Split phase motor/compressor contradict ohms law?

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Jimbro250

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San diego
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Service tech
If a motor or compressor has a split phase system (has a start winding and a run winding). Why is it that the start winding has more resistance/ohms than the run winding but also draws more amps than the run winding. Doesn’t this contradict ohms law where a load with less resistance/ohms draws more current and a load with more resistance/ohms draws less current?
If someone can please explain this to me in simple terms I am a newer student and am racking my brain over this one. Thanks!
 

gar

Senior Member
211016-1713 EDT

Jimbro250:

What is the definition of Ohm's law? How does your motor relate to that definition?

How does an AC induction motor start? What is basically necessary for this automatic start to occur?

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drcampbell

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The Motor City, Michigan USA
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Because DC resistance isn't the whole story.

The AC current draw will be proportional to the total AC impedance. I'm sure that if you were to calculate total impedance -- including resistive, inductive and capacitive contributions -- it will closely predict the actual current measured. (and if you use vector arithmetic, it will also predict the voltage-current phase relationship -- lead or lag, and how much)
 

ActionDave

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Where are you getting your numbers from? Don't confuse resistance that you read with an ohm meter with inductance of a winding when AC is applied. A start winding has to have current limited through it, either through a starting switch or capacitor or it will burn up.
 

Jimbro250

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San diego
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Where are you getting your numbers from? Don't confuse resistance that you read with an ohm meter with inductance of a winding when AC is applied. A start winding has to have current limited through it, either through a starting switch or capacitor o
Where are you getting your numbers from? Don't confuse resistance that you read with an ohm meter with inductance of a winding when AC is applied. A start winding has to have current limited through it, either through a starting switch or capacitor or it will burn up.
My teacher told us a quick way to troubleshoot a compressor is to to use your ohm meter across the leads. He said start winding will read higher ohms than the run winding.
are you saying my ohm meter won’t actually read total resistance? Then what exactly is my ohm meter reading and is there an instrument that will read total resistance including impedance?
 

Jimbro250

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San diego
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Service tech
My teacher told us a quick way to troubleshoot a compressor is to to use your ohm meter across the leads. He said start winding will read higher ohms than the run winding.
are you saying my ohm meter won’t actually read total resistance? Then what exactly is my ohm meter reading and is there an instrument that will read total resistance including impedance?
My teacher told us a quick way to troubleshoot a compressor is to to use your ohm meter across the leads. He said start winding will read higher ohms than the run winding.
are you saying my ohm meter won’t actually read total resistance? Then what exactly is my ohm meter reading and is there an instrument that will read total resistance including impedance?
As far as my current reading goes I guess I don’t have actual reading since this was just a quick example in class. But just with my experience having taken readings on single phase motors and compressors I’ve always noticed current draw ramps up for a few seconds as soon as it turns on then lowering to a more constant current. But if my ohm reading is higher on the start than the run I’m confused why. I would think ohm reading would be lower since it draws more current…
 

ActionDave

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My teacher told us a quick way to troubleshoot a compressor is to to use your ohm meter across the leads. He said start winding will read higher ohms than the run winding.
are you saying my ohm meter won’t actually read total resistance? Then what exactly is my ohm meter reading and is there an instrument that will read total resistance including impedance?
The run winding has very low resistance and a high inductive reactance whereas the starting winding has high resistance and low inductive reactance. Those are some big words to describe something that doesn't matter much when troubleshooting.

I don't how taking an ohm reading on the start and run windings is a quick way to troubleshoot anything. An easier way to tell the difference between the start and run windings is to look at the numbers on the motor leads. I don't know how you measure impedance, I've never done it.

I do know when it comes to troubleshooting I may use the ohm setting on my meter to do some basic continuity tests, but I'm much more interested in voltage and amp readings, checking the capacitors and the starting switch, listening for worn out bearings, looking for loose connections,,, ohm readings are way way down on the list.
 

gar

Senior Member
211017-0037 EDT

Jimbro250:

Impedance and resistance ohms are two different things. In the real world impedance will always include some resistance. However, in a very high quality capacitor you might ignore the resistive component.

Inductive and capacitive reactances produce a current that is shifted 90 degrees from a resistive component. A resistive component dissipates as heat power from the resistive component. Inductive and capacitance reactive components store and restore energy. The actual inductive or capacitive storage elements do not dissipate any current as heat power. But a physical inductance or capacitor does in reality contain some resistance and this will dissipate heat. The current for a given applied voltage, if resistance alone was used, the current would be much higher than the the series combination of resistance and inductance.

If you have both series capacitance and inductance, and are at resonance, then the series circuit just looks like the resistance alone.

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synchro

Senior Member
Location
Chicago, IL
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EE
My teacher told us a quick way to troubleshoot a compressor is to to use your ohm meter across the leads. He said start winding will read higher ohms than the run winding.
are you saying my ohm meter won’t actually read total resistance? Then what exactly is my ohm meter reading and is there an instrument that will read total resistance including impedance?
There are LCR meters that measure inductance, capacitance, and resistance. But along the sames lines as what Dave has said, they would not be all that helpful for troubleshooting motor problems.

