoes help to know some

33-Step-Step

PortablePower Tool

Tune-Upmotor basics.

Universal Motor First of all, thetype of motor used in a portable powetool is auniversalmotor. (See drawingbelow.) This is a lightweight motothat produces lots of torque and spinsat a high rate of speed.

To accomplish this, electrical current from an outlet travels throughtheplugandpower cordto aswitch

When you flip the switch, the currenflows through a pair ofbrushes tharub against acommutator. This sets upa magnetic field that spins an armature andthe shaft of the tool.

Three-Step Tune-Up At leas

thats how itssupposedto work. But itheres a break in the loop, thatswhen problems develop. The trick isfinding it. To do that, I use a simplethree-step tune-up which is explainedin the rest of this article.


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I N T H E S H O P

{ Check for Damage. To avoid the possibility of a serious electrical

shock, replace a plug if its missing the grounding prong (shown at left)

or a power cord with a break in the outer sheath (right).

If a tool runs intermittently (or not atall), check the obvious things first. Adamaged plug or power cord likethose shown at right can affect theperformance of a tool. Plus they can

cause a serious electrical shock, soits worth taking a few minutes toreplace (or repair) damaged parts.

REPLACING A PLUG

The first step in replacing a plug isdeciding which one to buy. As a rule,I get a replacement plug thats thesame type as the old plug.

Type of Plug For a double-insu-lated tool, this means getting a plug

with two prongs. (A double-insulatedtool typically has a plastic housing like

the drill in the photo on page 2.)But be sure to use a three-prong

plug if thats what the tool camewith. The grounding prong on thistype of plug is especially impor-tant in preventingstray current fromenergizing the case of ametal-bodied tool.

The drawing below shows a typ-ical, three-prong plug. Notice that theterminals are color-coded, so its easyto see which color of wire is connected

to which terminal. I also like how thecable clamp and clamp terminals keepthe wires from wiggling loose.

POWER CORD POINTERS

Its no wonder that a power cord getsdamaged. It gets tugged on, scuffedup, and wrapped around the tool likea tourniquet. If the damage is obvious(like a break in the outer sheath), itstime to repair or replace the cord.

If the rest of the cord looks okay,you can cut it off below the damaged

part and reconnect it to theswitch. If the cord is frayed or

cracked, its better tobuy a new one. Justbe sure to get a cord

with the same numberand gauge (diameter) of wires

as the existing cord.Strain Relief Either way, you

may have to get a new strain relief.This is a thick, rubber boot thats

1. VisualCheck-Up

{ Connectors. The wires from the power cord may be attached with

crimp-on connectors (left), wire nuts (center), or a spring-loaded mechanismin the switch (right). Just be sure theyre tight so they dont wiggle loose.

molded around the cord where itenters the tool. The strain relief pre-

vents the cord from getting akink that could bend (orbreak) the wiresinside. Note: You canget strain reliefs likethose shown at left atmany hardware stores.

Crack the Case Beforeinstalling the power cord, youll need

to crack open the case to provideaccess to the switch. The tip in themargin at right shows a handy way tokeep track of which screws go where.

Connectors Once the case isopen, you can see how the wires inthe power cord are connected to theswitch. As you can see in the photosbelow, there are several different typesof connectors that might be used. It

just depends on the tool.Color Cues Whats more impor-

tant is to know which color of wiregets attached to which terminal onthe switch. To avoid confusion, I leavethe old wires in place until Im readyto connect the wires in the replace-ment cord.

Install Power Cord To do this,start by slipping the replacement cordthrough the strain relief as shownbelow. Then remove the outer sheathof the cord and strip the insulation offeach wire. After slipping the cord andstrain relief into the case, connect the

wires one at a time.

{ To avoid

confusion when

reassembling a t

draw an outline o

shape on a piece

foam insulation.

Then stick screw

(or other parts) in

their proper loca


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the switch are exposed as in Step 1But if the wires go straight into theswitch, youll need to pull them outMost likely, the wires are held in place

with a spring-loaded mechanism. To

release each wire, stick a needle intothe slot next to it. Then push downand pull the wire loose.

Once youve located (or exposed)the ends of the wire, touch the probesto them to check for continuity in thepower cord. Remember, the metershould read zero if the cord is okay. Inot, replace it as shown on page 3.

TEST THE SWITCH

The next step is to see if the switchworks. Again, it may look fine. Buafter switching it on and off hundredsof times, a switch may wear out.

To test the switch, the procedure isbasically the same. This time, yourechecking whether the current fromthe meter flows through the switch

when its in the on position.A look at Step 2 shows an easy way

to do this. Start by attaching alligatoclips to the points where the wiresfrom the power cord connect to thethe switch. (These are the same two

points you touched the probes to inStep 1.)

Now locate the two wires thatleave

the switch and go to the motor. Touchthe probes to these two points, andpress the switch to the on position

Again, azero reading indicates theswitch is good. If its broken, its besto order a new switch from the manufacturer.

Meter Check Touching the two

probes of a multi-meter togethe

should produce a zero reading

just like when checking for continuity

1To check continuity in the cord, attach

clips to the prongs of the plug. Then touch

the probes to the terminals where the wires

from the power cord connect to the switch.

