# 27vDC 6a from 115vAC

#### colum

Jul 25, 2013
175
I need a variable power supply that will supply 27v DC at 6a from 115v AC to drive a small motor generator.. would this circuit need to be regulated in order to have a stable frequency at the AC generated...Thanks for your help and please keep in mind it needs to be variable ...Thanks...colum

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,652
would this circuit need to be regulated in order to have a stable frequency at the AC generated
That's a real conundrum to me. DC doesn't have frequency (or f=0Hz). What exactly do you want to achieve/regulate?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
No, it would require feedback to ensure that the frequency and voltage are maintained.

Essentially a regulator has feedback, but the feedback needs to come from the generated voltage, not the intermediate voltage.

What are you trying to do? Create a stable 115V from unstable mains?

#### solo2racr

Aug 21, 2013
142
I need a variable power supply that will supply 27v DC at 6a from 115v AC to drive a small motor generator.. would this circuit need to be regulated in order to have a stable frequency at the AC generated...Thanks for your help and please keep in mind it needs to be variable ...Thanks...colum

If I had to guess, I believe he is referring to residual AC ripple in the DC supply.

He may be able to get away with a LM338. http://www.ti.com/lit/ds/symlink/lm138.pdf

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,652
Thinking about the OP and the replies:
There is a DC source 27V / 6A. This source drives a DC motor. The DC motor in turn drives an AC generator. The output of the system is an AVC voltage from the generator.

The output of the AC generator will vary in voltage and frequency depending on the load. A high load will reduce the frequency and the voltage. A small loaw will increase voltage and frequency. Unfortunately voltage and freqeuncy do not necessarily follow the same load charactersitic. While controlling the RPM of the generator reliably controls the frequency, the voltage depends on otherfactors, too, like e.g. magnetizing current in the stator windings.

Therefore, you will
either have to accept stability in only one variable (V,f) while the other is unregulated
or you will need two control loops, one for frequency (via RPM of the generator) and another for voltage (via e.g. stator current).

I found this useful information on AC generators.

Another option you have is to forego the motor-generator setup completely and use an inverter.. This will be much more efficient than the motor-generator setup.

#### colum

Jul 25, 2013
175
That's a real conundrum to me. DC doesn't have frequency (or f=0Hz). What exactly do you want to achieve/regulate?

Hi Harald. I meant to say that a DC motor is rotating an AC generator and any voltage fluctuations in the DC to the motor will change the Hertz cycle rate of the AC output..colum

#### colum

Jul 25, 2013
175
No, it would require feedback to ensure that the frequency and voltage are maintained.

Essentially a regulator has feedback, but the feedback needs to come from the generated voltage, not the intermediate voltage.

What are you trying to do? Create a stable 115V from unstable mains?

Hi Steve. I'm trying to create a stable frequency from a DC motor rotating an AC generator and I know when the AC generated has a load it will change the speed of the DC motor and I think you are right it will only work if theres a feedback from the generated voltage and if this feedback is from the output Hertz then its not a variable frequency anymore. so I think I'm back to trying to regulate and vary the DC motor speed to maintain a frequency however imperfect and at 6 Amps + no load this may be quite a task..Thanks...colum

#### colum

Jul 25, 2013
175
Thinking about the OP and the replies:
There is a DC source 27V / 6A. This source drives a DC motor. The DC motor in turn drives an AC generator. The output of the system is an AVC voltage from the generator.

The output of the AC generator will vary in voltage and frequency depending on the load. A high load will reduce the frequency and the voltage. A small loaw will increase voltage and frequency. Unfortunately voltage and freqeuncy do not necessarily follow the same load charactersitic. While controlling the RPM of the generator reliably controls the frequency, the voltage depends on otherfactors, too, like e.g. magnetizing current in the stator windings.

Therefore, you will
either have to accept stability in only one variable (V,f) while the other is unregulated
or you will need two control loops, one for frequency (via RPM of the generator) and another for voltage (via e.g. stator current).

I found this useful information on AC generators.

Another option you have is to forego the motor-generator setup completely and use an inverter.. This will be much more efficient than the motor-generator setup.

Thanks again Harald. Its the frequecy that s most important.. colum

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,652
Its the frequecy that s most important
In that case you need a feedback loop that senses the output frequency and controls the DC power to the motor accordingly.

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