How to measure the dc current running through a dc motor for RC helicopter?

[efron]

Hi all,

I've being trying to measure the current delivered to a dc motor used to move the blades of my rc helicopter.

When activated through the rc emitter, the output of the rc receiver is giving approx. 7.4 volts without motor connected.

When I connect the motor, the output voltage drops to about 4 volts. I guess this is normal because of internal output resistance of the receiver and due to the current flowing out of the power circuit.

The motor speed is handled by changing the output current through a power mosfet.

However, I've tried to measure the current through the dc motor with a multimeter in current measurement mode (and in serial of course). When I do that, I feel how the current strongly decreases because the motor speed is slower than without multimeter (in this situation the multimeter is displaying 40mA).

I suppose that what is happening is that the multimeter's resistance in current mode, even if its effective resistance is very small, is somehow similar to the effective resistance of the motor.

How could I do to measure the real current through the motor?

Any idea is welcome,

Thanks

[G7PSK]

What type of multimeter are you using Digital or moving coil. If it is a coil meter and the current is pulsing it may be acting as an inductance.

[icon]

Hi

So you have a receiver connected to an ESC which then drives the motor (brushed or brushless?)? I'd be surprised if the ESC presents that much of a load to the receiver, to cause the output voltage to drop like that, though I admit I haven't looked at it. But how are you measuring? The output of each channel of a receiver is not a straight DC voltage - it's a variable width pulse. The pulse width varies according to the intended servo position (or ESC output, in this case). I can't remember the precise details, but it's in the order of 20ms. You'd have to be looking at a scope to see the pulse amplitude.

Similarly, the ESC is PWM'ing the power to the motor varying between pulses too short to actually overcome the inertia of the rotor, which just makes the motor 'sing' at the PWM frequency, to 100% duty cycle. The MOSFETs are fully on or fully off. Then the actual current drawn depends on the load on the motor - high angle of attack 3d stuff draws welding current. You couldn't measure that meaningfully with a multimeter.

I have an Eagle Tree data logger which records info over the course of a flight, and can tell you volts/amps/watts with time and average over the flight. Very useful for checking when things are going wrong, and for finding efficient pitch/throttle curves.

Sorry if this is all obvious.

John

[M. András]

some places says the signal from the reciver is 50hz 5 volt with varying pulsewidth . however if you want to measure the current to the motor, connect the meter or anything else before the esc not after

[ToBeFrank]

When I connect the motor, the output voltage drops to about 4 volts. I guess this is normal because of internal output resistance of the receiver and due to the current flowing out of the power circuit.

The voltage drop is when the motor is running I assume? If so, I'd bet the drop is from your battery. At 7.4V I'm assuming you're using a two cell lipo. Just as an example, if each cell has an internal resistance of 20mOhm, that's 40mOhm total. Using Ohm's law with 3.4V drop and 40mOhm resistance, the motor is drawing 85A, which is in the ballpark for a RC helicopter.

[PA0PBZ]

Or you just count the electrons?

https://www.eevblog.com/forum/general-chat/new-nanodevice-creates-a-near-perfect-electron-stream/

[M. András]

When I connect the motor, the output voltage drops to about 4 volts. I guess this is normal because of internal output resistance of the receiver and due to the current flowing out of the power circuit.

The voltage drop is when the motor is running I assume? If so, I'd bet the drop is from your battery. At 7.4V I'm assuming you're using a two cell lipo. Just as an example, if each cell has an internal resistance of 20mA, that's 40mA total. Using Ohm's law with 3.4V drop and 40mA resistance, the motor is drawing 85A, which is in the ballpark for a RC helicopter.

i guess you meant mOhm 

[ToBeFrank]

i guess you meant mOhm 

Yes! Corrected.

[icon]

the motor is drawing 85A, which is in the ballpark for a RC helicopter.

That's a pretty broad ballpark. My Mini-Titan hovers quite happily at 15-16A and even batting around doesn't draw more than 25A for brief periods. It would have to be a pretty sizeable heli. I would think it likely that the OP is using a separate regulated supply ("battery eliminator circuit") for the receiver.

No doubt the OP will be along to tell us in due course...

John

[dbinokc]

If you are measuring tens of amps, perhaps a current shunt would give better results. I bought one a while back, for measuring some motor currents, from ebay for about $10.

[T4P]

uhhhh ... DC current clampmeter?

[Psi]

The PWM current can't be measure with the DMM.
If i was you i'd forget the DMM and find an old analog meter out of something.
Calibrate it with a parallel resistor to get the scale right and put that in series with the motor.

[efron]

Hi,

The rc helicopter is a model T23 623 thunderbird (see attachment).

It has two dc motors (see attachment).

The MOSFET used in the power output circuit is a model STB80N (see attachment). According to its datasheet, it can handle up to 80A and 150W at 25°C. However, there is no heatsink on it and its thermal resistance Rthj-amb = 62.5 °C/W.

This limits its use without heatsink to a maximum power of about 2.5W (knowing that the motors are not stressed at 100% all the  time).

When the receiver is activated (at whatever level of ordered power by the rc emiter), the MOSFET is used as a ON-switch and the output is a dc output (with some noise). I've checked this with an oscilloscope but without motor connected.

I don't know the drop voltage in the MOSFET when it is ON. Let's imagine about 0.5V which I guess is really good. In this case, the maximum current must be limited to about 5A.

The battery nominal voltage is 7.4V (LiPO 2S) and 1500mAh. Do I have to assume that it is limited to 7.4x1500mA = 11W ??

All this seems to be a good estimation (on the paper).

In any case the current through the dc motor can't exceed the 5A for a long time or the MOSFET will be destroyed.

[icon]

Hi

The headline feature of the MOSFET in question is the R_DS(on) - 0.005 Ohms. Because the device is either fully on or fully off, it's just acting as a switch and not itself dissipating much power. For example if there were 20A flowing through it it would be dropping (10*0.005) 0.1V. The power dissipated would be 0.1x20 = 2W. And that's assuming there's only one - often there are a number in parallel. I would think the average current flow would be much less than that. (Someone will be along in a minute to tell me that's all cobblers)

The 1500mAH figure is just a nominal capacity - you should be able to draw 1.5A for an hour. In theory you could draw 3A for 1/2 hour, 6A for 15 mins, 12A for 7.5 mins... in practice the capacity diminishes rapidly with increasing current draw. My Mini Titan takes about 200W to hover. No doubt someone with better maths than me could work out, given the weight, what the theoretical power required to counteract gravity would be, and therefore how ridiculously inefficient model helicopters are.

John

[T4P]

I'm not going to do the maths but LiPo's ain't normal batteries
They're wicked monsters, they can allow you to discharge at many tens of multipliers without danger