MT30 / MR30 as phase wire connector

[momosfet]

Hi, long time reader first time poster here.

I have a quick question regarding MT/MR series of connectors (originally Amass).
In short: why are XT60, MT60 etc. very common while XT30, MT30 etc. are not. 30 series should 'in theory' suffice for most low power e-bikes / e-scooters?

Longer explanation:
I am currently replacing dead 250W controller for a new 350W one in a 36V ebike. Since power-wise it is small upgrade and ebike was quite nicely made I try to keep as much of original parts and wiring as I can. I am using cheap Aliexpress controller, so not much data is available, even less reliable. Label claims 24V/36V input, max. output 17+/- A. Now, usually I would solder XT60 for battery connection and MT60 for motor, but I can't quite fit the bulky MT60 in existing case. I have much smaller MT30 laying around, so I figured I would check its specs. Turns out that it should carry 15A continuously, and up to 30A peak, which should be more then enough for 350W controller. The AGW16 wires that are soldered onto the controller also fit this picture. But, outside of drone makers I did not see anyone use this line of connector neither for battery nor for phase connections. So I wonder, am I missing something?

All help would be welcomed!

 

[E-HP]

Remember that phase current can reach up to 3x battery current when under load or accelerating.

 

[momosfet]

That is a good point. Label on the ESC reads:
Cutoff voltage: 29V
Current output: 17A
Motor Power: 350W

I did assume that label given "output max current" would be max phase current. My back of envelope calculations:
Power / Current output = Vpwm (350/17=~20V), and it seemed believable.

Where the "x3" comes from? Assuming no power peaks (max. power 350W) at cutoff voltage battery current should be around (350/29=)~12A. So x3 that would exceed 30A limit of the connector. I guess that is good reason to use 60A version.

 

[amberwolf]

these typical controllers are not reading phase current, just battery current, so that's all they rate them with.

more advanced ocntrollers have current sensors on the phases themselves but these only have the shunt in the batteyr negative wire to the fets.

the phase current 'mulitplier' is because the controler plus motor is basically a form of switching power supply smps, and the currents and voltages on the output are 'disconnected' from those on the input, related by the total wattage being about the same minus losses in the controller. current fed into the very low resistnace and inductance of the motor coils can be very high compared to that at the battery side, chopped up into pulses that the inducatnce blurs together.

 

[momosfet]

So I guess that "max output" is actually "max draw". I was aware that phase current is higher then battery current, but wasn't expecting it could be 3x.

Controller I have definitely is the simpler one, it has R005 current sensing resistor. I guess I could solder leads to the resistor and try and measure current. If 3x max. draw rule of thumb is accurate tho then there is no point in doing so, as whatever I would measure it's probably either going to exceed or will be to close to 30A limit for comfort.

Thank you for your replies.

 

[rafalg]

im using MT30 (triangle) for phase wires in 250W motor and it works OK, no heating at all. So i think it's ok for small motors.
BTW i find it hard to explain how phase current can be higher than the current flowing from the battery, if the controller doesn't do any voltage conversion then the currents should all add up.

 

[Vbruun]

im using MT30 (triangle) for phase wires in 250W motor and it works OK, no heating at all. So i think it's ok for small motors.
BTW i find it hard to explain how phase current can be higher than the current flowing from the battery, if the controller doesn't do any voltage conversion then the currents should all add up.

It does convert the voltage. That's why they don't

 

[rafalg]

yep, just read a bit about that. So, it appears that the slower you go, the higher current is going thru the motor wires, and only when you're going at max speed the motor current will be equal to battery current.

 

[momosfet]

im using MT30 (triangle) for phase wires in 250W motor and it works OK, no heating at all. So i think it's ok for small motors.

I expect it to work just fine for 350W as well, but based on info provided here I will not use it until I have some reliable info about phase current.

 

[E-HP]

I expect it to work just fine for 350W as well, but based on info provided here I will not use it until I have some reliable info about phase current.

Remember that I stated up to. Cheap controllers provide less, good controllers more. You can have two controllers rated with the same battery amps, but one accelerates or climbs slower than the other because of that difference. Could be 2x to 3x depending on the design/quality. Programmable controllers allow you to set the max.