2Now test the switch. To do this, attach the

clips to the terminals where the power cord

enters the switch. Then touch the probes to the

wires leading to the motor and press the switch.

This device>

has an indicator

light that glows

to show continuity.

It costs about

four dollarsat an electrical

supply company.

the plug, as1 below. (Alligator

shown in Stepclips like the

ones shownabove are available at mostelectronic supply stores.)

After hooking the clips to theprongs, the next step is to locate theends of the wires that connect to theswitch. Thats easy if the terminals on

A visual inspection can pinpointsomeelectrical problems in a power tool.But what if everything appears to beokay, and the tool still wont run?

Here again, the problem is often

the same an interruption in theflow of electricity. Only this time, theinterruption in the loop may be hidden.

Continuity So how do you findout where the problem is? By testingforcontinuity. This shows whether

the electrical current travels in acontinuous path, or if theres aninterruption along the way.

Multi-Meter To do that, Iuse a simple testing

device called a multi-meterlike the one shown

in the photo above. This is a digitalmeter with several settings that canbe used to diagnose different prob-lems. But to check for continuity,

youll only need to dial in a singlesetting the one that meas-ures the amount of electricalresistance.

Ideally, the meter should readzero

resistance. In other words, the cur-rent flows unimpeded. To check

whether the meter itself is working,touch the two probes together. Thisshould also produce azero reading.

Editors Note: The meter shownhere (Model 22-806) is available from

RadioShack for about $24 . For a lessexpensive device that tests continuity

only, take a look at the photo in themargin.

Two Tests No matter whichdevice you use, there are two sepa-rate tests to make one for the power

cord and one for the switch.

CHECK POWER CORD

I usually start with the power cord. Iftheres a break in one of the wiresinside the cord, its not always visibleon the outside. The only way to findout is to hook up the multi-meter.

The idea is to check whether cur-rent from the meter (theres a smallbattery inside) flows from one end of

the cord to the other. This requireschecking conti-

nuity between theplug and the points

where the wires con-nect to the switch.

Alligator Clips Aneasy way to do this isto start by using a

pair of alligator clips tojump across the

two prongs of

T O O L T U N E - U P

2. Test for Continuity


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brushes wear downand dont make goodcontact with the com-mutator. This causesa loss of power or

excessive sparks.Remove Brushes

To inspect brushes forwear, youll need toremove them. Depending on the tool,access to the brushes is either on theoutsideorinsideof the case. (See photosabove.) Note: To ensure even wear,reinstall the brushes on the same sideof the tool they came out of.

After removing the brushes, youmay find theyre chipped or burned.(See margin.) But a more likely problem

is theyre too short. The brushes simply

Burned

Worn

An Ounce of Prevention

When it comes to taking care of theelectrical components of a tool, pre-

vention is the best cure.Blow Out Dust For example,

dust can prevent a tool from breathing.This causes it to overheat which candamage the motor. So I remove asmuch dust as possible by turning onthe tool and blowing air through theintake slots (Photo A).

Cord Wrap Even a simple thinglike how the power cord is wrappedaround a tool can keep problems fromcropping up (Photo B).

B. Cord Wrap. To reduce the strain

on a power cord, make a large loop

before wrapping it around the tool.

A. Remove Dust With the tool

running, blow air through the intake

slots to remove packed-in dust.

Okay, so what if theres continuity inthe power cord and the switch, butthe toolstillruns rough? Or not at all?Its time to inspect the brushes.

Carbon Blocks There are two

brushes. Actually, the word brushesis a bit deceiving. Theyre solid blocksof a carbon-graphite material that rubagainst the commutator.

To keep the brushes in contact withthe commutator, theyre attached tosprings. These spring-loaded assembliesfit into a metal brush holder, one oneach side of the commutator.

Brush Power A wire leading toone of the brushes (or holder) sup-plies power from the switch. When

you flip the switch, the brush feeds

power to the commutator. With use,

T O O L T U N E - U P

wear down past their useful life.Whats the minimum length of a

brush before it needs to be replaced?Sometimes theres an indicatormarked on the brush. If not, a goodrule of thumb is to replace thebrushes when theyreshorterthanthey are wide. Note: Replacementbrushes are available from the man-

ufacturer and some hardware stores.Final Check With new brushes

installed, the tool should run like a top.If it doesnt, there may be a moreserious problem with the copper wind-ings in the motor. If the tool has beenoverheated, the insulation on the wind-ings may have melted. This can causethe windings to short out.

To check, use an alligator clip tojump across the two wires leadingfrom the switch to the motor. Thenremove the brushes and place theprobes against the metal holders. Ifthe meter doesnt read zero, yourbest bet is to replace the tool or takeit to a repair shop.

Chipped

{Occasionally,

youll find a

chipped or burn

brush that needs

be replaced. Mo

often, the brush

worn to the poin

that its too short

3. Inspect Brushes

Brush Access A removable cap makes it easy totake out some brushes (left). The only way to

remove other brushes is to open the case (right).

power tune up

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