Problems with starting a 415v saw. Fires up in star, cuts out in delta

[speedscreed]

Bottom switch on off, top star delta. The motor spins freely when not connected via the drive belts, so too does the blade

 

[snowhead]

Has it previously worked normally?

Can you post a picture of the end showing the switches?

Normally there's a timer which automatically changes from start to delta, are there any instructions as to how long to leave it running before switching over?

When it cuts out what do you do to get it running again?

 

[marconi]

You might want to disconnect completely the motor during your starter investigation because at this stage it is not required in circuit and will be safer (and quieter). Always pause a few minutes before re-applying power or starting machinery to work through in your mind if you and others are safe and the machinery will not be damaged.

 

[speedscreed]

Has it previously worked normally?

Can you post a picture of the end showing the switches?

Normally there's a timer which automatically changes from start to delta, are there any instructions as to how long to leave it running before switching over?

When it cuts out what do you do to get it running again?

Yes. The machine has only been used once since new, albeit about two years old. The first and last time it was used was about a year ago after which it was turned off and parked up. The star/delta is a mechanical switch which the operator operates, the top switch in the box, as shown. I left it running for two seconds in delta, and started to smell something, so quickly onto delta. We disconnected the drive belts and the motor on its own spins ok. The point being that the blade is quite free running on its axis. A few times we have spun the blade by hand before switching into star, and even though the starting torque required is obviously considerably less, still the saw wont run. It tries for a split second and then trips out. I have tried adjusting the current reading on the relay, but not really sure what that does? I should add that my days as an electrical engineer in telephone exchanges finished 50 years ago, so am a bit rusty on all this.

 

[speedscreed]

You might want to disconnect completely the motor during your starter investigation because at this stage it is not required in circuit and will be safer (and quieter). Always pause a few minutes before re-applying power or starting machinery to work through in your mind if you and others are safe and the machinery will not be damaged.

I wouldn't know exactly how to test the star/delta transformation with my meter, to see if its effective and working. Since it is a mechanical change-over switch, and only been used once, I would assume this is ok. This is a wet-cutting saw, with an electric pump re-cirulating water onto the blade, from the water reservoir beneath the blade. I will take a small video, and try and post this. One thing for sure nothing is seized up, or difficult to spin.

 

[marconi]

If you have not done it - so apologies for teaching you to suck eggs if you have - please confirm that all three lines are present at the input to the isolator switch. Also check the lines are balanced by measuring the line voltages L1-L2, L2-L3 and L1-L3.

 

[Lucien Nunes]

Obviously without readings we are guessing. What trips? The overload relay?

The symptom 'runs OK in star but trips in delta' is very often a sign of incorrect connections between the starter and the motor. If one line lead of each of two coils are interchanged, the coil phasing is correct in star but two coils are in parallel and one short-circuited in delta. However, you say it worked OK before, so we should assume the wiring is correct (unless you think it has been interfered with?)

Another possibility is that one line connection is not closing in delta. This leaves the motor 'single-phasing' where it runs as a single-phase motor between two lines, while regenerating the 3rd. Obviously with a heavily loaded motor this creates serious overcurrent on the two connected lines and soon trips the O/L, but especially with a lightly loaded or idle motor it's unlikely to cause anything to burn up

The overload relay should protect the motor against damage by supply faults and mechanical overload, and generally against overheating. It's just like a very slow-acting thermal circuit breaker that mimics the thermal response of the motor and opens the control circuit if the main circuit is overloaded. If it is correctly set it should not allow the motor to burn out under any conditions, except where the winding is already damaged and has just a few shorted turns which can cause overheating without electrical overload.

The correct setting for the overload relay depends on whether it is in series with the supply or the motor windings. If the supply, it should be set to the motor full-load current, and it will protect in delta although not in star. Therefore most star-delta starters place it in series with one end of the motor winding, in which case it should be set to 0.58 x FLC and will protect the winding in both star and delta.

Anyway, diagnostics.

Yes, as Marconi says, check that all three lines are present. A motor usually won't start at all if one is missing, because there is no rotating field (it can't see neutral, therefore one line missing turns 3-phase into single-phase between two lines.) But check it anyway! Then, do you have or can you borrow a clamp meter with which to check the line currents? The most useful reading you can take from a malfunctioning 3-phase motor is the three line currents under the same conditions. I appreciate this can be hard if it immediately trips; do not jeopardise the motor by trying to get a reading.

If you can't measure the line currents, then with all switches off and the power disconnected, can you get resistance readings of the three motor windings? If the switches are indeed open, between all permutations of pairs of motor wires you should get exactly three readings, which should be similar and a few ohms.

If you need guidance to identify the connections, post good close-up pics of the control box. Or take the readings inside the motor terminal box if you can access it.

 

[snowhead]

Manual changeover Star to Delta seems to be a thing with masonary saws (may not be same manufacturer)