brickhead179

My Feedback: (1)

Hey guys! I have a serious problem and i need someone to please help! Okay, so here it goes:

I have 2 brushless motors each designed for a 64mm ducted fan unit. One motor has a rating of 3600kv and here are its specs:
RPM/Volt (Kv): 3600
Continuous Current: 46A
Maximum Burst Current: 55A (15 sec)
Speed Control: 60A brushless

Now i have a second motor thats rated at 5000kv and here are its specs:
Kv (RPM/V) 5000
fan size:50-64mm
esc needed: 40A

Now here is my dilemma: if i did the math correctly, then the motor with the higher Kv rating is actually more powerful that the motor with the lower one. So the 5000Kv motor that runs on a 40A esc is more powerful than the motor that has 3000Kv ad runs on a 60A esc. The higher the Kv and lower the amp draw the more powerful the motor. Am i correct?

Dr Kiwi

Kv and power output are not necessarily related. What you need to find out is the maximum continuous amp rating for each motor (the size of the recommended/suggested ESC is not necessarily closely related to max. amp rating of the motor).

Given the same voltage, a motor with a higher amp rating will be "more powerful" than one with a lower amp rating. Power (W) = V x I

brickhead179

My Feedback: (1)

Well how can i figure out the max continuous amps if the motor does not display it anywhere? And i thought that the higher the Kv the more powerful the motor

Dr Kiwi

Higher Kv simply tells you that the motor is suited to a smaller prop... [it may be a 10A motor or a 100A motor, but the mere fact that it is high Kv is irrelevant]..... or, in the case of an EDF, the higher Kv motor, run at the same voltage as a lower Kv motor, will draw more amps (my point is whether it can cope with those higher amps).

brickhead179

My Feedback: (1)

well then does that mean that my brushless motor thats has a higher Kv is actually more powerful than the other?

brickhead179

My Feedback: (1)

dr. Kiwi, if you are correct, then the motor with 5000kv and a constant draw of around 35A has a power equation like this: W= 5000*30 where W would equal 150,000.
And if the second motor has a constant draw of lets 45A and is 3600Kv then its equation would be W=3600*45 where W would equal 162000
So the more kv motor is actaully a little weaker then the other correct?0

Dr Kiwi

Multiplying amps by Kv gets you nowhere. Multiplying amps by volts gets you power-in......but what we do need to know is whether Motor A can cope with more or less than Motor B. At the same voltage, with the same prop, the 5000Kv motor (A) will obviously draw more than the 3600Kv motor (B)... but will that be too much for it?

Let's say we run both at 10.5v: say the 5000Kv motor (A) is rated for 30A/315W and the 3600Kv motor (B) is rated for 50A/525W...motor A will need a smaller prop than motor B to stay within its limit.

brickhead179

My Feedback: (1)

Well motor A is designed for primarily a 50-64mm fan unit and B is designed for a 69mm unit. So if I keep motor A in a 64mm fan I should be able to keep the motor within it limits while also having relatively the same power as motor B in it's 69mm unit.

Dr Kiwi

Well, motor A is designed for primarily a 50-64mm fan unit and B is designed for a 69mm unit

Aha... so now we are clearer! That makes sense.

brickhead179

My Feedback: (1)

But Dr. Kiwi, isn't 5000kv seem like a lot for a 65mm fan? Hobby-lobby is the one who distributes that fan and on their website they say its perfect for 65mm fans. Then right under that statement they say for up to 70mm fans! And when i called to ask about it, they transfered me to the tech guy, who didn't even answer! But why would they state on their site that a 5000kv motor is perfect for 64mm when they don't even knoe themselves!

KingCobra

My Feedback: (17)

Brickhead, Hobby-Lobby has been into electrics over 30 years (I'm from the Nashville region and I was in their shop back in '78-'79). They know what they are talking about with electrics; they test, build and fly their products. So if you call and get customer service that is responsible for orders and problems with orders, they will transfer you to someone with the technical expertise necessary to help you. So if that person is already on the line helping someone else, you can't jump to the conclusion they don't know what they are talking about just because they didn't answer your call immediately.

As Dr. Kiwi pointed out, the power consumption of a motor is related to the current draw (in amps, not milliamps) and the voltage applied. So for the first motor, you provided a continuous current draw specification of 46 amps, but you did not give the voltage specification. So you cannot perform the simple calculation to determine the power consumption of the motor on the information you've supplied. Let's suppose it is a 3-cell lipo, and call it 11.1V. Now we have 46A x 11.1V = 510.6 watts of power. So suppose you put this motor in a fan unit with a 3-blade rotor, and throttle up to full power. To determine the actual power consumption you will need something like an Astro Flight Whatt Meter which plugs into the battery leads and will display the current draw, voltage and power readings. Let's say you see a 40A current draw and 11.7V. This now says this motor in this fan unit on this battery uses (40 x 11.7) 468 watts of power.

