# How the DC inverter can ramp up to 90Hz?

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#### 112311

##### Newbie level 3
I refer to the variable refrigerant flow (VRF) system in air conditioning system. The DC inverter can ramp up to 90Hz. Can anybody explain to me how does it work? I thought we are using 50Hz supply, so the maximum limit can go also 50Hz?

#### chuckey

It is common for 3 phase ac motors to be driven from a VFD rather then the mains. A VFD, produces a PWM signal that drives the 3 phase motor, the PWM frequency can be adjusted normally 5 - 500 HZ. So the motors speed can be adjusted from .1 to X 10 of its faceplate value. A VFD contains a DC bus which is derived from the incoming mains, its the output circuit that generates the PWM - Is this what you are after.
Frank

112311

### 112311

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#### 112311

##### Newbie level 3
Tq Frank for your explanation. But how to calculate the motor rpm, for example at 200 Hz since it can go to 500Hz maximum. Let says the motor rpm is 1450 (4 pole motor) at 50 Hz. Is it 200x1450/50 = 5800RPM?

---------- Post added at 07:49 ---------- Previous post was at 07:47 ----------

Sorry Frank, is it applicable to 1 phase motor too?

#### chuckey

Yes!
Not really, depending on their type , some single phase motors (all the big ones > .5HP) use capacitors to start the motor and then to put a phase shifted voltage on a special winding. So putting a variable frequency into one of these, would cause all sorts of problems, over heating, low power etc. So a VFD could be used over a restricted range, say 50 +-10 Hz, more then this and the magic smoke will come out of the motor.
Frank

#### 112311

##### Newbie level 3
Thanks a lot Frank. You helped me a lot in understanding this concept.
But for your information, the VSD that i normally use to control my air handling unit's motor, its maximum frequency is 50Hz only, following the incoming frequency. Why they don't use that frequency inverter that can ramp up to 500Hz????

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