Booster Motor

[nornrich]

All,

I'm looking to do a power transmission unit for a lifting device. This unit is going to be used moving people up and down stairs. One of the things we are looking to do is provide battery backup in the event of a power failure. We thought through some options and one of the options is to couple a smaller HP motor moving at a slower speed to the back of the fan output shaft on the main drive motor. This will reduce the size of the batteries we need and will increase the cycles we can get out of a set of batteries. A couple of questions:

1. Has anyone done something like this in the past?
2. If the smaller motor is a gear motor, what reduction in efficiency can be expected if we are backdriving the smaller motor?
3. Any better suggestions?

Regards,

Rich......

Richard Nornhold, PE

 

[eromlignod]

I don't think I'd ever back-drive a gearmotor, especially at a higher speed than it's rated. You should probably use a clutch that spring-engages the smaller motor to the auxilliary shaft when power is lost.

Don
Kansas City

 

[cbrn]

Hi,
I'm with eromlignod. There are many cases you would even damage the small motor (well, not really the motor but the gearbox). Extreme: if the gearbox is screw+helical gear, it can't even be back-driven.

Regards

 

[TheBlacksmith]

I'm still trying to figure out how a smaller motor with a gearbox will allow you to both reduce the size of the batteries and increase the number of cycles. Work is work, you're still moving the same load over the small height.

 

[eromlignod]

Yeah, I was wondering about that too. He's doing the same work, but taking more time to do it (moving slower), so it takes less horsepower (work per unit time). But it seems you would still only get so many trips up the stairs from a battery with 'x' amp-hours of charge. Taking longer to make each trip would take less hp, but running the battery for the longer time would still deplete it in the same number of cycles.

Don
Kansas City

 

[cbrn]

Hi,
eh eh, yes, you're perfectly right. Using a smaller motor can be useful only if there is a current limitation in the design (I seem to know some batteries don't have a linear characteristic: over a certain current drain, the capacity drops). But then it would be far better to look for a different type of batteries!!
Thank you Blacksmith for waking me up...

Regards

 

[TheBlacksmith]

I looked at the post for two days before replying, wondering if I was missing something. Apparently not.

 

[dvd]

You also have the issue of the reflected inertia of the back-up motor/gearbox to contend with at startup. I vote for a clutch of some sort. If you can switch over to the backup manually, a chain coupling would be pretty low tech.

 

[sreid]

The most straight forward way of doing what you want would be to use an Uninterruptable Power Supply (UPS). These tend to be relatively inexpensive due to volume production.

 

[MintJulep]

Hydraulic motor + accumulator + small pressure pump.

No need for an electrical backup system.

 

[Tophinater]

My vote is for one motor with a simple ups. A small motor taking lifting 90kg for 5m (im guessing) in one trip would only require about(assuming about 85% efficiency) 1.5 amp-hours from a 12 volt battery. A few marine lead acid batts could easily send a chair up and down the stairs at least a dozen times depending on their capacity. If you want to go far enough you can even have the chair recharge the batts on the way down.

 

[ScottyUK]

There is some sense behind the idea of using the smaller motor - batteries can deliver more useable energy if the battery delivers a lower current for a longer period compared to delivering it as a high current for a short period. So although the work done may be the same, the efficiency with which the chemical reaction within the battery delivers that energy to do the work is dependent on discharge rate. Just one of the problems inherent in a battery! If you ever look at battery data you will see that the amp-hour (AH) capacity at a low discharge rate is significantly larger than for a high discharge rate.


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Sometimes I only open my mouth to swap feet...

 

[TheBlacksmith]

I agree with your statement regarding batteries Scotty. Perhaps if the OP had stated either "allow the use of smaller batteries" or "get more cycles", I wouldn't have commented, but to do both simultaneously??

 

[ScottyUK]

A little of each perhaps? The OP has vanished anyway!


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Sometimes I only open my mouth to swap feet...