How to be sure that there's no problem in winding of a motor of any type ?

[dineshdeshmuk]

How to be sure that there's no problem in winding of a motor of any type ?

By measuring line to line resistance ? what should be the values ?

 

[chuckey]

Measure their inductance. A single short circuit turn will show up dramatically. Or failing that use a low voltage AC voltage and measure the current or stick a suitable light bulb in series.
Frank

 

[munzir]

how can inductance be measured ?

 

[dineshdeshmuk]

I couldn't comprehend it , would you elaborate it more " A single short circuit turn will show up dramatically. Or failing that use a low voltage AC voltage and measure the current or stick a suitable light bulb in series. " ?

thanks a lot though all of ya

 

[danishdeshmuk]

i think whatever the resistance values are, but all the 3 line to line resistance values should be same .... for a good winding

also line to body resistance should not be shorted .... it should be open

As far as i know , leave it on the experts

 

[chuckey]

" A single short circuit turn will show up dramatically." If you have a lot lot of turns on an iron core as you have in a motor, it posseses a property called inductance which establishes the magnetic field. With AC currents flowing through the turns you get an alternating magnetic field, but the AC current is a lot lower then then the DC resistance would suggest. This is because the changing magnetic field is opposing the change in the current. If you have a single short circuit turn, this turn allows a huge current to flow around it, which comes from the voltage across the whole winding. This current and its magnetic field also overloads the magnetic characteristics of the core causing it to saturate which also causes overheating and a further increase in the current through the winding. So except for small shaded pole motors the DC resistance should be lower the a few ohms with motors typically being 0.3 - 1 ohm.
Frank

 

[dineshdeshmuk]

difference between the ohmic values of the AC & DC motors in their windings ?

 

[chuckey]

Electric drills use a universal motor so the winding resistance is the same. When an electric motor rotates it always acts as a generator, generating a voltage which opposes the applied voltage. So a three phase 440V, 3 KW motor might have a DC resistance of .9 ohms per phase, ( I/1.732 = 440/.9 =~ 700A per phase), but due to the fact that the windings are generating 420V, the current is only 5A.
In essence the design of the motor always keeps the winding resistance as low as possible, small DC rev to very high speeds to get the back EMF high to reduce the effective voltage driving the motor.
Frank

 

[munzir]

I couldn't understand your answer ....

 

[chuckey]

You have acess to the web, search WIKI on electric motors.
Frank

 

[kak111]

problem in winding of a motor of any type

Your short question includes material for a very thick book, i think.
Look this first...



At the request:
I give here short and simple instructions to check 3-phase induction motor with contactor control...

The motor is in operating place
- fuse(s) is blown or overcurrent relay tripped

1. Check voltages of all supply phases before fuses (phase to phase)
voltages should be same (+/- 2%)

Disconnect supply to motor control ( Main switch OFF )

Replace fuse(s) if there is blown one(s), reset overcurrent relay

2. check all motor contactors
-wiring connections are tight
-when closing contactor by pushing with hand
-- all contacts closes and closed resistance is zero ohm

3. check that motor overcurrent relay inside resistances in phases are equal
--check connections

4. all fuses are in place, overcurrent relay is not open

5. Disconnect motor cables. ( Now test for motor control and supply )

6. check that all connections and cables are safe for the start-up.

Connect supply to motor control ( Main switch ON )

7. Try to start motor.

8. Check that contactors are operating correct.

9. Measure voltages phase to phase in motor supply terminals.

10. If above is OK. ( Motor terminals have correct voltages )

Disconnect supply to motor control ( Main switch OFF )

11. check motor insulation to ground
- Dmm high-ohm range , measure phase wires to ground.
....should be megaohms.

12. Open motor connection box
- check all winding connections in motor

13. Check motor winding resistances (U-V), (U-W) and (V-W).
- Dmm lowest-ohm range ....all should be same (+/-2%)

14. Check that motor is free to run (not stucked)
-- if it is possible , remove the motor load.

15. Connect motor cables to contactors

16. check that all wirings are connected and
all connections and cables are safe for the start-up.

17. all fuses are in place, overcurrent relay is not open

18. Try to start motor.

19. If motor dont start, STOP immediately .... >> goto 21

19a. if motor start and run
-- measure all phase currents U,V and W
--measure all phase to phase voltages in motor connection box (U-V), (U-W) and (V-W).

20. Phase-phase voltages should be equal (+/-2%)
-if difference is higher ,
- check difference before main fuses
- check difference before contactors
- check difference before overcurrent relay
- check difference in motor cable supply side

Phase currents should be equal (+/-5%)
- if phase-phase voltages are not equal in motor connection box >> currents are not equal
- if phase to phase voltages in motor connection box are equal and currents are not >> some failure in motor

If not fail, connect load to motor and test again...

21. Fuse(s) blown >> high current short circuit or breakdown in wiring, contactors, cabling or in motor
-Overcurrent relay tripped >> if contactors and cabling are supplying all 3 phases >> some failure in motor
-Motor is stucked.

