Fried servo cable while trying to measure load current of RC car

[mechkbfan]

I'm sure there's a logical explanation because I've just shorted it but I can't quite wrap my head around it

Attached two photos immediately after

Basically I just bought a Power Analyzer, 4.8V to 60V 130A High Precision Watt Meter off Amazon with goal of doing some motor load measurements

Connected the Load to Motor
Connected the Source to ESC
Connected servo cable to Channel 4 of Receiver

Did a test, got a good measurement, then figured using the RC power switch would be the safest

Wrong, immediately the receiver cable that was connected from analyzer to receiver started to fry / smoke

It does my head in because I figured that switch would power off everything.
i.e. ESC would have no power, i.e. no power via analyzer cables and no power via servo cable

Can someone walk me through it?

Next time I'll pull out receiver cable then power off the car, unless there's something obvious I missed in this scenario?

 

[Jerold]

I believe these are meant to be placed between the battery and the ESC?

Just guessing here. I don't know what's actually in the ESC or the Watt meter. But I see the ground wire is fried, which is a big clue.

It's a DC motor and the +/- legs are not always positive and negative. Polarity is relative to the direction of the motor. If you reverse the motor from the ESC, the "GND" become the plus side.

Normally this doesn't' matter, because true (receiver) ground is isolated from the (+/-) high voltage motor wires. I'm guessing inside the Watt box, ground is always ground. Which means the high voltage/high current motor driver now could have a common ground with the receiver.

The second problem is, the switch is not like a light switch; it's not an inline break. It goes to some internal circuit that tells the ESC to shut down. It might be grounding a pin or completing a loop, but it's not disconnecting the power. I'm pretty confident in this one, becasue you can't push 500W (or whatever) through the 20 AWG wire and that switch. It's like a 1 amp swtitch at most.

So what shorted. Inside the ESC there are lots of capacitors on the high voltage motor side, that would normally discharge between the isolated +/- legs of the motor. But we lost that isolation in the Watt meter. So it is possible that the high voltage caps, inside the ESC, discharged into the receiver ground.

But that also means disconnecting the battery might do exactly the same thing, because the caps still need to discharge somewhere.

Again just a theroy, not fact.

 

[mechkbfan]

It's pretty reasonable theory

Yes, you're correct about it should be between battery & ESC. Double checked instructions and didn't see ESC -> motor as an option

And interesting enough they used external power sources, but I wonder if powering via receiver is still valid as external or if I'll end up in same situation

I'll just use a 9v battery with an adapter next time to be safe

 

[CertifiedMike]

Why are you using an external power source rather than the battery in the car?
I think this is your issue.
2 different power sources, 2 different voltages and it smoked.

I am just guessing here. I don't own one of these meters.

 

[Jerold]

It's pretty reasonable theory

Yes, you're correct about it should be between battery & ESC. Double checked instructions and didn't see ESC -> motor as an option

And interesting enough they used external power sources, but I wonder if powering via receiver is still valid as external or if I'll end up in same situation

I'll just use a 9v battery with an adapter next time to be safe

What are you testing?

If they used an external DC source, the negative side of the motor is probably grounded. I don't know of any bench top supplies that can dynamically invert power and ground like the ESC. Something might be out there or perhaps a high voltage signal generator could do that, I dunno?

The ESC limits the current and filters the noise. You could connect it to a battery, but I'm not sure that's a good idea. Doing that gives the motor 100% unregulated current draw from the battery instantly. You will probably get sparks when it connects. There are a bunch of coils in there that can produce back EMF (reverse currents) that will use your battery as a filter. Now it becomes a limit of the battery and it may not be good for the battery. Again I dunno? With low power motors and alkaline batteries it's not a huge deal, because they don't explode.

Also it may not be that accurate. The brushes in the motor are noisy and run better with capacitance to smooth that out, which means the current might be a bit spiky and the reading might be off. Not to mention the second you connect the battery, the voltage starts to drop and you will get the Wattage ever decreasing.

If you have an adjustable bench top power supply, you can set the voltage and current limits, which means you can get or calculate Wattage directly from supply. There is no ESC or RX involved. Start at 0V or off, plug it in, then adjust the voltage and/or current limits. But a 50A adjustable power supply is quite expensive though.

 

[RustyUs]

 

[RustyUs]

Are you sure you hooked things up correctly?
I have the 150A version of that watt meter and never had an issue with it.

 

[RustyUs]

This...

