Tractorsport Flowbench Forum Archive

Posted: **Thu Aug 23, 2007 5:41 am**

I did see the blower unit under construction. Nice job!

I wasn't all the interested in this thread before but after seeing the blower and discussing it with Bruce, I'm wanting one for a new bench or to adapt to mine.

Getting so i can't hear like I once could......Blowers are the way to go IMO.

Larry C

Posted: **Thu Aug 23, 2007 5:58 am**

Posted: **Thu Aug 23, 2007 7:29 pm**

Posted: **Thu Aug 23, 2007 7:49 pm**

Posted: **Thu Aug 23, 2007 7:58 pm**

A good investment might be an ac amp meter so you can actually see what is going on, and judge how much load is really there.

Posted: **Fri Aug 24, 2007 3:54 am**

Posted: **Fri Aug 24, 2007 1:42 pm**

As Tony said a tong tester (ammeter) would give you a lot more insight.

Wow- I haven't heard them called Tong testers in over thirty years! I'll have to run that by some of the "kids" in the shop :p

Posted: **Fri Aug 24, 2007 6:11 pm**

If you guys are going to be playing around with large blowers, some means of measuring motor current is an absolute necessity. There is simply no other way to judge how heavily loaded a motor is. It is remarkably easy to overload a motor without realizing, or have a motor running at only a fraction of it's full load capability. The motor rating plate, and an ammeter will tell you exactly where you are.

Posted: **Sat Sep 15, 2007 7:39 pm**

Well I'm getting back to my old-self and spent sometime today in the shop working on the blower project, here are some updated pics on the hub assembly:

Posted: **Sat Sep 15, 2007 8:38 pm**

The Tong tester is how we discovered the 20 amp
feed to the 110 vacuum motors that were supposed to be drawing 10 amps. LOL

Tom V.

Posted: **Sat Sep 15, 2007 9:47 pm**

A proper tong tester is probably the most convenient way, another is to buy a direct reading ac panel amp meter, the sort of thing often fitted into power boards.

A third way is to use a current shunt, and just measure ac millivolts across the shunt with an ordinary cheap multimeter.

Love your rotor hub Bruce, you do some really beautiful fabrication work.

Posted: **Sun Sep 16, 2007 2:02 pm**

Posted: **Sun Sep 16, 2007 6:09 pm**

A current shunt is a low value precision resistor that can be wired in series with your power circuit. Due to the resistance of the shunt, there will be a voltage drop created, which will always be exactly proportional to the current flowing.

By knowing the resistance, and being able to measure the voltage drop, the current flowing can easily be measured and then calculated. Both ac and dc currents can be measured this way. The shunt will always have stamped on it the rated maximum current, and the number of millivolts dropped at that current.

It is an especially convenient way to measure very high currents with an ordinary digital multimeter.

Shunts come in all shapes and sizes and cost from around ten dollars upwards new on e-bay.

*HINT* When selecting a shunt try very hard to get a 1:1 or 1:10 relationship between rated current and millivolts. For example 75mV at 75amps, or 50mV at 50amps, or maybe 35mV at 350amps.

That makes it very easy to read with a digital multimeter if millivolts equal amps exactly. If you get something strange like 30mV at 75amps, you will have to do some calculations to figure out what the current actually is. Getting something that reads directly is vastly better, but not many shunts are actually like that, but it is well worth the search finding one.

Just connect the current to be measured through the two large bolts, and connect your digital multimeter across the small screws. Shunts are available to measure from a few amps to thousands of amps.

Posted: **Sun Sep 16, 2007 6:42 pm**

A shunt is a small resistance placed in series with your motor/s.

amps = volts / resistance

The volts are measured across the line and load terminals of the shunt and the resistance of the shunt must be known.

The amps are then calculated from the volts and resistance.

The resistance needs to be small so that voltage drop across the shunt does not greatly eat into motor volts costing motor performance.

Based on a 5 amp rated motor and 0.1 volt shunt measurement, a resistor of 0.02ohm would be required with a 0.5 watt power capacity.

R = V / I
0.02 ohm = 0.1v / 5a

P = V I

0.5w = 0.1v x 5a

Be warned the shunt terminals ARE at mains voltage. For 240V mains the shunt line would be 240V and the shunt load would be 239.9V.

For those countries with a Multiple Earthed Neutral, the shunt could be placed in the neutral leg which is much closer to ground potential. Typically the neutral voltage at your test point would be above ground potential by the voltage drop in the neutral wire back to a good MEN point.

The shunt method is really only useful if you already have a suitable voltmeter to measure with, otherwise it would be cheaper and easier to by a meter to read the amps directly.

EDIT: Tony's hit the submit post before me.

I'd also like to be clearer about the dangers of measuring mains supplies, IT CAN KILL YOU. Before doing this you need to be competent, have suitable test equipment and personal protective equipment. Proceed at your own risk.

Posted: **Sun Sep 16, 2007 6:47 pm**

Tractorsport Flowbench Forum Archive

Blower design