I would be interested to see Documented tests.
My alternator on my vehicle is rated at 150amps, I imagine how long it would last if subjected to this for an extended period.
I have played somewhat with these types of motors, although not this high a current, and the numbers tend to be misleading.
Max.

Yes, of course the if we run the alternator at full rated capacity its gonna be dead soon. Looks like I might have to get some motors of eBay and test it myself at different loads to see how reliable it was.

 

Yes, of course the if we run the alternator at full rated capacity its gonna be dead soon.

Not the commercial grade ones. Most of those can take running at 100% load for days on end without damage. I've seen them do it many times.

 

Not the commercial grade ones. Most of those can take running at 100% load for days on end without damage. I've seen them do it many times.

Hi,
But wouldn't running it at full capacity cause overheating and damage. Yes while the vehicle is running the forced air supply will cool things down a lot faster. Also, alternators run at full capacity only occasionally when the alternator is charging a very low battery and running lot of electrical devices. I seen people upgrade their alternators when they add custom high current devices on their vehicles to keep up with the load demands.
Thanks.

 

Not the commercial grade ones. Most of those can take running at 100% load for days on end without damage. I've seen them do it many times.

I am talking GM Delco.
You only have to inspect it physically to get the picture.
Obviously if industrial and built for it should endure.
Max.

 

I am talking GM Delco.

The larger frame ones like the 22 - 28SI sizes of units most often can take 100% duty cycle operation unless they are extremely high current special application units. Same with the older <50 amp rated 10/12SI units that were common on older farm and industrial machinery as well.

The cheat with the old 10/12 SI units seems to be that the lower amp rated ones had the windings configured in Wye connections, Vs the more common Delta that the higher current ones are set up with now, so that they could put out the proper voltage at lower working RPM's which with many non automotive applications was more important since it was common for them to be on slower turning heavy service type engines.

 

Also, alternators run at full capacity only occasionally when the alternator is charging a very low battery and running lot of electrical devices. I seen people upgrade their alternators when they add custom high current devices on their vehicles to keep up with the load demands.

Depends on the application. In most typical automotive applications the alternator is sized to not need to run at more than ~1/2 - 2/3's it peak rating while keeping up with all typical electrical loads. However on industrial service applications it's common for some machinery to have electrical loads, like high powered lighting or additional various electrical systems, that can take near or even beyond 100% of the alternator's capacity for extended run times.

Then there are applications with generic power plant engine systems where whatever alternator the stock engine came with is used for whatever load the machine that engine is used for. In the oil fields it was common to see a something like a Caterpillar, Cummins, or Perkins industrial power plant unit engine with a stock 60 - 100 amp alternator (on some special application industrial process rig) that had a substantial electrical load when in us that was actually at times beyond 100% load capacity of the alternator in normal rig operation.

Same with genset units in such applications. Many of the machines I worked around in the oil fields had mixed voltage systems that might have a 30 - 50 KW single or three phase diesel genset with a stock 60 amp alternator powering a combination of systems and loads that more often than not took more power off the low voltage DC side of the system than the stock alternator could support so a big AC to DC step down power supply was used to supplement whatever the alternator couldn't keep up with.
Most often the biggest problem with those setups was the alternators typically put out ~14.4 VDC and the power supplies put out ~13.8VDC or less which meant that the alternator had to be running at 100% duty cycle before the second power supply started to carry anything.

 

Yes, of course the if we run the alternator at full rated capacity its gonna be dead soon. Looks like I might have to get some motors of eBay and test it myself at different loads to see how reliable it was.

Back to your original question, these motors can handle so much current because the windings are stationary, there is no commutation to a rotating member. Carbon, copper, or precious metal brushes all cause sparks and high resistance connections that evaporate metal through sparks or abrasion. The current must flow through a coil on the rotating part of the motor to make a magnetic field that makes torque by opposing the magnetic field of stationary permanent magnets on the frame of the motor.

On a brushless motor, the permanent magnets are the rotating members and the coils are on the stationary frame of the motor. A speed controller is used to sequentially energize the coils and generate torque with the magnets on the frame of the motor. No brushes means high current can flow though hard-wired connections. The complexity of the switching circuitry is well with the power gain for a minimum of weight and a simplified motor design.

 

The main drawback is that these motors are outrunners, the heat is in the internal coils which is hard to cool, and they cannot be cooled by an exterior heat sink as in a BLDC with rotory magnets and the coils in the stator.
Max.

 

The main drawback is that these motors are outrunners, the heat is in the internal coils which is hard to cool, and they cannot be cooled by an exterior heat sink as in a BLDC with rotory magnets and the coils in the stator.
Max.

Hi,
Max isn't a BLDC motor actually an AC motor infact? Like the power source for a BLDC motor is DC which is converted to AC by the ESC right?

