The TONDAR NIROU Company

 provides a variety of  selling in soft starter and drives and engineering services including installing soft starters and drives and implementing various industrial projects and troubleshooting specialized electronic devices

What's the difference of variable frequency drive and soft starter

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 (50 Hz or 60 Hz ) into a variety of frequency AC power, to achieve electric motor variable speed operation control. There are several ways to define a VFD. Base on main circuit working methods, it can be divided into voltage type VFD and current VFD; Base on switching methods, it can be divided into PAM control drive, PWM control drive and high carrier frequency PWM control drive; Base on the working principle ,  it can be divided into V/F control VFD, slip frequency control and vector control VFD , etc.; Base on usage, it can be divided into general-purpose VFD, high-performance dedicated VFD, High-frequency VFD, single phase VFD  and three-phase variable frequency drive.

Soft starter is a set of motor soft start/stop, light-load energy saving and various protection functions devices to control motors.

Soft starter uses three opposite parallel thyristors as regulator, plug it into the power source and motor stator.  When using soft starter to start the motor, the thyristor output voltage increases gradually, and the motor accelerates gradually until the thyristor is turned on completely. The motor operates at rated voltage to achieve a smooth start, reduce starting current and avoid start overcurrent trip. When the motor reaches rated RPM, the startup process is completed, the soft starter uses bypass contactor to replace thyristor to provide rated voltage to the motor, in order to reduce the thyristor heat loss, extend the soft starter service life and improve efficiency, also avoid harmonic pollution to the power grid.

Fast Fuses

We have to use semiconductor fuses to product IGBT and   Thyristor in VFD and Soft starters electrical panels.

Semiconductor fuses are a type of power-limiting fuse using specifically-shaped silver elements in a silicon/sand environment. Because of their power-limiting characteristics, they can protect some semiconductor devices from excess power. The power-limiting characteristic of semiconductor fuses is also useful for reducing the amount of damage done by an arcing fault; hence, the fault current is effectively reduced.

For Variable Frequency Drives (VFDs), there are two power semiconductor technologies employed: bridge rectifier and the output stage.

The bridge rectifier is made up of power rectifiers which are connected directly to the supply and drive a capacitor bank, either directly or via inductors. By design, then, the rectifier of the VFD is less likely to be damaged by a short circuit on the output or in the motor. The rectifier is susceptible to damage by a voltage surge or input transients. Due to the capacitive load on the rectifier, even a relatively low-magnitude fast impulse can cause high-charging current through the rectifiers into the capacitors (which may damage them). The use of AC line reactors or DC bus chokes reduces the vulnerability of the rectifiers to such transients. The semiconductor fuses may be used on the input to give some protection to the rectifiers, though this is not a common practice.

The charged capacitors are connected to a 3-phase inverter bridge, usually made up of modern IGBTs. The output of the inverter is then connected via a cable to the motor. Modern IGBTs are very rugged and able to be controlled. If an overload happens, it's normally possible to turn OFF the devices before damage occurs. The capacitors, however, store a large amount of energy and if any fault occurs, the energy is not limited -- even if the AC supply is removed -- until the capacitors are discharged. If there is an insulation breakdown, or an IGBT fails, then the resulting damage can be very severe and difficult to repair. The use of power-limiting fuses between the capacitor bank and the output inverter will significantly reduce the damage, but will not protect the output transistors. The output devices are often damaged, not by overload currents, but by peak voltages due to arcing, switching or similar events. Fuses can't prevent this.

As a rule, one or, better yet, two semiconductor fuses are used in the DC bus between the capacitors and the output bridge, but rarely used elsewhere.