Newbie from NZ

[rjsutton]

Doddy
Im a little bit concerned
the motor ive brought is a 120w 12vdc 13A motor ( see pic )
the controller i brought was
http://go.skimresources.com/?id=2051...bf6eee96225584

it says its 30A . ive just burnt out a 60 A variable speed controller
https://www.ebay.com/itm/PWM-Motor-S...IAAOSwhBVdTlhI
on another 12v motor (200W 9-12 amp)) and hoping that the new motor wont do same to the 30A wifi controller. I dont understand the amp-watt connection but i would think that a 60A controller should be able to handle the 9-12 A. am i missing something
Thanks]

[Doddy]

EDIT:
One very quick question to OP - are you confident that the wiring/polarity to the battery was correct?, it's the easy way to destroy a controller if reverse connected.

I was going to suggest also adding a TVS across the motor but this article https://www.modularcircuits.com/blog...fety-features/ adds some interesting information. For reference, the h-bridge is essentially the output from the controller.
END-EDIT:

Rambling reply alert

In theory, nope, you're not missing a lot, apart from understanding the stall-current of the motor.

Before we get too involved, can I ask you to characterise how the controller failed? Did it fail immediately in a puff of smoke?, or did it fail after prolonged use? Was the motor output under significant load at the time, or just turning air through the gearbox? If it was under heavy load was the motor straining, or stalled? All this may help to understand what may have gone wrong with your first controller.

Before I forget to include this: sometimes s**t happens and a device will fail very quickly after initial power-up/test. Particularly power-devices such as those in your controller. I had this with a Seig mill brushless motor controller and a IGBT exploded within 20 minutes of operation on a brand-new mill - an otherwise expensive repair which was fortunately covered by the supplier. So, yeah, particularly with Chinese-sourced boards s**t sometimes does happen. Anyway, back to the main thrust of this reply...

The motor will often be described with two ratings for current draw (I noticed that with the earlier 90W motor). The lower of the two (around 1.5A?, from memory) would be the no-load current, and the higher value (6A?, from memory) would be the current draw under a quoted load (so many Nm... I think around 4, from memory?). Beyond that load the motor will draw more current. As the load increases, it will slow the speed of the motor and the current will increase. At the point that the motor stops (stalls) the current will be at a maximum, the stall current. That will be (significantly) higher than the rated power under load and will likely result in damage to the motor coils if power is not quickly removed. At the same time it's presenting more of a load to the speed controller which may be similarly stressed.

There's a slightly confusing labelling on the motor in the attached image, with V=12V, I=13A, and P=120W. I expect that the "P" output here is the mechanical output power at the rated voltage, current and RPM, somewhat lower than the computed 12*13 = 156W and the ratio to the rated power indicates the motor efficiency (120/156 = 77% efficient). Be warned, there will be further losses through the gearbox.

What you can do is measure the resistance of the rotor winding of the motor isolated from any supply - i.e. just on the bench-top. This is likely to be a rather low number of Ohms, but if you can measure this then that allows you to calculate the stall-current (it's essentially I(stall) = 12V / R(static)). This would tell you the maximum possible load that the motor would present if you stall it... or on start-up. Normally, of course, the motor would quickly come to speed and the current drawn would drop from the stall-current to the current appropriate for the mechanical load presented to the motor. For this reason a poorly designed, or under-specced speed controller may fail at the instant of turning the motor on. But, it must be expected that the motor has to start from stationary position in all but a few cases, and the speed controller should be designed to handle this instantaneous current surge.

