Sizing VFD's

[frank_n]

I started a new job last week and found out that one of the previous electricians and the HVAC mechanic ordered VFD?s for some older pump motors for the boilers. They (HVAC mechanics) are having problems with the pipes shaking at startup and they also want to control the amount of flow. I believe the VFD?s were sized according to their horsepower. I?ve never sized VFD?s before and I?m concerned that these VFD?s will fail if installed.

These motors are 25 years old, and here is the information of the name plate:

Volts 230/460 (we are running at 460)
Amps 74/37 (the motors are currently drawing between 41-43 amps, but the HVAC said this is because they have a valve controlling flow. If he opens the valve, the amps will go back to 37)
HP 30
3-phase 60 hz

1. First will the VFD?s enable the motors to start slowly and rev up? If it?s possible, will the motors burn out quickly?
2. Can the motors be run below 50 hz? If not, I believe it will severely limit the amount we are able to control flow. Am I correct?
3. Does the VFD take the place of the starter? Is any additional overload protection required?
4. Is the sizing of the VFD?s and the control wiring usually sent to engineer, or does the electrician and HVAC mechanic usually size it?

Thanks in advance,
Frank

 

[petersonra]

I believe the VFD?s were sized according to their horsepower.

That would generally be how they are sized.

These motors are 25 years old, and here is the information of the name plate:

Volts 230/460 (we are running at 460)
Amps 74/37 (the motors are currently drawing between 41-43 amps, but the HVAC said this is because they have a valve controlling flow. If he opens the valve, the amps will go back to 37)
HP 30
3-phase 60 hz

Presuming they are 1.15 SF motors, this is probably OK. Its unusual though that a system pulls FLA all the time.

1. First will the VFD?s enable the motors to start slowly and rev up?

Yes.

If it?s possible, will the motors burn out quickly?

Old motors may not have insulation compatible with VFDs, so they may fail faster than they would otherwise.

2. Can the motors be run below 50 hz? If not, I believe it will severely limit the amount we are able to control flow. Am I correct?

centrifugal pumps run at lower pressure when their speed is reduced. the lower pressure results in lower flow. Its hard to tell just how low you can run them without having them fail due to overheating issues. The cooling fans on them will pull in less and less air as the speed goes down.

3. Does the VFD take the place of the starter?

yes.

Is any additional overload protection required?

normally it is built into the VFD.

4. Is the sizing of the VFD?s and the control wiring usually sent to engineer, or does the electrician and HVAC mechanic usually size it?

Most simple installations do not require an engineer. A knowledgeable electrician is quite adequate to the task.

 

[frank_n]

Peter,

Thanks for the quick response. I'm trying to learn as much as I can about these VFD's because I'm concerned about the motors and VFD's failing.

I read an article in EC&M Magazine and they said that VFD's should be sized according to FLA not HP. Do you ever size according to FLA? The article also says it's sometimes necessary to oversize the VFD's. Do you recommend this since the motor is running at full FLA? (The VFD's max is 40 amps)

Finally, I've read a lot about the importance of using induction motors with VFD's. Since these motors are not induction motors what consequences should I expect?

Thanks again,
Frank

 

[sii]

If not induction, then what type of motor are they? Brand, Hp, Catalog # if possible. Also, are they TEFC (totally-enclosed, fan-cooled), TENV, ODP? If you have an ODP motor for example, you will burn it up very quickly running at or near FLA due to the lack of a cooling fan.

Edit: FLA is generally consistent with Hp on modern motors, although not totally. What I mean is a three-phase motor of similar construction will usually have the same FLA from one manufacturer to another or at least very close. I can't speak to whether this true of older motors or not. Also, the VFD will probably list Hp and FLA in the specs.

 

[Jraef]

...
Amps 74/37 (the motors are currently drawing between 41-43 amps, but the HVAC said this is because they have a valve controlling flow. If he opens the valve, the amps will go back to 37)
...

I would suspect this is exactly the opposite of what he has told you (unless the pumps are Positive Displacement, which is unlikely on boiler feed pumps).

In a Centrifugal pump, the power consumed varies by the cube of the flow through the pump. So when you close the valve, the flow decreases and so does the current. So most likely the pumps were designed to always have the flow restricted a little by the valve and only when they open the valve too much does the current rise above the nameplate FLA.

With that in mind, IF THEY MUST have that full flow at times, then this is a good time to use the "Size the VFD by the FLA" method, but in fact size it by the FLA times the Service Factor. A "30HP" VFD may be sized by the manufacturer to allow "typical" FLA of a 30HP motor, roughly 36A, plus a small fudge factor. But if the VFDs were sized based on what are called "Variable Torque" requirements, that fudge factor is practically non-existent and then if the motors are expected to run into the service factor, you are way too high. If they need the full flow, the motors will try to draw that 43A value and the VFD may not be capable. So make sure the VFD is rated for 43A, regardless of what the HP says on it and you will be fine.

Either that, or leave the valve in there and trim the maximum flow (with the VFD at full speed) so that the current does not exceed the VFD max. amp rating. Then put a sign on the valve saying "Do Not Change Setting".

Welcome to the world of VFDs... Over time you will pick up on issues such as this and your knowledge will make you more valuable. The world is changing and VFDs are going to be even more commonplace than they are now, so people who understand how they work are going to remain in demand.