... is not this...

Your battery should be the only thing hooked up to the SOURCE side of the meter.

 

[RustyUs]

Also, your motor will not be under a full load if just doing some sort of bench testing with no resistance to the drivetrain. Strap that analyzer onboard and let 'er rip.

 

[Jerold]

Are you sure you hooked things up correctly?
I have the 150A version of that watt meter and never had an issue with it.
View attachment 270344

I think this is the setup. The dotted black line is my assumption that the input/output ground is the same at the RX ground. This would be fine with a battery. The blue (assuming low side) is not really a ground it's a relative voltage to the yellow and can be + or -.

Anyway, follow the direction you posted.

 

[RustyUs]

I think this is the setup. The dotted black line is my assumption that the input/output ground is the same at the RX ground. This would be fine with a battery. The blue (assuming low side) is not really a ground it's a relative voltage to the yellow and can be + or -.
View attachment 270356

Anyway, follow the direction you posted.

The receiver battery input next to the main battery input, on the SOURCE side of the meter, is probably where the confusion lies.

 

[WRM]

It looks like a Futaba "J" jumper connected to that "thing", and that's the wire that fried?...

Major failure, somewhere. I question why an "Ammeter" needs an umbilical to the RX to measure current in those cables? The Rx throttle should be connected to ESC, not the meter. As they say, it shouldn't matter if you hook it up between Bat & ESC or ESC & motor (same "power" w/ESC losses that create heat)....

(Edited to add) Perhaps you need J for Ammeter power? Still major failure.

 

[RustyUs]

It looks like a Futaba "J" jumper connected to that "thing", and that's the wire that fried?...

Major failure, somewhere. I question why an "Ammeter" needs an umbilical to the RX to measure current in those cables? The Rx throttle should be connected to ESC, not the meter. As they say, it shouldn't matter if you hook it up between Bat & ESC or ESC & motor (same "power" w/ESC losses that create heat)....

(Edited to add) Perhaps you need J for Ammeter power? Still major failure.

The analyzer has that Futaba J style three wire input only as a battery input option. It was never meant to be connected to a receiver. Instructions never show receiver connected to power analyzer other than on the LOAD side.

Yes, I'd hook up the ESC/receiver/servos as you said...

 

[RustyUs]

I'm sure there's a logical explanation because I've just shorted it but I can't quite wrap my head around it

Attached two photos immediately after

Basically I just bought a Power Analyzer, 4.8V to 60V 130A High Precision Watt Meter off Amazon with goal of doing some motor load measurements

Connected the Load to Motor
Connected the Source to ESC
Connected servo cable to Channel 4 of Receiver

Did a test, got a good measurement, then figured using the RC power switch would be the safest

Wrong, immediately the receiver cable that was connected from analyzer to receiver started to fry / smoke

It does my head in because I figured that switch would power off everything.
i.e. ESC would have no power, i.e. no power via analyzer cables and no power via servo cable

Can someone walk me through it?

Next time I'll pull out receiver cable then power off the car, unless there's something obvious I missed in this scenario?

I should have taken the time to cut/paste speed control into photo earlier. I pretty sure this is how you wanted to hook things up...

I hope this makes sense.

 

[mechkbfan]

2 different power sources, 2 different voltages and it smoked.

Instructions do say I can use aux power (thanks to those that posted above)

Sorry about lack of replies, sick kids again

 

[mechkbfan]

What are you testing?

Admittedly doing it from the battery would have sufficed

It was more out of lazyness that I went from ESC

My motor had the bullet plugs to battery, and I didn't have any female 5mm ones to use (I've since ordered)

While my ESC to motor, I had plenty of spare connectors to solder to watt meter

If you have an adjustable bench top power supply, you can set the voltage and current limits, which means you can get or calculate Wattage directly from supply.

Yes, my goal was to find out how much amps certain motors were drawing, and how much they're impacted by load and gearing

e.g. There's a lot of information out there that doesn't seem backed by data

 

[mechkbfan]

Also, your motor will not be under a full load if just doing some sort of bench testing with no resistance to the drivetrain. Strap that analyzer onboard and let 'er rip.

Yes, I have a project there too. I'm using some longboard trucks & wheels strapped to a plank of wood that's 257mm apart (TT02) then putting some barriers to stop it from falling off

Eventually I'll put an metal disc on inside of each wheel + magnets to apply resistance

Still a lot to be worked out but first step was just getting things measured