So, the controller that failed is described with two primary ratings, a Voltage range 0-55V and a Current rating off 60A. Read through the advertising blurb and immediately you find that the 60A is a peak rating, and the continuous rating is 40A. So, a little bit of optimistic headline advertising there - to all intents that controller would be designed for 40A operation. But that doesn't explain the failure. What you might uncover if you stripped the controller down is that the rated current capacity is that in the manufacturer's data sheet for the power control devices (the output MOSFETS, or whatever devices are chosen), whereas the rest of the design could be poorly designed (for example, PCB traces could be too thin [localised heating, vaporisation of the PCB foil], inadequate heat sinking [the power devices would get too hot under load, overheat and fail], or inadequate protection from back-EMF from the motor [with PWM drives the motor will generate large voltage spikes which can damage semiconductors if not clamped]). All this comes under the banner of fitness for purpose. And, without wishing to appear too xenophobic or casting stereotypes, Chinese advertising. But, let's not kid ourselves, most of the electronics will come out of China and we, the consumer, drive the quality down by the price we want to pay.

Enough of the hypothesis, posturing and blame. Let's get practical.

I'm surprised that you managed to burn out the controller, but possibly not for the reason that you might expect. I'm going to throw some random numbers together to explain why...

*** ALL NUMBERS BELOW ARE PLUCKED OUT OF THIN AIR FOR EXAMPLE PURPOSES ONLY ***

Let's make a bold assumption on that motor. Rated 12V, an average of 12A, I'm going to make a rough-arsed guess that the rotor resistance (including brushes and terminals) comes out at 0.5 Ohm. That would give you a stall current of 24A. Still way below the maximum current that the controller is advertised as capable of sustaining, even under the stalled (or starting) condition.

But, the motor is only one component in the whole drive chain here. We need to consider the internal resistance of the battery, the resistance of the cable from the battery to the controller (both positive and negative supplies) and the resistance of the cable from the controller to the motor (again, both lengths). And in amongst that lot, the resistance of the terminals/connectors being used, and the internal resistance of the controller. Some not unreasonable numbers....

R(motor-stalled) = 0.5R
R(bat) = 0.05R
R(2m cable) = 0.2R. Imagine that you have four such lengths - source/return battery->controller, and controller->motor
R(controller) = 0.15R
R(terminal) = 0.0 - unrealistic, but it's just drawing out the numbers unnecessarily a this point

Your overall resistance would be 0.5 + 0.05 + 4x0.2 + 0.15 = 1.5R, and so the current draw from the battery would be 12V/1.5 = 8A (thereabouts). Definitely well within the advertised rating of the controller.

Further, you might expect the controller to be suffering under the short-load of 8A at 12V (around 100W of dissipated power), but it's not. Because of the internal resistance of the battery the battery terminal voltage has dropped...

V(terminal) = V(cell) - I * R(bat) = 12-8*0.05 = 11.6V

and then you have the voltage drop across the supply wires to the controller...

V(controller) = V(terminal) - I * R * (R(2m cable) * 2) = 11.6 - 8 * (0.2 * 2) = 8.4V at the controller, so the maximum power dissipated by the controller/motor/cables-between-these would be only 8.4 * 8 = 70W.

What I'm getting at here, is that with low voltage, high current designs, you get significant parasitic losses throughout the system that tend to degrade the performance significantly but in the same breath tend to be self-protecting.


I'm rambling now, and not coming to a conclusion...

My thoughts are with the advertised controller and motor, on paper they should work fine, and you've either been unlucky with a unit destined to fail due to manufacturing flaw, or that its design is either flawed, or underrated for the advertised specification. I'd approach the Chinese supplier for a refund or replacement.

For the new controller and motor... tread with caution. If you use a long supply wire from the battery to the controller (increasing the resistance) you should introduce a level of protection that you can use to test, and then start to reduce in length as you gain confidence that the controller isn't overheating etc, with the motor under load. Ultimately you do want that cable length to introduce as low an electrical resistance as possible to allow you to attain the power through the motor that you expect.

