Hello everyone,

This is the first time I have ever written on a forum and this is my first electronic project,
My name is Timothee I am an electrical hobbyist and I am working on a project where the goal is to create a functional electric outboard motor, I am trying to create an electric controller for a brushed DC motor (80amps continuous output)
The problem ?
Everytime I try using the controller in my garage with the controller everything is fine it can run forward and backward and the RPM changes accordingly to the PWM signal send by my potentiometer (see schematics). But once in the water i start accelerating slowly to not overload the motor the 4 parallel N channel mosfets (IRFZ44N, Id=49Amps continuous, Vdss=55 volts) explode... And I do not understand what is wrong with my schematic (very simple with no programming I am still learning progressively).
My assumptions:
Water creates more resistance and evidently overload the motor making it consume more amps (but since I have a BMS it lets out max 80amps) and in a perfect world each mosfet should have and Id=80/4=20A BUT I guess there has to be an increase in the junction temperature that creates a snowball effect and makes them overheat even though i used a big heatsink to counter this...
I do not know how to properly scale my heatsink to counteract the increasing junction temperature so any help would be greatly appreciated !
Also I was wondering if IGBTs where better in that case or if mosfets would just do the job ? Like if someone is working in power electronics for those application what would be the best ?

PS: PWM switching frequency is around 1kHz
Battery voltage goes from 18V to 29,4V when fully charged
Vgate/source goes from 1,2V to 13V when full throttle

If you have any question ask me i will be reactive ! Thanks a lot

Timothee

 

I didn't have the time to analyze your circuit, but I have some experience of designing DC motor controllers for electric flight. I don't think four IRFZ44s in parallel will be enough. You will probably need twice that many to handle the high current the motor will draw when it is fully loaded in the water.
Have you checked the battery voltage under full load? How does that affect the amplitude of the MOS FET gate pulses?

 

A quick look at your circuit suggests the FET gate drive is only about 14mA per FET. So the FET turn on time may be excessive, resulting in over-heating of the FETs. R304 seems unnecessary. Have you considered using a dedicated FET driver instead of a 555 ?
If the motor current at full running speed is 80A then it will be a LOT more when starting up or under a heavy mechanical load.

Id=49Amps continuous

That rating is for a FET case temperature kept at 25C. Can you heatsink achieve that?

 

The check in the garage running the DC motor without any load does not pass nearly as much current as when the motor is running at full loaf, 80 amps. So Alec is right!! And also the heat sink is not nearly large enough, nor the control system able to deliver 80 amps.

 

I have a BMS it lets out max 80amps

Are you sure it actually limits current to 80A, or is it merely rated for 80A maximum in use?

 

What is the stall current specification for the motor?

 

Are you sure it actually limits current to 80A, or is it merely rated for 80A maximum in use?

The fact that the supply you use for testing is capable of providing 80 amps does not mean that during the tests the motor is drawing 80 amps.
Within the limits of a power supply, the load current drawn depends on the load requirement, not the supply rating.. I have used my 50 amp rated power supply to test small light bulbs that do not draw even one amp.

 

I have a 30A very simple speed controller for 12 or 24V motors and it uses a single IRFB7537PBF FET, they are rated at 60V, 173A, 2.75mΩ and are designed specifically for brushed DC motor drives and similar. They are much better suited than the IRFZ44.

Another I've used is the FDP047N08 which is rated at 75V, 164A, 4.7mΩ and also the IRF1405, 55V, 169A, 5.3mΩ. There are many others, all only a couple of dollars each.

 

I didn't have the time to analyze your circuit, but I have some experience of designing DC motor controllers for electric flight. I don't think four IRFZ44s in parallel will be enough. You will probably need twice that many to handle the high current the motor will draw when it is fully loaded in the water.
Have you checked the battery voltage under full load? How does that affect the amplitude of the MOS FET gate pulses?

Hi,
Thanks a lot for all these replies thats my first experience on a forum and I didn't imagine that some people would care nough to respond to my problem ahah !
Anyways yes of course the battery voltage must be droping while a high current is drawn from it, I havent measured the internal resistance of the battery pack I made but I am guessing that at max load the voltage must be droping by a couple of volts, but as you can see on my schematics I used a linear voltage regulator to output a constant 15V (yes very ineficient to convert 18-30V into 15V but I thought that was good at first) now I am thinking of using a DC to DC step down buck converter to efficiently convert the battery voltage into a stable 15V or 12V. But for now my 15V output is stable and so the FETs gates should be seeing a stable voltage even if the motor is under load dont you think ?
I am still a beginner in electronics and I might be missing something

 

A quick look at your circuit suggests the FET gate drive is only about 14mA per FET. So the FET turn on time may be excessive, resulting in over-heating of the FETs. R304 seems unnecessary. Have you considered using a dedicated FET driver instead of a 555 ?
If the motor current at full running speed is 80A then it will be a LOT more when starting up or under a heavy mechanical load.

That rating is for a FET case temperature kept at 25C. Can you heatsink achieve that?

Hi Alec,
Thanks for the reply
I'm not sure I understand how you managed to calculate the gate drive current to be around 14mA ? Would you be kind enough to explain it to me ?
Yes well when i'm full throttle the PWM is basically a continuous DC signal (13V) and the FET should be on all the time but if i look at the datasheet i'm seeing that at Vgs=10V you should have 100A output ? (see file) But does that mean that I can't actually apply 13V to the Gate ?
Ok for R304 but any particular reasons ?
No I was learning so the 555 timer was something I understood (regarding the internal working principle) so I went with it but what would be the advantages of a proper mosfet gate driver in my case ? I'm still a student and already spent like 400$ on the build so I dont know about doing big changes to the model but I am still noting your info and opened about any proposition to make my build better !
Yes for the heatsink I was trying to implement a watercooling heatsink that would stick to the side of the controller where the mosfets are but that was my last option because I didnt know what was the problem and why my mosfets kept exploding. It is still an option but I feel like you guys on the forum will help me a lot !
Thanks again

 

The check in the garage running the DC motor without any load does not pass nearly as much current as when the motor is running at full loaf, 80 amps. So Alec is right!! And also the heat sink is not nearly large enough, nor the control system able to deliver 80 amps.

