Using Watt's Up; Where Do You Connect Watt's Up; Maximum Current Capabilities; Usinga Watt Meter With Inductive Loads Like Motors - RC watt's up WU100 User Manual

Using Watt's Up

7 USING WATT'S UP
7.1

Where Do You Connect Watt's Up

You can think of the Watt's Up like a set of jumper wires. Connect it in the same way and
you can measure what the jumper wires are connected to. Like Jumper wires, the Watt's Up
is essentially a direct connection between correspondingly colored SOURCE and LOAD
wires, i.e. both the "SOURCE" and "LOAD" leads of the Watt's Up are electrically "hot"
when a battery is connected to either side.
Example use #1 -- Testing Loads; Battery on SOURCE side, Electronic Speed
Controller (ESC) and motor on LOAD side. With the ESC on, the Watt's Up shows the
current into the motor, voltage and power at the battery and accumulates the Ah and
Wh while the motor is running.
Example use #2 -- Battery Charging; with a battery charger on the SOURCE side and
battery pack on the LOAD side (using gender changing jumpers), the Watt's Up shows
the charging current into the battery, the voltage and charging power at the battery and
accumulates the charge (Ah) and energy (Wh) into the battery.
7.2

Maximum Current Capabilities

Current flowing through Watt's Up and its supplied wires generates heat due to the
resistances of the wires and of the precision internal current shunt used for current
measurements. Though very low ( ~ 0.004 Ohms in 14 gage wires and 0.001 Ohms in
shunt), these resistances are finite and at high currents the heat generated becomes
noticeable. This is because heat is created with the "square" of the current. I.E.:
heating power (W) = current
So doubling the current increases the heat produced four times.
At 75 Amps, approximately 28 Watts of heat is produced – mostly in the Watt's Up's wires.
This doesn't only happen with Watt's Up. All wiring, connectors, etc have resistance
subject to the same resistance based heating.
The high current handing capabilities of Watt's Up will be maximized when the Watt's Up's
SOURCE and LOAD wires are kept short and cool. Cut the wires as short as is convenient
for high current operation. A fan blowing over the wires will help their cooling. At
moderate currents there is little heating. Using a "three-wire" connection will also lower
heat generation. Please see our Internet website connection diagrams and FAQ for more
information on connections.
7.3 Using A Watt Meter with Inductive Loads Like Motors
Loose connections and long wires on circuits switching high currents can damage electrical
components. The problems can be largely eliminated by ensuring that connections are
secure/not intermittent (e.g. not just twisted together) and to a lesser degree by keeping
wire lengths short and by twisting wire pairs together to reduce their inductance. Here's a
little more about why this all happens.
Wires exhibit a property called inductance. Inductance is an indication of the amount of
energy stored in a wire's magnetic field due to the flow of current.
If you interrupt the current flow, say with a switch or pulling apart a connector, the
magnetic field collapses and induces a voltage in the wire to try and oppose the drop in
current. Depending on the specifics of the wire, current and materials near the wire (like
iron) the induced voltage can be quite high. In fact, it can give you a mild shock and
damage electrical components still connected to the wire by generating voltages that
exceed their ratings. This is one of the reasons capacitors and diodes are needed on brushed
motors in addition to reducing RF noise.
Always disconnect the meter from the highest inductance wiring first. Usually this is
on the LOAD side.
"Watt's Up" & "Doc Wattson" Watt Meter and Power Analyzer User's Manual
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(A)  resistance (Ohms)
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RC Electronics, Inc.