Standby motor in AHU
Standby motor in AHU
(OP)
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 ?
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





RE: Standby motor in AHU
RE: Standby motor in AHU
RE: Standby motor in AHU
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
RE: Standby motor in AHU
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.
RE: Standby motor in AHU
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.
RE: Standby motor in AHU
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
RE: Standby motor in AHU
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.
RE: Standby motor in AHU
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.
RE: Standby motor in AHU
HVAC68
RE: Standby motor in AHU
RE: Standby motor in AHU
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.
RE: Standby motor in AHU
I really don't think a standby motor is the right solution.