How Do I Determine Minimum Allowable Dynamic Braking Resistance for PowerFlex 750 Series Drives?
How to use the PowerFlex 750 series technical data document to determine minimum allowable dynamic braking resistance?
When selecting a dynamic braking resistor for a PowerFlex 750-Series drive, one of the first values that must be determined is the minimum allowable dynamic brake resistance for the drive.
The minimum resistance is determined by the specific drive rating and should not be confused with the resistor's wattage or duty-cycle rating.
Rockwell Automation publishes the minimum dynamic braking resistance for PowerFlex 750-Series drives in the PowerFlex 750-Series AC Drives Technical Data, publication 750-TD001.
PowerFlex 750-Series Technical Data – 750-TD001
[PowerFlex 750 Series Technical Data]
Why Minimum Resistance Matters
When the motor is decelerating, regenerative energy can cause the drive's DC bus voltage to increase.
A dynamic braking resistor allows this energy to be dissipated as heat. The drive's dynamic brake transistor switches the resistor across the DC bus when braking is required.
The resistance of the braking resistor determines how much current can flow through the dynamic braking circuit.
Lower resistance = Higher braking current
Because the drive's braking transistor has a maximum allowable current, there is a limit to how low the connected resistance can be.
For this reason, never select a braking resistor with a resistance below the minimum value specified by Rockwell Automation for the drive.
Finding the Minimum Resistance
Open the PowerFlex 750-Series AC Drives Technical Data – 750-TD001.
Search the document for:
Minimum Dynamic Brake Resistance
The Technical Data contains dynamic braking tables organized by drive voltage and rating.
Locate the table corresponding to the voltage class of the drive, such as:
- 400/480V AC
- 600V AC
- 690V AC
Then locate the specific drive rating within that table.
The table provides the minimum dynamic brake resistance in ohms (Ω) for the applicable drive.
Selecting the Correct Drive Rating
Do not select the minimum resistance based only on motor horsepower.
Use the actual PowerFlex drive rating/catalog number to identify the correct row in the Technical Data.
Two drives used on similar horsepower motors may have different current ratings or drive sizes and therefore may have different minimum allowable braking resistance values.
The recommended process is:
- Obtain the complete PowerFlex drive catalog number.
- Determine the drive voltage class.
- Determine the drive rating/current code.
- Open publication 750-TD001.
- Locate the applicable dynamic braking table.
- Find the specific drive rating.
- Record the published Minimum Dynamic Brake Resistance (Ω).
Selecting a Braking Resistor
Once the minimum resistance has been identified, the selected braking resistor must have an equivalent resistance that is:
Equal to or greater than the drive's published minimum dynamic braking resistance.
Example
Assume the Technical Data lists the minimum dynamic braking resistance for a particular drive as:
Minimum Resistance = 20 Ω
Acceptable examples would include:
20 Ω – Acceptable
25 Ω – Acceptable
30 Ω – Acceptable
A resistor rated:
15 Ω – NOT acceptable
would be below the drive's published minimum resistance and could allow excessive current through the dynamic brake transistor.
Lower Resistance vs. Higher Resistance
A lower resistance allows more current to flow through the braking circuit and generally provides greater instantaneous braking capability.
However, the resistance cannot be reduced below the drive manufacturer's minimum value.
Increasing the resistance reduces braking current. While a higher resistance protects the braking transistor from excessive current, selecting too high of a resistance can reduce the amount of braking torque available.
Therefore, the minimum resistance value should be considered the lower limit, not necessarily the resistance that should automatically be selected for every application.
Minimum Resistance Does Not Determine Resistor Wattage
The minimum resistance rating and resistor wattage rating are two different requirements.
Resistance – Ohms (Ω)
Determines the current that can flow through the braking circuit.
The selected resistance must not be lower than the drive's published minimum resistance.
Power – Watts (W)
Determines how much braking energy the resistor can dissipate as heat.
The required wattage depends on the application, including:
- Motor and load inertia
- Deceleration time
- Frequency of stops
- Braking torque required
- Length of each braking event
- Duty cycle
A resistor can have the correct resistance but still be undersized for the application if its wattage or duty-cycle capability is insufficient.
Multiple Resistors
If multiple braking resistors are connected together, use the total equivalent resistance seen by the drive when comparing the resistor bank to the drive's minimum resistance.
For example, two equal resistors connected in parallel will result in an equivalent resistance that is one-half the resistance of either individual resistor.
Always verify the equivalent resistance of the complete resistor bank before connecting it to the drive.
Important
Never connect a braking resistor or resistor bank with an equivalent resistance below the minimum dynamic braking resistance published for the drive.
Doing so can exceed the allowable current of the drive's dynamic brake transistor and may result in drive faults or damage to the braking circuit.
The minimum resistance table in 750-TD001 establishes the minimum resistance the drive can safely use. It does not, by itself, determine whether the resistor has sufficient wattage or duty-cycle capability for the application.
Quick Reference
When selecting a braking resistor for a PowerFlex 750-Series drive:
1. Identify the exact drive.
2. Open 750-TD001.
3. Search for "Minimum Dynamic Brake Resistance."
4. Locate the drive's voltage and rating.
5. Record the minimum resistance in ohms.
6. Select a resistor or resistor bank whose equivalent resistance is equal to or greater than that value.
7. Separately verify that the resistor's wattage and duty-cycle ratings are adequate for the application.
ReferenceRockwell Automation Publication 750-TD001 – PowerFlex 750-Series AC Drives Technical Data
This publication should be used to determine the minimum allowable dynamic braking resistance for the specific PowerFlex 750-Series drive being applied.