Yes thanks for the reply I totally agree

 

Are you sure it actually limits current to 80A, or is it merely rated for 80A maximum in use?

This is the BMS i'm using : https://fr.aliexpress.com/item/1005..._main.256.528c5e5b2BvPLS&gatewayAdapt=glo2fra

I think it is rated for max 80A output in use, here are the specs:

 

What is the stall current specification for the motor?

Hello,
Is the stall current the current at no load ? If it is then it's 3,2A here are the motor specs (E30-400-24 the third one from the left):

 

The fact that the supply you use for testing is capable of providing 80 amps does not mean that during the tests the motor is drawing 80 amps.
Within the limits of a power supply, the load current drawn depends on the load requirement, not the supply rating.. I have used my 50 amp rated power supply to test small light bulbs that do not draw even one amp.

Hi,
I'm not sure if i miss understood your question or if I was not clear in my explanations, I am using a battery pack that can provide 120amps continuous MAX, then comes the BMS that allows 80amps MAX then comes the controller then the motor so basically when unloaded the motor must draw around 3amps at full throttle but in the water it can draw max 80amps since the BMS isnt allowing it to draw more ? Am i speaking logic or complete nonsense ? I think that's how it works but I could be wrong.

 

I have a 30A very simple speed controller for 12 or 24V motors and it uses a single IRFB7537PBF FET, they are rated at 60V, 173A, 2.75mΩ and are designed specifically for brushed DC motor drives and similar. They are much better suited than the IRFZ44.

Another I've used is the FDP047N08 which is rated at 75V, 164A, 4.7mΩ and also the IRF1405, 55V, 169A, 5.3mΩ. There are many others, all only a couple of dollars each.

Hi Boggart,
Thanks a lot for that input, I am very curious to know and understand why this IRFB7537PBF FET is more suited for these kind of low frequency switching applications than the IRFZ44N I amjust missing some knowledge and if you could explain it would be great !
Wouldn't that be an overkill to have a 173A continuous for a 30A application ? just curious
So in my case 1 or 2 in parallel would do the job ? But using parallel FETs has drawbacks like for example if I checked the datasheet correctly the gate threshold voltage is between 2.1 and 3.7V meaning that at 3V I may have one FET that is on and not the other ones so do I need to buy like 10 of them and test which ones how the same (or close) gate threshold voltages ?
How do I properly scale my FETs for the application, and also in your case how did you calculate the Heatsink you needed for the proper power dissipation of your single FET ?
Thanks again help a lot, I am considering in buying more appropriate FETs like you told me
Timothee

 

I'm not sure I understand how you managed to calculate the gate drive current to be around 14mA ? Would you be kind enough to explain it to me ?

When the 4 FETs are off their Vgs is 0 and the gate capacitances are discharged. When the 555 output goes high (about 13.5V) it drives current via R304 through four 100Ω resistors (R305,7,9,11) effectively in parallel. So the effective load resistance seen by the 555 is 200Ω+100Ω/4 = 225Ω. The 555 output current is 13.5V/225Ω = 60mA. That divides four ways, so each gate capacitance charges initially at 60mA/4 = ~15mA.

 

Your specs show a resistance of 0.089Ω.

At 20V the stall current is 20/0.089 = 225A.

A soft start is necessary, given your BMS.

But I don’t think that is the problem. I think the problem is mechanical. At 5700 what is the torque required to spin the prop in water?

You most likely need to gear this down to run a propellor of any reasonable size.

 

Certainly the torque load is much greater with the prop in the water. That is how power to move the boat is delivered. 5700RPM is not a reasonable speed for a prop. The secondary benefit of providing a serous reduction ratio is the ability to get the motor farther away from the water.

 

When the 4 FETs are off their Vgs is 0 and the gate capacitances are discharged. When the 555 output goes high (about 13.5V) it drives current via R304 through four 100Ω resistors (R305,7,9,11) effectively in parallel. So the effective load resistance seen by the 555 is 200Ω+100Ω/4 = 225Ω. The 555 output current is 13.5V/225Ω = 60mA. That divides four ways, so each gate capacitance charges initially at 60mA/4 = ~15mA.

OK thanks a lot for the explanation ! And so you think this is too high of a current for the gates ?

 

Your specs show a resistance of 0.089Ω.

At 20V the stall current is 20/0.089 = 225A.

A soft start is necessary, given your BMS.

But I don’t think that is the problem. I think the problem is mechanical. At 5700 what is the torque required to spin the prop in water?

You most likely need to gear this down to run a propellor of any reasonable size.

Hi,

Thanks for the reply ! But the stall current is only when you are starting the motor right ? Because when you apply 20V to the motor it's not actually going to draw 225amps right ? And yes I implemented a small soft start with the capacitor bank (charged through a 10ohm 10W resistor) and then the relay switches when the capacitor bank voltage reaches a certain point. And yes this isnt the problem. I don't think its mechanical either because when in the water the drag causes the current to rise which is normal but even if the RPM is too high and the torque too small it shouldnt mean that my controller fails ...? It will only lead to an inefficient propulsion which is obviously not desired but shouldnt be the cause of failure ! Am I clear ?
The implementation of a gear box is quite impossible right now...