# 2020 - Excalibur Electric Racing kart

#### Functional Artist

##### Active member
I added & am testing a DIY precharge circuit. It's just a 10W 1KJ resistor & a momentary (micro) switch that is mounted across the contacts of the circuit breaker.

Here is a wiring diagram of it * Notice how the switch is positioned/connected on the (VRC) or "restricted voltage" side of the resistor  I did a video to help explain & show how it works
...& also, a quick demo of it "in action" #### Functional Artist

##### Active member
After getting most everything connected up, I precharged the system, engaged the circuit breaker & gave 'er a try
...but, the wheels went backwards So, I got out some of my old notes (for when I had ta switch the direction of rotation, on the 48V 1,800W Boma brushless motor)
...& did some re-arrangin' of the wires Yup, that seems to work better #### Functional Artist

##### Active member
How to "do the math" to "size" a precharge resistor.

Let's use the 10W 1KJ resistor , that I installed on Excalibur, as an example Current:
First, you have to figure out how much current will be flowing?
The basic equation is Volts = Amps x Resistance, V = I x R; In this case we know V and R, so we can calculate for current.
The basic equation is Current I = V/R= 60volts/1000 Ohms = 0.06 Amps. (the voltage divided by the resistance)
This will be the current flowing thru the resistor during inrush to fill the capacitors.

Power:
Next, is the resistor power rating adequately sized to handle this much current?
The basic equation is Power = I² x R , (the current squared, times the resistance)

Using the current as calculated above, and also, knowing the resistance, we come up with,
Power = (0.06 Amps) x (0.06 Amps) x 1000 Ohms = 3.6 Watts.

The numbers to precharge this 60V system would be
60V / 1,000 Ohms = 0.06A (current)
0.06A x 0.06A x 1,000 Ohms = 3.6W (power)

So, according to "the math", this 10W 1KJ resistor should be adequately sized to precharge this system. • landuse

#### Functional Artist

##### Active member
Next, we need to know how long it takes to precharge the cap's in this system. ...so, we know how long to "hold the button" since this is a manual precharge circuit
...& to be sure that the required duration is not too long.

If it is for a short period, then it might be okay
...but, if it is a long time then the resistor will get hot.

How to calculate this?

Knowing R and C, we can calculate the Inrush time ~= 6 x R x C.
...the R is the resistance of the resistor (1,000 Ohm)
... the C is the total capacitance of the capacitators in the SC (940uF)
(which are in micro Farads, uF & requires you to divide by a million. 1uF = 0.000001 Farads)

*Remember, when we looked inside of the controller, there were 2 blue caps in parallel with an unpopulated spot for a 3rd. (2x 470uF = 940uF)

Here is what the math would look like:
T = 6 x R (1,000) x C (940) / 1,000,000
...& then doing the math, 6 x 1,000 x 940 / 1,000,000 = 5.64 seconds

So, now we know the precharge needs to be engaged at least 6 seconds, to adequately precharge this system
...& if only "used" for short periods like this it shouldn't get too hot either #### Functional Artist

##### Active member
Well, that's enough technical babble, for a minute First, let's get some RPM data on this 60V 2,000W brushless motor
...then, we'll get some MPH data

So, I got the RPM meter out & mounted the sensor on the frame, next to the sprocket
...then, stuck a magnet on the sprocket, to "signal" the sensor

Then, I precharged the circuit
...switched the breaker "on"
...switched the system "on"
...switched the RPM meter "on"
...switched the (3) speed switch into "low"
...activated the throttle pedal
...& let 'er "rip"  Yup, I did a video to record the data * Donuts are @ the end #### Functional Artist

##### Active member
After reviewing the RPM data for this 60V 2,000W brushless motor
...& running @ a 6:1 GR (10T motor & 60T sprocket)
I wanted to record the data that I came up with * I took the readings off of the sprocket (axle)
...so, we have to multiply those RPM reading, by the gear ratio, to calculate the motor's speed (RPM's)

Low: 580 3,480
Med: 762 4,572
High: 942 5,652

Low: 500 3,000
Med: 675 4,050
High: 830 4,980

(max or peak)
...so, watching the video, I also, looked for & recorded the top (max) RPM reading for each speed 