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btn7971b pwm frequency notes

Why is my PWM efficiency about 50%?

Ask QuestionAsked yesterdayActive yesterdayViewed 130 times2

I’m using a BTS7960 module with an Arduino to adjust the voltage for my DC motor.

When I check efficiency there is a problem. PWM efficiency is supposedly about 90% but mine is only about 50%.

I have a 7S3P 29V li-ion battery and a 12V DC motor. While I was using it 50% PWM mode, I calculated the efficiency of power consumption and I realized that I almost lost half power in PWM.

How I measured the efficiency:

  • I put a clamp meter between the battery and the BTS7960 and I have check voltage “B +” and “B-” terminals. Voltage is 29V and current is 11 amperes so my battery is giving 320 watts.
  • Then I checked the consumption of the DC motor. I put a clamp meter between the BTS7960 and the DC motor and I measured voltage “M+” to “M-” terminals. Voltage is 13,6 and current is 13,8 amperes so as I understand my DC motor consumes 187 watts.

Either my PWM module efficiency is very bad or I’m doing something wrong.

The BTS7960 module became really hot while testing.

If you ask why I dropped 29 volts to 13 volts and drive the DC motor, it is because I have a 7S3P li-ion battery and I have a 12 volt Dc motor. If I can overcome the problem, I will drive a 24 volt DC motor with a 7S3P battery.

What is the reason for the high power loss?

(edit)Additional information: My clamp meter is Unit UT210E. Also you can reach datahseet this link:

UT210E English Manual

Also I measured current with multi meter(Unit UT39C)

Lastly I made same circuit with dc-dc convertor. The energy loss in the measurement was very low and I saw this as normal aslo With dc-dc converter, the battery and cables was not very hot, but in pwm modulation, the battery, cables and bts7960 integrated was very hot.

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arduinopwmpower-electronicsdc-motorShareCiteEditFollowFlagedited yesterdayStainlessSteelRat6,22022 gold badges1313 silver badges2828 bronze badgesasked yesterdayoğuz kaan çomoğlu2144 bronze badges

