hi

although i have never faced such kind of problems, but you first check the condition of pump i.e. impeler and then check the bearing of the motor. some time because of jaming of bearing it required some exteranal staring torque which is in the your case.

regards

divyesh patel

The assembled machine turns freely by hand.

Where is the rotor shorted? At the rings or where the field wiring leaves the generator? It sound like some kind high resistance problem in the rotor circuit, like an open in a coil or at the slip rings. I am assuming that this is a three phase machine, modified to run in an asynchronous mode. I know someone will correct me if I'm wrong, but torque increases as speed decreases in such machines unless something is inhibiting current flow in the shorted rotor.

If the rotor field wiring is shorted in the terminal box as compared to the slip rings, it may be possible to measure current and observe what happens when this problem occurs. A low reading that increases abruptly upon manually spinning (to start as you described) would indicate that there is some kind of partial or intermittent open circuit problem.

Thanks for your repsonse

the rotor is shorted at the rings. You are correct in assuming this is a 3 phase motor. Yes the torque increases as the speed decreases.

A wound rotor motor normally uses resistors in series with the rotor windings to acheive incredible amounts of starting torque and also variable speed control. When full spped is reached the windings are essentially shorted at the drum controller. we were using it for speed control. Variable speed control was no longer needed in this application so we had the rings shorted to operate at full speed (1775 RPM) at all times.

I tend to agree with you, there is an open or shorted coil in the windings. I have a few other motors that have been shorted and have been running for years without incident. I will test rotor influence on at least on of them tomorrow and post the findings.

I'm going to have to slightly disagree with your assumptions regarding starting torque versus secondary resistance. The main use of slip ring wound rotor motors is to be able to insert resistance into the rotor circuit to both limit the starting current and starting torque. The wound rotor motor is a good choice for high inertia loads such as conveyors and large pumps. In the case of conveyors it is possible to increase starting torque above design figures to pull a bogged conveyor away by temporarily shorting some of the resistance (push up stage 1 or 2 shorting contactor). The risk is damage to the conveyor belt by ripping a splice apart. Yes I have seem it happen.

Now to your fault. There can be a couple of possibilities. One winding can be open circuit or there can be shorted turns in one or more windings. To determine which use a welder to inject current into the rings in turn (A to B, B to C, C to A). Measure both the current and voltage at each injection and compare. If the culprit is an open circuit either AC or DC will work OK, DC will show Ohmic resistance (in milli or micro ohms) and AC will show inpedance at the applied frequency (expect a much higher figure). For shorted turns AC is likely to be the better option as some turn to turn voltage is likely to be needed to initiate the turn to turn short.

The other cause can be single phasing on the stator windings. Spinning the rotor in one direction or the other will allow it to start with torque increasing as speed builds. Be very careful, as the motor can easily be started in the wrong direction, if this is the fault. The damage to the pump can be catastrophic if the impellor is threaded to the shaft.

Whatever the case you have a fault, it is not inherent to the motor design. Personally I would never recommend DOL starting of such a large pump and would reinstate the secondary resistor starting system. Alternatively VVVF drives in your voltage range are now available, whether you could get funding is of course another matter, given the type of "short cut" already taken.

I cannot test the windings as you described due to the windings being shorted at the rings on the shaft without disassembling the motor. As of now, disassembly is not an option

The stator windings are not single phasing. I confirmed that the voltage across the phases are balanced, when the motor starts the current is balanced and running below FLA on nameplate. The stator windings test good. The motor operated perfectly while the resistor banks were still in service.

Unfortunately I cannot go back to the resistor banks. The unit had been in service since 1955 and parts were getting hard to come by. New switch gear has been installed and engineering overlooked the motor. The project is over budget and the retrofit was supposed to be a quick fix.

I am thinking if there was a bad coil or winding short in the rotor before being converted the motor would have had trouble starting then. or it would have shown up in vibration analysis.

i had to sign in on order to attach documents.

the upper graph is the rotor influence test that was performed on the motor after the rings were shorted. This is the first time i have seen the 3 phase sine wave without some type of seperation. The anamoly between positions 11 and 14 looks like a bad coil. i dont think this would keep the motor from starting.

the lower chart is what a typical ac induction motor rotor test looks like.

I'm still leaning toward an open circuit problem in the rotor circuit. If one of the coils had a short, a magnetic field would still be present in the other coils and I believe the rotating magnetic field of the stator would still pick it up and it would still start. However, if the coil had a high resistance connection, or an intermittent open, or combination of these, it might explain why the unit did not start on it's own occasionally. However, it does not explain why just manually bumping it would be enough to start it unless the shorting done to the rotor was done on the ring brushes instead of the ring itself. In such a case the brushes would be the problem.

I can't blow the charts up enough to read the detail. If this is a chart of the stator currents it is similar to a classic single phasing waveform. I have also seen one like this before where one supply phase was open circuit from its normal supply (say C phase) but supplied by another phase (say B phase). It was a real trap as current was flowing in B and C unlike a normal open circuit induced single phasing. From memory there were 2 faults one being a phase to phase cable short and the other an open at the contactor.

Another thing that gives a funny reading is when start and finish for one winding is transposed currents can look almost normal but torque is affected and it can be difficult to get the motor to run in the correct direction. This sometimes happens during attempted modifications (6 wire terminal block).

Are you 100% sure the windings were properly shorted when the mods were made, experience tells me the more poorly thought out the job (no money to do it in the first place) the greater the odds it is a stuff up.

the chart is built from inductance readings i obtained measuring the stator windings phase to phase. rotated the shaft a few degrees and took another measurement, This was repeated until the shaft made one full revolution. I typically use this test to look for casting voids or broken rotor bars in squirrel cage motors that are not operating as they should. no current or voltage was introduced to the rotor windings. I cannot seperate the rotor windings to get an accurate test due to the way the rings have been shorted without disassembling the motor.