Overspeeding Powerflex 525

mylespetro · Oct 3, 2020

Hey everyone,

Working on a project replacing a whole bunch of various AB drives (1336, Powerflex 40 etc) with Powerflex 525s. The motors are 3HP on conveyors, so we had specified 3HP drives, but we found out a couple days ago that the original VFDs are 5HP, and also that the max frequency is set to 90Hz, meaning that once the output frequency goes above 60Hz, the drive goes into constant power mode as I understand, decreasing torque to maintain 100% horsepower as the speed increases. I believe at 90Hz we should have 2/3 of the available torque.

I know that it’s common practice to oversize the VFD by one size, but if we were to have used 5HP drives we would have been setting them for a 3HP motor through the parameters anyways, so is there any risk of the drive not having enough torque at higher speeds? My understanding is that the current output available would be lowered to that of a 3HP drive anyways, essentially derating the 5HP to a 3. Just as a side note, we do have the additional fan kits installed so cooling shouldn’t be an issue as far as I can tell.

Thanks in advance!

thingstodo · Oct 3, 2020

mylespetro said:
Hey everyone,

Working on a project replacing a whole bunch of various AB drives (1336, Powerflex 40 etc) with Powerflex 525s. The motors are 3HP on conveyors, so we had specified 3HP drives, but we found out a couple days ago that the original VFDs are 5HP, and also that the max frequency is set to 90Hz, meaning that once the output frequency goes above 60Hz, the drive goes into constant power mode as I understand, decreasing torque to maintain 100% horsepower as the speed increases. I believe at 90Hz we should have 2/3 of the available torque.

I know that it’s common practice to oversize the VFD by one size, but if we were to have used 5HP drives we would have been setting them for a 3HP motor through the parameters anyways, so is there any risk of the drive not having enough torque at higher speeds? My understanding is that the current output available would be lowered to that of a 3HP drive anyways, essentially derating the 5HP to a 3. Just as a side note, we do have the additional fan kits installed so cooling shouldn’t be an issue as far as I can tell.

Thanks in advance!

Above 60 Hz, you are in the constant power mode. In my experience, you can't push much more current into a motor at higher frequency unless you have more voltage. IE - if you lie to your VFD and say it is a 5 HP motor, it will try to push rated current into the motor at higher frequency but the impedance of the motor is higher, so the current just won't go ...

The over-size for VFDs WAS mostly if you are running lower speed, so the electronics don't overheat. Or that was the rule of thumb in the old days. Electronics running cooler = more reliable operation.

If the conveyors are USING more power, moving more product, etc and you need the torque to speed them up while loaded, etc your 525s should allow for about 30 seconds (from memory) of 150% load ... under 60 Hz. If you are using more than 2 HP of torque (2/3 torque, as you mention) at 90 Hz, I don't believe that over-sizing the drives to 5 HP will help you.

You might want to verify that the '3 HP' drives are rated for 100% amps 100% of the time. I have not checked lately ... other manufacturers have their marketing departments involved and they mention things like '20% duty cycle' and such. Just a suggestion!

mylespetro · Oct 3, 2020

thingstodo said:
Above 60 Hz, you are in the constant power mode. In my experience, you can't push much more current into a motor at higher frequency unless you have more voltage. IE - if you lie to your VFD and say it is a 5 HP motor, it will try to push rated current into the motor at higher frequency but the impedance of the motor is higher, so the current just won't go ...

The over-size for VFDs WAS mostly if you are running lower speed, so the electronics don't overheat. Or that was the rule of thumb in the old days. Electronics running cooler = more reliable operation.

If the conveyors are USING more power, moving more product, etc and you need the torque to speed them up while loaded, etc your 525s should allow for about 30 seconds (from memory) of 150% load ... under 60 Hz. If you are using more than 2 HP of torque (2/3 torque, as you mention) at 90 Hz, I don't believe that over-sizing the drives to 5 HP will help you.

You might want to verify that the '3 HP' drives are rated for 100% amps 100% of the time. I have not checked lately ... other manufacturers have their marketing departments involved and they mention things like '20% duty cycle' and such. Just a suggestion!

Great, I had suspected as much. I’ll also look into the current/duty cycle thing, that’s a good catch. My understanding was that the majority of the time, the drives are running at around 30Hz, but they just need the capability to go over 60Hz occasionally. Thank you so much for the reply!

rQx · Oct 3, 2020

I've never seen duty cycles mentioned with VFDs before, I would be surprised if it was so

thingstodo · Oct 3, 2020

rQx said:
I've never seen duty cycles mentioned with VFDs before, I would be surprised if it was so

Sorry - The duty cycle thing was for the Brake resistors

The derating for the VFDs is for over-current and how long. The Super Light Duty only does 120% current for 3 seconds and 110% for 30 seconds ... with the lower 40C ambient temperature. We end up using the HD column - 250% for 3 sec, 200% for 30 sec and (not listed) 125% continuous with 50C ambient.

Ambient means the temperature inside the VFD cabinet, since the VFD could be mounted in open air as well.

This is for VFDs that we use (not Rockwell, but Mitsubishi. I gave up on Rockwell drives during the Powerflex 700 and Powerflex 70 era)

NetNathan · Oct 3, 2020

When you overspeed a motor, you need to check what maximum speed you can go to. This is related to the bearings used and sometimes motor construction.

rQx · Oct 4, 2020

NetNathan said:
When you overspeed a motor, you need to check what maximum speed you can go to. This is related to the bearings used and sometimes motor construction.

I would say that often you can go 100Hz without problem on a 1500rpm motor since from what I know they use the same parts for the 3000rpm motors.

Overspeeding Powerflex 525

mylespetro

Member

thingstodo

Member

mylespetro

Member

rQx

Lifetime Supporting Member

thingstodo

Member

NetNathan

Lifetime Supporting Member

rQx

Lifetime Supporting Member

Similar Topics