Strange Motor Configuration

The maths and theory for this is waaaaaay too far back in time for me to remember specifics, but the following GUESS might be a starting point.

The 3 x 0.6 uF on the input for power factor while the 3 x 1 uF on the operator side influential in starting and direction change with some effect also on switching due to the RC value with the 90 Ohm resistors.

Remember that switching voltages and such will be induced at the switching end of each motor coil.

But why? Why not just use a reversing starter with just two contactors reversing the two of the phases and connect the motor either delta or wye depending on the voltage?

also, if one of the capacitor is damaged, what do you think would happen?

The capacitors are for power factor correction and the resistors are bleed off for the capacitors to prevent damage if a capacitor were to remain charged with the wrong polarity during reversal.If the resistors go bad, or if the motor is reversed too quickly,(to fast for the capacitors to bleed off) damage to the motor starter can occur and fuses can blow.If a capacitor goes open,power factor will drop.If one shorts out,the result is obvious.Distributed capacitance is the most efficient method to improve power factor.I have seen this method applied where there are multiple motors (1500 in one particular case)requiring frequent stop/starts,not just reversals.

The image resolution is not too great,but all capacitors look like the same value to me.

Durguy,

"Why not use just two contactors?".

If I am right, reverse is star connection & reverse is easier work than forward, see my post #7; I guess the following...

1. There is a gearbox in the motor drive.
2. Gearboxes do not like reverse motion.
3. Star gives 1/3 the torque of Delta, for same supply volts.
4. For the angular shift between forward & reverse gear mesh, compared to delta, 1/3 torque is 1/3 energy to lose when gears etc bang together in star. Less stress to wear out drive.
5. The contactors have easier work at star currents.
6. Winding voltage is about halved in star which makes less insulation stress over life.

For a motor/drive continually changing direction, the reduced stress & better life/reliability may be worth an extra contactor.

Perhaps you could let us know what contactors, connections & control sequence you have found by further examination.

67model

The 1's of U-V-W windings are connected to the contactor on right side and the 2's are to that on the left side. It seems the polarity of winding is reversed whenever the drive rotation needs to be reversed.

For example, initially, the right contactor is used to create star point and left contactor is off with supply coming to the windings through winding terminals marked-1. When the rotation needs to be reversed, the left contactor shorts out 1's forming the star point and the power to the motor windings is coming through terminals marked-2.

I don't have much knowledge in this kind of motors and am just guessing with y limited knowledge.

Here's how I'm thinking it works...

Assume the left contactor is closed and the right contactor open, and just consider one phase. The current splits with part going to the resistor and capacitor on the left side and the rest going to the motor.

A 1uF capacitor at 60 Hz has an impedance of 2652 ohms (current leading), so very little current goes that way. The motor winding is an inductor (current lagging) in series with the resistor and capacitor on the right side. If the inductive impedance of the motor winding is about equal to the capacitive impedance (resonance at 60 Hz), the two will cancel out resulting in a larger current passing through the motor.

Series Resonance Circuit

Resonance occurs in a series circuit when the supply frequency causes the voltages across L and C to be equal and opposite in phase.

https://www.electronics-tutorials.ws/accircuits/series-resonance.html

Of course, the same analysis holds if the right contactor is closed and the left contactor is open.

Yeah, my calculations show that 1 uF and about 7 Henries would be resonant at 60 Hz.

2πF = 120 π = 377

X_C = 1/2πFC = 1000000/377 = - 2652j Ω

X_L = 2πFL = 7 x 377 = 2639j Ω

Maybe they are reversing through 2 sets of contacts rather than 1, to improve contact life and/or allow operation with contacts with lower rating, they are PCB mounted? The surge/arc suppression networks also indicate concern with the contactor rating. It’s nice a 6 lead motor is not uncommon.

I think you will find the answer will be explained by the way the contactors are connected to each other ...the bit missing from the drawing...the bit you did not have time to look at...

Durguy, the answer may be in the contactors. If the contactors fall out/break to a short over the motor windings on either side. Activating/ one contactor will run the motor one way in Star. Activating the other contactor will reverse the motor in star.

