Condenser fans 3 ph internal overload?

[g.mccormick]

Is anyone familiar with condenser fans for air cooled condensers?

I'm looking at taking over control of some refrigeration equipment, in looking at the condenser controls I noticed that there are 12 contactors for the 12 fans, but no overloads. I'm assuming that the overloads are internal to the motor.

I may be looking to add VFD to drive the fans (one vfd powering all of the motors). Would you suggest that I add additional overloads to atleast be able to disable the vfd if overload trips?

 

[OkiePC]

I have seen this before. I used to work for a place where three condenser fans were all wired to one contactor with no overload and my research found that each motor had internal overloads.

I would be leery of driving that type of motor with a VFD without doing the homework to make sure the type of overload in the motors (if they exist) can handle it, and of course, separate overloads for each motor would be better.

 

[jraef]

It would be somewhat unusual for 3 phase motors to have internal "Klixon" type overloads inside, because they must break all 3 (or at least 2) phases to shut down the motor, then when they cool off, they restart either automatically or when you press a reset button on the back of the motor, so the contacts must be rated for the starting load. It's not unheard of, it's just uncommon. I've only seen that on fractional HP hermetically sealed compressor motors. If yours have that feature, the motor mfr is required to state it on the motor nameplate, although it is sometimes a little cryptic in that they may abbreviate it, something like "TP" for Thermally Protected.

If you have them on the condenser fans, retrofitting those with VFDs is not the best idea, only because you can damage a VFD by opening power contacts on the output side. It might not be immediate, but the damage to the transistors is essentially incremental and can lead to premature failure. If you could somehow use control contacts in the motor to turn the VFD off first, that would be fine, but that's not how those integral overload protectors work, they are only embedded in the power circuit of the windings, usually at the Wye (Star) point inside.

Here's a drawing of what they look like inside.

 

[g.mccormick]

I have seen this before. I used to work for a place where three condenser fans were all wired to one contactor with no overload and my research found that each motor had internal overloads.

I would be leery of driving that type of motor with a VFD without doing the homework to make sure the type of overload in the motors (if they exist) can handle it, and of course, separate overloads for each motor would be better.

Okie.
I was thinking that I would add extra overloads so that the contacts of them would be in series with the drive run command (or external fault).

With multiple motors behind 1 vfd, if a motor overloads and goes off line, is there any detriment to the vfd?

 

[g.mccormick]

It would be somewhat unusual for 3 phase motors to have internal "Klixon" type overloads inside, because they must break all 3 (or at least 2) phases to shut down the motor, then when they cool off, they restart either automatically or when you press a reset button on the back of the motor, so the contacts must be rated for the starting load. It's not unheard of, it's just uncommon. I've only seen that on fractional HP hermetically sealed compressor motors. If yours have that feature, the motor mfr is required to state it on the motor nameplate, although it is sometimes a little cryptic in that they may abbreviate it, something like "TP" for Thermally Protected.

If you have them on the condenser fans, retrofitting those with VFDs is not the best idea, only because you can damage a VFD by opening power contacts on the output side. It might not be immediate, but the damage to the transistors is essentially incremental and can lead to premature failure. If you could somehow use control contacts in the motor to turn the VFD off first, that would be fine, but that's not how those integral overload protectors work, they are only embedded in the power circuit of the windings, usually at the Wye (Star) point inside.


Here's a drawing of what they look like inside.

We went and got some information. Attached is picture and information from Baldor. I appears it has internal automatic reset thermo protection as you guessed. I guess the VFD on this may be more hassle than it is worth. Staging of the fans via contactors will be easier to stick with probably.

 

[shooter]

put them all on one VFD, as the safety is in motor.
it can give some strange thing in the VFD though.

you do not know how many motors are running.
and a VFD can control the discharge pressure pretty simple.

 

[g.mccormick]

put them all on one VFD, as the safety is in motor.
it can give some strange thing in the VFD though.

you do not know how many motors are running.
and a VFD can control the discharge pressure pretty simple.

Jraef pointed out that the internal overloads tripping is bad on the vfd. An option is to add additional overloads infront of the motor and set them lower than internal overloads. The NC contacts could all be series together to the drives external fault input.

The discrete staging may just be easier to deal with.

 

[DickDV]

The benefit with internal motor overloads over externals is that motor heat is actually measured instead of simply estimated. That equates to better protection usually.

Not all VFD manufacturers speak against contacts opening in the motor circuits on their latest versions of drives. You may want to research that a bit before ruling out a drive with multiple motors. Just remember, you cannot just add up the FLA's of the motors and size the drive ampacity to match. I would size the drive with about 150% continuous output amps compared to the total FLA. Otherwise, you risk nuisance overcurrent faults when starting due to the exceedingly low impedance of 12 motors in parallel.

 

[g.mccormick]

The benefit with internal motor overloads over externals is that motor heat is actually measured instead of simply estimated. That equates to better protection usually.

Not all VFD manufacturers speak against contacts opening in the motor circuits on their latest versions of drives. You may want to research that a bit before ruling out a drive with multiple motors. Just remember, you cannot just add up the FLA's of the motors and size the drive ampacity to match. I would size the drive with about 150% continuous output amps compared to the total FLA. Otherwise, you risk nuisance overcurrent faults when starting due to the exceedingly low impedance of 12 motors in parallel.

Thanks DickDV. I will check with manufacturer. Thanks for heads up on overrating drive.

 

[g.mccormick]

The benefit with internal motor overloads over externals is that motor heat is actually measured instead of simply estimated. That equates to better protection usually.

Not all VFD manufacturers speak against contacts opening in the motor circuits on their latest versions of drives. You may want to research that a bit before ruling out a drive with multiple motors. Just remember, you cannot just add up the FLA's of the motors and size the drive ampacity to match. I would size the drive with about 150% continuous output amps compared to the total FLA. Otherwise, you risk nuisance overcurrent faults when starting due to the exceedingly low impedance of 12 motors in parallel.

DickDV (or anyone else) do you know of a drive that would be OK with this action? The manufacturer for the drive I was looking at said not a good idea.