halting motor reverse rotation

[ImissGarcia]

I am looking for suggestions to solve a condenser motor problem in an air cooled chiller. The unit has 12 condenser fan motors – all with a common air intake. Motors are chronically failing – after forensic inspection of a couple units, we have determined electrical grounding after bearing failure. We also see a design flaw with the unit – there is a common air intake without baffling between motors. Motors that aren’t in use rotate backwards, and are under stress when called to start.

The manufacturer has addressed this flaw with proper baffling in newer models, but there is no retrofit kit for this older chiller. Any suggestions to halt reverse spin? thanks.

 

[MikeHalloran]

Centrifugal pumps that run in parallel or that feed the bottom of a reservoir have the same problem.

Mechanical solutions from the pump world include sprag brakes or wrap spring brakes. They may not be an easy retrofit on condenser fans, but take a look.

Electrical solutions include wiring the motors to all run at once or all stop at once, or electrically braking each motor for some time before attempting a start. You will need an EE to properly advise you about braking.

Pneumatic solutions include automatic (spring or gravity loaded) dampers to shut off the discharge flow of each fan when the motor is not running.

Baffling between the fans may not be as effective as other methods, but may be the cheapest to try, if you have a good tin-knocker on the premises. Assign an electrician to lock out/ tag out the fans he must be near.


Mike Halloran
Pembroke Pines, FL, USA

 

[jraef]

Are there VFDs on the fans? If so, many VFDs have the ability to perform what is called an "anti-windmilling" routine prior to accelerating the motor. When you give the drive a Run command, it first does a DC Injection Braking routine to stop the fan motors, then once stopped, begins accelerating them.

If there are no VFDs already, trust me, MikeHalloran's suggestion of finding a local tin bender to make you some retrofit dampers is the least expensive solution in the long run. Been there, done that, got the T-shirt and the hat!


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[MatthewDB]

How about hitting the motor with DC for a few seconds prior to start? If you size a DC power supply to hit nominal current on two phases, the motor will come to a near dead stop. You could try a reversing contactor pair (mechanically interlocked) so that you can't accidentally send line power to the power supply output.

 

[crshears]

Are there VFDs on the fans? If so, many VFDs have the ability to perform what is called an "anti-windmilling" routine prior to accelerating the motor. When you give the drive a Run command, it first does a DC Injection Braking routine to stop the fan motors, then once stopped, begins accelerating them.

Might another option be to gang the VFD controls so that all motors run together and at the same speed? This would eliminate any reverse flow problems [and, depending on the prevailing climate, preclude any issues with ice buildup on the dampers/flaps during colder weather].



CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[ImissGarcia]

Great suggestions!

Only 2 of the motors run on VFD’s as required for coil temperature demand. My HVAC vendor suggested adding vfd’s to all remaining motors – which would resolve the hard start-up, but seemed too drastic cost-wise for an older unit – and I didn’t think would give much payback.
I looked at the mechanical brakes on the Warner site – that looks like a reasonable option. And i'll ask my EE service vendor - perhaps there is a way to draw some current to brake Much Thanks!

 

[waross]

Over the years I have encountered several problems caused by the failure of one or more fans in parallel. I am more concerned with the inefficiency and waste caused by the air circulating back through an un-powered fan. On the evaporators of a blast frezer, the solution was to stop all the motors in a group for a short time and then re-start them together. Yes I know, this is easier from a process point of view with evaporators than it is for condensors.
How about running all the motors at the same speed on one large VFD? That may be cheaper than an electrical solution that requires all the starters to be replaced with interlocked reversing starters to apply an electrical braking method.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Strong]

Have you considered a mechanical anti-reverse rotation device, such as this one from Gates.com?
"Fan drives can rotate backwards when they are turned off, generally from air movement across the blades. Draftguard anti-rotation device secures the drive from spinning backwards during maintenance operations as well as protects the entire system from extreme shock loads during start-up, causing unnecessary wear of the motor components."

Walt

 

[ScottyUK]

One large VFD has potential to becomes a maintenance nightmare: if the VFD fails then all fans are down, compared with the current scheme where the unit is likely to be operablke with one fan in a failed state.

 

[crshears]

Compromise, then: two VFDs, with half the fans on each, and keep one of the old DOL starters in place as a back-up supply so if one VFD fails, there would only be a short duration outage while the fans that would be out of service by configuration are re-wired into the DOL starter box. With the jury-rig in service, half the fans run at full speed while the others modulate via the remaining VFD to maintain correct condenser pressure.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[jraef]

Ah come on. VFDs don't fail...


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington