Woung rotor slip ring motor principle

T

Thread Starter

Tauck

Hi,

I would like to know characteristics of slip ring motor upon speed Vs Load. I have a case that slip ring bow truster motor is getting high current by 25% (125%) than rated at both stator and rotor winding. I did check all the source and resistance. the current and voltage for both stator and rotor were pretty balanced.

So i am thinking of possible case that causing high current.

1) External loading effect- those gearbox are freely turn able. (no distortion)

2) Rotor resistance - Can it be effect by changing of wire size combination at rotor winding ( 3 X 1 = 2 X 1.5)?

Lastly , Is this possible that at same rotor resistance, the stator current should be nearly maintain regardless of loading change and only varied in speed?

Please help me to explain, if you have any experiences.

Thanks
 
Tauck... first some general comments. You understand that both the standard induction motor and the wound-rotor induction motor have squirrel-cage rotors, and the torque developed is proportional to current circulating in the rotor. The principle difference is that for the former its rotor-resistance is fixed, while for the latter, the rotor-resistance can be varied by connecting an external resistance via slip-rings. Thus, the motor speed can be controlled without varying applied voltage.

1) Regarding your question, "Can rotor-resistance be affected by wiring size?" The answer is yes!

2) Regarding your statement that although the "gearbox" turns freely, the bow-thruster motor is overloaded! My question is, "Always! That is from start to full speed?"

3) Has there been a change in stator-current and rotor-current measurements at "start?"

4) Has there been a change in stator-current and rotor-current measurements when all of the external resistance is "cutout?"

5) Was the bow-thruster tunnel or tube changed, allowing a greater volume of water to flow

6) Was the bow-thruster tunnel or tube equipped with a water-flow restrictor, that somehow may have been eliminated?

7) This question may seem odd, but has frequency of the supply voltage been increased?

Regards, Phil Corso
 
Hi Phil Corso,

Thanks for your post. Further more info that i did measure the rotor current & voltage, stator current & voltage , frequency and resistance of both winding by phase to phase(using mOhm meter), external resistance as well. They are pretty balanced.

For the control, there are three steps of resistor tapping.Say 75%,85% and 100%. The current is getting 130% at highest speed ( shorted rotor )both rotor and stator.

Again, we tested for free load. around 40% of rated current at stator and a few current at rotor. At the free run, both current are same for three(every) step. I think that is true, because Wound rotor is controlling torque.

For foreign distraction , we have checked the tunnel by under water inspection and clear.

Best Regards,
Tauck
 
Hi,
The rated frequency of motor is normal 50Hz. And We read at 50.2Hz. the voltage is 400VAC. Further investigation, we found that Rpm of motor rotor is getting higher.

Additional info that the rotor coils was have been rewind. The re-winder claim that the motor is in good condition.

Thanks,
Tauck
 
Tauck... several conditions will change the full-load current (with resistor-bank excluded from the starting circuit.) They are:

A) A higher supply-frequency than rated.

B) A lower supply-voltage than rated (increases current.)

C. A lower rotor-winding resistance than original design (thanks for the rotor rewinding info.)

C) A larger tunnel-volume than the original.

D) A higher water density than the original design.

E) A change of propeller diameter.

E) A change of the wire-size connecting the slip-rings to the resistor-bank. This is especially important because the effective resistance always remains connected to the rotor’s resistance.

In closing, some of the factors, taken alone or in combination, will tend to increase rpm! And, because of the affinity-law associated with fluid movement, the horsepower increase is proportional to the cube of shaft speed (or propeller diameter.)

Phil Corso
 
Dear Sir Phil Corso,

I have also the same problem encountered with the rotor wound motor. I am assign to investigate the cause of frequent damaged (burning) of slip ring of Ball Mill Motor - (equipment is used to grind slags). I measured the parameters like voltages, primary and secondary current (already at 93% full load) and rotor resistance and the result seems alright.

During my investigation, I found out that there is an increase of 50% of the feedrate to the ball mill. When I analyze the motor using motor analyzer, the motor is operating at 93% full load (with 50% increase in feedrate). The ball mill is operating continuously, 24/7.

When I monitor the the slip ring temperatures,it is already at an average of 110-150C...

My questions are;
1. Is the High temperature at Slip ring has something to do with high secondary current?

