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ThunderDragonCy

Multimeter motor current measuring help

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Help needed? I would like to measure the current draw on my touring car motor to close to 6 amps to get the timing right, but i am a complete dunce about electricty. If i buy a basic multimeter, do i just touch the positive and negative points on the meter to the aporopriate outlets on the battery and open the throttle at full load? Is that the current draw I need to measure? If it isn't can one of you learned people tell me what is? Thanks. 

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The meter needs to be in series with the motor in the circuit.  Batt+ --------- METER+ --------- METER- --------- Motor+ ---------- Motor- ---------- Batt-

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44 minutes ago, LongRat said:

The meter needs to be in series with the motor in the circuit.  Batt+ --------- METER+ --------- METER- --------- Motor+ ---------- Motor- ---------- Batt-

Thanks! 

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When you go shopping for the multimeter, make sure it has a setting for 10 amps or 20 amps of current.  Usually multimeters have more than two jacks on the faceplate for different functions; you use the basic jacks for volts/milliamps/resistance/continuity/diode check.  Then you use a different jack for the 10 amp/20 amp current measurement.  If you try to measure amps of current in the 200 mA setting, there's a good chance you'll blow a fuse inside the meter.  That's why there's a dedicated jack and a specific meter function setting for amps of current; it's a different internal circuit.  LongRat's diagram is right, but instead of Meter+ it's usually the 10A/20A jack.

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Electric motor also needs to be put under normal working load for it to draw normal working current.

Its like dyno'ing your WRX, you need to run it under drag load on a rolling road dynometer - you don't just freerev the engine which tells you nothing about its power.

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I'm going to echo what @WillyChang said; without some sort of load your numbers will be meaningless.

I'd maybe give this a try since it's only $23 and will log the peak amperage while under load:

https://www.amazon.com/dp/B07LBDGM4J/ref=sspa_dk_detail_1?psc=1&pd_rd_i=B07LBDGM4J

It has an inductive sensor, so you don't even need to contact the wires for an accurate reading.

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The 6amp figure is based on absolutely no load (pinion removed or motor out of car), and while it is not the most accurate way to tune the timing it will at least get you in the ballpark (+/- 5 degrees). The most accurate way to set timing is with a dyno to measure peak power. A less accurate way is to measure lap times but this introduces a problem as you're always testing with the same gear ratio so you will arrive at the most optimal timing setting for your current gear ratio, but a different gear ratio and a different timing setting may actually be faster because adjusting timing adjusts not only power output but also the kv of the motor and kv can compensate for gear ratio.

Putting any kind of wired multimeter in series with the battery will change how the system behaves under load as the resistance of a typical multimeter and leads on a 10A range is in the ball park of 0.1ohms. That may not seem like much until you realise that even a mild motor like a 21.5t brushless draws around 20-30A in it's power band. 30A*0.1ohm = 3V drop across the multimeter! Measuring the peak current while driving will be meaningless for tuning timing as it will just be as much current as your ESC and battery can muster when the motor is stalled i.e. when you punch it from a standstill, peak current will occur. This will likely be 80A+ for 21.5t on 2S with the punch turned up, which obviously does not work if you have a 0.1ohm multimeter in the circuit as the multimeter will drop a tonne of voltage and prevent 80A from ever being achieved. At 6A the multimeter is only dropping 0.6V which while not ideal is somewhat more managable and introduces minimal effect on setting the timing. Ideally you want a DC clamp meter, which measures the current going through a cable without needing to break the circuit.

 

 

 

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On 9/16/2019 at 4:43 PM, OCD said:

I'm going to echo what @WillyChang said; without some sort of load your numbers will be meaningless.

I'd maybe give this a try since it's only $23 and will log the peak amperage while under load:

https://www.amazon.com/dp/B07LBDGM4J/ref=sspa_dk_detail_1?psc=1&pd_rd_i=B07LBDGM4J

It has an inductive sensor, so you don't even need to contact the wires for an accurate reading.

The inductive sensor is for AC only.

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On 16 September 2019 at 4:02 AM, ThunderDragonCy said:

Help needed? I would like to measure the current draw on my touring car motor to close to 6 amps to get the timing right, but i am a complete dunce about electricty. If i buy a basic multimeter, do i just touch the positive and negative points on the meter to the aporopriate outlets on the battery and open the throttle at full load? Is that the current draw I need to measure? If it isn't can one of you learned people tell me what is? Thanks. 

OP doesn't quite say whether he's fiddling with a Brushed or Brushless motor either... ;) 

No idea what motor setting would draw 6A with motor freerevving unloaded (pinion removed, no drivetrain drag) but IMHO that's a fair bit of energy (30-40W?) being dissipated doing no work. Imagine a 40W incandescent lightbulb, that'll be similar level of heat generated.

Assuming it's a brushed motor, IMHO that's either in very bad shape or it's got an internal short circuit to draw 6A unloaded.

I used to test freshly rebuilt motors on a bench powersupply, with no load they'd barely draw 1A at 6.0V if memory suffices.

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6A is for brushless 540 motors. Most brushless 540 motors will be set for 2-4A no load current out of the box and it only requires a handful of degrees adjustment to get them up to 6A. 

If you adjust the timing to achieve minimum current draw, you've adjusted the motor to 0degrees timing, which is not where the motor produces peak output power. 6A no load is a rule of thumb that generally corresponds to peak output power (give or take 5-10%) but definitely not peak efficiency. A good 21.5t motor at moderate timing produces about 120W of mechanical power at about 50-60% efficiency (80-120W as heat) at half it's max rpm, or about 80W of power at 75-80% efficiency (20-25W as heat) near three quarters of it's max rpm. At 0rpm it produces no mechanical power, uses hundreds of watts of power and has 0% efficiency. That's why a motor runs HOT when you put long gearing in the car, because it spends more time at lower rpm where it has poorer efficiency. 

Increasing the timing also increases the Kv of the motor, which has the same effect as longer gearing, therefore it will run hotter. As long as your no load current isn't ridiculous (like >15A), and you readjust your gearing to compensate for a change in Kv, the reduction in efficiency of the motor should be manageable. Adding no load current just adds a static amount of heat output at every rpm. If you're already chucking away an average of ~50W as heat at 0deg timing, it's not a big deal to make that ~70-80W if it gives you 20%+ extra power on track. If all you care for is reducing temps, just set to 0 deg. 

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Ah ok, brushless. Interesting RoT. 

So that 6A current is only measured between battery & ESC.

Can't stick meter between motor & ESC, no idea what you'll get - probably Smoke! :) 

 

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