Phase Convertor - Capacitance Bleeding Resistor

[Guest]

I'm building a rotary phase convertor for a home workshop. My idler motor is 15HP. I'm using a bank of oil-filled motor run capacitors as the motor start caps - about 1500 mfd. The start caps are pulled into/dropped out of the start circuit by a contactor controlled by a potential relay. I want to install a resistor across the cap bank to bleed off any residual power left in the caps when the caps are dropped out of the start circuit.

What's the formula for calculating the resistance and watts rating for this application? I know that I don't want a low resistance because that would bleed off too much power when the motor is starting. Would a 10K-ohm, 50 watt resistor work?

Thanks, Rob

 

[mikeackley]

Re: Phase Convertor - Capacitance Bleeding Resistor

Rob:
I?ll take the first wack at this one, just to get something started and to give others here something to expand on.

Starting with the easy formula first (for calculating the resistor?s required watts rating); basic OHM?s Law with give you this one. I(current) = V/R; then P(watts) = V * I.

Now for the harder one: Calculating resistance required to discharge a capacitor (I hope you are familiar with differential equations), the only formula I know is the basic R-C Circuit Formula for Discharging a Capacitor:

V (Capacitor at time t) = V (Capacitor initial) e^ (-t/RC).

Where:
V = Voltage
e = natural log (base e), a constant of 2.7182818?
t = time in sec.
R = Resistance
C = Capacitance in Farads

Rob, if I knew what the capacitor?s voltage is [ V (Capacitor initial) ] at the time they are dropped out of the starting circuit, I could run a few scenarios for you using different values of R.

Hope this helps?and if anyone knows an easier, more straight forward formula to use, I would be interested in seeing it as well.

[ March 22, 2003, 10:27 AM: Message edited by: mikeackley ]

 

[Guest]

Re: Phase Convertor - Capacitance Bleeding Resistor

The potential relay I'm using is a GE 3ARR3FJ3BR3 (Grainger #6X550) with a pickup voltage range of 166 - 180 volts. I know that's the voltage between the 3rd/generated leg and one of my input single phase legs. Can you use the average of that, or 173 volts as a SWAG at what the voltage in the caps will be or is it something closer to 240 volts?

 

[mikeackley]

Re: Phase Convertor - Capacitance Bleeding Resistor

Rob:

Here are 4 time slices depicting capacitor voltage drop assuming:
A 1500 mfd Capacitor; Capacitor voltage of 173V (immediately after dropping out of the starting circuit); and a ?bleeding? Resistor of 10K ohms.

[All timing begins when the capacitors drop out of the start circuit]
After 30 seconds, capacitor voltage drops to 23.4V; after 60 seconds, cap. voltage down to 3.2V; 90 sec., down to .4V; and finally, after 120 sec., cap. voltage will be at .058V (call it zero).

Is this what you were looking for?? If you want to try some different assumption values, I don't mind pushing the buttons.

 

[dereckbc]

Re: Phase Convertor - Capacitance Bleeding Resistor

I have never heard of a bleeder resistor used for a run capacitor on a AC motor.

I do know bleeder resistors are used in DC power supplies. They are calculated to draw 10% of the load current of the DC supply. As the name implies it is to bleed off the DC charge after the supply is turned off, but it is a DC charge.

By putting a resistor in parallel with a motor run capacitor you could defeat the purpose of the capacitor. I am no motor expert, but I have never heard of it being done.

What about using a extra set of contacts on the relay to switch in the resistor after the relay is disengaged? You could use a 500 ohm resistor and discharge the capacitor within a second or two. But the voltage would really depend on where in the cycle when the power was turned off.

[ March 21, 2003, 09:25 PM: Message edited by: dereckbc ]

 

[Guest]

Re: Phase Convertor - Capacitance Bleeding Resistor

Mike,

Your calculations are exactly what I was looking for. That tells me the caps would be safe to handle 2 minutes after starting the phase converter. If I need to work on the caps, say to add additional ones because the idler motor isn't syncing up quickly enough, all I have to do is wait 2 minutes before I start messing with the cap wiring. I could probably use a resistor with a lower impedance, but that would get to Derek's point.

Derek,

While I'm no EE, it seems that using a resistor with a relatively high impedance (in this case like 10K) shouldn't impact the start caps. Not much power will bleed off to the resistor during the idler motor startup. If I were using a resistor with a lower rating and bleeding the power off in, say 5 seconds - that would be a different story.

As far as using an additional contact on the contactor that pulls the start caps in, that's a good idea. I'll see if the contactor I have for that (run of the mill SqD 90 amp contactor) can take additional contacts.

Thx for the help. Rob

 

[Guest]

Re: Phase Convertor - Capacitance Bleeding Resistor

Followup - I may need to lower the impedance. Discovered that 460-6(a) says the "Stored Energy" needs to be reduced to a nominal level in 50 seconds.

What does that mean - a 4-5K ohm resistor?

 

[don_resqcapt19]

Re: Phase Convertor - Capacitance Bleeding Resistor

460.6(A) says that the voltage shall be reduced to 50 volts or less within 60 seconds.
Don

 

[dereckbc]

Re: Phase Convertor - Capacitance Bleeding Resistor

RIRCT, It takes 5 time constants (TC) to fully charge or discharge a capacitor. One TC=R*C. So to fully discharge a capacitor in 50 seconds is pretty strtaight foward. 1 TC equal 10 seconds. R = TC/C = 10/.0015 = 6666 ohms.
Hope that helps

 

[Guest]

Re: Phase Convertor - Capacitance Bleeding Resistor

Thank you all for the assistance.

Rob