Braking Resistors in Variable Frequency Drives

Braking Resistors are used in Variable Frequency Drives to dissipate the energy released by the motor into the power system.  The Braking Resistors perform the duty of absorbing the power from the rotor when the VFD reduces the speed to zero and preventing rotor heating.  They also prevent the rotor from exceeding the synchronous speed set by the Variable frequency drive.

A variable Frequency drive consists of three main components - the rectfier which converts the AC supply into DC, the DC busbars and the inverter which converts the DC supply into a variable AC supply.  The VFD varies the speed of the motor by varying the frequency of the AC supply applied at the motor terminals. 

When the motor is required to be stopped suddenly, the Variable frequency drive reduces the supply frequency to 0 HZ.  In this condition, the rotor is rotating at speed higher than the synchronous speed.  This causes the motor to behave like a generator and send power in the reverse direction, into the DC bus bars.  During this time, the voltage across the DC busbars can rise to very highlevels.  The braking resistors absorb power in this situation and prevent the voltage from rising beyond limit and damaging the Drive.     

The value of the resistances determines the rate of fall of the motor speed (braking). 

Special provision is made for cooling the resistors which can generate a huge amount of heat when in operation. 

Resistance Temperature Detectors

Resistance Temperature Detectors or RTDs are sensors which measure the temperature by altering their resistance. >The Resistance temperature detector consists of an element made of a metal such as platinum  located in a metallic casing.

When the temperature increases, the resistance of the sensor increases (positive temperature coefficient of resistance) This increase in the resistance is measured through a wheatstone bridge. The relationship between temperature and the resistance is linear.  Thus, the temperature can be deduced from the measured resistance.

Platinum and Nickel are two metals used to construct the sensing elements.  

Some common types of RTDs are the Pt-100 and Pt-1000. 

The Pt stands for Platinum while the number 100 stands for the ohmic value at 0 degrees Celsius.

The resistance increases linearly with temperature.

For example, the Pt100 has an ohmic value of 100 ohms at 0 °C and a value of 161 ohms at 160°C  

Advantages of RTDs

Accuracy

Long Term Stability

Ability to withstand shock and vibration

Disadvantages of RTDs 

Errors due to lead resistance,

Slow response

Internal Self heating

2 Wire, 3 wire and 4 wire RTDs

One of the disadvantages of the RTD is the error caused by the lead resistance.  That is, the indicating device which measures the sensor resistance to calculate resistance also measures the resistance of the leads connecting the sensor to the device.  This is unavoidable, though the error can be minimized by running a wire in parallel to one or both the leads. (Refer diagram)