# Run Capacitor Measured Voltages for an Induction Motor

Discussion in 'General Electronics Chat' started by Hrishikesh Rokade, Mar 25, 2016.

Mar 25, 2016
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Hi Everyone,

I am working on a project which needs an Induction Motor with a RUN capacitor. I did some measurements when i was operating the Motor and i realized the voltages measured across the capacitor are as follows. For Case 1 of connections, i apply a CW and CCW command using a single phase supply. The measured voltages at Pt. A and Pt. B for Case :1 of the connections are as follows (Please refer the attachment)

CW : Pt.A = 380Vrms , Pt. B= 0
CCW : Pt A = 240 Vrms, Pt. B =0

N is the Neutral Connection.

Question: How can the voltage at Pt. A of the capacitor be 380Vrms when the supply voltage is only 230Vrms ?

Warmly,
Hrishi

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2. ### recklessrog Member

May 23, 2013
338
102

Mar 25, 2016
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Thanks recklessrog,
But this doesnt give any clear explanation about "why" ? Could you help me decode this ?

Jul 18, 2013
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Mar 25, 2016
18
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But i am afraid, if you have a look at the schematic i have attached in my thread with the connections made as in Case:1, you will notice winding-1 and winding-2 ( which should be inductors in series) are in parallel to the capacitor C.
So in effect this schematic would become a parallel LC circuit and hence the explanation provided in the above link would be untrue as its for series LC circuits.

Thanks,
Hrishi

6. ### recklessrog Member

May 23, 2013
338
102
The maximum voltage in the series R, L, C (r being the resistance of the windings) will make the largest PD develop across the capacitor as it has the highest reactance, As max headroom says, study LCR circuits and Phase relationship for a better understanding.
In practical terms, the run capacitor should generally have a voltage rating of at least 1.5 x the supply voltage and more often than not it is 1.75 to 2 x the supply voltage.
Look at the circuit more closely, the supply is in series with one coil and the capacitor, and the capacitor is across both coils, until you understand series and parallel LCR it will not make sense.

Last edited: Mar 25, 2016

Jul 18, 2013
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Not true.
The connection configuration of a 1ph Cap start run is one inductor across the supply, the second is a capacitor in series with an inductor also across the supply, they are considered totally separate for this discussion.

Max.

Mar 25, 2016
18
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Yes, you are correct , this is the correct way of connecting a single phase induction motor using a split phase cap. However, please note, in my schematic for Case:1 i am doing something different. The two windings are in series and the cap is in parallel to the series combination. I know this is a wrong connection, but i am doing some experiments to understand what happens to the motor and the cap when such connections are made. During this experimentation i observe the voltage is higher at 380Vrms and these are not series LC circuits ( as the two windings are in series with their resultant in parallel)

Hrishi

Jul 18, 2013
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I am not sure of the point, the motor is not going to run, at least not properly, if you want to study voltage vectors across LC circuits it is an odd choice of experiment components.
Don't forget you only have two supply conductors with a choice of three termination points.
Max.

Mar 25, 2016
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What happens when we make connection like that of Case:1. N is for Neutral. CW and CCW are 230VAC commands for rotation Clockwise and Counter Clockwise. When CW command is given , winding-1 and winding-2 are in series and the resultant is in parallel to the cap C. Yet, i see voltage of 380Vrms at Pt. A. This astonishes me !

Jul 18, 2013
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You have to define the point of reference and it should be constant, in case 1 it says T3 is open leaving the supply on T1 and T2 which is supply across W2 with W1 in series with C ?.
You need to define your 'State Chart' a little better.
Max.

12. ### tcmtech Well-Known Member

Nov 4, 2013
2,041
1,673
Think of an induction motor as a multi-phase transformer with a rotating core.

There are winding ratios plus phase angle ratios between different windings which will give you odd voltage readings depending on how the power is applied to a specific winding set and the motor is mechanically loaded.

Mar 25, 2016
18
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Max,
Terminal -1 is Open , Terminal -2 is connected to Neutral , Terminal-3 is connected to 230Vac (rms). I believed the voltage at Pt.A should be around 230Vac only but I get 380 Vac on my multimeter .

Hrishi

Jul 18, 2013
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For one a modern DVM is designed to measure RMS of a pure sine wave, what I suspect you have at that point you are measuring is a more of an effect of the capacitor in circuit rather than what you would see at the service power source, this would be a parallel tuned circuit where at anything other than resonance would be very high impedance.
Capacitors are used in this manner with very high inductive loads to correct the power factor of a system.
I suspect you are seeing a combination of any or all of the above.
You would need a scope and some kind of power factor meter to see the results properly.
Max.

Jul 18, 2013
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I forgot to ask, What is the motor doing at this point? surely not rotating correctly, if stationary it could be drawing excess current.
Max.

Mar 25, 2016
18
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Max,
Indeed the motor rotation for CW (Terminal -3) command is erratic . I haven't made any current measurements but I agree the motor should be drawing more current.
If we were to use some mathematical equations to correlate the observed voltage would that be possible ?

Hrishi

Jul 18, 2013
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The LC parallel circuit could be measure and the results predicted, but Not sure how you would evaluate the effect of the shorted turn secondary/rotor in this, but also if the windings are in series then they would be anti-phase rather than in phase with each other meaning there should be some cancellation of field?.
Max.

18. ### recklessrog Member

May 23, 2013
338
102
Because we are literally guessing as to what you have, and the way the motor windings, stator and rotor are configured, + the inductance of the windings and a host of other parameters, it is impossible to give a definitive answer other than, "don't know"
What you are trying to do is use the motor outside of its design spec which even if that were known, it would take considerable time and effort + suitable test equipment to compile a table of parameters from which some conclusions could be drawn.
Do yourself a big favour and read some of the education articles published on this site that deal with Ac, Motors, phase etc.

Jul 18, 2013
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In light of the subsequent posts, I am beginning to wonder what this project results are aimed at, other than some perverse academic exercise?
Max.
.

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