Torque and speed of a BLDC when we exceed the specified DC bus voltage  - Page 1
Forum section: electronics / projects

author: alderbaran748159
author_rank: Newbie
timestamp: March 22, 2023, 01:43:19 pm
content: Hi all,I would like to understand the implications of using a DC bus at 80V instead of 48V. Although the winding is likely isolated to higher than 80V, I am concerned it may shorten the motor's lifespan. Is there any other concern? I am aware that increasing the DC bus will result in higher speed or stronger torque, which could potentially cause mechanical problems with ball bearings and mechanical constraints.The datasheet provides Rated torque and Rated Speed at 24V and 48V (see attachment). What would be the best approach to estimate the values at 80V?My primary goal is to size the DC cable for the motor drive. Therefore, I would like to obtain the rated power at 80V and estimate the electrical power and DC current based on the motor and motor controller's efficiency. PI attempted to use the following formula to calculate mechanical power: Pmech = τ * ω = (((Kt * V) / R) - Km^2*ω)*ω.Where:τ represents output torqueω is angular velocity,Kt is torque constant,Km is the motor constant,R is the resistance of the windingV is the voltage applied to the armature.However, when I used this formula with the values given for 48V, I did not get the same mechanical power result.Thank you

author: Siwastaja
author_rank: Super Contributor
timestamp: March 22, 2023, 03:20:12 pm
content: If the motor is rated for 48VDC nominal, 80VDC on the DC bus is not that much. 48VDC rating on the motor means theminimumyou need to get the rated performance. At higher DC bus voltage, the duty cycle the motor sees just gets lower. Current rise dI/dt = V/L, where V = (Vsupply - Vbemf) gets faster, but this is rarely a problem because inverters are designed for fast current sensing.If your controller is correctly designed, it will regulate phase currents actively (simplistic old-school BLDC controls may not, but they must have input voltage feedforward, then) - it's not a problem. In other words, if there is a problem, it is fixable and should be dealt with.If you are redesigning an existing inverter, that is going to need full evaluation of everything, of course; MOSFET ratings, capacitor ratings, Vin measurement resistor dividers, etc.. And the firmware needs some modifications. You are correct that increasing the voltage allows lowering the DC current, as per Ohm's law, for given power. Motor wires still need to be dimensioned for the motor current., which will stay the same.

author: alderbaran748159
author_rank: Newbie
timestamp: March 22, 2023, 03:48:30 pm
content: Thank you for you reply Siwastaja it makes sense.Yes the motor controller is a fancy one with fast current sensing.As I said my main goal is to size the DC conductor. With no risk I could use the same gauge as the three phases (20 AWG) but my goal is to reduce it as much as I can.Is it correct to say we have the resistance of the coil 1.6 Ohm, peak current in the datasheet is 11.1Arms, this gives P=RI^2=200W ish, so if I report this power in amount assuming an efficiency of the motor of 80% and 80% for the inverter, the power a the input of the system would be Pin=200/(0.8*0.=308WThen if my DC bus is now at 100V this give me a I=308/100=3.08A so now I can choose the smaller gauge for 3A ->22AWGThank you

author: johansen
author_rank: Super Contributor
timestamp: March 22, 2023, 04:34:06 pm
content: You can run smallbwires to the motor controller if the capacitors are adaquate in the controllerAs for the motor leads they must be rated for the motor current.Think of the motor controller as a buck converter.

author: alderbaran748159
author_rank: Newbie
timestamp: March 22, 2023, 05:45:52 pm
content: The stator comes with 20AWG phases wires, I assume it it sized for the rated power.

author: botvink
author_rank: Newbie
timestamp: April 03, 2023, 11:42:19 am
content: Quote from: alderbaran748159 on March 22, 2023, 03:48:30 pmHow fast that current sense alghoritm? Now with higher bus voltage that peak signal glitches will be faster,Also many circuits designed for sub 55V voltage levels, Giving 80V to 48V circuit may not be good idea.May I know that ESC brand?