A capacitor/resistor question

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

 

[Tom Biasi]

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