Power for motor

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hawkeye23

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I like to ask you guys for a way to solve a problem before it happens. We have a 3/4 hp motor 120 volt that needs to be running at all times. Should we lose power just from the feeding cb that feeds the building this motor is in could cause a major problem for the company. There is a generator on the switchgear but that only would come on if power is lost to all the switchgear.
What is the best way to protect critical equipment when only parcal power is lost? There are about 12 main switchgear breakers.
It could be atleast 1 hour before someone can get to this 3/4 hp motor to make a fix should it lose power. Would a ups work ?
 
120803-2343 EDT

An appropriate UPS with a sufficiently large battery could be used. This would not be a small computer UPS. Appropriate means it could properly drive the motor without hurting the UPS or motor.

A brushless DC motor (really a synchronous AC motor) with its drive, or a VFD of some sort designed for your motor, and a large battery added to the the DC bus of the drive might work.

For 1 HP assume 1 kWh of energy to provide your 1 hour requirement. An ordinary large auto battery has about 12 * 50 = 0.6 kWh of storage. Maybe somewhat less.

There are other details I am not getting into, and the above values are very much ballpark estimates.

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If the motor is that critical, it needs its own transfer switch so that when the motor circuit fails, the generator will start. What about other modes of failure? Loss of power is not the only thing that will stop the motor from running.
 
don_resqcapt19 said:
If the motor is that critical, it needs its own transfer switch so that when the motor circuit fails, the generator will start. What about other modes of failure? Loss of power is not the only thing that will stop the motor from running.
Click to expand...

Yeah, I think you need to have a backup identical unit that can be switched-in if the online equipment fails. We call that "redundancy".
 
Besides, or in addition to the generator the transfer switch could use a source from the other switchgear bus that is still energized. You would have three sources & the generator start could stay as it is.
 
hawkeye23 said:
I like to ask you guys for a way to solve a problem before it happens. We have a 3/4 hp motor 120 volt that needs to be running at all times. Should we lose power just from the feeding cb that feeds the building this motor is in could cause a major problem for the company. There is a generator on the switchgear but that only would come on if power is lost to all the switchgear.
What is the best way to protect critical equipment when only parcal power is lost? There are about 12 main switchgear breakers.
It could be atleast 1 hour before someone can get to this 3/4 hp motor to make a fix should it lose power. Would a ups work ?
Click to expand...

If you go with a UPS, make sure that you use PF correction caps on the motor because most if not all UPSs are only capable of a small amount of reactive power. For example, I have an APC smartUPS 3000 at home on my computers. Its load ratings are 2200 watts or 3000 VA.

If you go the UPS route, I would go with at least a 5kVA unit. You need something capable of the inrush currents and the bad power factor while having sufficient battery power to keep it running until the generator can start.

Just for a data point, I ran an old 1/2 HP washing motor on it and found that the this particular unit is barely capable of running the motor on battery but can only start it if the UPS is on AC.

Here is what I would do:
Find the power factor of the motor by measuring it. Calculate and install enough capacitance on the motor to get the power factor at a value of 1 or unity. Get a UPS that is at least 5 kVA. Get a generator that will automatically start and a transfer switch. You may have to put the the transfer switch AFTER the UPS because some UPSs will not accept the generators power due to frequency and voltage fluctuations. Wiring it this way is different from how its done on larger systems but may be needed in this special case.

The idea is that the UPS would hold the motor while the generator starts and gets up to speed. Once its online, the transfer switch would switch from the UPS to the generator. The UPS would then either idle on battery power or shut down until AC is restored. Once this happens, the UPS switches to AC, the transfer switch moves the load to the UPS and the generator shuts down.

Sorry for writing a book. If you have any questions, let me know.
 
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