As far as my current reading goes I guess I don’t have actual reading since this was just a quick example in class. But just with my experience having taken readings on single phase motors and compressors I’ve always noticed current draw ramps up for a few seconds as soon as it turns on then lowering to a more constant current. But if my ohm reading is higher on the start than the run I’m confused why. I would think ohm reading would be lower since it draws more current…
Remember that both the start winding and run winding are in parallel during the starting period, and so both are drawing current during the starting interval. As Dave said above: "The run winding has very low resistance and a high inductive reactance whereas the starting winding has high resistance and low inductive reactance." This is done to create a phase shift between the start and run windings so that there is a rotating component in the stator's magnetic field in order to produce torque on the rotor for starting purposes. The higher resistance in the start winding is achieved using a smaller diameter wire and so it dissipates significant power, but this is acceptable because the start winding is only connected for a short time.

Both the start and run windings draw more current initially until the motor speeds up. This is because the shorted turns in the "squirrel cage" of the rotor act just like having shorted turns on the secondary of a transformer, where the start and run windings are acting as the primary of this "transformer". But as the rotor speeds up it is "slipping" less and less relative to the rotating magnetic field that's being applied by the stator. This means that the rate of change of the magnetic field as seen by the rotor is being reduced as the motor speeds up, and that reduces the voltage across the rotor windings (via Lenz's law) and therefore also reduces the current flowing through them. As a result the current in the start and run windings are also reduced because they are primary windings in the transformer analogy above,

There are other ways to look at this such as a generated "back EMF" that increases as the motor speeds up and effectively subtracts from the voltage applied to the motor, therefore resulting in a reduced current.



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GoldDigger

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If a motor or compressor has a split phase system (has a start winding and a run winding). Why is it that the start winding has more resistance/ohms than the run winding but also draws more amps than the run winding. Doesn’t this contradict ohms law where a load with less resistance/ohms draws more current and a load with more resistance/ohms draws less current?
If someone can please explain this to me in simple terms I am a newer student and am racking my brain over this one. Thanks!
How are you determining that the start winding draws more current than the run winding? Are you using an ammeter connected in series with each individual winding?
As noted by synchro both windings are drawing current in parallel while the motor is starting and both windings will probably draw more current during starting than during running. (up to the time the start winding turns off)
 
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jim dungar

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Ohm's law is often misapplied when using data obtained from multimeters. It does not work well with AC circuits, because of non-resistive components. It assumes the voltage is linear with the current and resistance which is not true in most motor circuits, especially during sudden load changes like starting.
 

Carultch

Senior Member
Location
Massachusetts
My teacher told us a quick way to troubleshoot a compressor is to to use your ohm meter across the leads. He said start winding will read higher ohms than the run winding.
are you saying my ohm meter won’t actually read total resistance? Then what exactly is my ohm meter reading and is there an instrument that will read total resistance including impedance?

What a simple ohm meter does, is it applies a sample DC voltage across its leads, and measures the corresponding current that flows through the component it measures. The ratio between the two, is the read-out in Ohms that it gives you. It will also tell you whether you have continuity (finite ohms) or discontinuity (infinite ohms). You can only extrapolate this reading to the behavior in a circuit, when it is a pure resistive component you are measuring, and it is a component where you can expect a linear relationship between current and voltage. In other words, an ohmic resistance.

Ohm's law is not V=I*R as you have been told all your life. That is an algebraic re-writing of the definition of resistance. Ohm's law is that resistance is a constant for components and materials that obey it. It doesn't universally apply to every passive component, because other factors come in to play.

Inductance and capacitance, which are properties that are involved in a motor's circuit elements, can translate to a corresponding reactance in Ohms at a given frequency. This combines in a Pythagorean theorem formula with resistance in Ohms to get total impedance. Reactance refers to storing energy in a component, and releasing it at another part of the cycle. Resistance refers to dissipating energy in a component to another form of energy. Reactance depends on frequency in the opposite manner for inductors as it is for capacitors, while resistance of resistors is independent of electrical operating conditions.

An ideal inductance or electromagnet winding like you'd see in a motor, will measure zero ohms with an ohm-meter. The ohms you really measure are the parasitic resistance, that you get in a real inductor.

An ideal capacitor will measure infinite resistance with an ohm-meter, once the capacitor fully charges in the steady state, since there is no continuity across the gap between the plates. It will initially measure finite resistance that starts out at zero and grows to infinity with time, because there will be current in the circuit as a capacitor charges. You get initial continuity with a capacitor that eventually settles to a lack of continuity.
 

GoldDigger

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In the case of a motor it is further complicated back a series voltage, called the back EMF, which is created by the rotating magnetic field and depends on the rotation speed of the motor. The relation between current and motor terminal voltage cannot be described by an impedance, even with reactive components taken into account.
 
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