  • 5Your clamp meter may not like reading pwm and will lie to you. The BTS7960 is expected to get hot if there’s 11Amps flowing. It is not a perfect switch. – Kartman yesterday
  • 1Does your clamp-on meter measure DC amps? Please edit to include the make, model and link to datasheet. – Transistor yesterday
  • @Kartman Yes, I thought so, but then I measured it with a multimeter. The result turned out to be the same. – oğuz kaan çomoğlu yesterday
  • 2Agree with @Kartman, you must be making an errant measurement. I can’t think of any scenario where you could be this inefficient. You would have to be dissipating 320-187 or 133 watts in your circuitry. – John Birckhead yesterday
  • @Transistor My clamp meter is “Unit ut 210e”, also I measured current with multimeter(Unit ut39c) – oğuz kaan çomoğlu yesterday
  • @Transistor drive.google.com/file/d/0B4Jyby-tjH5oNnlQQmY1RktSUnc/view datasheet for clamp meter – oğuz kaan çomoğlu yesterday
  • If you were dissipating all of that extra 133W in the driver circuit then it would be hot enough to cook on. It’s far more likely that you clamp meter is not capable of giving accurate current readings on the PWMed motor side. – brhans yesterday 
  • 2Please edit all the information into the question. Don’t sprinkle it through the comments where people have to search for it. – Transistor yesterday
  • @JohnBirckhead I want to tell you about a situation I observed, I tried the same circuit with a dc-dc converter. The battery and cables didn’t get too hot but when I used pwm modulation battery and cables became really too hot. So if the measurement was wrong in the first method, wouldn’t the battery be at the same temperature when I used a converter? – oğuz kaan çomoğlu yesterday
  • 1@oğuzkaançomoğlu again, please edit your question to include all information you gave here in the comments into your question. Thank you! – Marcus Müller yesterday
  • @oğuzkaançomoğlu no, battery temperature has little to nothing to do with all this. – Marcus Müller yesterday
  • 1) Cables getting hot can be cured by using thicker cables – what gauge wires are you using for the high-current paths? 2) What is the current rating of the DC-DC convertor that you used? Did you check its output voltage during the test? – Andrew Morton yesterday 
  • #oğuz kaan çomoğlu, I found your question interesting. I happen to have a BTN79xx driver module and a Rigol DS1054 50MHz scope to display the V-I waveforms. So perhaps I would spend this gloomy lock down Saturday afternoon to do some PWM efficiency evaluation. Now a couple of questions: (1) I read the UT201 clamp meter manual and I am surprised to read that it is for both V-I of AC and DC, as accurate to the order of 1mA. / to continue, … – tlfong01 1 hour ago   
  • / continued, …Or please let me know how you calibrate you meters. (2) I only have US$5 cheapy multimeters with the max current range of only 10A. So I will be doing small currents tests under 10A. I wonder if you have already done low current tests OK, and only found problem with bigger currents? Comments and counter suggestions welcome. Cheers. – tlfong01 1 hour ago   
  • #oğuz kaan çomoğlu, (3) BTN79xx handles huge current, so it is important to sense current for alarm and security, (also for speed feedback control). You might like to skim my answer to the following question, to get a rough idea of how to do hardware fiddling for current sensing. (3a) Anything wrong with connecting L298Ns in parallel? electronics.stackexchange.com/questions/556772/…. (3) Are you OK to do DIY soldering work, like what described in the above question? – tlfong01 20 mins ago   
  • #oğuz kaan çomoğlu, Your yr2004 7960 is similar to my yr2008 7971b, except a couple of years older. (4) Do you have any reason to prefer one over the other? If not, I would write an generic answer applicable to both drivers. (5) 7971B’s PWM freq is max 25kHz. I usually start off with a very low frequency, say 1kHz. (5) Do you have any particular preference on PWM frequency? If not, I will start with 1kHz, 50% dc. BTW, I for reliability/repeatability, I will us a cheapy manual/uart control digital sq wave sig gen for PWM test. Ah, lock down lunch time. See you later. Cheers. – tlfong01 1 min ago   Edit   

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1 Answer

ActiveOldestVotes4

Your multimeter and your clamp meter are both not meant to observe PWM going through an inductive devices.

You need an oscilloscope with a high-bandwidth method of sensing current. I don’t know your PWM frequency, but rule of thumb: estimate how fast the transition from fully on to fully off has to be, take the inverse of that time (yielding a frequency), and take 5, better at least 7 times that frequency as minimum bandwidth if you really want to see what happens on that cable. It’s not DC, by any meaning of that word.

Current measurements can be quite tricky, so you might end up getting a hall effect sensor IC with something like 400 kHz or 1 MHz of bandwidth, and just live with current components that you cut off through that not-really-sufficient bandwidth.ShareCiteEditFollowFlaganswered yesterdayMarcus Müller67.3k44 gold badges100100 silver badges179179 bronze badges

  • I check the pwm frequency with oscilloscope and it was 500HZ and duty cycle was %50. I just want know what is my real power consumption ? Or can I say is my real power consumption can measure between the battery and the BTS7960 (29 volt * 11 amper) because there is no modulation on this cable. And lastly how I can measure current after pwm modulation ? – oğuz kaan çomoğlu 14 hours ago 
  • 1. 500 Hz sounds very low for a PWM frequency. 2. it’s not about the frequency, but about the bandwidth, which is defined (mostly) by how steep the edges between on and off are. – Marcus Müller 13 hours ago
  • 1- Yes I read somewhere about this, it’s making noise and I check my hedge trimmer it’s working with 50KHz pwm(%75 duty cycle). 2- You mean duty cycle ?(I’m sorry I am asking because I do not have enough information. Maybe I can bother you) – oğuz kaan çomoğlu 13 hours ago
  • no, I don’t mean duty cycle. I mean edge speed (as I said three times now), the time it takes to go from “on” to “off” (and vice versa). – Marcus Müller 13 hours ago

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