halting motor reverse rotation
halting motor reverse rotation
(OP)
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.
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.





RE: halting motor reverse rotation
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
RE: halting motor reverse rotation
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
RE: halting motor reverse rotation
RE: halting motor reverse rotation
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]
RE: halting motor reverse rotation
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!
RE: halting motor reverse rotation
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
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"Why not the best?"
Jimmy Carter
RE: halting motor reverse rotation
"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
RE: halting motor reverse rotation
RE: halting motor reverse rotation
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
RE: halting motor reverse rotation
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington