Motor Protection Relay (MPR)

A soft starter reduces the voltage applied to the terminals of a motor at start-up so as to reduce the inrush current.

A variable frequency drive varies the frequency of the supply to the motor so that, in theory, it can start at 0rpm and accelerate from there.

Motor protection relay does the function of protecting the motor from abnormal parameters like current,voltage,temperature during the operation of motor.

Starter does the function of starting the motor from o speed to designed speed .

Both devices are different does different functions.

MPR- Its main function is to protect the motor from any fault and keep safe the motor. It has certain features like Thermal Overload, Instantaneous, Earth fault, negative phase sequence, Stall current etc with amny other features.

SS- Its main function is to apply low voltage to motor terminal during starting(to limit the starting current) and then gradually increases the voltage to attain the full speed or max. torque.

VVVF- Torque is proportional to flux. By applying VVVF method, flux is maintaining constant in all values of voltage. This provide constatnt torque to motor from starting to max speed.

Very much different !

MPR is mainly used for protection and can be a part of motor starter. A motor starter is used to start a motor and can have energy saving capabilities. A soft starter is used where the application needs to smoothly start without a jerk. Some of these starters are also believed to be energy saving devices.

A VFD (aka VVVF i.e. Variable Voltage Variable Frequency) is an energy saving device primarily used in applications requiring control of rotational speed by variation of frequency of supply. It is recommended to be used with a filter / choke (possibly inbuilt) to negate the effects of Harmonics - one of the most serious system pollutants.

Cheers,

RRV

www.econserve.in

Also, to add to the answers already given:

MPR is a monitoring device only, can be used to disconnect the supply

Soft Starts and VFD are control devices which also may have different monitoring and protective functionality

It should be noted that most VFDs and some high-end soft starters will provide most if not more protection features found in many Motor Protection Relays. In the case of VFDs, most inexpensive MPRs are incapable of accurately protecting the motors because they cannot correctly interpret the output current waveform coming from a VFD, so the VFD will actually do a better job or providing protection. There are exceptions to this rule as you go to higher priced MPRs and lower priced VFDs of course.