# Power Supply Sizing

I have 6 motors total, 4x 24V and 2x 48V and I am trying to come up with current for the PSU. I am looking into stringing together several PSU to get the right current, but I’m running into issues with finding the current values.

I have the peak current draw for all of the motors and I could use that to size the various PSU together, but I’ve been reading online to not equate the motor current to the PSU current. Does someone have some resource I can use to help with sizing the PSU’s? I’d like to use the fewest possible to reduce cost. Also, I do know the peak torque required of each motor that I could use to find the overall peak power required (but this is pretty close to the peak current values of the motors.)

Thanks in advance for the help.

The best option that you have is to try putting in a bit more work to try to get a better understanding of what your performance requirements are. Without knowing the peak torque and speed requirements of your application the next best thing is to size for the peak power draw of the motors. Just be aware that sizing for the peak capabilities of all of your motors will likely be a very conservative estimate and result in a much bigger supply than what is required.

The conservative approach is to take the peak power rating for each of your motors (usually available on the motor spec sheet) and add all of those up. You then take that value and divide it by whatever your power supply voltage is. According to the equation for calculating electrical power this gives you current:

Power = Voltage * Current
Current = Power/Voltage

This current value that you calculate is what your power supply must be able to provide and what your DC+ and DC- conductors should be sized for (assuming you will be powering all of your different odrive boards from a single supply).

There will be some losses in the odrive that this method doesn’t take into account, but you will likely already be drastically oversizing your power supply at this point, so it should be fine to ignore the losses.

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I am using these motors:

And they don’t seem to give a peak power rating. They give a max power output at stall torque. I am told that power at the motor may not be the same as power at the supply. So is this number useless for choosing a power supply? Or do I take the input values they list and get the req’d input power from that? 12V * 105A = 1260W?

Energy (and thus power) has to be conserved, so the only way power at the motor would be different than power at the supply is if there are losses in the inverter or you had other things running off of your power supply (such as another motor). Realistically there are inverter losses, but they can typically be ignored.

If you are sizing the supply based off of only the motor requirements then I would use the theoretical peak output power rating from the data sheet for sizing your power supply. Realistically you probably won’t ever get to that level of power draw without burning up your motor, so sizing for that will give you a bit of margin.

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There is a much more practical approach.
Take the max rpm for your application and divide that by the kv rating of the motor.
P=(max.rpm/kv)xIpeak
You can use hp server psus