 

Hi,
Max isn't a BLDC motor actually an AC motor infact? Like the power source for a BLDC motor is DC which is converted to AC by the ESC right?

It is a bit of a semantics issue.

The brushless DC uses DC but the esc pulses power to two of the three coils at just the right point in the cycle of the motor. There are some nice .gif animations online of you look.

But, even though dc is pulsed to different coils, few people would call it AC. The supply is specifically DC.

 

Hi,
Max isn't a BLDC motor actually an AC motor infact? Like the power source for a BLDC motor is DC which is converted to AC by the ESC right?

The AC synchronous P.M. motor and the BLDC are virtually physically identical, the difference is in the commutation.
The former is AC true 3ph powered, the BLDC has only two winding's powered at any one time, and is usually commutated through hall sensors. Hence the DC motor turned inside-out.
See PDF for BLDC.
The AC wave form seen is the generated phases from a back rotated motor.
Max.

 

The brushless outrunner motors in my RC airplanes are driven with PWM and have 3 wires without any hall effect sensors. I think the frequency determines the RPM and the pulse width determines the power because they make whistling and whining sounds that change their pitch with RPM change. The motors are tiny and in the cooling airflow only get warm at full power.

 

The brushless outrunner motors in my RC airplanes are driven with PWM and have 3 wires without any hall effect sensors. I think the frequency determines the RPM and the pulse width determines the power because they make whistling and whining sounds that change their pitch with RPM change. The motors are tiny and in the cooling airflow only get warm at full power.

They are known as sensorless brushless DC. The position is sensed by back-emf is sensed by the ESC when power is not applied to a given pair of leads.

 

I notice Wikipedia have this explanation.

Brushless ESC systems basically create a tri-phase AC power output of limited voltage from an onboard DC power input, to run brushless motors by sending a sequence of AC signals generated from the ESC's circuitry,

But the example stripped version of an ESC shows only 3 devices, If this were true AC 3ph I would expect 6 devices?
Max.

 

Back to your original question, these motors can handle so much current because the windings are stationary, there is no commutation to a rotating member. Carbon, copper, or precious metal brushes all cause sparks and high resistance connections that evaporate metal through sparks or abrasion. The current must flow through a coil on the rotating part of the motor to make a magnetic field that makes torque by opposing the magnetic field of stationary permanent magnets on the frame of the motor.

On a brushless motor, the permanent magnets are the rotating members and the coils are on the stationary frame of the motor. A speed controller is used to sequentially energize the coils and generate torque with the magnets on the frame of the motor. No brushes means high current can flow though hard-wired connections. The complexity of the switching circuitry is well with the power gain for a minimum of weight and a simplified motor design.

Hi,

I know I had asked this question back three years ago, but before I mark this as complete I thought I'll give a response. After reading all the replies and specially yours I had this thought in my mind for two days. I had obtained a brushless and a brushed 540 35T motor during that period. So, I had obtained one of those cheap brass ebay drill chucks and bits to drill very fine precise holes in thin metal sheets. I didn't want to use those bits with my cordless drill as I had broken those tiny needle like drill bits in the past. So I was planning on making a small hand drill with wires to a 12V SLA so that I could feel the pressure I except on the tiny bits. I found that the brushed motor did get hot faster than a brushless. But the BLDC also gets hot if a larger bit is used and if there is resistance on the drill bit as its being drilled into a material.

As, I was traveling in my car the next day on the highway and when I opened the window there was a sudden rush of air. I made me thinking that these RC cars also run from 20 - 40mph usually and the expensive motors+esc can put around 60mph too. But again these are usually driven in bursts and not continuously in a straight road. So, there is some time to cool off plus the high speed will make sure there is enough air flow on the motors while driving.

Thanks everyone for the valuable comments!
Cheers

 

RC cars and planes have modest powerdraw compared to RC Boats.
I race them so have some idea lol.
Eg, a Mono hull powered by a 5000Mah 4S Lipo, pulls approx 90 A in the straights and up to 150A in the corners. a 1.5km race over 5 laps takes approx 1m 25secs.
Both the Speed control and motor are water cooled.

 

RC cars and planes have modest powerdraw compared to RC Boats.
I race them so have some idea lol.
Eg, a Mono hull powered by a 5000Mah 4S Lipo, pulls approx 90 A in the straights and up to 150A in the corners. a 1.5km race over 5 laps takes approx 1m 25secs.
Both the Speed control and motor are water cooled.

I guess that the RC boat motors have more resistance from the water on the propellers to be overcome compared to a car or airplane.

But what still intrigues me is how are these tiny motors with tiny winding and small wires able to handle 90A or 150A through them? I mean those Lipo packs are around 11.1V to 14.8V mostly. The higher volt ones may consume less current than the former. But still those currents are similar to those of a big car starter. It makes me think these currents might be given as bursts like a PWM to prevent the wires from burning up?