AC > DC > Serial Motor - Equivalent Series Resistance??

[jeffewil]

Hi,

I need to power a series 90V DC motor, from a rectified AC supply.

The terminal resistance of the motor is 3.6 ohms.

I will use a bridge rectifier over the output of the AC supply, then smooth it.

I have calculated I need a 5600uF smoothing capacitor over the bridge rectifier output , but considering that it will have an equivalent series resistance gives me a problem.

The ESR if I have my numbers right = 0.04( or that magnitude), which when you consider it will effectively be in parallel with the motor will mean an enormous current > 1000A will be pulled.

What am I doing wrong, surely this can't be right?


I am assuming ESR = DF/(2 x PI x 50 x 5600uF )
DF = disapation factor = 0.15


Also I assume for teh smoothing circuit t=CR, if t = 1/f, should f = 50Hz, or 2 x 50Hz as its a full wave rectifier?

Please help

 

[waross]

Forget the capacitor. Use a free-wheeling diode.
A series motor takes a lot of current at low speeds.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[RajeevCell]

waross, the bridge itself should provide a free-wheeling path. I agree that a cap is not needed.

 

[RajeevCell]

jeffewil, you can run the series wound dc motor with a variac and bridge rectifier. Use a fuse and a switch (or better a circuit breaker) before the variac.

So the circuit is

115 Vac -> fused disconnect or circuit breaker -> variac -> rectifier -> motor

 

[jeffewil]

Just need to find a variac now then....

Thanks for your advice, eBay here I come.