VFD; is power still power

[Macbeth]

I see people refencing V/Hz (Scalar Control) this application would be a Vector Control. In Scalar Control the stator and rotor magnetic fields are not held orthogonal as they "Slip" from the optimal 90° to each other, the Motor Torque is Under driven "Slip" or Over driven. In Vector Control the angle of the stator and rotor magnetic fields automatically rotated to maintain required Torque necessary to maintain speed. The angle is adjusted not necessary the Current or Voltage.

In Scalar mode at 0Hz the motor would have 0V and be allowed to free spin. In vector mode 0hz Flux could still be applied to the motor preventing it from turning.

 

[Shaneyj]

I see people refencing V/Hz (Scalar Control) this application would be a Vector Control. In Scalar Control the stator and rotor magnetic fields are not held orthogonal as they "Slip" from the optimal 90° to each other, the Motor Torque is Under driven "Slip" or Over driven. In Vector Control the angle of the stator and rotor magnetic fields automatically rotated to maintain required Torque necessary to maintain speed. The angle is adjusted not necessary the Current or Voltage.

In Scalar mode at 0Hz the motor would have 0V and be allowed to free spin. In vector mode 0hz Flux could still be applied to the motor preventing it from turning.

Thank you for that explanation.
I am still hunting for how the current maths are related to output.
Can you please explain what you mean by "not necessary the current or voltage"?

 

[winnie]

I absolutely agree that vector control provides better control of the motor than scaler control.

I can't comment on this particular application as needing vector control or not.

However even when using vector control you will find that at any given speed in the steady state the voltage supplied to the motor will still approximate constant V/Hz operation. Slower speed means lower voltage. The drive voltage is not directly controlled with vector operation, but the vector control must still adjust for the physics of the motor, which is dominated by stator inductance.

-Jon

 

[Macbeth]

I may have poorly stated that misleadingly. In V/hz mode the output of the VFD is fixed to a V/Hz curve. For every Hz a X-Volts is applied. Always the same, regardless of load.

In vector control the Stator Flux and Armature Flux are controlled separately to create Torque to maintain RPM. It does not raise Voltage/Current unless it needs to.

Torque(nm) = HP x 7127 / RPM

107nm @ 500rmp = 7.5hp
107nm @ 1000rmp = 15hp
107nm @ 2000rmp = 30hp

30hp @ 500rpm = 427nm
30hp @ 1000rpm = 214nm
30hp @ 2000rpm = 107nm

 

[winnie]

Agreed, vector control does not raise voltage unless it needs to.

If you took your table of torque, speed and power and added voltage (as determined by the vector control system) then in the steady state you would find higher voltage associated with higher speed, and if near full rated torque that voltage would be pretty near the V/Hz value.

Think of it this way: V/Hz is a pretty good approximation of what the motor needs to operate, vector control actively measures what the motor needs to operate. The results from vector control cannot be wildly different from V/Hz control because if they were then V/Hz wouldn't be a reasonable approximation.

Jon