Now suppose we take that same motor and put in the same size fan unit but with a 4-bladed rotor. This will impose a greater load on the motor as it will try to move more air. Lets say you see a 50A draw and an 11.0V reading. This now calculates for a 550 watt power consumption, but we are over the continuous current rating and you risk damage to the motor if run full throttle for too long.

The same concepts can be applied to the second motor you commented on. With that motor, you did not provide the battery/voltage spec or the continuous current spec, so there is no way to even do a basic power consumption calculation for comparison. Personally, I like to have an ESC which is rated maybe 10% higher than my actual current draw, so if the second motor requires a 40A ESC, I'd limit current draw to 35A. So if it too were using a 3-cell lipo, then 35A x 11.1V = 388.5 watts power consumption.

With a prop plane, we can change prop size and pitch to tune the power for the plane, flight characteristics desired, and electronics on board. To the best of my knowledge, you do not find fan units with a variety of different rotors for tuning performance to a motor of interest. For the EDF, in my limited experience, we have to rely on manufacturers specifications and suggestions for motor applications, and we have to hope they've done their homework correctly on it. And H-L is usually pretty good on doing that.

CC

brickhead179

My Feedback: (1)

kingCobra, im not bashing hobby-lobby believe me. I like their products. Im jst trying to figure out why on their site they have two different statements about what size edf unit can be used. And also why customer support didn't know if their own motor could work. i didn't say they don't know what thier talking about, i just meant to say they didn't know their own product to say that it would go in a 64mm duct, then in a 70mm duct.

And the motor has no specs other than what esc (40) what duct (50-64) and its weight. So i cannot provide any info on the watts or continuous amp draw.

Swift427

At least half the battle is knowing how to phrase your questions without coming across as misinformed or confrontational to a tech rep (or customer support). As you've stated above a tech rep would wonder why you'd think a motor/fan combo that requires a 40 amp ESC would be more powerful than a motor/fan combo requiring a 60A esc (assuming each motor/fan has similar internal/external efficiency). In a way a tech rep. might be scratching his head wondering exactly where you're coming from with your reasoning. "Powerful" = Power is expressed in watts.

40A x 11.1V = 444W
60A x 11.1V = 666W

Most tech reps. don't have the time to explain all the basics/variables over the phone to someone just beginning their learning curve journey. You need to visit a few LHS on a slow week day, ask a lot of questions while being considerate. You'll get differing answers and some may seem conflicting. You don't learn all the stuff you'd like to know in one week or even one month. The learning curve can be frustrating (e.g. Wilbur and Orville Wright).

One thing that's questionable is the impression from Hobby-Lobby that a 5000Kv $20 outrunner with 40A ESC applies equally to a 64mm fan as it does to a 50mm fan. The 3000Kv with 60A ESC is 'generally' a 64mm specification. The 55 & 64 fans 'generally' use motors around 4200-3600Kv for best efficiency. As a general rule the more efficient fan size for a 5000Kv is a 50mm fan.

Another consideration is do you really want to go that fast (5000Kv) when you're only asking for trouble (crash)?

Dr Kiwi

Give us the details of these two motors (brand, size, etc) and we may be able to hunt up more detailed specs... they may have been tested and their specs entered into one of the calculators.

Dr Kiwi

Just for fun I put a Wemotech 65mm fan on a Feigao 130L ((4834Kv)... that draws over 40A on 3s. [problem is the motor is only rated for 20A max.... so I'm wondering how your 5000Kv motor, if it is in any way similar, is going to manage 40A+]

Motor: Feigao 130L 12T (1308412L); 4834rpm/V; 1.5A no-load; 0.0387 Ohms.
Battery: Thunder Power TP3200 (G4 ProRace 40C) (40C); 3 series x 2 parallel cells; 3200mAh @ 3.7V; 0.003 Ohms/cell.
Speed Control: Castle Creations Phoenix 60; 0.0012 Ohms; High rate.
Drive System: WeMoTec Mini Fan 480; 2.7x3.66 (1.3in hub); (Pcoeff=0.506; Tcoeff=0.807).
Airframe: Anything you want.
Conditions: Sea Level, 29.92inHg, 59°F

Motor Amps = 42.9
Motor Volts = 10.6
Input (W) = 456.0
Fan RPM = 36183
Thrust (oz) = 31.7
Efflx (mph) = 125.4
Time (m:s) = 8:57

brickhead179

My Feedback: (1)

I found all the detailed specs for the 3600kv motor but here is the 5000kv motors specs as they are displayed by its website:

Kv (RPM/V) 5000

Fan Size 50-64mm

Battery Range 3 LiPo

Controller 40A

Shaft 3mm

Motor Weight 51g


Oh and the brand is 'eRc'

Dr Kiwi

Well, I guess it is possible for that purpose-built outrunner EDF motor to cope with 400W+, but I think that is a LOT to expect of a 51g motor! I'd try it in a 50mm fan and see if it can cope with that, before testing it with 64mm/65mm/70mm