# Switching frequency of BLDC inverter?

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#### treez

If a 650W BLDC motor is rotating at 8000rpm (133 revs per second), then does that mean that the switching frequency of the inverter is 400Hz?
133 revs per second seems very high.....is a pump motor likely to rotate this quickly?

#### kam1787

I don't see a relationship between RPM and the switching frequency of an inverter

treez

### treez

Points: 2

#### treez

if its 8000rpm, and its a 3 phase BLDC.
then the bottom group of bridge transistors switch three times for every rev (once each)
8000 rpm = 133 revs per sec
so its 7.5ms per rev.
so each transistor switches with a period of 2.5ms......this is 400Hz

#### chuckey

" .is a pump motor likely to rotate this quickly?" you say it rotates at 8K RPM. Does seem high, but machinery commonly rotates up to 50 K RPM and up to 150 K RPM for specialised grinders for optical lens grinding. The typical AC water pump motor I have seen is a 2850 RPM.
Frank

#### FvM

##### Super Moderator
Staff member
The typical AC water pump motor I have seen is a 2850 RPM
Because it's driven by a 50 Hz (single pole-pair) induction motor. DC motor pumps run often at higher RPM and are respectively noisy.

#### chuckey

The BLDC motors I was thinking about are the 15KW tool rotating motors on CNC "milling" machines. The reason for BLDC motor is that the tool rotation speed is under computer control, so when the tool is automatically changed, the motor speed can be changed to the correct speed for that tool, also when machining complex shapes the tool's speed can be continuously changed as that of the workpiece rotating mechanism.
In general I do not see this degree of control being required for a water pump unless there is some requirement for a constant pressure head when the flow is variable. Traditionally this would be done with a pressure valve and a bypass loop.
Is it because the motor has to be a funny shape? - long and thin to go down a bore hole?.
I think it would help if we knew the motor type number and manufacturer, - what do they recommend as a driver?
Frank

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