2. Is high secondary current has something to do with overload?

3. what should be the standard %full load for a continuous motor, operating 24/7? Is 93%FL advisable?

Thanks!
Ben
 
Ben… Following are a series of questions/actions to aid your investigation:

1. What are the motor’s rated parameters as given on its nameplate?

2. What were the measured variables given with the Motor Analyzer?

3. What are the motor’s operating parameters (V, A, kW, Speed, Temp)today?

4. What are the individual rotor slip-ring currents? Pls note frequency will be substantially lower than line-frequency so most clamp-on ammeters may not be accurate. However, it is important the currents have about the same magnitude!

5. What is the ambient temperature at its location?

6. What is age of motor?

7. Is this a relatively new problem?

8. Did the problem exist before the feed-rate was increased?

9. Can you provide comparable operating data before the problem started?

10. Do you have another machine of comparable size in the plant?

Several tests on the rings and brushes will help pinpoint the problem:

a) Is arcing present? If not visible in day-light, I suggest you use an ordinary AM radio (detuned) held in the vicinity of the brushes. Arcing emits RF.

b) Do the three rings exhibit the same temperature readings?

c) Have you checked rings for eccentricity?

d) Do the brushes move freely in their holder?

e) Have you checked brush pressure?

f) Do the brushes chatter?

g) Have you measured rotor-winding resistance and inductance for unbalance?

h) Have you measured resistor-bank resistance for unbalance. Measurement should be from slip-rings with brushes lifted?

i) Is the resistor-bank totally bypassed at full-load?

Regards, Phil Corso
 
1. What are the motor's rated parameters as given on its nameplate?

Vrated: 3.3kV
Irated: 92A
Outputrated: 400kW
Sec Voltage: 740V
Sec Current: 325A

2. What were the measured variables given with the Motor Analyzer? What are the motor's operating parameters (V, A, kW, Speed, Temp )today?

Below are the latest readings of Motor Analyser<pre>
Name Plate Measured
Output Power [Hp] 536.19 492.26
Speed [RPM] 870 883.57
RMS [V] 3300 3335.16
RMS [A] 92.00 81.60
pf [p.u.] N/A 0.82
Nema Derating [p.u.] 1.0 1.00
Torque [ft-lbs] 3238.42 2927.43
Efficiency N/A 94.89
Percent Load N/A 91.81
Test Value Status
Voltage Level (Over) [%] 101.07 Good
Voltage Level (Under) [%] 101.07 Good
Voltage Unbalance [%] 0.36 Good
THD [% of fund.] 2.00 Good
Total Distortion [% of fund.] 2.14 Good
Current Level [%] 89.55 Good
Current Unbalance [%] 1.05 Good
Load [%] 91.81 Good
Ef. Service Factor [p.u.] 0.92 Good
Rotor Bar [db] -69.10 Good
Op. Point [%] 0.00 Good</pre>
4. What are the individual rotor slip-ring currents? Pls note frequency will be substantially lower than line-frequency so most clamp-on ammeters may not be accurate. However, it is important the currents have about the same magnitude!

Based on my data gathered (average) rotor currents are u=307A, v=298A, w=301A.

5. What is the ambient temperature at its location?

Ambient temperature ranges from 35-40°C

6. What is age of motor?

Motor is already at 20+ yrs

7. Is this a relatively new problem?

Nop, from 2005, record shows that they replaced at least once a year. (maximum of 2 slip rings/year)

8. Did the problem exist before the feed-rate was increased?

I don’t have a concrete idea since Im new to the plant, but according to my people in the line, there was no problem encountered in the past....

9. Can you provide comparable operating data before the problem started?

The available data with respects to motor parameters like slipring temp and secondary currents
were not monitored. It was just monitored lately when this problem occurred.

10. Do you have another machine of comparable size in the plant?

Yes,.. We have tertiary ball mill with motor rating lower than primary ballmill at 255hp, connected in parallel. We also did an analyses in the motor using motor analyser and resulted to 68% full load. No problem encountered for the past 5 yrs regarding slipring ring burning. Temperature at slip ring rings are lower at 60-80C only.

a) Is arcing present? If not visible in day-light, I suggest you use an ordinary AM radio (detuned) held in the vicinity of the brushes. Arcing emits RF.