Ok so another look at the machine and I got the following:

-the mechanism that the motor is moving forward and backwards has an electric brake to stop it at the exact point which the operator needs.

-there are three contactors, the 6 leads that are showed in the drawing are connected to the same contactor. The contactor has 2 sets of terminals on top of each other, each 3 leads are connected to 1 set. I did not check the terminals so I do not know if the 2 sets are both normally open or one of them is normally closed.

- there are 2 more contactors beside the afromentioned contactor. I did not check the connection because, as I said, the contactors are mounted on a PCB and all the wires coming out of them are going to the PCB, so I have to remove the PCB to follow the tracks. I did this for the capacitors/resistors PCB and I got the drawing but the contactors are on a different PCB and connected together with plugs.

- the motor is 3 phase 380V 50Hz, a normal 3 phase motor with six leads coming out marked u1, v1, w1 and u2, v2, w2 so nothing special here.

during transportaion of the machine a couple of months ago, it fell and one of the capacitors was damaged (0.5microf 850v quite big in size). They just removed it and the machine worked fine for some time, and then the main transformer (one big transformer that supplies about 10 different voltages to the machine) was burned, so they rewired it and I installed it. They are now afriad to operate the machine without the damaged capacitor becuase they think this will damage the transformer again which is unlikey because the motor is fed directly from the 3phase supply and not from the transformer.

this all the information I have at the moment.

This is old equipment & the insulation will not be as strong as new. Not a good idea to leave out a spark suppression capacitor.

One of the often forgotten factors in surge/spike suppression & EMC susceptibility is that if excess energy is not dissipated it will find somewhere to go & do damage. The inductors & capacitors in filters reflect energy back - signal filter designs require resistive termination at usually both ends, at least one end. The resistor in the R-C network burns off the energy & cuts the duration of the over-voltage.

When it comes to mains supplies, measurements find impedances of a few ohms to kilohms according to frequency, location & installation with fluctuating resistance/reactance values most commonly 50 to 200 ohms (most cables lie in this range & EMC test specs require resistive line simulation networks of these values).

If you disconnect a motor, the rotor current does not stop instantly, the stator generates voltage. This can add to the voltage when contactor opens/closes for reversal or star/delta connection. This means it sparks over at higher voltages, more often & for longer - these surges occur on both motor & installation side of contactor.

Spikes & harmonics in supplies go for the end windings in motors & transformers, the extra voltage & heating is localised.

I would suggest adding some R-C networks & voltage dependent resistors particularly at supply connection to electronic equipments. My stock item was RIFA metalised paper R-C assemblies with integrated carbon composition resistors 0.5μF + 22 ohm [PMR209 0.22μF + 47R nearest now] & 20mm diameter Littlefuse VDR to protect capacitor against overvolts & clamp surge voltage (275V rms rated can absorb 300 Joules, 700V clamp @ 25 amps).

Put 2x 1uF 450V caps in series in the place of the damaged one.

You'll need two 2 uF capacitors, to hold the original desired rating...

No, doubling the thickness of the capacitor insulation only 1/2 the capacity and doubles the Voltage.

So 1/2 microfarad total...looking for 1?

Rwilliams he wrote that the damaged cap is 0,5 micro's 850V. To replace that will be 2x 1 micro's 450 or 500V in series if he can find the space for them.

If difficult to remember. It is just the opposite of resistor circuit values.

If you would take a look at the three phase conductors looking from left to right you will notice that the conductors are reversed to allow the motor to move in a back and fourth manner and the circuits with the resistors and capacitors are there to give the motor a fluid motion. This is a common circuit configuration in old washing machines.

AH, analog. The world before computer programmable VFD's. Such a mystery.

1. Hi, as for my understanding refering to the drawing given. There's 2 motor, one on left and the other right side. Right side motor is getting full or mains voltage but the left gets voltage from the TX. As it is, you need to confirm it by way of the name plate. By looking at the drawing carefully, the RHS motor connected in star or scotts, this is to operate at 415/1.732, meaning 240v 3 phase.Whereas, the LHS motor which is having an operating voltage of lower than the mains voltage, so 2 motors operating at different voltages.check name plate of both motors and step down voltage of TX

Thanks for sharing, such useful info, great piece