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A Simple Fix for a Vexing Problem

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Blog entry by Paul Stoops posted 182 days ago 855 reads 3 times favorited 4 comments Add to Favorites Watch

One of the most favorite tools in my shop is my Performax (Jet) 16-32 Plus drum sander. I use it on almost every project. However, this power tool has one characteristic that has proven to be a great vexation to me over the last 15 years – the tendency to trip the motor circuit breaker while sanding. This one part blog describes my journey to find a solution to this problem.

The 1 1/2HP Jet drum sander drive motor has a thermal circuit breaker rated at 15 Amperes. Exceeding this limit for only a limited time can cause the breaker to trip, cutting the power to the drum motor. The load on the motor is a function of a number of factors, including the depth of cut, speed of the conveyor belt, grit of the sanding belt, condition of the sanding belt, hardness of the wood being sanded, length of the drum contacting the workpiece, etc. The difficulty arises in not knowing how much current is actually being used by the motor for any given sanding pass.

The depth of cut and speed of the conveyor are the two major variables which are usually adjusted by the machine operator. The amount of drum height change between passes and the speed of the conveyor are usually arrived at by trial and error. Since the amount of material that can be removed in one pass is very limited, drum sanding is commonly used for such tasks as resaw surface cleanup, finish sanding or minor panel flattening. Unfortunately, many folks, including myself, tend to become impatient and take too deep a cut and/or run the conveyor too fast, inadvertently overloading the motor, frequently tripping the motor circuit breaker. This event significantly slows down an already time consuming operation which usually requires a substantial number of sanding passes.

The folks at Jet recognized this problem and have come up with an electronic adaptive control circuit on their newer models, which they call SandSmartTM. The folks at SuperMax have a similar adaptive control system which they call IntellisandTM. These adaptive control systems essentially monitor the motor current and automatically adjust the conveyor speed to reduce the load on the motor so the motor current will stay under the maximum rating. These systems perform very well as long as the depth of cut is within reasonable limits. Watching one of these systems perform seems kind of magical – the conveyor speed automatically slows down as necessary to limit the motor loading. To minimize sanding time, some folks set the conveyor speed at maximum and let the adaptive control adjust the conveyor speed as required.

Unfortunately, a lot of us who do not have these newer sander models with these “magical” adaptive controls still have to guess at the depth of cut and conveyor speed settings. Ideally, we would like to set these values as high as possible in an attempt to speed up this very time consuming machine operation.

While investigating possible solutions to my circuit breaker tripping problem, I ran across a video online in which a gentleman was showing how to upgrade his Performax 16-32 Plus to add the SandSmartTM feature by replacing the main controller circuit board and a few other components. Aside from the difficulty and skill required to alter the machine circuitry, the major drawback to this approach is that the new control circuit board and other parts require expenditure upwards of $350 – about a third of the cost of a new machine!

Since that approach didn’t fit within my budget, I decided to take a different and much simpler path, which I have seen described in other forum posts – simply add an AC Ammeter to permit monitoring the motor current in real time, giving the opportunity to adjust the conveyor speed manually to limit the motor current to a level just under the 15 Ampere limit. In some of the forum posts I have seen, folks installed an analog (needle-type) meter on their sanders for this purpose. Some commercial wide belt and other sanders have built-in current meters for monitoring motor current.

I prefer digital meters to analog meters because the readout is an easy-to-read discrete number, rather than having to interpret a needle position on a meter scale. On eBay, I was able to purchase a low cost digital meter, similar to this one, which has the current sensing coil built-in to the circuit board, recommended on one of the forums:

“http://www.amazon.com/Digital-Combo-Panel-Voltmeter-Ammeter/sim/B00AQAQHQU/2/ref=pdcphisexpl”

Though I was interested primarily in monitoring Motor Current, this particular meter displays both Line Voltage and Current. This meter is suitable for a wide range of applications, including 240VAC equipment with currents up to 100A. I recently installed one of these meters on a 240VAC, 3HP dust collection system to allow monitoring of motor current. There are also other low cost meters available on eBay and elsewhere that would be equally suitable for this application.

Since I didn’t want to modify the original circuitry, I fabricated a simple plug-in junction box to install between the motor and its power receptacle on the Performax (Jet) 16-32 Plus controller box using the meter and commonly available component parts. The junction box plugs into the Performax control box receptacle where the motor is normally attached, and the motor plugs into a receptacle installed on the junction box. Essentially, this approach simply extends the power cord to the motor allowing a Current Sensing Coil and Meter to be applied to the motor circuit.

This approach does not require any modification of the original sander or motor control circuit. In addition, the original 15 Ampere circuit breaker continues to provide circuit protection.

As shown below, this simple < $25 addition requires only about a half dozen parts and a couple of fasteners:

1. Digital AC Volt/Ammeter with Current Sensing Coil
2. Coil Mounting Block (Shop-made)
3. AC Panel Receptacle
4. AC Line Cord with Plug (with same wire size as the motor)
5. Cable Connector
6. Electrical Box (22.5 Cu. In.)
7. Box Cover

The wiring diagram is as shown below:

The actual connections to the meter and coil are determined by the meter you have. They are not all the same. Be sure to follow the instructions or graphic labeling included with your meter. As with all electrical devices, be sure to follow safe and accepted wiring practices when fabricating this device.

Inside the junction box, one wire of the Line Cord passes through a Current Sensing Coil, which provides the signal input to the meter, and is attached to the AC Panel Receptacle. The other two Line Cord wires also attach to the AC Panel Receptacle. The Meter itself is powered by the line voltage to the motor. The Coil Mounting Block, attached inside the box with a couple of fasteners, secures the Current Sensing Coil in position with one conductor of the Line Cord passing through the Coil.

