ECN Forum Forums Technical Discussion Electrical Theory and Applications Capacitor-Start Motor Question

When a large capacitor-start motor on an air conditioner compressor starts, sometimes lights will dim slightly. The AC people say adding a "kick-start" kit will help the dimming problem. A "kick-start" kit is a capacitor and a relay combination. The relay is designed to remove the capacitor from the circuit after the motor is up to speed. This added "kit" is placed in parallel with the existing start capacitor on the motor.

Would someone please explain how this extra, momentary capacitor helps mitigate the light dimming and if there could be any possible motor damage to the compressor motor from using one of these "kits".

Capacitors are devices that store Voltage, where as a coil essentially stores current. To some degree this is a simplification but I believe it will suit the purpose for your question.

When an electric motor starts the internal resistance is considered to be very low (no Counter EMF so to say), This absence of opposition to current flow will allow a considerable amount of line current to flow until rotation is developed to create (generate) internal resistance (CEMF).

The time it takes to reach operating RPM is increased when starting high torque loads. Compressors must push a cylinder which must pump against gas to change it’s state to liquid (lots of work).

Well any way the locked rotor current is high for any motor (generally) and high line current creates voltage drop at the business end of the circuit. E drop = I line X R line. Know if you have high LRC and additionally reduce the applied voltage to the terminals of the compressor, it would seem to me that the unit would take a longer time to start and aggravate the Voltage drop on the premises wiring. This causes other equipment to see a reduce voltage and you get lights dimming.

Back to the capacitor, it stores voltage between the Zero points of the cycle of an AC sine wave (charge and discharge). At those times when the voltage falls it will supply voltage to the compressor to compensate for the dip at the unit. This reduces the excessive line current and diminishes the dimming to other equipment in the system.

I hope this helps I’m sure some one will be more technical,

Charlie

Thanks for your reply, Charlie. I know you said coils store current but doesn't your explanation require the capacitor to discharge its stored current (while the voltage drops during start-up) in order to maintain the voltage across the motor? If I understand you correctly, the capacitor stores enough energy to supply current as the voltage drops while the motor is starting. I would assume that these are large capacitors in order to be able to lessen the voltage drop. Please correct me if I misunderstood you.

The primary type of single-phase motor used in refrigeration is PSC—permanent split-capacitor type. Study case 4 at URL www.lmphotonics.com/single_phase_m.htm The compressor locked-rotor current will be on an equipment nameplate, to give an idea what sort of load it presents when started. With the unswitched run capacitor, shaft torque is dynamic in that phase shift [voltage angle] between the run and auxiliary windings is low at the initial zero-speed condition. A start capacitor increases the voltage angle for a moment until the kit’s relay senses high-enough voltage across the auxiliary wining.

One relationship of start-to-run currents can be understood by comparing compressor steady-state running impedance in ohms at full-load current to the DC resistance (in ohms) of the main stator winding. The kit may not reduce starting-voltage drop so much as reduce the time it exists. [No free lunch.] Understand also that there are two types of start kits. Most common it the all-in-one type that is more of a “fits-all” variety, versus a discrete electrolytic {starting} capacitor and separate potential relay that is specified by the compressor OEM. A third type is the two-lead PTC resistor, but has limited effectiveness and popularity.

In hermetic compressors, a popular mechanical style gaining ground is the “scroll” compressor versus older reciprocating type, that takes less effort to start. An unavoidable problem is that sometimes humans are sensitive to changes in voltage to varying degrees with incandescent lighting, so it bothers some folks more than others. On a 120-volt base, a standard minimum is 110 volts, but in some cases, 106 volts.

Let the guy install the hard-start kit if he will guarantee results to your satisfaction. Be well aware that if the start kit’s relay contacts stick or weld, your compressor stator will be cooked in short order.

I think Bjarney has said it all don't forget that Voltage is a unit of measurement, and it quantifies a difference in potential (the amount of stored electrons). If it makes since that the discharge of a capacitor is the release of current you are quite right. the capacitor sort of stores a charge (voltage) by storing a quantity of electrons. so in mechanical since it's pretty much the samething, but bjarneys explanation is right on and rather concise.

I take that as a compliment, Charlie. Your statement that “Capacitors are devices that store Voltage, where as a coil essentially stores current” is an important correlation in electrostatic {electric} versus electromagnetic properties in electrical systems, and resurfaces again and again—as does Ohm’s Law. Let me caution that an opinion is that, and one’s slant on declaring pat “rules” to things electrical can be treacherous and occasionally embarrassing.

My deep-seated contempt for single-phase motors was formed decades ago repairing such {sometimes melted/incinerated} equipment. I avoided sure insanity by “graduating” to more complex systems, but fundamentals of resistance, inductance and capacitance and their interaction with voltage and current cannot be forgotten and should never be considered rigidly obvious. Their application often has subtle variations that can shoot you out of the saddle declaring one’s version of so-called universal truths.

Often, too, it’s a lot easier to accept fundamentals applied in the real world ‘fitting a pattern’ than try to impatiently explain them to others. Also, it is a grave error to assume learning is no longer necessary—that you have it all figured out.

Be aware that “small potatoes” characteristics of things electrical may apply to much bigger ones. Often I thought of single-phase transformers as “nickel and dime”—unimportant and inconsequential for all-but-the-very-smallest applications—when assuming so can bite you in the butt. Where three single-phase 5kVA transformers could be applied to smaller 3ø loads, so can single-phase 333,000kVA transfomers in bulk-transmission duty. In some cases four identical 1ø transfomers are built and delivered at significant expense on cross-country rail cars, with the last one sitting in the yard aside the other three as an easily rebussed spare—for in an unfortunate emergency a replacement could be a year out.

For motors and capacitors, there is an ongoing discussion of capacitor-assisted starting duty for a 30,000hp synchronous motor on a weak {large starting voltage-drop} feed. One number estimated was 137,500kVAR for the required cap bank.

In our area in the old mills, it was common to supply 3 phase delta loads with three single phase transformers affording the option of open delta operation in the event of a single unit failuer. Many of them were still in operation not too long ago!!