Standby motor in AHU 2

[HVAC68]

We have an application with a standby motor in AHU.

Is it a standard practice to keep the belt "on" in the standby motor also, so that in case of airflow failure detected through the BMS, the standby motor takes over without the need for a technician to go and put the belt.

OR

Is it a standard practice to just keep the belt of the standby motor without connecting and put it on as and when required ?



HVAC68

 

[Wilbur55]

First, why is there a standby motor? How much delay is tolerable before starting the second motor? How much wear might a standby belt experience in spinning the idle motor? Is this wear any worse than when the motor sheave is driving? Might a standby belt be too worn to bear the load if called on to spin the fan? If all belts will remain installed, will the motors run in alternating lead-lag to equalize wear on the belts? This manufacturer expects the belts to be moved to the standby motor:

http://www.toroaire.com/spec/supremedmnl.pdf

 

[Wilbur55]

This manufacturer expects all belts to remain installed:

http://www.baltimoreaircoil.com/english/products/cooling-towers/series-1500/custom-features-options

 

[HVAC68]

Thanks for the response.

There is a scheduled weekly change-over through BMS from duty to standby motor.

This is a 24-hour application and hence in case there is a breakdown in the night, there may not be operatives available for changing the belts. This is the reason they want the belts to be on the standby motor.

The standby motor is there, since it is a critical application. They want the unit to start as soon as possible without the need for manual intervention in case of failure of either motor or belt of the working set.

HVAC68

 

[Wilbur55]

Since "there may not be operatives available for changing the belts" and "without the need for manual intervention in case of failure" is an operational requirement, what is "standard practice" would seem to be less important than what is possible.

If your AHU supplier can do this, it sounds like you want a standby motor with all belts installed at all times.

If your AHU supplier cannot do this, it sounds like you need to ask other vendors.

Make sure you have enough motor power to drag the idle motor and belts. Also make sure you have enough motor torque to get the fan, idle pulley, and idle motor up to speed.

 

[KiwiMace]

Not that this answers your question, but...

Critical applications should have a standby fan or AHU. This sounds a bit pointless to me. The bearings are just as likely to fail in the standby motor as in the duty motor, and you have to shut it down to service the spinning standby anyway.

 

[HVAC68]

Thanks. Does any code talk about standard practice of allowing the belts to run on the idle motor as well. I understand the need to additional torque and oversizing the motor to take care of this.

My question is the other way round. If something is not a standard practice, then, customer pays for it !

Concern is regarding the life of motor bearing and belt installed on the standby motor.

HVAC68

 

[Wilbur55]

Would not someone else's "standard practice" be trumped by the customer's directive that the they don't want to rely on "manual intervention in case of failure?"

The customer pays for whatever is installed: standard practice or not.

As is often the case, the customer may not realize the implications of their request. They can't have everything they want, such as the combination of:
1. A backup motor in the same unit, and
2. Reduced runtime on bearings, and
3. No manual intervention to move belts to the standby motor.

Are they receptive to a list of pros and cons? Or do they want what they want? Will they only be annoyed if presented with more information, and ask why the unit hasn't been ordered yet?

What are the costs both ways?
What are the reliability issues both ways?

How has the AHU supplier seen backup motors applied? With or without belts on both motors?

Is the backup AHU suggested by KiwiMace out of the question due to cost, space, or other issues? If possible, this seems like a better option. Then they can have reduced bearing runtime on a unit that can be started without manual intervention.

The decision is ultimately the customer's. They are not obliged to conform to some other company's standard practice.

 

[SAK9]

HVAC68,

It is not recommended to have the standby motor running idly as there will be back EMF generation within its stator windings.With my limited knowledge in electrical engg I am not able to explain the effects of this back emf generation in an idle motor with no impressed vlotage across its terminals.If you repost this at the Electrical forum,they may be able to explain it further.

 

[HVAC68]

The customer wants what he wants. Period. Having said that, we feel that it is our duty to tell the customer the pros and cons of both. Again, with limited knowledge on the subject, customer is not buying the argument unless backed up with "standards"or "codes"and hence my request in this forum. I have also heard regarding the EMF generation or somebody calling it as dynamo effect (whatever that means !!!), but I need to give sufficient back-up data in terms of a written standard or code by an authority. The manufacturer gets little respect, unfortunately. Don't ask me why - that's how it goes with this particular client. With a not-so-strong consultant, my task becomes that much more difficult !

HVAC68

 

[hollandhvac]

Not the answer to your question, but what I have seen in the past is an AHU fitted with two complete fans installed within the AHU and connected to the ducting. The two fans (with motor) were in duty standby condition inorder the system to run continues.

 

[MNMaint]

In future applications, I'd suggest considering one AHU with two or more direct drive fans in parallel with dedicated VFDs for each fan motor. Direct drive = no belts to maintain or break.

The fans do not have to be sized for 100% airflow each at 60Hz either. As long as the drives are allowed to ramp up beyond 60HZ, either to meet a CFM or static set point, the remaining operational fans will speed up to do the job. Make sure the motors are sized for the max possible amp drawm NOT the design condition.

 

[tys90]

I second what MNMaint said. Look at something like a FANWALL system too. We've put up to 9 fans in large AHUs, all on independent micro-drives (small VFDs) and motors. If one fails, the rest speed up to cover the lost airflow until you can replace it. You can even size the fan arrays so you can cover multiple fan failures.

I really don't think a standby motor is the right solution.