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Topic: Oscilloscope tests with a driver (Read 4040 times) previous topic - next topic

Serge_Brien

Hi everyone,

  I have been using my Arduino in order to control a 350W Electric Motor 24v trough PWM (speed and direction). But before I assemble joystick + Arduino + driver + dc motor I wanted to check each component..
  By the way, I already saw with the oscilloscope that the code is doing what I wanted (desired frequency and duty cycle at the output of the Arduino), but since I don't want do burn the driver or damage the motor.. I wanted to check what values are coming out of the driver.
What tests would you advise me to do just to be sure everything will work fine when I assemble the motor?

Thanks in advance!

P.S - My only idea is when I connect the arduino and the driver, add a resistance next to the output pins of the driver and see the values on the oscilloscope 

Erdin

#1
Apr 26, 2013, 07:23 pm Last Edit: Apr 26, 2013, 07:26 pm by Erdin Reason: 1
The driver will perhaps work with 6V or 12V.
Use a simple 12V power supply with a low current, 500mA or so, and a tiny 12V motor.

Next step is to use a light bulb in the 24V line.
If there is a shortcut, the light will turn on. Perhaps a halogen light bulb of 24V, 150W. Those can be as cheap as 2.50 euros. If you want to increase the current, use two light bulbs parallel.


MarkT

If you have access to an adjustable current-limited bench power supply this is useful for
testing at low current, gradually cranking it up as confidence is gained.  Any issues can
be spotted before a component melts/explodes!
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Serge_Brien

Thanks for the answers Erdin and MarkT. Sorry, but I was only able to check them  today because I had a lot of work this week..

Erdin, the driver I have has an operating voltage of 15~24V. I have a small dc motor from a toy and a power supply that can achieve 24V. So, this weekend I will definitely try that suggestion and see if the small motor speed and direction changes!
However, I didn't understand the light bulb test. The purpose of that test is: if there is a shortcut and consequently the light turns on, it means that there is current "leaving" the driver?

MarkT unfortunately I don't have current-limited bench power supply.

Erdin

A light bulb is only to prevent big shortcut currents in case something is wrong.

MarkT

Yes, a lightbulb is an easy-to-locate substitute for a high-power load resistor - with the added bonus
you can see if a lot of current is flowing or not.

Basically any means of current limiting is good when commissioning a high power system - if something
goes wrong its less likely to overheat rapidly and burn out.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Serge_Brien

Now I understand  :) Thanks again!
I will tell you afterwards if the driver is working.

Serge_Brien

The driver works with a small motor (6V and 12V). Although, when I used the 12V dc motor I barely noticed a difference while changing the speed from a (128) duty cycle to a (255) duty cycle.
However, when I tried with a 36V DC motor, it didn't rotate..
I'm not sure if the question I'm going to make fits in this post. The timer2 of my arduino is using 31KHz, and I was supplying my driver with 15V (operating voltage of 15~24V). Also I read somewhere that it's not a good idea to suddenly change the direction of a powerful motor like this.. Sorry if it's a basic question, but it's the first time I'm doing a project like this.
Do you have any idea what could be the problem? (should I write this on another topic)

afremont

31kHz might be too high for the 36V motor.  Inductance will slow the build-up of current thru the windings.  If the windings have too much inductance for the frequency you are using then the current may not build to sufficient level to turn the motor, before it (the current) turns off each cycle.  But at 100% duty cycle it should still run just fine if it's a regular DC motor, unless the voltage is just too low for it.  Depending upon the controller you are using, there could be significant voltage drop in the driver transistors.
Experience, it's what you get when you were expecting something else.

Serge_Brien

Thanks for the quick response afremont!
Please correct me if I'm wrong, like I said before I'm still new on this subject. I have been doing some research and I think the problem is not related with the frequency (31KHz).
- I used 31KHz because the technical parameters said this, Input PWM signal: Duty Cycle 0 ~ 98% (not more than 98%), the standard 16KHz highest 100KHz. And I though the frequency was only important for "activating" the driver, because we can only see current and voltage at the output of the driver. However you're telling me that the frequency affects the inductance, and consequently it interferes with current.
- What about the inertia moment, that is considerably higher on the 36V dc motor than on a 6 or 12V dc motor. This associated with the fact that I was using a power supply with 15V and the current was lower than 1A, could explain why the 36V motor wasn't moving (and the 12V motor wasn't running as quick as the 6V motor)

I think if I use 2x12V batteries in parallel I would have enough current for the 36V dc motor.. what do you think?