The photo below shows the completed junction box ready for installation.

Although there are other suitable mounting locations, I chose to mount the meter box on the lower side the motor capacitor cover. This location is out of the way, reducing the risk of inadvertently damaging the meter. Because the motor capacitor is mounted at an angle to vertical, the back-lighted Meter face is tilted upward toward the operator, making easy viewing. Although there are several ways the meter box could be mounted, I chose to put double back tape between the capacitor cover and the back of the box along with a long tie wrap around the capacitor cover and the front of the box below the Line Cord and the Panel Receptacle. This mounting method secures the box in place, does not require any modification of the sander, and provides the opportunity for easy removal at a later date for repair, should that become necessary, or restoration to the original configuration.

The following photo shows the meter box installed under the start capacitor on my Performax (Jet) 16-32 Plus drum sander. The tie wrap is clearly visible.

The Meter is only powered up when the drum motor is running. The conveyor motor is on a different controller circuit and does not affect the Meter reading.

I discovered that this stock Jet 1 1/2HP motor operates at about 10.5 Amperes under no-load conditions, which doesn’t provide a great deal of head room under the 15 Ampere rating! In my opinion, this restricted range of operating current accounts for most of the circuit breaker tripping problem because there is no way to know the actual operating current level without a meter or similar device.

The photo below shows the no-load Motor Current.

The upper number on the Meter display is the Line Voltage to the motor; the lower number on the display is the Motor Current. By changing the conveyor speed control setting, the Motor Current can easily be kept under the maximum rating of 15 Amperes.

After installing the Meter, I found it quite easy to adjust the conveyor speed while the machine was operating to maintain the Motor Current level within the 15 Ampere rating, virtually eliminating the circuit breaker tripping problem!_ Very quickly it was easy to establish a conveyor speed that would work under a variety of conditions with only minor manual adjustments of the conveyor speed dial.

This same meter box addition can also be used on a variety of machines having AC operating voltages up to 240VAC and currents up to 100 Amperes, particularly since no modification of the machine circuitry is required. However, the Line Cord wire size and the Plug and Receptacle may have to be changed to suit higher motor voltages and currents.

This has been a really worthwhile, cost-effective, addition to my Performax (Jet) 16-32 Plus drum sander, which I am sure will enhance my appreciation of this marvelous tool – as well as removing the annoying frustration of the frequent circuit breaker tripping!

-- Paul, Auburn, WA



4 comments so far

View HerbC's profile

HerbC

1154 posts in 1460 days


#1 posted 182 days ago

Great accessory for the sander.

Excellent documentation of a well designed project.

Keep up the good work.

Be Careful!

Herb

-- Herb, Florida - Here's why I close most messages with "Be Careful!" http://lumberjocks.com/HerbC/blog/17090

View KS_Sparky's profile

KS_Sparky

22 posts in 224 days


#2 posted 182 days ago

It looks like you’ve gotten your problem under control, but I’d like to add a word of caution. You really should be using a 20A circuit for your sander; the fact that the 15A overcurrent device is tripping is your clue. That is the first dangerous situation. It would also save you from watching an ammeter instead of your power tool and workpiece. That is the second potentially dangerous situation.

The National Electrical Code says that cord and plug equipment shall not exceed 80% of the branch circuit ampacity. So a 15A circuit should not be loaded more than 12A. A 20A circuit allows 16A. This is to prevent those 14awg copper wires from glowing and starting a bonfire in your walls. This is the reason so many house fires are started by small electric space heaters…they get switched to high, they exceed the branch circuit rating, and eventually the breaker fails and the guys with shiny red trucks get called.

You have a really neat setup, though, and it can give a good indication of how hard the tool is being worked. Many tools could benefit from this sort of thing. Just be careful.

-- apprentice Electrician, IBEW L.U. 226

View rdwile's profile

rdwile

137 posts in 713 days


#3 posted 182 days ago

Paul,

A well written and well thought out solution to this problem. Like you, I use my 16-32 on virtually every project I build and have the same challenge. My solution to date has been to listen to the motor starting to drag and then adjusting the belt speed until it sounds right. Wearing hearing protection muffles this quite a bit and sometimes I miss it before it trips. This provides a nice visual cue that the load is increasing.

I will be looking into doing something like this as it seems the only time this puppy quits is on the last pass on a board and the gouge it leaves when it quits is sometime impossible to remove without missing my target thickness.

Thanks, will be shopping for the bits today.

Rich

-- Richard D. Wile, http://richard-wile.blogspot.ca/

View Paul Stoops's profile

Paul Stoops

321 posts in 1162 days


#4 posted 182 days ago

@Herb—

Thanks for the kind words. It’s wonderful that we have such affordable high quality digital cameras available today to document our projects.

@KS_Sparky—

Thanks for the “80% rule” reminder regarding circuit and circuit breaker loading. In this case, my sander is connected to a 20A circuit, but as I stated, the sander motor is protected with a separate 15A thermal circuit breaker.
I would much rather admire the “shiny red trucks” from a distance, rather than in my driveway…........ :-)

@Rich—

Thanks for your comments. I find it hard to hear the sander over the noise of the dust collector when wearing hearing protection, so the visual meter display is especially useful for this machine application. The circuit breaker trip out on the final pass is a bummer—and I suspect it is more likely to occur then since we are usually using a finer grit abrasive belt which probably produces more friction, translating into higher motor loads.

-- Paul, Auburn, WA

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