VFD || Variable frequency drive || operation

In an industry, there are so many induction motors which are used for different purpose or work. For different work, they need to rotate at different speed, operate different condition like power, voltage etc. but motor's manufacture company will not manufacture all variation speed of motor. In that case, we have a solution for that which is known as VFD. Here we are going to discuss about VFD || Variable frequency drive || What is VFD? Its operation

What is VFD (Variable frequency drive)  ? 

Variable frequency drive is a drive that is used to control the motors by varying the frequency of supply to the motor. As we know that the motor speed is depends on frequency of supply. i.e.

motor's speed (Ns) = (120  *  f  /  P )

From that formula, we can conclude that the speed of motor is dependent on frequency for a  motor with specific pole. At 50/60 Hz (British/American) as power system (power supply), the motor runs at full speed depending on the load. So we need to operate the motor less than or equal to that system frequency. 

VFD's operation || How it works?

Let's us consider, a motor having 8pole of induction motor, then its slip is 0.06 then than speed of motor for 50 Hz system is given by,

synchronous speed, Ns = (120 x 50 / 8) = 750 rpm

Actual speed, Na = ?

we know, s = (Ns - Na) / Ns

            or, Na = (1-s) x Ns

                        = (1 - 0.06) x 750

                        = 705 rpm

This induction motor will runs at 705 rpm at 50 Hz but it will run less than that at different frequency as given below,

At 40 Hz, Na = 564 rpm,

At 30 Hz, Na = 423 rpm,

At 20 Hz, Na = 282 rpm,

At 10 Hz, Na = 141 rpm,

As per requirement, the frequency can be changed and it will varies the speed of motor. Generally in an industries, VFD is used mostly for controlling the no of motors and one VFD will control one motor. Also, VFD is used to change the voltage of supply as per voltage requirement for the motor.  A VFD rating can be in its power i.e. in KW or HP. Before installing it, its operating voltage, frequency, power, current etc should be known. The above figures are VFD (Variable frequency drive) and its rating given by manufacture. 

Operating principle of VFD

For VFD, it need to change the supply frequency so firstly input AC supply is converted into DC and again inverted into AC of required frequency, voltage. AC supply is converted into DC by using rectifier circuit and DC source is converted into AC by using inverter circuit. 

AC Supply → Converted to DC →  Inverted to AC → Electric Motor

We know that VFD is used for motor speed control so let's have a look on that how motor is connected to the VFD. For simple connection,

          Supply →  VFD → Motor

This is a very simple connection for knowledge purpose only where supply is given to the VFD and we can set the motor speed from frequency in VFD and it will control the motor. But in real practice, this is not happened. There are more component in the connection such as PLC (programmable logic control) , MPCB (motor protection circuit breaker), MCCB (molded case circuit breaker), relay, contactor, sensor etc. are connected in the circuit for controlling the system. In real system, for VFD frequency change we can use PLC with sensor or manual connection so it will be easy to control the frequency and get required result.