100HP VFD direct drive fan motors

Designer69

Senior Member
I am helping on a project with VFD controlled, 100HP fan motors and I do not have too much experience with VFD installations.

480/3ph



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this will be set in V/hz mode for ramp up.

1. how can I determine current draw for the configured ramp up time?

2. can you guys also provide some input in important things to consider when designing VFD control?

Thanks!
 

petersonra

Senior Member
I am helping on a project with VFD controlled, 100HP fan motors and I do not have too much experience with VFD installations.

480/3ph



Uploaded with ImageShack.us

this will be set in V/hz mode for ramp up.

1. how can I determine current draw for the configured ramp up time?

2. can you guys also provide some input in important things to consider when designing VFD control?

Thanks!
This kind of application is pretty simple. I would not be sweating it too much.

The current draw is going to be whatever it takes to keep the fan accelerating at the currently applied frequency.

The biggest thing I see is people expect the VFD to somehow just know what is the optimum frequency to run at. You will need some means of determining just what frequency is appropriate.

Many times it is quite acceptable to do so empirically. Just set a fixed frequency and let it always run that way. Other cases you may want to have multiple fixed settings available and some way to select which frequency is currently appropriate.

You may need to have some kind of PID type control so the frequency can be controlled based on some measurement. Many VFDs have this capability built right in to the drive.

The most common problems I see in these kinds of installs are usually because people worry too much about the hardware and not enough about just what it is one is trying to accomplish.
 

Jraef

Moderator
Staff member
From an installation aspect you just have to realize that by code, the circuit is sized for the VFD's maximum input amp rating, the connected motor load is irrelevant. So you need to look up that Benshaw VFD data sheet and see the Max Amp rating, then you are required to size the conductors feeding the VFD at 125% of that VFD max amp rating. You do not have a choice in this matter (other than over sizing). then when the conductors are sized, yu size the OCPD accordingly. You also must read the VFD manual to make sure they don't have a specific maximum size OCPD that can be put ahead of the VFD. Most do.
 

petersonra

Senior Member
From an installation aspect you just have to realize that by code, the circuit is sized for the VFD's maximum input amp rating, the connected motor load is irrelevant. So you need to look up that Benshaw VFD data sheet and see the Max Amp rating, then you are required to size the conductors feeding the VFD at 125% of that VFD max amp rating. You do not have a choice in this matter (other than over sizing). then when the conductors are sized, yu size the OCPD accordingly. You also must read the VFD manual to make sure they don't have a specific maximum size OCPD that can be put ahead of the VFD. Most do.
Good point. Some VFD manuals make finding the VFD's input current rating kind of hard. I read one not too long ago that listed the maximum current draw at about 8 times what the maximum current output of the VFD is. Sort of like the locked rotor current rating of a motor is real high, but you don't really size the OCPD or wire to that. They also had a maximum OCPD listed for each size, so I just sized the incoming power wires to that. It didn't really matter much. I think the biggest wire ended up being #10 that way, and the difference between 20 feet of #10 and 20 feet of #12 was not worth worrying about to me.
 

Besoeker

Senior Member
I am helping on a project with VFD controlled, 100HP fan motors and I do not have too much experience with VFD installations.

480/3ph



Uploaded with ImageShack.us

this will be set in V/hz mode for ramp up.

1. how can I determine current draw for the configured ramp up time?

2. can you guys also provide some input in important things to consider when designing VFD control?

Thanks!
As petersonra has said, it is a fairly straightforward application.
In my experience, the usual reason for using a VFD on a fan is for energy savings. If you can reduce the speed by 10% and still get enough airflow, the energy consumption drops by 30%.
A couple of points from that.
The speed demanded is likely to be a signal from the system in which the VSD is being used. Maybe the project engineer or specification can provide detail. Possibly it will be 4-20mA and most VFDs have analogue input capability.
The second point from that is about speed range. Typically, fans on VFDs don't need to run much below 70% of rated speed so the minimum operating speed of the VSD may have to be set accordingly.

A few other points in no particular order.
The motor ought to be rated for VSD operation. That's rather important.
The length of the conductor run from VSD to motor maters. A long run may require filters. The VSD installation manual should have information on this.
How the conductors are arranged is also important. I'm from a different part of the world to you and we have different practices. We use steel wire armoured cable for installation. You might use conduit. I can't offer any advice on that other than don't stick signal cables in the same conduit as the motor power cables.
 

Besoeker

Senior Member
Good point. Some VFD manuals make finding the VFD's input current rating kind of hard. I read one not too long ago that listed the maximum current draw at about 8 times what the maximum current output of the VFD is.
Shouldn't be.
Usually the input current is lower than the output current - it's generally at a better displacement power factor.
Maybe there's some cheap and nasty stuff out there with no pre-charge for the DC link capacitors but I don't imagine that to be the case for a 100HP (75kW) drive.
 

petersonra

Senior Member
Shouldn't be.
Usually the input current is lower than the output current - it's generally at a better displacement power factor.
Maybe there's some cheap and nasty stuff out there with no pre-charge for the DC link capacitors but I don't imagine that to be the case for a 100HP (75kW) drive.
These drives were low end and smaller than 100HP. Maybe 10HP tops. I think the number they were citing was probably some instantaneous value. It did not make a whole lot of sense to me when I first looked at it.
 

Besoeker

Senior Member
These drives were low end and smaller than 100HP. Maybe 10HP tops. I think the number they were citing was probably some instantaneous value. It did not make a whole lot of sense to me when I first looked at it.
If you can find it, I'll take a look at it.
 

Jraef

Moderator
Staff member
I would have looked at the Benshaw drive data for you earlier, but they have flagged my home IP address (I'm an ex-employee) so I can't download anything from them. Now that I'm at the office I can download it.

If it's a VT rated drive, a 100HP version is rated 152A max., if it's the standard CT rated drive it's rated 139A max. Most likely it's the VT version on a fan so you'll have to base the line input conductors on 125% of 152A. the motor leads can still be sized for the motor per the NEC charts in 430, I think table 430.52?
 
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