Oversped Motors and MOP Value

[JC30]

Hi all,

We have an air handling unit that runs (11) supply air fans, each of 11FLA, through a single, internal VFD. The VFD commands the fans to 76Hz. In suspect of this causing each fan to draw more current, possibly totaling higher than the air handling unit breaker is sized for (as found, the VFD does not utilize current-limit setting), we recorded the VFD amperage at this command speed of 76Hz and found that it was only about 110A to run all the fans, which was less than I expected [I guess I expected it to be at least 121A (11 fans x 11 FLA), if not more due to overspeeding the fans past 60Hz.] This isn't particularly my strong suit - can someone help me understand intuitively why this would be the case? As 'fan loads,' shouldn't we expect much higher power required if we're running at 76Hz? ((76/60)^3 times more)?

Also, a more general (but still related) question - is the nameplate maximum overcurrent protection (MOP) of a mechanical unit (such as an air handling unit) calculated under the assumption that the fans won't be run past 60Hz? If, operationally, it is desired to run these fans past 60Hz, do we need to reconsider the sizing of the HVAC unit feeder breaker?

Any guidance would be greatly appreciated. Thanks much!
Jay

 

[petersonra]

Presumably the VFD current you are looking at is the input to the VFD. It will be at a PF close to 1.

The output current will likely be higher because it will be at a lower PF.

In any case, the amount of power used reflects how much work the fans are doing, not how fast they are running.

 

[ptonsparky]

Curious. Does each fan have its own overcurrent protection?

Axial flow fans will operate a bit differently than centrifugal. The small axial flow fans I've worked with increase current as restriction is applied. The centrifugal will reduce but I've never attempted to over speed one in combination with reduced flow.

 

[JC30]

Thank you both for the replies!

Bob - Got it. So, my understanding then: the frequency alone won't always alone dictate the expected power draw, but rather it depends on if, even at speeds higher than 60Hz, if the fan is even doing more work? Is it fair to say that in this case, running the fans beyond 60Hz and up to 76Hz lowers the torque (from reduced V/Hz ratio), but not in so much a way that it increases slip and causes the fan to do more work (and so drawing more than its FLA)?

Tom - Each fan only has its own overload protection (thermal switch). Can you elaborate on what you meant in, "as restriction is applied"?

Much appreciated!

 

[ptonsparky]

For the centrifugal to do more work it must move more. Air or water. Restricting the flow reduces the work. Running it at a higher speed won't increase the work it is doing if it doesn't move anything.

 

[Jraef]

The affinity law states that the load ON THE MOTOR of a centrifugal machine (fan) will increase at the cube of the speed change. So yes, in theory running a motor at 76Hz (127%) will increase the motor load to 200% of what the motor load would be at 60Hz.

But if the motor is ALREADY over sized, the load might still be UNDER the motor nameplate FLA. So with 11FLA, assuming 460V, these are 7-1/2HP motors. But if the fan only needed 3HP at 460V, then running the fans at 76Hz will only be 6HP, still under the motor rating.

 

[JC30]

Great explanations, definitely making more sense to me. Thanks folks!

Any thoughts regarding the MOP question?

 

[ptonsparky]

The MOP should already have the motors in it's calculations at max design load. Individual overloads will protect the motors.

 

[Jraef]

The MOP should already have the motors in it's calculations at max design load. Individual overloads will protect the motors.

And if all of the loads come off of the one VFD, the MOP will be what the VFD is listed with.

 

[paulengr]

Hi all,

We have an air handling unit that runs (11) supply air fans, each of 11FLA, through a single, internal VFD. The VFD commands the fans to 76Hz. In suspect of this causing each fan to draw more current, possibly totaling higher than the air handling unit breaker is sized for (as found, the VFD does not utilize current-limit setting), we recorded the VFD amperage at this command speed of 76Hz and found that it was only about 110A to run all the fans, which was less than I expected [I guess I expected it to be at least 121A (11 fans x 11 FLA), if not more due to overspeeding the fans past 60Hz.] This isn't particularly my strong suit - can someone help me understand intuitively why this would be the case? As 'fan loads,' shouldn't we expect much higher power required if we're running at 76Hz? ((76/60)^3 times more)?

Also, a more general (but still related) question - is the nameplate maximum overcurrent protection (MOP) of a mechanical unit (such as an air handling unit) calculated under the assumption that the fans won't be run past 60Hz? If, operationally, it is desired to run these fans past 60Hz, do we need to reconsider the sizing of the HVAC unit feeder breaker?

Any guidance would be greatly appreciated. Thanks much!
Jay

You would need the FAN curves to do what you are doing. Fan power draw is also proportional to air density. Usually the fan curves are given based on SCFM which is at STP (standard pressure and temperature) and assuming ZERO static and dynamic pressure. Contrary to popular belief if we restrict air flow, the power draw goes down. In most cases this last factor drives everything. So the name plate number is truly a maximum, not a minimum, at 60 Hz. A lot of factors go into the final result.

