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Author Topic: Using a AC generator to power a DC motor  (Read 1566 times)

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Offline DaaniiTopic starter

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Using a AC generator to power a DC motor
« on: March 17, 2012, 11:29:16 PM »
We have a 48 Volt DC motor hooked up to our wheels that we now use batteries to power. I would like to have the option of powering the motor using a portable generator.

The generator I have in mind puts out 120 Volt single phase AC current, at a maximum of 14 Amps. Can I hook up a 120 Volt to 48 Volt AC transformer, followed by a full-wave bridge rectifier, and get DC power clean enough to put in the motor controller for the motor? Will I need other components in the circuit?

It may be a lot more complicated than I am thinking. Since I could not find anything on the Internet similar to this, I'm posting these questions.

Thank you.

UPDATE: I understand that I will have some voltage loss in the rectification, but I'm not sure how much. So I guess I will need to have a transformer that puts out more than 48 Volts AC.
« Last Edit: March 17, 2012, 11:34:33 PM by Daanii »

Offline Soeren

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Re: Using a AC generator to power a DC motor
« Reply #1 on: March 18, 2012, 09:35:37 AM »
Hi,

The generator I have in mind puts out 120 Volt single phase AC current, at a maximum of 14 Amps. Can I hook up a 120 Volt to 48 Volt AC transformer, followed by a full-wave bridge rectifier, and get DC power clean enough to put in the motor controller for the motor? Will I need other components in the circuit?
If you peak-rectify an AC voltage, it will be sqr(2) higher (minus the diode voltage drops) as the caps charge to the peak voltage and the AC voltage is RMS.

For 48DC, you need: (48+2xdiode drop)/sqr(2)
Assuming the diode drop (for a single diode) at the current in question is say 1.5V (you need to check a datasheet or measure), you'd need:

 (48+2x1.5)/sqr(2) = 51/1.41 = 36VAC

The current OTOH will decrease by sqr(2).

As I understand your setup, it's: 120V @ 14A max -> 120V:36V transformer -> diode bridge -> cap -> motor control and this equates to...
Max current: (14A x 120 / 36) /sqr(2) = 33A

This will give a DC with some ripple, depending on the value of the cap. The cap needs to be huge for this - at least 66,000F (and with a 60V+ rating) following the rule of thumb saying 2,000F/A, which should give a ripple of around 3V).

How much ripple your controller can handle, you have to see in its datasheet/manual.
Regards,
Sren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline DaaniiTopic starter

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Re: Using a AC generator to power a DC motor
« Reply #2 on: March 18, 2012, 05:13:52 PM »
Thanks, Soeren. That's very helpful.

Offline DaaniiTopic starter

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Re: Using a AC generator to power a DC motor
« Reply #3 on: March 19, 2012, 02:49:36 PM »
Let me ask a follow-up question. I would like to hook up both the output of the generator and the output of some batteries to the motor controller. Would that work?

In other words, can I have the generator provide power to the motor when it needs it, and otherwise be charging the batteries? The batteries are deep-discharge lead-acid batteries.

Offline Soeren

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Re: Using a AC generator to power a DC motor
« Reply #4 on: March 21, 2012, 03:49:11 PM »
Hi,

I would like to hook up both the output of the generator and the output of some batteries to the motor controller. Would that work?

In other words, can I have the generator provide power to the motor when it needs it, and otherwise be charging the batteries? The batteries are deep-discharge lead-acid batteries.
For charging an "48V" lead-acid battery, you need an end-of-charge voltage of between 54V and 60V (depending on charge and use methodology) and you need a bit higher voltage to overcome contact resistance, lead resistance and loss in an eventual charge regulator.

If you connect both the motor and the charger to the battery, the charger will supply the current for the motor as far as it can and the motor will take any above what the charger can supply from the battery.

But... What kind, if any, of charge controller do you use and where in the chain is it placed?
This may be what makes or breaks the idea.
Regards,
Sren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

 


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