measure a voltage across of a dc motor

[mariomoskis]

hello,i am trying to measure a square wave(pwm) with amplitude 3V, so when i try to measure it with a multimeter across my motor,should i watch the next:

Vmultimeter=3*sqrt(duty cyle)

is it correct? because sometimes i am watching more than 3V on my multimeter across the motor about 3.7V but i think that it shouldn´t be higher than 3V.
i think that it can be caused about the inductance of the motor but i try to put a zener diode of 3V at the output of the AO but the problem is still happening.

somebody could help me with that? the Vcc=6V and the multimeter is set between the output of the power operational amplifier and ground

thanks

View attachment 63321

 

[cachehiker]

What is the frequency of the PWM involved?

Most multimeters aren't designed to be accurate with high crest factors or at high frequencies associated with PWM inverter drive outputs. Even the fancy Flukes here at work have issues accurately measuring the output of a VFD. We use an oscilloscope, download the raw data to a flash drive, and analyze it in Excel or Matlab when accurate average or RMS voltages are absolutely essential.

 

[mariomoskis]

the frequency is between 100Hz and 500Hz,it won´t be higher

ok,i have check the measure at the input of the AO and i have 1.5V with duty cycle 50%,and 3V with duty cycle 100%, so with the multimeter i am measuring avegare or effectuve voltage?

i think that the formulas are: Vaverage=Vmax*(duty cycle) ,is it correct? and what about the Veffective? which is the different between both voltages(average and effective)?

 

[alec_t]

Vmultimeter=3*sqrt(duty cyle).... is it correct?

No, as your measurement shows.

i have check the measure at the input of the AO and i have 1.5V with duty cycle 50%,and 3V with duty cycle 100%, so with the multimeter i am measuring avegare or effectuve voltage?

The average voltage at the opamp input is indeed 1.5V and 3.0V for 50% and 100% duty cycles respectively. So if your meter indicates those values then it is clearly measuring average voltage ok.
If the motor voltage measured by that meter is > 3.0V then either the spikey waveform is distorting the reading or the diode/opamp combination is not fast enough to suppress fully the motor back-emf spike.

 

[mariomoskis]

maybe the the diode/opamp combination is not fast enough to suppress fully the motor back-emf spike because with the occilloscope i saw a wave which is in 3V(high state) and 1V(low state) and it should be 0V in low state,yes?
how can i solve this? because the frequency isn´t higher than 500Hz,it shouldn´t be a problem i think.

 

[alec_t]

it should be 0V in low state,yes?

No. When the motor drive is switched off the motor continues running but is acting as a voltage generator.

 

[KeepItSimpleStupid]

One question: Are you using a TRMS meter?

 

[cachehiker]

1V(low state)

That TCA0372 saturates at about 1V above the negative supply rail. Under those conditions, the motor is generating a voltage and the op-amp is attempting to sink it to ground, decelerating the motor.

 

[mariomoskis]

That TCA0372 saturates at about 1V above the negative supply rail. Under those conditions, the motor is generating a voltage and the op-amp is attempting to sink it to ground, decelerating the motor.

so the 1V which i see at the output is about it,
and my supply is 6V and ground
and this low state should be 0.7V about the diode?

i am using a multimeter like that (in mode voltage continue)
https://www.google.es/imgres?q=mult...41&start=24&ndsp=28&ved=1t:429,r:4,s:24,i:157

 

[mariomoskis]

and one thing that i forgot to say:

the increase of voltage across the motor isn´t linear with the increase of duty cycle,because i saw that when i increase duty cyle from 50% to 75% the motor increase its speed more than when i increase the duty cycle from 75% to 100%
why is that? should it happens?

 

[KeepItSimpleStupid]

The manual does not mention TRMS at all, it's probably a "average responding, RMS reading meter". They work by full-wave rectifying the input and applying the result to a capacitor so that it gets averaged. They can use a precision rectifier. Then it get multiplied by a fudge factor to read RMS of a sine wave input only. You change the waveshape within the frequency bounds and all bats are off.

 

[audioguru]

It looks like a very cheap meter. It can measure DC or the 50Hz or 60Hz sine-wave from electricity mains.

Don't you understand that your opamp acts like a brake when its output is low on each cycle of the PWM?

 

[KeepItSimpleStupid]

That amp is designed for a servo. Break the line from the output to your motor/diode combination. Replace that wire with a diode where the (+) of the diode is toward the motor.

When the amp is at 0V, the motor is trying to operate into zero volts. That effectively "stops" the motor from rotating.

 

[mariomoskis]

if the data sheet of the motor says that the maximum current no load is 0.3A is there a problem is that current for my case is 0.4A?? even if the data sheet says too that at maximum efficiency that current can be 1A

 

[alec_t]

if the data sheet of the motor says that the maximum current no load is 0.3A

It doesn't say that. 0.3A is the specified current when the motor is running under no-load conditions. It doesn't say anywhere what the maximum current is (which would be when the motor is stalled, e.g. at the instant of start-up).

 

[mariomoskis]

and i thought about to set a power resistor 1ohm or 0.5ohm ,between the output of the AO and the parallel wire (motor diode),
and i will know the current which is really at the output of the AO
to do it,i will check the voltage across the resistor with the osciloscope and later calculate the I, so i will be able to know aproximately the current which is really at the output, what do u think about it?

 

[alec_t]

It's a good idea to measure the current by using a scope to determine the voltage across a series resistor, but if you can find one less than 0.5Ω it would be less likely to affect the motor performance.

 

[mariomoskis]

it is difficult to see because the waves are distorsionated about the inductance of the motor,but more or less i saw a different of voltage between the output and the wire parallel of 0.4V,with a power resistor of 0.5ohm,so tyhe current is about 0.8A,no load

so maybe when i use the gearbox this current will be higher,and my AO only can have 1A at its output,so i am sure that i am going to have a higher current, then this is why the AO is getting hot, i think
is there a way to don´t have this problem?

 

[alec_t]

is there a way to don´t have this problem?

Yes. Use a driver amp with greater current rating, or a less powerful motor.

 

[mariomoskis]

and one thing more is that the average current consumed for the motor no load, is higher than 0.3A because i saw 0.36A,so i must reduce this current if i don´t want to burn the motor

i tried a power resistor of 1ohm in with the motor the motor and it got,but of course the AO is hot too,maybe that the AO is hot,isn´t a problem if i don´t hurt it so much