Capacitor Size

[vcand]

Hello,

I am trying to find out what size capacitor is needed, for a friend. He has a 240v, 25amp, 50hz? motor that he wants to power for 5+ seconds after power is removed. Does anyone know what size capacitor would be needed? I've tried the formulas I found online, but I don't quite understand them, so any help would be appreciated.

Thanks.

 

[nsaspook]

If you're talking about a standard AC motor capacitor, forget the capacitor idea, it's maybe needed for starting the motor but provides no power.

 

[vcand]

Yes, I believe its AC... So the likelihood of removing power and the motor and it continuing for a few seconds is unlikely. I'm guessing the voltage drop would be too quick? What about if it was a DC motor?

 

[4pyros]

You may be better off trying to fix why he needs to to run for 5 more seconds after power is removed. Seems like a vary odd request.

 

[Diver300]

A capacitor can't store AC power. Also the amount of energy that you are looking to store would need a huge capacitor.

You are looking for an uninterruptible power supply (UPS) which uses a battery. However, a 25A one, about 5 kW, will cost a lot.

 

[unclejed613]

the other option that may work better is a mechanical one. a big flywheel. depending on the load on the motor, it may not run for 5 seconds though.

i agree with 4pyros, try to fix the "why" of the additional run time. or add a turn-off delay to the motor circuit.

 

[crutschow]

Yes, I believe its AC... So the likelihood of removing power and the motor and it continuing for a few seconds is unlikely. I'm guessing the voltage drop would be too quick? What about if it was a DC motor?

Voltage drop has nothing to do with it. You just can't store AC energy on a capacitor, only DC.

You could use a capacitor for a DC motor but it would be HUGE capacitor to store that much energy.

 

[crutschow]

What is the load on the motor?

 

[nsaspook]

Yes, I believe its AC... So the likelihood of removing power and the motor and it continuing for a few seconds is unlikely. I'm guessing the voltage drop would be too quick? What about if it was a DC motor?

You need to store the energy of a 7hp motor for 5 seconds. That takes real money to do.

 

[jpanhalt]

You don't need to store AC, just store DC and convert to AC. Now, 25A for 5 seconds is 125 Coulombs. Assume a 60% overall efficiency so "all" you need is 208 Farads at 240 volts. That's a walk in the park. Just call DigiKey. It can help you.

John

 

[unclejed613]

You don't need to store AC, just store DC and convert to AC. Now, 25A for 5 seconds is 125 Coulombs. Assume a 60% overall efficiency so "all" you need is 208 Farads at 240 volts. That's a walk in the park. Just call DigiKey. It can help you.

John

as was said earlier, an inverter (or UPS) for that load will be very expensive. the OP still hasn't got back to us as to why the strange requirement. from what little he did say, it sounds like a power fail situation, where the motor needs to move something into a convenient position when the power fails.

 

[vcand]

i agree

I agree that its a weird request. My friend 'thinks' he's an inventor. He has given me very little details, only a picture of the motor label and that he wants a cap to power it. Obviously, by the response, he's in daydream land in regards to powering a motor off of a cap. I suggested to him a battery or some sort of backup power source, but he said its no good. I tried to pry more details out of him, but nothing. My 'assumption' (based upon my online searches), I can only assume that he's trying some sort of perpetual energy thing, but I could be wrong. But all of your information has been helpful to get him off my back

Thanks

 

[nsaspook]

Perpetual energy things need this: https://www.oreillyauto.com/site/c/detail/EB00/121G.oap?keyword=121g

 

[RCinFLA]

You need a timer switch. You can design a 555 timer that is re-triggered for every applied AC peak so it stays on. When the refresh is removed it can be set to time out 5+ seconds later, driving a relay to supply the motor AC power.

 

[4pyros]

You need a timer switch. You can design a 555 timer that is re-triggered for every applied AC peak so it stays on. When the refresh is removed it can be set to time out 5+ seconds later, driving a relay to supply the motor AC power.

The OP wants it to run after power is removed. Not a delayed turn off, thats easy.

 

[RODALCO]

A flywheel option would be best, as already suggested.
In the power industry tap changer drives have a heavy flywheel on the motor which is brought up to speed first, then releases the geneva to do the actual tapchange.
This is to avoid getting the contacts stuck between the resistive path in case power is lost during the event.
The actual process takes 5 to 7 seconds.