Dyanamik Braking Resistor Calculations.

[rahuljadhav1]

 

Dear all,

 

Please explain in detail that,

 

How to calculate Braking resistor Kw ,Ohm ?

 

Any general thumb rule exsist?

 

Take care

 

 

 

 

[marke]

When the variable speed drive causes the motor to decelerate, the kinetic energy in the load needs to be dissipated somewhere. If the rate of deceleration is slow enough, the energy is consumed as work energy, but as the rate of deceleration is increased, the flow of energy from the load becomes higher than the flow of energy out the motor shaft. This energy is typically dumped via a chopper circuit into a bank of one or more braking resistors.

The resistance determines the maximum rate at which the energy can be taken from the load.

The rating of the brake chopper transistor also determines the maximum current that can be drawn from the load.

The drive manual will specify the minimum resistance that can be used as a brake resistor. If you use a resistor less than that value, you will damage the brake chopper transistor.

The power rating of the resistor is dependent on the power that needs to be dissipated.

There are two issues, the average power and the peak power dissipation. The peak power can be determined from the deceleration torque required. If you need 100% braking torque while braking, then the peak power rating is equal to the motor input power rating.

The average power rating is related to the peak power and the time spent with the brake applied.

 

Best regards,

[KBrown]

You need to calculate the braking power required to decelerate the motor and connected inertia over the delta RPM of interest.

 

I am going to make the assumption that your decel rate is Linear. The following approach should give you reasonable results. You will need to know a number of things about your system. Also - since I am referencing an old spreadsheet I developed that has proven to work well in applications I have dealt with in the 480V drive realm between 5HP and 250HP - I can't vouch that it is 100% applicable to your application if your parameters are much different than mine.

• DC Bus Voltage trigger point at which the Braking Circuit Turns ON (Volts)
• Motor and Load Inertia (lb-ft^2)
• RPM at start of Decel (RPM)
• RPM at completion of Decel (RPM)
• Minimum Brake Resistor Resistance recommended by the drive manufacturer (Ohms)

 

Start by calculating the Braking Torque. . . Braking Torque (lb-ft) = Rotational_Inertia (lbxft^2) x Delta RPM / (308 x decel time in seconds)

 

Peak Braking Power (HP) = Braking Torque (lb-ft) x Max RPM / 5250

Min Braking Power = Braking Torque (lb-ft) x Min RPM / 5250 (obviously if you are braking to a stop, min braking power = zero)

 

Average Braking Power (HP) = (Max - Min) / 2

 

You also need to know the time between Decel Events in seconds.

 

Typical minimum braking resistance = V_DCBUS^2 / (Average Braking Power (HP) x 746) Ohms

 

Select a resistor that has a Lower Resistance than the Minimum Braking Resistance - YET a higher resistance that that recommended by the drive manufacturer.

 

Once you pick a resistor - check to ensure that it has a high enough peak power rating by using the following equation.

 

Peak Power (kW) = V_DCBUS^2 / (Resistance (Ohms) x 1000)

 

Also - make sure the resistor can dissipate the Average Power (kW) = Decel Time (sec) / Time between stops (sec) x Peak Power

 

This should get you in the ballpark. There are a number of good references for this method and alternate methods on the Emerson Control Techniques website.

 

[rahuljadhav1]

You need to calculate the braking power required to decelerate the motor and connected inertia over the delta RPM of interest.

 

I am going to make the assumption that your decel rate is Linear. The following approach should give you reasonable results. You will need to know a number of things about your system. Also - since I am referencing an old spreadsheet I developed that has proven to work well in applications I have dealt with in the 480V drive realm between 5HP and 250HP - I can't vouch that it is 100% applicable to your application if your parameters are much different than mine.

• DC Bus Voltage trigger point at which the Braking Circuit Turns ON (Volts)
• Motor and Load Inertia (lb-ft^2)
• RPM at start of Decel (RPM)
• RPM at completion of Decel (RPM)
• Minimum Brake Resistor Resistance recommended by the drive manufacturer (Ohms)

Start by calculating the Braking Torque. . . Braking Torque (lb-ft) = Rotational_Inertia (lbxft^2) x Delta RPM / (308 x decel time in seconds)

 

Peak Braking Power (HP) = Braking Torque (lb-ft) x Max RPM / 5250

Min Braking Power = Braking Torque (lb-ft) x Min RPM / 5250 (obviously if you are braking to a stop, min braking power = zero)

 

Average Braking Power (HP) = (Max - Min) / 2

 

You also need to know the time between Decel Events in seconds.

 

Typical minimum braking resistance = V_DCBUS^2 / (Average Braking Power (HP) x 746) Ohms

 

Select a resistor that has a Lower Resistance than the Minimum Braking Resistance - YET a higher resistance that that recommended by the drive manufacturer.

 

Once you pick a resistor - check to ensure that it has a high enough peak power rating by using the following equation.

 

Peak Power (kW) = V_DCBUS^2 / (Resistance (Ohms) x 1000)

 

Also - make sure the resistor can dissipate the Average Power (kW) = Decel Time (sec) / Time between stops (sec) x Peak Power

 

This should get you in the ballpark. There are a number of good references for this method and alternate methods on the Emerson Control Techniques website.

 

 

hi Kbrown

 

thx for help.

 

take care

[edingwall]

Hi Marke,

I've got a question regarding Dynamic Braking. How does the Dynamic brake actually stop the motor from rotating?

I understand that Dynamic Braking is the process of converting the kinetic energy of the rotating motor into electric power, then applying a resistor bank to that power to dissipate it as heat to allow the energy to be dumped into a brake resistor.

With DC injection braking, a counter force is exerted on the rotor when the magnetic fields are aligned (N to S and S to N) as it rotates through the static stator flux. My understanding is clear with DC injection braking as I understand what's actually happening with the rotor & stator fields. 

I lack the understanding of Dynamic Braking. Could you please kindly shed some light if possible of how the Dynamic brake actually stops the motor from rotating?

Thanks again Marke,

Kind regards,

Ewan 

 

 

 

[marke]

Hello Ewan

If we apply a variable frequency variable voltage waveform to a three phase induction motor, the rotor will try to spin at the frequency of the applied voltage. - The induction motor is a pseudo-sychronous device.

If the rotational speed of the magnetic field is higher than the motor speed, there will be an acceleration torque that will accelerate the motor up to synchronous speed. The motor will draw energy from the VFD. - current will flow from the DC bus, through the inverter to the motor. The DC bus will be charged by the applied rectifier and supply.

If the frequency applied to the motor is less than the motor speed, there will be a breaking torque to slow the motor to synchronous speed (speed of applied waveform) This will cause the motor and the driven load to slow down.
As the motor and load is slowed down, we are reducing the rotational kinetic energy. This energy is transferred to the DC bus and will cause the DC bus to rise.

There is only a small amount of allowable DC bus, so we apply a Brake Chopper that pulses a connection to a large discharge resister. The excess energy from the rotational kinetic energy, is dissipated in the break resistor.

The sizing of the resister is important. The resistance of the brake resister must be low enough to soak up the energy at a fast enough rate, but must be high enough to limit the discharge current to less than the maximum current of the brake chopper transistor.

The resister must be large enough to absorb the total excess energy without overheating.