On most slipring trouble, arcing and burn odor were observed and smell before the breakdown.

b) Do the three rings exhibit the same temperature readings?

Nope, they don’t have the same reading... sometimes one ring is relatively higher than other temps. There was a time that we replace one carbon brush due to very high temp as compared to the other 2.

c) Have you checked rings for eccentricity?

Nope, we haven’t check it... Whats the purpose of checking?

d) Do the brushes move freely in their holder?

Yup, but last dec 14,2011 (latest breakdown), i found out that some of the carbon brushs stuck-up at holder. We difficulty of removing the carbon brush to its holders.

e) Have you checked brush pressure?

We don’t monitor and measure the spring pressures. This could be one of the causes.

f) Do the brushes chatter?

Yes... I measured the vibration at rotor side of the motor (slipring side) and found out that the vibration is 3.75mm/sec. Is this level of vibration acceptable or in favour to slipring operation? What should be the vibration limit of slipring to operate normally?

g) Have you measured rotor-winding resistance and inductance for unbalance?

Yes, rotor-winding resistance is ok!

h) Have you measured resistor-bank resistance for unbalance. Measurement should be from slip-rings with brushes lifted?

Nope, we have not checked it...


i) Is the resistor-bank totally bypassed at full-load?

Yes , all the resistors during start-up were short-circuited. But currently , there is a running resistor installed in at rotor winding. According to my boss, the purpose of the running resistor is to help minimize the temperature at slip ring. Is this valid?
 
Ben... this problem is not going to be solved with an exchange of posts. The numbers you have produced so far are not indicative of a serious electrical problem.

That said, please send me photos of the motor installation, especially the slip-ring compartment:

cepsicon[at]AOL[dot]com
 
The following comments address your 18-Jan answers:

5. Can you provide motor temperature (while ambient is 40C) by embedded T/C, embedded RTD, or carcass measurement?

a) I should have said “sparking” instead of arcing! I meant the continuous sparking (it may not be visible) between the brush face, and its slip-ring. I am not referring to the observed arcing and odor just prior to breakdown.

b) Can you provide a guesstimate of temperature difference between one ring and the others? Is the delta-temperature between slip-rings significant, i.e., 5, 10, 30C?

c) Slip-ring eccentricity, at operating speed, can produce visible arcing if brush movement is restricted so that the brush pressure against the slip-ring surface varies, resulting in current-density changes!

d), e), f) I apologize for not being more vigilant with these 3 points. I should have included the following:

i) Where does the air for brush compartment ventilation come from?

ii) Can you provide a visual description of the slip-ring surface?

iii) On a scale of 1 (O.M.G.) to 10 (B.T.E.W) how would you rate brush maintenance?

I eagerly await your photos, Phil
 
Ben,

some questions further to phil's:

1. You stated that the motor is 20+ years. When was the last time you changed the brush springs, brush holders?

2. You change the slip rings pretty often. Is the job done in situ, or the rotor is taken off the motor, slip rings changed and slip rings machined in situ on the rotor on a lathe or something similar?

3. What type of bearings are used to support the rotor, journal or Ball bearings? If journal bearings, have you checked their clearance lately?

4. Combining 2 and 3, i.e. assuming that you machine slip rings out of motor and then install, are you sure that the centre-line used for machining corresponds with the final shaft orbit when the motor is in service? I have seen cases where the two did not agree and even though slip rings are machine perfectly on a lathe, they end up eccentric when placed in the motor.

5. Is your environment or the ventilation air reaching the motor aggressive (i.e. corrosive gases running around) or relatively clean?

6. Are you using the same grade of brush as originally designed, or have you changed grade?

7. Is the motor designed for 45C ambient temperature or is it designed for a lower ambient temperature?

8. When you said that the last time brushes failed you found them stuck in the brush holder, were they stuck due to dust and possible oil ingress, or were there signs of arcing between brush and brush holder?

9. Do you have lubricant (oil or grease) leaking from the bearing and possibly reaching the brushgear assembly, contaminating it?

10. Is the cooling air reaching the brushgear of the same temperature as the ambient temperature, or is the motor sucking hot air from other plant close by?

11. Is the motor equipped with some sort of air filters on its cooling air or is the air movement unrestricted?
 
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