Cooling radiator, multi motors, vfd or contactors?

We are going to be putting in place a large radiator for cooling water. The radiator is going to have some number of fans on it. I don't yet know number of or hp rating of these fans. Lets assume 5 5hp motors. I will control the fans to control the outlet temperature of the coolant leaving the radiator. As load increases, fans will need to be brought online. As demand lessens, they will be shutdown.
I'm pretty sure I don't want to put a vfd on ever motor due to cost, complexity, etc. I'm thinking of either 1 vfd running the 5 motors (with individual contactors and overloads), or just running the motors across the line and controlling the contactors based on demand.

Q1. What would be your preferences?
A. 5 vfds for 5 motors
B. 1 vfd and 5 contactors/overloads
C. 5 contactors/overloads

Q2. If using the vfd, I would plan to bring on the motors in stages, would you expect anything funny to happen to the vfd when throwing a motor load such as this on it? Obviously the higher the freq. when the next motor is called for, the higher the loads since this is a vt fan application.

Q3. I'm assuming that I'd need to oversize the drive. If it is going to be 5 5hp motors, I'd probably need to go larger than 25hp drive correct? How oversize to go? I'm thinking 125% based off wild *** guessing.


Anyone with insite?


Thanks
Garrett

If you want to do it economically then put 1 VFD on one motor and put across the line contactors on the other four motors. Divide your CV into five segments, each motor represents 20% of your control. The VFD will scale 0-100% of speed for each segment. When the CV gets to 20% (VFD 100%) stage up the next motor and drop the VFD speed. Do the same at 40%, 60% and 80%. Do the reverse when staging down.

If you use one VFD for five motors I wouldn't try staging the motors on and off. If you switch on a contactor with the VFD running you'll fault the VFD. If you use the single VFD route just place all five motors on separate OLs without contactors and run them all at once.


Just curious, what are you cooling? A radiator isn't going to be able to cool much unless you have very hot process water. Have you considered a cooling tower?

Speaking of cooling towers, we have discussed them several times here and various control schemes for multi-fan towers. Search the forum for cooling tower, you'll find some ideas.

TConnolly said:

If you want to do it economically then put 1 VFD on one motor and put across the line contactors on the other four motors. Divide your CV into five segments, each motor represents 20% of your control. The VFD will scale 0-100% of speed for each segment. When the CV gets to 20% (VFD 100%) stage up the next motor and drop the VFD speed. Do the same at 40%, 60% and 80%. Do the reverse when staging down.

If you use one VFD for five motors I wouldn't try staging the motors on and off. If you switch on a contactor with the VFD running you'll fault the VFD. If you use the single VFD route just place all five motors on separate OLs without contactors and run them all at once.


Just curious, what are you cooling? A radiator isn't going to be able to cool much unless you have very hot process water. Have you considered a cooling tower?

Speaking of cooling towers, we have discussed them several times here and various control schemes for multi-fan towers. Search the forum for cooling tower, you'll find some ideas.

Click to expand...

Thanks TConnolly
I was wrongly thinking that I could bring a motor on/off line while the vfd was running. SInce i cannot do that, I'm thinking either 5 contactors, or 1 vfd driven fan and 4 across the line like you mentioned. I would have 1 control loop running, and have the output to the vfd scaled so control loop of 20% is vfd running 60hz.

In this case, do you think the vfd is even needed? Would you do it or just 5 contactors?

The radiator is providing cooling to 2 very large engines. Cannot use cooling tower since need to be pressurized closed system running water/glycol mixture.

Thanks

If this is for engines then I'd just use contactors. I can't imagine that precise temperature control is required.

If precise control is needed, then I'd let the VFD modulate the control in between each step. Eg, on rising temp, VFD modulates between 0 and 20%. After the first contactor is engaged, the VFD works between 20 and 40%. After the third it is between 40% and 60%, etc. You'll need to build in some hysteresis between steps so that the contactors aren't being rapidly switched on/off.

TConnolly said:

If this is for engines then I'd just use contactors. I can't imagine that precise temperature control is required.

If precise control is needed, then I'd let the VFD modulate the control in between each step. Eg, on rising temp, VFD modulates between 0 and 20%. After the first contactor is engaged, the VFD works between 20 and 40%. After the third it is between 40% and 60%, etc. You'll need to build in some hysteresis between steps so that the contactors aren't being rapidly switched on/off.

Click to expand...

Thats what I was thinking. The engines may or may not have thermostats in use. I will also have a 3way mixing valves for controlling coolant temp at the engines. I am going to wait and see what hp the fans are going to be. If 5hp or so, I will probably throw a vfd on the first fan.

Thanks for the help

I'd use 5 contactors with O/L's and five motors. Precise enough, most likely.

If you want continually variable control, look into ABB's Pump/Fan Macro for controlling multiple motors with a single VFD. It really works nice and is economical too.

Aside from the starting configuration issues, you should also consider whether these fans are going to bump up the demand charges from your electric company. You can often pay for a VFD just in reduced electric costs.