V/F mode:

1. A standard VFD will sometimes be referred to as a "V/Hz" drive to differentiate it from a vector drive. It maintains a certain V/Hz ratio to the motor at all times. For example, a 230 Vac, 60 Hz motor wants to see a V/Hz ratio of 3.83 (230/60 = 3.83). When a V/Hz drive changes speed (frequency), it also changes the output voltage to keep the ratio constant. So, at 30 Hz (half speed), the same 230 Vac motor will only see 115 Vac output from the drive.

2. For most applications this works great, as long as you operate from about 6-60 Hz (10:1 speed range). Below 6 Hz, a motor on a V/Hz type VFD can't generate much torque, because at those low speeds, the V/Hz ratio to achieve maximum torque is different than at higher speeds. If you try to run at those low speeds with a V/Hz drive, you will typically see the motor shaft "cogging" as it tries to turn.

Vector Control:-

1. Vector drives can separately manipulate the voltage and frequency to always produce the optimum V/Hz ratio for maximum torque. In this way, vector drives can generate more starting torque, and can provide full torque down to 1 Hz or less, while V/Hz drives are limited to about 5-6 Hz.

2. There are basically 2 types of vector drives: closed-loop, and open-loop or sensorless.

3. Closed-loop vector drives require special motors with encoder feedback (hence the term "closed-loop") that provide motor shaft position info to the drive. The drive uses this information to constantly alter the V/Hz ratio to produce maximum torque. This type of VFd is very high performance and very expensive.

4. Sensorless vector drives work with standard motors, but because there is no encoder feedback, the performance is not quite as good as closed-loop, but still much better than V/Hz drives. Instead of getting real-time info from an encoder, a sensorless vector drive estimates what is going on by creating a motor model within it's software. This model is created using motor info that the user manually enters, along with other info that the drive measures when it is connected to the motor.

Vector drives (more accurately referred to as "Field Oriented Control" drives) do not explicitly solve for a V/Hz relationship. Rather, they solve for two specific currents within the stator simultaniously. The V/Hz relationship is solved implicitly according to flux density from the vector sum of current components described below:

The first CURRENT is the Magnetizing current. Since there is no magnetic field within the rotor of the motor, it must be induced using electromagnetism (Hence Induction Motor). A DC current is controlled in the stator to induce an electromagnetic field in the rotor. As the rotor is turned (eg motor spinning) this magnetic field is rotated synchronously with the rotor to maintain a static orientation of the magnetic field relative to the rotor.

The second CURRENT is the Torque Producing Current. This current is controlled in the stator to produce another electromagnetic field within the stator itself that is oriented 90 degrees to the magnetic field simultaniously induced in the rotor as described above. As the intensity of the torque producing current increases, the torque applied to the rotor (eg motor shaft) increases.

It is the VECTOR SUM of these two currents in the stator that is solved for by the drive in a Vector drive application. Hence "Vector Drive"