[rjsutton]

Doody
hi
the controller just stopped after about 15 seconds
no puff of smoke -no load on motor lead from battery was 300mm and had correct polarity
had worked 10 times but i was only running for a few seconds each time to test motor setup. Just stopped
Didnt buy from china but the nz supplier did as they are identical to ones on ebay etc.
ill try and digest your ather comments. i just didnt want to destroy the new controller as well

[Doddy]

Have a read here... https://medium.com/jungletronics/dc-...f-589d8ed174cc, you might consider this a pragmatic approach.

[rjsutton]

Morning.!
Doody i want-(NEED) to protect these motors and electronics so thought i could use relays to protect each high amp motor circuit
would https://www.ebay.com.au/itm/MICTUNIN...frcectupt=true
something like this work ?
or this?https://www.trademe.co.nz/electronic...45063f2ae1-003

im not sure HOW large the amps-watts etc need to be but i just want to protect the electronics.
OR do i simply hardwire a 10-15-20 amp car fuse in the power supply feeds? this may be messier
trying to get smallest-neatest setup so i could fit them all in a protective case somewhere.
thanks if you can help
Richard

[Doddy]

Richard,

I need to understand your thinking. How do you perceive a relay will protect the motor and/or electronics? A relay is effectively a switch, so my question is really what quality is actuating the switch? Are you talking thermal overload? (a thermistor somewhere in the speed controller?) Some form of limit control? (already accommodated with the motor speed controller)? Sorry, I don't understand where you're coming from for "protection".

Similarly the timer module that you have there. From the outset I'd argue a couple of points - that module is likely (from a glance at the image of the board) to fry with the current draw from your motor (and I'm only talking of the principal motor in this thread). And in terms of "protection" you're only looking to introduce a timed on-state with that, and the only protection this effectively offers is against prolonged use (the quality here being the probability of over-temperature of the power devices in the speed controller). That presupposes that particular failure mechanism.

For the new controller and motor... tread with caution. If you use a long supply wire from the battery to the controller (increasing the resistance) you should introduce a level of protection that you can use to test, and then start to reduce in length as you gain confidence that the controller isn't overheating etc, with the motor under load. Ultimately you do want that cable length to introduce as low an electrical resistance as possible to allow you to attain the power through the motor that you expect

This is what I recommended to avoid overheating. It's a heath-robinson solution but it's robust.

There is another potential failure mechanism of back-EMF from the PWM output to the motor frying the electronics (if poorly designed - pure speculation on my part) - which

Have a read here... https://medium.com/jungletronics/dc-...f-589d8ed174cc, you might consider this a pragmatic approach

was intended to address. Here, add one or three capacitors across the motor terminals, or (I prefer, but seems to have limited traction in a number of articles) a TVS to suppress the back EMF.

[rjsutton]

Hi
i guess im just trying to stop any of the electronics from frying. MY THOUGHT is that if i put relays in the switches( realize a relay is a low voltage switch) then it MAY stop the electronics getting zapped by to much V or Amps when switched on. Maybe im not heading in the right direction.
On my unit i have a 200W geared motor with a 60 Amp fwd-rev-speed controller
, a 120w geared motor with the 2 proximity switches with the 30A speed-reversing controller
and 10A linear actuator.
Non of these will be run simultaneously they will go in sequence so the current draw is lower.
the whole units in the boot of the vehicle so i imagined running a fairly decent sized awg feed direct from battery to the boot and feed of a contact there . ( lessening power drop)
Nothing runs for more than 40 secs at a time either...
being overly cautious perhaps but this is a prototype and i cant-dont want weak points that could cause returns -claims etc.
at the moment the battery is on the floor 1m away from the electronics but obviously when installed it would be feed off the car battery ( with motor running) which gives slightly higher V. and could possibly cause problems?? ( or not)
Thats really where i am. it will probably fine with a few inline fuses but i dont know .During trials ive never over stressed any motor or controller as when i fried the last one it costs $ to replace ( $ which are running out)
I do need to put pressure on all parts as who knows what the purchaser may stupidly do. It needs to be pretty foolproof.( or protected)
Cheers