question about 60V 650W motor at 48V

[middriveebike]

I recently came across a dual controller that is good for both 48V and 60V.
The current is limiting at 28A.
It is marketed as a 500W controller.
I am thinking of using it on a 650W 60V brushless 3 phases hall effect sensor motor.
My question is that when I am using this motor at 48V, would the torque generated be similar to a 48V 500W motor?
What is the effect on the efficiency on the system as a whole?

 

[dogman dan]

The torque would be exactly like 28 amps at 48v. 1500w ballpark, if you are able to put enough load on it to pull all 28 amps.

48v= about 53v

53x28=1484 watts. You'd have that in the first 5 feet from a stop sign, then it would drop as you get going. 30 mph is typically possible with 1000w. So if your top speed is 30, that's more or less what you have at cruise.

Efficiency depends on many things. Lug a motor to a too slow rpm under 1500w load, and you could be making 800w into heat. At cruise, it should be more like 80-90% efficient.

 

[middriveebike]

I did some more research and come up with something.

P = VI = I square R = V square /R

From here, we can see that if Voltage is reduced by 20% as of 48V instead of 60V,
P=(0.8V) square /R
= 0.64V square /R
In other words, P is reduced by 36% when voltage is reduced by 20%
A 650W motor with 48V battery will give you a 416W power.

T= kt x I where T is torque and kt is a constant
V=IR
I is reduced by 20% when V is reduced by 20%.
As a result, T is reduced by 20% when battery is lower to 48V.

In the above situation, we assumed that I=V/R. However, in reality,

I=(Vs-Vemf)/R where Vs is the source voltage and V emf is the back emf voltage generated by the motor when it is spinning
Vemf = Ki x w where Ki is constant and w is the angular speed.

So when the motor is at stall (w = 0), we have max current and when running at max speed, we have I=0
From this equation, we can see that current can range from 0A to 28A depending on the angular speed.
I=(Vs - ki*w)/R

So, the above calculation about P is reduced by 36% when V reduced by 20% is not correct.
The true max electricial power is , as suggested by dogman
P=VI
= 53 x 28
=1484 W

Assuming 90% efficiecy at crusie , P mechanical is 1484 x 0.9 = 1335.6W

In theory, my modification should work.

I think china has a different way of rating the power of motor and controller than us.
We usually use the max value, while they use an unknown way to rate.
They rated this controller at 500W.
I am yet to find a way to find out the reason how they come up with this value.

 

[amberwolf]

Many (most?) of the "ads" for various ebike parts, either Chinese or not, seem to just make some sort of approximation values for many of the specifications. Some of them are apparently even just made up out of thin air. Others are outright lies.

There's not really any way to know for sure what specs are for any individual controller, motor, or battery until you have it in your hands to test it, unless there's hard data out there by someone else that's already done it for that specific item.

You can use all the electrical formulas you like on the specs they do give you, but since those specs are not trustable, the results from the formulas are equally "valid".

 

[major]

From here, we can see that if Voltage is reduced by 20% as of 48V instead of 60V,
P=(0.8V) square /R
= 0.64V square /R
In other words, P is reduced by 36% when voltage is reduced by 20%

This would be true if you were working with a circuit with only a fixed resistance. This doesn't apply because you have a controller which limits current to 28A, regardless of the battery voltage. So a 20% lower battery voltage results in a 20% lower peak power. 60V*28A=1680W and 48V*28A=1344W,peak power. 1680W-1344W=336W lower. 336/1680=0.2 or 20% lower.

The actual numbers will vary due to the actual battery voltage which depends on the SOC and SAG(voltage drop due to internal resistance). I used nominal battery voltage for illustration. The 20% figure is likely to be pretty close.

 

[middriveebike]

Thanks everyone for all the inputs.
I will test it when I get my hands on the parts.

 

[kris]

Hi I brought this motor with the 60v controller I a noob and need to find out the dischearge rate,and would like to use rc lipo batterys as only got about 300 left spend

 

[middriveebike]

Hi I brought this motor with the 60v controller I a noob and need to find out the dischearge rate,and would like to use rc lipo batterys as only got about 300 left spend

Which motor and controller did you buy exactly?
If you are talking about the controller I am talking about, the max current is 28A.
72v x 28 = 2016W.
So the maximum wattage would be 2016W.
$300 is not even enough for the 10Ah battery pack.