How Do I Select Between Normal Duty and Heavy Duty Ratings for a PowerFlex Drive?
PowerFlex drives may have both Normal Duty (ND) and Heavy Duty (HD) ratings. This article explains the difference between these ratings, why the same drive can have different horsepower ratings depending on duty classification, and what should be considered when selecting a drive for an application.
Yes. I’d also tighten it up a bit so it reads more like a KB article and less like a training document.
How Do I Select Between Normal Duty and Heavy Duty Ratings for a PowerFlex Drive?SummaryPowerFlex drives may have both Normal Duty (ND) and Heavy Duty (HD) ratings. This article explains the difference between the two ratings, why the same drive can have different horsepower ratings depending on the selected duty, and what should be considered when selecting a drive for an application.
What Are Normal Duty and Heavy Duty Ratings?
Normal Duty and Heavy Duty ratings describe the amount of continuous current and temporary overload current a drive is capable of supplying.
A single PowerFlex drive may have both a Normal Duty and Heavy Duty rating.
Because Heavy Duty applications require greater overload capability, the drive's Heavy Duty horsepower rating is typically lower than its Normal Duty horsepower rating.
The physical drive has not changed. The difference is how much of the drive's available current capacity must be reserved for temporary overload conditions.
Normal Duty
Normal Duty (ND) is generally used for applications with relatively low overload requirements.
Typical applications include:
- Centrifugal fans
- Centrifugal pumps
- HVAC equipment
- Other variable-torque loads
These applications generally do not require large amounts of additional torque during acceleration or normal operation.
Because less overload capacity is required, the drive can typically be applied at its higher published Normal Duty horsepower rating.
Heavy Duty
Heavy Duty (HD) is generally used for applications that require greater overload capability.
Typical applications include:
- Conveyors
- Mixers
- Crushers
- Extruders
- Positive displacement pumps
- High-inertia loads
- Applications with frequent acceleration or deceleration
- Other constant-torque or high-overload applications
Heavy Duty operation reserves more of the drive's current capability for temporary overload conditions. As a result, the same drive will typically have a lower horsepower rating when applied as Heavy Duty.
Why Does the Horsepower Rating Change?
A drive has a limited amount of current it can safely produce.
In a Normal Duty application, more of that capability can be used continuously because less current needs to be available for temporary overloads.
In a Heavy Duty application, additional current capacity must remain available when the application temporarily requires more torque.
For this reason, the same drive may be rated:
Higher HP at Normal Duty
and
Lower HP at Heavy Duty
For example, a drive may be rated for a 10 HP Normal Duty application but only a 7.5 HP Heavy Duty application.
Do Not Select a Drive Based on Horsepower Alone
Horsepower is useful for initially identifying a drive, but the final selection should also consider the motor's nameplate full-load amps (FLA) and the requirements of the application.
When selecting a drive:
- Identify the motor horsepower.
- Identify the motor nameplate FLA.
- Determine whether the application requires Normal Duty or Heavy Duty operation.
- Verify the drive's continuous output-current rating for the selected duty.
- Verify that the drive's overload capability is sufficient for the application.
The selected drive must be capable of continuously supplying the required motor current while also providing the overload capability required by the application.
How Do I Know Which Duty Rating to Use?
Consider the characteristics of the driven load.
A centrifugal fan or pump with relatively low starting and overload requirements may be suitable for Normal Duty.
An application with constant torque, high starting torque, high inertia, frequent acceleration/deceleration, or significant temporary overload requirements may require Heavy Duty.
The actual overload requirements of the application should be compared with the Normal Duty and Heavy Duty ratings of the specific PowerFlex drive being selected.
Check the Actual Overload Rating
Normal Duty and Heavy Duty overload capabilities can vary between PowerFlex drive families and ratings.
When selecting a drive, verify:
- Continuous output current
- Normal Duty or Heavy Duty rating
- Short-term overload capability
- Duration of the allowable overload
- Application-specific derating requirements
Do not assume that the same overload percentages and durations apply to every PowerFlex drive family.
Normal Duty vs. Heavy Duty Example
Assume an application uses a 10 HP motor.
A particular PowerFlex drive may be rated:
10 HP – Normal Duty
7.5 HP – Heavy Duty
If the 10 HP application requires Heavy Duty performance, that drive should not be selected simply because it is advertised as a 10 HP drive.
The next larger drive may be required to provide the necessary Heavy Duty continuous-current and overload capability.
Replacing an Existing Drive
Duty rating is also important when replacing or upgrading an existing drive.
Do not assume:
Old Drive HP = New Drive HP
means the replacement drive is correctly sized.
When selecting a replacement, verify:
Motor FLA + Application Duty + Required Overload Capability
against the ratings of the replacement drive.
A drive correctly sized for a 10 HP Normal Duty application may not be correctly sized for a 10 HP Heavy Duty application.
Quick Reference
| Normal Duty | Heavy Duty |
|---|---|
| Lower overload requirements | Higher overload requirements |
| Common with variable-torque loads | Common with constant-torque/high-overload loads |
| Centrifugal fans and pumps are common examples | Conveyors and mixers are common examples |
| Higher HP rating for a given drive | Lower HP rating for the same drive |
| Use Normal Duty current ratings | Use Heavy Duty current ratings |
Key Takeaway
When selecting a PowerFlex drive, determine the required duty rating before finalizing the drive selection.
Horsepower identifies the general drive size.
Motor FLA determines the continuous current requirement.
The application determines the required overload capability.
Always verify the Normal Duty or Heavy Duty current and overload ratings for the specific PowerFlex drive being selected.