Thanks in advance!

afremont

Are those the technical parameters for the 36V motor?  I don't think adding another battery in parallel will do it, but you can try because it pretty much can't hurt anything.  I think you need more voltage, not more current.  Can your motor controller take 24V, if so you should be able to put the batteries in series.
Experience, it's what you get when you were expecting something else.

Serge_Brien

Input PWM signal: Duty Cycle 0 ~ 98% (not more than 98%), the standard 16KHz highest 100KHz These are the technical parameters of the driver.
- Yes, the driver I have has an operating voltage of 15~24V.
- You're right, I meant to say 2x12V batteries in series  ;)
Even though the driver only supports 24V, as long as I'm careful enough and don't forget to disconnect the driver while the batteries are charging I think won't face any problems (such as burning the driver)

Sorry to insist, but I still don't understand the relation between frequency and inductance? how that affects the motor, since I'm only receiving current and voltage from the driver.

MarkT

Inductance as it relates to PWM is fairly simple - for lower inductance windings you will need a higher
frequency to reduce current ripple (one source of power loss and unwanted heating).

If the frequency is too high then iron magnetic losses will start to dominate (these are proportional to
field reversal frequency and other factors).

The basic equation is   L dI = V dt 
(or put another way rate-of-change of current is  V / L)
This means that higher voltages or lower inductances allow faster rates of current change.

For PWM you want the current change to be small (a ripple superimposed on the main current),
so higher frequency allows less time for current to change).

For a DC or brushless motor to be able to turn quickly you want low inductance windings so the winding current can
change fast enough as the rotor spins.  For a standard DC motor that applies to the armature windings, for a brushless
or stepper motor it applies to the stator windings.

However even low inductance windings are inductive enough for PWM in 4kHz and up range I reckon.
16kHz is often used as its out of the range of most adults' hearing.

Faster PWM can mean more losses in the driver if its not fast-switching.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Serge_Brien

Thanks, I'm really learning a lot! but I'm also a bit confused.

Quote
If the frequency is too high then iron magnetic losses will start to dominate (these are proportional to field reversal frequency and other factors).


I understand that at higher frequencies (such as: 31KHz, like I was using) DC motors have significant iron losses.I though that at the output of a driver would only came out continuous values of voltage and current: di/dt = V/L = 0 (pure d.c. voltage, because the motor's inductance keeps the current essentially constant during the switching cycle).

Quote
However even low inductance windings are inductive enough for PWM in 4kHz and up range I reckon.16kHz is often used as its out of the range of most adults' hearing.


Like I mentioned before, the driver I'm using has an Input PWM signal: Duty Cycle 0 ~ 98% (not more than 98%), the standard 16KHz highest 100KHz. This means that the driver high-power N-channel MOS transistor (a single full-bridge) will switch at 16KHz? I read on some articles that a frequency around 20 kHz is used to avoid noise from the motor in the audio range. I think if I use a frequency between 16KHz and 20KHz the motor will work just fine.

Quote
Faster PWM can mean more losses in the driver if its not fast-switching.


Can you tell me if my logic is correct? The switching voltage of the MOS transistor requires high transient current. Since this driver doesn't have a datasheet, I don't know the value of that high transient current.. Is there a nominal current value to these driver's? the technical parameters only said that the rated/peak current is 40A/80A, and the Supply voltage signal output: 0 ~ 5V corresponding to 0 ~ 26V; Supply current signal output: 0 ~ 5V corresponding to 0 ~ 91A.

By the way, the brushed DC motor I'm going to buy for my project has this specifications (I only used the 36V dc motor because it was the closest I had)
•Operating voltage: 24 volt
•Power: 350 Watt
•Rated Current: 18.7 A
•Rated Speed: 3000 rpm

(sorry for the long post and full of questions, thanks for your patience ;) )

Serge_Brien

Do you advise me to create a new topic?
Since I'm no longer writing questions about oscilloscope tests, but asking questions about the driver electronics..
- Is an high-power driver N-channel MOS transistor ( single full-bridge, standard frequency of 16KHz) fast-switching?
- Usually these driver's allows reverse current? For instance, when the motor is braked. (I'm using a joystick to control the speed and direction of the motor, and I'm afraid I will damage the motor if I suddenly switch the direction)

I really didn't want to buy a motor without being sure of all this..

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