No, No this could be expensive!

[steamer]

I remember the day I started old Norm up in top gear in a cool shop...and he tried...and then the magic smoke came out of the motor...and the electrical box...all at the same time....seems I blew and shorted the motor caps which fried the box....I ended up rewiring the machine AND putting in a new motor!....sounds like you got off easy Jo.

Dave

[Don1966]

Jo, sorry to hear about the smoke and glad you have the convertor fixed. But I hate to sound like a broken record. Make sure to amp check any motor you install right when you turn it on. Failing to do this can result in a burnt motor.

Regards Don

[Jo]

New motor wired up. I have spun her up off the machine and no nasty smells 



Spinning over she is taking  very little current, so next job mounting the motor in the lathe I am hoping that the existing holes will line up  .

Jo

[Jo]

 

I also took off the cover on the old motor.. spot the burnt out winding 
 


Time to put the workshop back together. I still have not worked out why the 3HP Colchester takes less current than the 2HP Harrison under no load  .

Jo

[Don1966]

Looks like you loss a leg and single phased. Amp check all legs and this would not of happen. Bummer to have a burnt motor.

Don

[AOG]

Time to put the workshop back together. I still have not worked out why the 3HP Colchester takes less current than the 2HP Harrison under no load  .

Jo

A motor like that draws the most current when starting. As the motor speeds up it generates back EMF that reduces the apparent current required to run the motor. When the forces hit balance the motor stops accelerating and runs at a steady speed. When you put it under load you will see the current increase.

Tony

[Jo]

Thanks Tony 

I was more intregued that the larger motor was in no load condition taking less amps. I also noted that the new 3HP Colchester motor needs a lower boost setting on the 3 phase converter than the old one used to .

Jo

[Admiral_dk]

Hi Jo - with you technical background I'm sure that you know this (but have forgot), so since your asking.

I repair quite a number of guitar / PA Amps over time (and still do some) + have wound my own transformers and I have seen the text book confirmed quite a number of times, so I'm sure this applies to electric engines as well.

When applying power to a newly repaired Amp (or a checking a bad one), technicians normally use a Variac (variable transformer) to start at zero Volts and then slowly increase the voltage while checking the current consumption - this is a very good way to avoid catastrophes if you overlooked a serious fault. Doing the same to just an unloaded transformer and plotting the resulting current, you'll discover that a few volts over the designed max voltage, you'll get a sharp rise in the idle current - from almost nothing to destruction in a small distance.

I've also learned that you sometime design a new transformer, only to discover that you can't quite fit all the copperwindings in the bobbin and that the next size up can't fit the overall design .... what to do ?.... You decrease the number of turns on all the windings by IE.: 1-5% and try again. In most cases it works OK, but you will increase the idle current and loss in your transformer - I'm as stated sure that this applies to engines as well.

[Jo]

(A_dk sadly your country is one I have yet to visit  ...One day (on company expenses  ))

I don't have a variac.... But maybe you are right the replacement motor is younger than the original and hence it is requiring less starting/no load current.

I think I will start looking for a new 2HP motor for the Harrison and in the meantime I must learn not to say no to any opportunities that present themselves

I hope that this thread will be of use to others should they have the miss fortune of their own three phase converters contactors fail so that they many manage to avoid burning out their electric motor in the way this has, which could have been so so much more expensive than the fault itself if I had not already had a replacement

Total repair cost = £27.  It could have been cheaper if the contactor had been brought from that far eastern country ...

Jo

[Don1966]

Thanks Tony 

I was more intregued that the larger motor was in no load condition taking less amps. I also noted that the new 3HP Colchester motor needs a lower boost setting on the 3 phase converter than the old one used to .

Jo

Jo, does your name tag on the motor have a dual voltage rating? Your motor connect is a Y connection. For a six lead motor and dual voltage rating it should be Delta. I have also seen motors have what's called Y start, Delta run on very small motors. Which is not really required for fractional HP. This could explain the low run current at no load, in which case loss of HP.

Time to put the workshop back together. I still have not worked out why the 3HP Colchester takes less current than the 2HP Harrison under no load  .

Jo

A motor like that draws the most current when starting. As the motor speeds up it generates back EMF that reduces the apparent current required to run the motor. When the forces hit balance the motor stops accelerating and runs at a steady speed. When you put it under load you will see the current increase.

Tony

Sorry Tony no disrespect here but that best describes a DC motor not AC. An AC motor in simple terms works like a transformer. You have the primary, " the stator" , the secondary winding, "  the rotor. The difference being that the secondary winding on the motor is shorted. The rotation is caused by the induced currents opposing the current that produced them. Referred to as Lenz law.  So the rotor is trying to catch up with the rotating magnetic field but will never reach synchronous speed it will always be behind because of slip. When the rotor is locked it is just like taking the secondary of a transformer and shorting the secondary together it will pull a lot of current. This short is reflected back into the primary and will burn it.
When the motor is in a still position we have to over come the forces to get it turning and it will draw three to four times it's normal current to do this and reach full speed which is almost the speed of the rotating magnetic field. Slowing the speed of the motor with loads, the motor has to pull more current to produce more torque to return to it's fixed speed.

Regards Don

[Jo]

Don, She is a dual voltage rating so she could be wired in star or delta. I have left it as star as that was the way the original motor was wired and how the Colchester Manual shows.

I am very pleased that I have got everything up and running again    I just hate to think what that new Colchester motor would have cost  if I had not already had a spare (maybe I will give my ex one brownie point for acquiring it  ) .

Jo

[Stuart]

Jo

as I was for a year during my apprenticeship ( at the local iron works so it was heavy power not house bashing ) I did armature and stator winding

as to the off load running amps a lot of factors come into play
no. poles
star/ delta connection  to get the correct supply volts , it depends on how the motor was designed
the copper iron ratio
dose the motor have a fan and the other not
condition of the bearings

in all unless the difference is great do not worry there are to many variables

glad you have it sorted from my experience (limited ) with static converters is that they do not produce a output that is exactly balanced in both volts and phase difference

get it back together and enjoy your machines

Stuart

[Don1966]

Don, She is a dual voltage rating so she could be wired in star or delta. I have left it as star as that was the way the original motor was wired and how the Colchester Manual shows.

I am very pleased that I have got everything up and running again    I just hate to think what that new Colchester motor would have cost  if I had not already had a spare (maybe I will give my ex one brownie point for acquiring it  ) .

Jo

Jo, if your motor is dual voltage and you are running it on 240 volts you need to connect it Delta. Otherwise you are running at half voltage and low HP. This explains the low current draw you had. Under heavy loads it will draw heavy current even at half voltage trying to give you what you need. If you are happy the way it is and doesn't bog down with heavy cuts leave it as is by all means. But I would amp check it with heavy cuts just to see.The name plate should show two current ratings and it's current rating is now the higher voltage current rating which is half with this connection. Anything over this is overload
Jo I am just trying to help here and I do do this for a living.

Regards Don

[Jo]

Thanks Don , I am running it on 415V 3 phase. Nothing (even 5mm deep cuts in steel  ) has ever managed to stall the Colchester.

The Harrison current demands seems to have settled down so I am assuming the oil in her gearbox was cold. Both machines are now happily starting on a lower boost setting than before 

Jo

[Don1966]

Great Jo, I am glad to here that.


Don