Selecting PWM frequency - BLDC motor control

"Also for 3 wire BLDC (U,V,W output) if we measure L & R between two wires then it will be resistance & inductance of both phase combined & we have to divide it by 2 to get L,R for individual phase, is it right ?"

line-line and line-neutral are both valid representations of Ls and Rs, it just depends on how they are going to be used.

There isn't a formula to determine the ideal PWM frequency for your motor.
it depends on the motor (primarily R/L which effects max frequency) and the power stage (minimizing switching loses and dead-time effects).

in general, for low voltage, high frequency motors you are going to use as high of a PWM as possible - usually limited by the driver and MOSFETS (typically 20 - 100 KHz)

for high voltage, low frequency motors you are going to use a very low frequency PWM to limit switching loses (typically 2 - 16 KHz)

Thanks Chris for your reply.

You are taking about two terms :--
1> low voltage, high frequency motors
2> high voltage, low frequency motors

Here by frequency you mean to say speed of the motor, right ?
If right then till what speed motors are considered as Low/high frequency ?
low frequency motors = ? RPM
high frequency motors = ? RPM

What Voltage range motors are considered as Low/high voltage?                                                                                                                                                                           Low voltage motor = ? volts                                                                                                                                                                                                                                             High voltage motor = ? volts

Is there some application note from TI which specify motor under various catagory (low voltage, high frequency  or  high voltage, low frequency  motors)?

I have bought one BLDC from local market its chinese made & only specification i have about it is :--
Motor Condition : General
Rated Voltage : 60 V
Power Capability : 850W
Rated Speed : 3000 RPM

I was refering application note from TI "25 Energy Flow and Modulation.pdf" on page number 32, there are three diffrent type of motors shown.
If i have got it right then my motor use to lie in middle motor category ? What kind of motor is mine ?

Please suggest.

Regards,
Dinesh

your motor is 60V and <20A and looks relatively slow speed. you didn't mention the # of poles, but with 3 KRPM 16 poles is only 400 Hz, which is not particularly high
I would guess it has an inductance of a few hundred mH, unless the Rs is very, very small.
If so, a PWM in the 16-24 KHz range is most typical for this type of motor.

Thanks for your reply.

>>but with 3 KRPM 16 poles is only 400 Hz

relation between motor pole and rpm &  frequency is :--

RPM = (120 * Frequency) / # of poles in the motor

If i am right than this is the maximum frequency of sin wave we can pass to BLDC u,v,w phases, right ?

Now what is the role of  this frequecy(which is calculated by above formula) & ratio R/L &  driver & dead time here, to calculate MCU PWM ?

Also one thing i knows, The highest frequency of the human audible range is considered as 15~20 kHz, basically depending on age. Thus, if we choose higher PWM frequency than 15~20 kHz, we cannot hear the motor noise caused by PWM fundamental frequency.

>> I would guess it has an inductance of a few hundred mH, unless the Rs is very, very small.If so, a PWM in the 16-24 KHz range is most typical for this type of motor.

Depending upon estimated value of R & L & frequency, How you made conclusion that PWM should be 16-24 kHz . Any math behind this ?

I am bit confused on this please suggest.

Regards,

Dinesh

"There isn't a formula to determine the ideal PWM frequency for your motor.
it depends on the motor (primarily R/L which effects max frequency) and the power stage (minimizing switching loses and dead-time effects)."