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Will, President & Small Business Owner
Category: Electrical
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Experience:  IBEW apprentice to Journeyman, 30 years experience Owner Operator
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# I would like to install an R L C circuit across the start winding

### Customer Question

I would like to install an R L C circuit across the start winding of the compressor on my central home air conditioning unit out side the house, as a surge uptake to save money on my electrical bill, over time. What are the values and watt and amp ratings and other specs for 220 v ac at 40 amps 60 hertz. ?
Submitted: 2 years ago.
Category: Electrical
Expert:  Will replied 2 years ago.

Will :

.Where is:
f0 - frequency, L - inductance, C - capacitance

The capacitors and inductors used won't be free of losses. The largest "handbrake" for the electric current is the ohmic resistance of the inductor's wire. It can be symbolized by an ohmic resistor being connected in series.

[IMAGE][SRC][/SRC][ALT][/ALT][WIDTH]246.3235294117647[/WIDTH][HEIGHT]100[/HEIGHT][STYLE][/STYLE][/IMAGE] Electrical circuit consisting of an ohmic resistor (R), an inductor (L) and a capacitor (C). That's why an arrangement like this is called a RLC circuit.
The resistor converts permanently electrical energy into thermal energy which is extracted from the system. Hence the maximal voltage respectively the maximal current through the devices decreases permanently until the whole energy is transformed into heat. For the resonance frequency we get:

[3.43]

Where is:
fr - resonance frequency, L - inductance, C - capacitance, R - resistance

The resonance frequency is defined as the frequency at which the impedance (=resistance) is at it's minimum when the circuit is connected to an (mostly sinusoidal) AC-voltage.

Will :

There are other considerations to deal with here. The first, many 220 volt single phase motors use a 110 volt start circuit, always on a dual voltage rated motor. Obviously a dual voltage motor rated at 115/230 cannot have a 220 volt start circuit.

Will :

For manufacturing purposes, most manufacturers tend to use the same winding configuration be it single voltage rated or dual voltage rated. It is rare to find a 220 volt start circuit, most are tapped off the run half way.

Will :

Next the actual voltage should be used in the formula. As voltages tend to vary by location and even time of day, or day of week, the best way is to measure the voltage. But again, that voltage value may change during the day, and in the summer, it can drop, depending on areas of the country. One reason for the intentional rolling black outs are put in play for the purpose of not delivering low voltage to everyone. It is not always a complete lack of power, that is anticipated, it is the often detrimental low voltage that can cause all kinds of issues, mostly with inductive devices, mainly electric motors.

Will :

The next problem, is motor A manufactured by Company A, same horsepower, same speed, and frame, will react differently to voltages below and above nameplate ratings. So there are a lot of assumptions by basing a circuit on nameplate data. The locked rotor values are also different from motor to motor, these values are effected by the type and quality of the steel laminations in the stator, and the rotor, the rotor bar casting materials, the resistance end rotor configurations, the amount circular mills in the wire to handle the rated current.

Will :

While the starting of an electric motor is the highest electrical draw, from locked rotor, you might be able to save a bit, but I doubt it will be anything that would add. I can give you some assumed values, but to produce any savings the values should be taken from the motor under power. You can measure the start winding current by clamping on a lead in front of the start cap, or caps. And you should be able to measure the start winding voltage by measuring the incoming voltage. The watts would be a formula of those values.

Will :

IF you are really concerned with power consumption, an energy efficient motor, sized correctly to handle the filter[s] and restrictions is probably your best bet. Also using the largest filter you can keeps the motor startups to a minimum. As a matter of fact, have you measured the current on your pump motor as it is, and compared it to the nameplate? Are you low or very close to nameplate current? If you are low, and have a sand filter, adding a coating of DE will add restriction that you have room for if the current is low, and would help filter, thereby reducing the need for starting. Most residential pump motors are sized incorrectly. Because motors are built with 1 HP, 1.5, 2, 3, 5, 7.5, 10, 15, 20 there are is a lot of GAP between off the shelf sizes. Or if you have not measured the running current, and find it low, if low enough, I have seen applications where a 2 HP can be replaced by a 1.5. power savings that would never be obtained by any starting modifications.

Will :

Another approach that has a lot of merit, is to use a single to three phase convertor, with frequency control. By using a much less expensive and much more efficient three phase motor, savings can be accomplished in that manner. If you have 220 volts and 40 amps you have one large pump, somewhere around 7.5 HP. Also with a frequency setup, you can gain the advantages of a two speed single phase, but with complete control of the speed.

Will :

I apologize I was doing two problems at once, and yours is an air conditioner not a pool pump, my mistake! I thought wow what a pool, so disregard the pool references but the remainder applies. The variable speed three phase would be a great modification, on an AC unit. I would still be curious as to the current draw as it sits.

Will :

Again if you want starting amps, you will need to clamp the start winding. I could not give you the actual starting current too many parameters. And again it is highly likely the start winding is 110 volts. It is possible but if the motor is rated for dual voltage it has a 110 start winding. If you contact the manufacturer of the motor, they may supply you with more data than is normally provided, it depends on who it is , how proprietary they think the data is, and which engineer you happen to get. Actually if you could provide me with the starting current, and voltage, I could run this starting issue through our electric motor engineers who quite possibly could provide us with a better sized capacitor, and resistor or inductor value. Again I would be surprised if the motor is rated for 220 volts and you have utility voltage of exactly 220.

Will :

Do you know if the motor is a psc motor? A capacitor start capacitor run motor, uses an oil cap for assisting the run, at 7.5 HP it could be a cap motor, hard to say. I would like to have the full motor nameplate data if you could snap a photo or take it off the data plate, I would like to run this by our engineers who do nothing but mods to motors. If there is significant savings to be had, they will be a huge help in determining the correct values, of any modified circuits. But the best as I described is to convert to three phase, controlling the ramp up, and soft starting the motor, other considerations will need to be taken into consideration.

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