Proper VFD sizing

This may be a dumb question, but do you try to nail the VFD size on the head or go up a size?

I can see the value in going up a size of VFD in case you need a little more muscle for an application and then you don't have to buy a new drive. However, I think there are some merits to sizing the VFD directly to the motor size.

For one, the smaller drives have less of a heat loss and multiple small drives (such as three 1/2 HP motors) won't supply enough of a heat loss to worry about panel cooling, but three 1HP drives would double that.

If you size up one, do you size the rest of your circuit for the larger drive? (IE - wire size?) I understand that the VFD provides some overcurrent protection, and the manufacturer typically calls out a recommended fuse size per drive anyway.

For me it depends on the drive application.

For example, we have transfer presses that create regenerative voltage. By increasing the size of the VFD required for the motor, the regenerative voltage is absorbed by the IGBTs without the need to have more components such as a regenerative brake resistor. VFDs sized to the motor in these applications tend to burn up very quickly.

For other applications such as conveyors, pumps, etc, the VFD sized to the motor tends to do the job just fine.

We try to buy VFDs based on motor hp. The manufacturers try to build them based on load current.

Glitches' application is unusual. Most of the time you should be comparing the motor FLA to the drive rating as a double-check. The type of load matters too. If it is a variable torque load like a fan or centrifugal then the hp and amperage ratings are usually sufficient and I don't see an advantage in over-sizing. If it is a constant torque load like a conveyor or a positive displacement pump then the current dictates and a higher rating drive is usually required.

You should run the application past your VFD supplier.

The VFD overcurrent limit is typically adjustable and should be matched to the motor full load amps. Line side fuses are dictated by the drive. Load side fuses are dictated by the load. Ditto wiring. Oversized wiring may waste some money but won't cause problems. Oversized fuses are a no-no; they won't protect the motor.

We use Siemens softstarters and VFDs. For sizing these we use a Siemens tool called SIZER (appropriately).
When using this tool we are sometimes shocked as to how much we must over-size the drive (when comparing to merely matching the motor name plate values to the VFD nominal values).
Maybe they just want to sell us a bigger drive ..... but we usually follow the recommendation.

With drives, it’s really all about torque. A motor is rated for a given amount of torque at a given speed. A VFD allows the motor to deliver that torque at any speed (up to rated). So if the motor is sized properly, there is no reason to over size the drive UNLESS you need for your motor to deliver MORE than it’s rated Full Load Torque for longer than the drive can handle it.

So for example if the application required Locked Rotor Torque, the motor will want around 150% current. A VFD rated as “Heavy Duty” or “Constant Torque” can generally handle that for 60 seconds. The motor can actually periodically deliver LRT for longer than that even though you are pushing it into the range of overloading it. So if you need that ability, you would need to over size the drive. In addition the PEAK torque a motor can deliver is called the Break Down Torque and is typically around 200% of the FLT. Again, a HD drive can deliver that, but only for about 2-3 seconds and would need cooling off time. So there too, if you NEED to stretch the motor’s capability, you may need to over size the VFD to be able to deliver it.

But 70% of all AC motors are used on variable torque loads such as centrifugal pumps and fans. On those loads, if the mechanical engineer has properly selected the motor, the nature of a centrifugal machine means you CANNOT make it draw more current unless something is wrong, do there is no point whatsoever to over sizing the VFD. And that’s why you will see that a “Normal Duty” or “Variable Torque” rated drive has a significantly lower over load capability compared to a HD drive.

And while we’re at it, motors with Service Factors will always state that if you use a VFD, the Service Factor becomes 1.0, so there is no point in sizing the VFD for the Service Factor Amps, unless you are OK with voiding the warranty on your motor.

jraef said:

<snip> But 70% of all AC motors are used on variable torque loads such as centrifugal pumps and fans. On those loads, if the mechanical engineer has properly selected the motor, the nature of a centrifugal machine means you CANNOT make it draw more current unless something is wrong, do there is no point whatsoever to over sizing the VFD. And that’s why you will see that a “Normal Duty” or “Variable Torque” rated drive has a significantly lower over load capability compared to a HD drive. <snip>

Click to expand...

And never size the drive just by HP & V, always double-check the FLA. One area you will get burned regularly is submersible pumps. For some reason, most of them have much higher FLA than the NEMA tables show for a given HP. (I can only guess that it is to make up for the voltage loss of long leads)

We typically buy them one size over as standard procedure. We've found it's just not worth it to size them exact.

Gene Bond said:

And never size the drive just by HP & V, always double-check the FLA. One area you will get burned regularly is submersible pumps. For some reason, most of them have much higher FLA than the NEMA tables show for a given HP. (I can only guess that it is to make up for the voltage loss of long leads)

Click to expand...

Excellent point and one I should have made. Thanks.


I once had a customer install 6 drives for "250HP pumps" that they bought on their own from a "bargain basement" on-line reseller. I got called out because they couldn't get them to program for the motor FLA and so they kept tripping on OL. Turned out the motors were 12 pole (565RPM) with a motor nameplate FLA of 396A, whereas a typical 250HP motor is 302A, so the standard "250HP" VFD was sized for 302A. They actually needed 350HP rated drives (2 sizes larger). That was an expensive mistake for them...

My initial thoughts were "wow, 6x 250HP sounds expensive."
Then I'm like, wait, what's the lead time on 6x 350HP drives?
Is the room that was purpose built for 6x 250HP drives big enough to fit 6x 350HP drives?
If they got the wrong drive size, did they get the wrong motor cable size?
Are there additional room cooling requirements?

"Hey boss, I made a mistake..."

We use some motors with a 1.25 service factor - continuous.
So I always order by current....not HP.

Depending on the application, Size the drive according to the manufacturers HP rating. But be mindful they have both a standard duty and heavy duty rating...
The ND or HD rating refers mostly to how the drive reacts during an intermittent overload..

NetNathan said:

We use some motors with a 1.25 service factor - continuous.
So I always order by current....not HP.

Click to expand...

Did you notice my last paragraph in the above post? Motor mfrs will tell you that if using a VFD, all motors are 1.0SF.

AustralIan said:

My initial thoughts were "wow, 6x 250HP sounds expensive."
Then I'm like, wait, what's the lead time on 6x 350HP drives?
Is the room that was purpose built for 6x 250HP drives big enough to fit 6x 350HP drives?
If they got the wrong drive size, did they get the wrong motor cable size?
Are there additional room cooling requirements?

"Hey boss, I made a mistake..."

Click to expand...

The physical size of the VFDs wasn't an issue, but all of the other things were. it was truly a huge fuster cluck.

jraef said:

Did you notice my last paragraph in the above post? Motor mfrs will tell you that if using a VFD, all motors are 1.0SF.

Click to expand...

That is why I order by current and not HP.