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A capacitor/resistor question

Started by Unknown July 17, 2013
Greetings All,
In the book "Electric Motors and Control Techniques" a circuit is
illustrated that is for the dynamic braking of a squirrel cage motor.
This circuit discharges a capacitor through the field windings to slow
the motor. The capacitor is connected across the AC power to the motor
in series with a 50k resistor and a diode. I under stand why the diode
is there. The resistor must be there to slow the rate at which the cap
is charged. Is this done just to limit the current when the cap is
charging? When the cap is discharged through the windings it is
connected to the windings only and does not discharge through
resistor.
Thanks,
Eric
On 7/17/2013 11:29 AM, etpm@whidbey.com wrote:
> Greetings All, > In the book "Electric Motors and Control Techniques" a circuit is > illustrated that is for the dynamic braking of a squirrel cage motor. > This circuit discharges a capacitor through the field windings to slow > the motor. The capacitor is connected across the AC power to the motor > in series with a 50k resistor and a diode. I under stand why the diode > is there. The resistor must be there to slow the rate at which the cap > is charged. Is this done just to limit the current when the cap is > charging? When the cap is discharged through the windings it is > connected to the windings only and does not discharge through > resistor. > Thanks, > Eric >
You have a grasp on it. A capacitor when discharged will present a low impedance (or resistance)at first and then look more like an open circuit as it charges. The resistor is to prevent a large inrush current when power is first applied. The cap is charged to DC and when released to the coil will brake the rotation. A little more to say but this is the basis of what you asked. Tom
On Wed, 17 Jul 2013 14:12:11 -0400, Tom Biasi <tombiasi@optonline.net>
wrote:

>On 7/17/2013 11:29 AM, etpm@whidbey.com wrote: >> Greetings All, >> In the book "Electric Motors and Control Techniques" a circuit is >> illustrated that is for the dynamic braking of a squirrel cage motor. >> This circuit discharges a capacitor through the field windings to slow >> the motor. The capacitor is connected across the AC power to the motor >> in series with a 50k resistor and a diode. I under stand why the diode >> is there. The resistor must be there to slow the rate at which the cap >> is charged. Is this done just to limit the current when the cap is >> charging? When the cap is discharged through the windings it is >> connected to the windings only and does not discharge through >> resistor. >> Thanks, >> Eric >> >You have a grasp on it. A capacitor when discharged will present a low >impedance (or resistance)at first and then look more like an open >circuit as it charges. The resistor is to prevent a large inrush current >when power is first applied. The cap is charged to DC and when released >to the coil will brake the rotation. A little more to say but this is >the basis of what you asked. > >Tom
Thanks Tom, It's nice to know when I'm thinking about things the right way. Eric