Plus you simply cannot use Amps = Power. That’s completely wrong and I have no idea why so many people screw this up. A motor draws two currents. The first is a magnetic flux current. As the magnetic field rotated around the stator there is some magnetic resistance so it draws a current. The magnetic flux is almost constant at all speeds and is close to no load currents. It is 90 degrees lagging because it does no work. The other current produces torque and turns the shaft. It is doing work so it has power factor 1.0 and is drawing power (kw). Especially with OEM fan motors expect poor power factor (high flux). So amp draw may not be related to power draw very well. The fan law that power is related to the cube of speed is based on MECHANICAL power.

This only relates to what is going on at the motor. Now we need to address the VFD itself. At or below name plate we have HP = torque x RPM. As we reduce RPM if we kept torque constant which we do in a VFD, power decreases. This is even more so with fans because the fan power draw drops with the cube of speed. Over on the electrical side as I said earlier torque is related almost directly to current but as we reduce the power needed by decreasing frequency, thus means that voltage decreases, too. Fans are often run using a VFD control algorithm known as V/Hz mode. We keep
The voltage to frequency ratio constant except at low speeds. This is true at up to name plate speed.

Above name plate speed though we have a problem. The inout voltage is already at name plate so without exceeding the fan rating we can’t go up on voltage. Thus we hold voltage constant while increasing frequency only. The consequence is that current is now limited and decreases linearly with frequency. We call this the constant power region since mechanically HP is now fixed and RPM and torque are now inversely related, on a relative scale. So although the fan mechanical power draw continues to increase the flux field is decreasing (field weakening). Don’t get me wrong power demand by the load and amps still increases but it’s not by any means a simple relationship.

So simply put your math is completely wrong. On top of that the VFD monitors and trips on overloads way before the breaker should, unless the breaker is set up incorrectly. The breaker should be set to 125% of the VFD rating. AND the VFD has a current limit printed on it. You can exceed this for short periods of time right up to the limits of the electronics, usually 150%, regardless of what the software attempts to do. With overload protection off the heat sink protection still applies and that’s not user adjustable. In an application with multiple motors and only one VFD normally there are overload relays for each motor because you can stall out one motor entirely becoming a fire hazard while not exceeding overload settings on the VFD since only 1/11th of the motors has an issue. In this situation using the normal electronic overload settings assuming 1 motor per VFD is violated so it’s easier to just disable it. Current limiting and stall protection similarly is kind of pointless.

 

[Jraef]

Well, he has a packaged air handling UNIT so one would expect that the manufacturer has taken all of that into consideration. I didn’t think he was trying to design one, just to understand the issues with regard to why running the motors at higher speeds was not causing an overload condition. But that’s good info for anyone who reads this later and wants to replicate it from scratch.

 

[ptonsparky]

I know this is OT a bit but what would be the advantage for an OEM to design equipment to operate at higher rpm?. A larger range of operation for that unit?

 

[Jraef]

I know this is OT a bit but what would be the advantage for an OEM to design equipment to operate at higher rpm?. A larger range of operation for that unit?

Thats the only reason I’ve seen it done. At some point in the fan curve it cannot overcome static pressure so the fan turns but no longer moves air. This is your minimum speed, which changes with the fan blade size. Most likely someone looked at the fan curves and decided that to be able to keep moving air at a low speed, they needed these smaller fans, but couldn’t get the max flow they wanted without over speeding them, which meant using a larger motor. I’ve only seen this done on blast coolers where the maximum air velocity was an issue, but I imagine it could be the same for an HVAC air handler. Air handling design is an art in itself because they often have to balance cost, performance, air balancing and physical restraints as to the overall equipment size and/or weight to fit in a pre-existing space. When they get that wrong, it can be pretty spectacular.

 

[paulengr]

I know this is OT a bit but what would be the advantage for an OEM to design equipment to operate at higher rpm?. A larger range of operation for that unit?

Realistically you are mistaking how it most likely works. They are probably measuring and controlling ACFM or SCFM or some kind of pressure. The speed is a consequence. Or the HVAC company simply adjusted the drive as needed and that’s where it runs at. It is common for this since the back pressure is site dependent.

There is nothing magic about 60 Hz when you have a drive other than the transition to field weakening at the peak power point. With fans in particular it has little meaning.

 

[ptonsparky]

Realistically you are mistaking how it most likely works. They are probably measuring and controlling ACFM or SCFM or some kind of pressure. The speed is a consequence. Or the HVAC company simply adjusted the drive as needed and that’s where it runs at. It is common for this since the back pressure is site dependent.

There is nothing magic about 60 Hz when you have a drive other than the transition to field weakening at the peak power point. With fans in particular it has little meaning.

Nobody said how it was controlled, but I see some kind of pressure control or feedback at some point Otherwise they just as well use a ATL and use the smallest motor/fan possible.