VARIABLE FREQUENCY DRIVES (QUESTION &ANSWERS)

1)     What are electrical drives?

ANS) In very simple words, the systems which control the motion of the electrical machines, are known as electrical drives. This drive system is widely used in large number of industrial and domestic applications like factories, transportation systems, textile mills, fans, pumps, motors, robots etc. Drives are employed as prime movers for diesel or petrol engines, gas or steam turbines, hydraulic motors and electric motors.

2)What is Variable frequency drive?

ANS) Motors speed can be changed by changing its frequency. Therefore VFD’s are actually motor controllers that controls its speed by changing its frequency and voltage. Other names for VFD’s are adjustable speed drive, micro drive, AC drive etc.

3)How VFD’s works?

VFD’s consist of four important components:

1)     CONVERTER: Converter circuits converts the incoming AC supply to DC.

2)     SMOOTHING CIRCUIT: This circuit smooth’s the pulsating included in dc

3)     INVERTER: Inverter converts DC to AC

4)     CONTROL CIRCUIT: This circuit is used to control inverter part.

4)In VFD first we convert AC to dc by converter and then we again convert Dc to Ac by an inverter? What’s the logic behind it?

ANS) First we convert the incoming 220v Ac supply which is at 50 hertz frequency. Then DC To AC is done again because we want to produce the waveform at our desired frequency. Whatever frequency is required to control the speed of our motor we can produce waveform of that particular frequency.

Secondly, it’s difficult to do frequency conversion directly (from incoming 50 hz supply from k electric to our required frequency) and more easy to convert it to dc and then produce waveform of our desired frequency

5)     How VFD’s can save energy?

·        Majority of the devices such motors used for pumping waters in our homes cannot change their speed by themselves and during majority of the time they do not need to be run at full speed. During those times if the speed of that motor can be reduce than energy it consumes can be reduce drastically. So VFD when connected to these motors can control its speed by changing its frequency.

·        During startup machines draw huge current (inrush current) which is much higher than normal rated current. So to reduce that current we use VFD in combination with a soft starter to reduce the current and protect the equipment from being damaged.

6)     Can I put a VFD to control any motor?

ANS) The speed of any motor is dependent on its applied frequency. However some AC Motors are just not suitable for variable speed operation such as single phase motor because generally speed single phase application are fixed speed.

To control the speed of 3 phase IM, Synchronous motors VFD’s are generally used

7)     Difference between soft starter and VFD?

ANS ) Soft starters and variable frequency drives are two different purpose products. VFD is for AC motor speed control, it's not only change the output voltage but also change the frequency; Soft starter is a regulator actually for motor starting, just changing the output voltage. Variable frequency drive has all the features of soft starters, but the price is much more expensive than the soft starter and the structure is much more complex. 

Variable frequency drive is converting power supply (50Hz or 60Hz) into a variety of frequency AC power, to achieve electric motor variable speed operation control. Where as Soft starters are basically used for reducing starting current of motors which is normally 5 times of full load current and they stay in circuit for less than 30 sec and then are bypassed.

8)     Drawbacks of VDF’S?

·        The high-frequency PWM switching that VFDs use to generate the output voltage waveform often carries high-frequency transients out onto the wires going to the motor. Often it is recommend of using special shielded cable between the VFD and the motor, which can be more expensive and harder to work with (in conduit, etc.). If not properly shielded, this transient noise can cause interference with other low-voltage electronic devices that may be nearby.

·        Another major problem that may arise is repair and maintenance of a VFD. General technicians can’t provide necessary technical support in case of any drawback. The problem increases if more than one VFDs are networked together.

·        VFD’s cost will increase as the system size increases.

9)     Are there other methods to control speed other than vfd?

ANS) The most widely used large motor is the AC induction motor and for them, the VFD is the best way to control speed, electronically speaking, however you can use gears and variable speed drive belts…

Previously load banks were used to control speeds of motors by reducing the voltage across the motor. They are a bank of resistors and basically create voltage dividers such that one is across the load bank, and the other across the motor. The bad thing about them is they waste a lot of energy. The good thing is that they last a long time.

10)     Can single vfd be used to control multiple motors?

ANS) Yes single vfd can be used to control multiple motor but there are certain condition that must be met first:

FIRST CONDITION: Each motor must have same speed. If we have one VFD for each motor than there is no issue of running motors at same speed but since we have only one VFD for multiple motor than they must be operating at same speed

SECOND CONDTION: Our system will become less reliable because if that one VFD which is controlling all the motor fails than our whole system will shut down.

Various VFD bypass schemes can be used to overcome this limitation, but these schemes all add cost and complexity to the system.

THIRD CONDITION: care must be taken when operating the motors. To minimize VFD size, all motors need to be started up simultaneously. The VFD will ramp all the motors up to speed at a controlled rate, minimizing the inrush current required by each motor at startup.