Disconnect supply to motor control ( Main switch OFF )

22. Disconnect all connections from motor windings ( cables and shorting plates)
Check insulation between windings
- Dmm high-ohm range , measure winding to winding. (U-V), (U-W) and (V-W)
....should be megaohms
Much more reliable results are obtained when High Voltage Insulation tester is used. ( Testing with 500V...750V)

23. Check resistances of windings.
- Dmm lowest-ohm range , measure windings. (U1-U2), (V1-V2) and (W1-W2)
....all should be same (+/-2%)
Much more reliable results are obtained when comparing windings inductances.

Because motor winding resistance is usually very low ,
normal DMM cant usually give reliable value for comparison windings.
One good way to test motor winding for comparison is....
-smaller ac-voltage can be used too



Here is good pictures from failures in motor windings...

Three Phase Motors - Typical Causes of Winding Failures
http://rescuemotors.com/motor windings.html

 

[dineshdeshmuk]

Check insulation between windings
- Dmm high-ohm range , measure winding to winding. (U-V), (U-W) and (V-W)
....should be megaohms

1) How it would be in megaohms ? if i would measure this way (U-V), (U-W) and (V-W) then it would be (say 20 ohms), (20 ohms) and (20 ohms) something like that ....... means it would be line to line resistance


Much more reliable results are obtained when comparing windings inductances.

2) How to compare windings inductance ? How to calculate & measure the windings inductance ?


One good way to test motor winding for comparison is....
-smaller ac-voltage can be used too


3) How to use smaller ac voltage for motor winding comparison ?

thanks a lot , you must be exhausted , so sorry

 

[munzir]

What do you mean by these ?

i) 3. check that motor overcurrent relay inside resistances in phases are equal , how to check that ?

ii) if it is possible , remove the motor load. Remove everything like pulley , belt etc ???

iii) -- all contacts closes and closed resistance is zero ohm . It won't be zero ohms but it should be 0.4 ohms or even 0.3 ohms that's what my DMM shows (measures), would it be consider as alright ?

iv) What's overcurrent relay ? Is it a O.L (Overload) present or connected with the contactor ?

v) How to suspect for mechanical problem (failure) in the motor ? If its stucked while trying to turn it by hand, then, its a mechanical failure, right ? But are there any other symptoms of mechanical failure ? like any sound (as of sound of bearings etc)

thanks very very great stuff

 

[kak111]

Check insulation between windings

2) How to compare windings inductance ? How to calculate & measure the windings inductance ?
3) How to use smaller ac voltage for motor winding comparison ?

small error above in text "lets say we get current 0.8A" , must be "lets say we get current 0.9A"


continue in post #16...

 

[kak111]

3. check that motor overcurrent relay inside resistances in phases are equal , how to check that ?
What's overcurrent relay ?

if it is possible , remove the motor load. Remove everything like pulley , belt etc ???

The purpose of "removing motor load" is test the motor run
in such a way that the motor is running at idle
ie. motor does not move the mechanical running power to a device or machine.
( Only remove mechanical connection of motor to the load . )

-- all contacts closes and closed resistance is zero ohm . It won't be zero ohms but it should be 0.4 ohms or

I will say , that it is normal when measuring very low ohms with multimeter , it show "something near zero value".
Test meter by shorting measure leads, note reading and if measured result is same value ,
you can say it is "zero", tested with this meter.
Remember that multimeter measuring voltage is 1.5...3Vdc. Even smaller oxidation or dirt in contact surfaces is noticed.
Situation is truly different when connecting 400Vac = 560Vpeak voltage to the contacts.
On the other hand , if true resistance in closed contacts is 0.5 ohm and current 100A , power dissipation in contact points
is I^2 * R = 5000W and result is melted or welded contacts in contactor.
If you want to know real contact resistance , measure voltage over closed contact and measure current in contacts
then true resistance over contacts is U/I.

How to suspect for mechanical problem (failure) in the motor ?

Good readings....

Maintenance and Troubleshooting of Electric Motors
http://www.maintenanceworld.com/Art...intenance-troubleshooting-electric-motors.htm

http://machineryhealthcare.com/pdfs/Electric Motor failure modes.pdf

http://www.slideshare.net/dmanney/why-electric-motors-fail

 

[kak111]

Simple motor winding test for 3-phase motors....



This test can also be carried out by measuring the currents in windings.

 

[munzir]

For a 3 phase induction motor of 5.5 KW, what should be the current limiter value & its wattage ?

thanks for a whole lengthy doing

 

[dineshdeshmuk]

What would be the indications of the variations in the inductance of the windings & how the partial short circuits in the windings be detected ?

Thank$ for your extensive & immense support & co-operation

 

[kak111]

For a 3 phase induction motor of 5.5 KW, what should be the current limiter value & its wattage ?

Simplified calculation model part 1.



If you will try some calculations or need motor current values...
The following table shows the load current drawn by three phase motors
of various power ratings and supply voltages.

http://www.lostock-electrical.co.uk/html/motor_rating.html

 

[kak111]

What would be the indications of the variations in the inductance of the windings & how the partial short circuits in the windings be detected ?

The partial short circuits in the windings can be detected
by measuring inductive reactance in each phase
and comparing these values.

Here is info...