Troubleshooting

Q: We have a 2250HP Synchronous motor used for compressor application. It was observed that IR value in on phase was very low0.007 while the rest of the phases were about 50 Mohms. We cleaned the stator with solution, heated the windings to approx. 60deg. But even then faulty phase did not show an improvement in IR. Although rewinding seems to be an option, what needs to be done to pin point the fault?

A: When working with motors or xmfrs, especially MV, you need a megger/IR tester ...

Shaft voltage is directly attributable to two different things: dissymmetry in the magnetic path, and generation of a static charge.

Internal to the machine, the construction of the magnetic circuit (laminations and frame) and the layout of the conductors (winding) contribute to a potential dissymmetry in the flux path. After all, most coils are hand-made - which leads to marginal, but noticeable differences.

External to the machine is the source waveform. If it is disto ...

With larger motors it is normal to insulate the NDE bearing. The normal reason is that the majority of movement will come from the DE. This is done by machining out the end shield to take a sleeve into which your bearing will sit. Material selection is vital. Please be aware that this insulating requirement would include any lubrication, vibration, temperature monitoring equipment and the like. If your motor is very big the insulation material sits between the base and the pedestal but again ...

Variable frequency drives should be protected by fuses. Why? Because the VFD itself has all the protection you require for your motor so the only item you are protecting is the Rectifier and DC bus. Then you will find it is actually really impossible to protect it as when it has a fault it is generally catastrophic and all you wish to do is to remove the faulty VFD from the network to enable the rest to continue. This must be as fast as possible to limit the damage and MCBs don’t really ...

Operating on a VFD means there is a LOT of high-frequency distortion of the waveform, leading to imbalance in the magnetic circuit - which in turn leads to the development of shaft potential. If the shaft is higher potential than the ground plane, then current will flow - directly across the bearing, usually. So get a good grounding method (actual brushes riding on the shaft are by far the best over the long haul, in my experience). How do you know it's happening? The failure mode is scoring, ...

Case: A fan motor of 315kw running on VFD. Its operating speed is 795 rpm but its winding temperature gets heated like R phase temperature is 126.3 Y phase temperature is 125 and B phase temperature is 120. The motor is surrounded by cooling fans still temperature gets raised. Is there any alternative or have to change the motor to 400kw?

Heating effect can be caused by improper load match of the motor and mechanical VFD. Look at the temperature rise of the motor on the na ...

Every motor is to be protected with back up MCB / fuse and overload and single phasing protection. The motor is to be earthed properly. Once protective relays are connected in the system the system get tripped when the live terminal of any phase get earthed. Once tripping happens proper Megger check to be carried out for cable and Motor, to find the cause of tripping. The tripping can be due to Earth fault.

First, the motor body should have a "hard" connection to ground to keep the ...

Q: I have a conveyor application. There we have a Motor connected to Gear Box. Motor is driven by a variable frequency drive (VFD). Details are as follows
Motor Power - 2.2 kW, 415V, 50 Hz, 1460RPM
VFD - 2.2 kW
Gear Box - Ratio = 58.25, RPM - 25 Rev/Min ,

According to the Above details, gear box output is 25 Rev/Min When the Motor Runs at 50 Hz at 1460 RPM. But I need to increase the Gear box output (Conveyor Speed) from 25 RPM to 53 RPM. So, as a t ...

What is the only case that makes the single line-to-ground fault bigger than the 3-phase short circuit fault?

The L-G fault current will exceed the 3Ø fault current whenever you are near the terminal of a D-Y transformer. The Delta winding effectively blocks the zero sequence impedance contributed by the source. Thus Z0 source is zero and the only Z0 contribution to the impedance is the transformer impedance. As you move out from the transformer, the zero component should qu ...

While reading a generator AVR operation description: "When a transformer is used to boost the generator voltage, the transformer impedance provides the DROOP characteristic (voltage drop) and so compensates for any reactive cross current flow. In these circumstances, the AVR does not provide a DROOP characteristic".

AVR of generator has a circuitry which senses reactive component & hence called quadrature droop compensation. When we operate multiple generators in parallel exci ...

A very common problem affecting submersible motors is over-temperature. Causes for over-temperature include pumping hot water, overloading of the motor by the pump, loss of cooling flow past the motor, ochre or scale buildup and frequent motor starts and stops.

Submersible motors somehow must cool themselves. This is accomplished almost universally by transferring the motor's internally generated heat to the water that is flowing past the motor and into the pump. Most standard wate ...

Case: I am restoring a 1960s analog audio tape recorder for high quality audio use, the tape recorder is a USA model operating on 120VAC at 60Hz, the mains voltage in Singapore where the machine will be operated is 230VAC but 50Hz. Other converters that convert 230VAC/50Hz to 120VAC/60Hz have been unsuitable -- I hear high distortion and square wave noise to break through in to the audio in ...

Q: I have a situation where our UPS servicing data center rack systems are failing repetitively after replacements.

We know our mains transformer is undersized in terms of load capacity and under voltages are measured going to the input of the UPS that is providing back up power to data center in a rack mount. However these voltages measured were 110VAC and still within operating range. In another facility with the same rack systems, colleagues are saying they're seeing "brown out ...

Q: We have 02 motor generator sets each of same KW rating and speed etc running in parallel and having pure inductive load. Thrice it happened that 01 set got tripped on reverse power protection. 01 set operates with 0.65PF lagging while other runs with <0.5PF leading. After tripping of MG set, remaining set's current decreases from 42A to 35A. If we want to synch again two MG sets while plant is running at 100% power, is there any danger of tripping?(if remaining set got tripped, plant wi ...

I have a 7KVA isolation transformer and having issues where it's tripping the circuit breakers. I have two circuit breakers, one main and one acting as a cutoff switch both installed on the primary side. Transformer manufacturer specification says 235A of inrush current with 7KVA purely resistive load. I have two questions:

1. I am fearful to load the transformer with equipment, so will the load on the second ...

Low Voltage (400V-440V) Earthing
This discussion applies to the low voltage side of a (6.5/.400 or 11/.400 or 33/.400 kV transformers). Since your neutral is solidly grounded, the earth fault level will be sufficiently high, to trip the over current (50/51) elements. You will not need any earth fault protection. If you are running long low (really long) voltage cables you may need to provide earth fault protection.

We never design low voltage systems where the ...

I have an interesting dilemma at work. We have never had the ability to monitor each individual feeder emanating from the substations. I recently installed a modern recloser on one circuit which allows detailed measurement of line characteristics and discovered that we have 1) a serious imbalance among the phases, and 2) a terrible power factor on this feeder - .33 on one phase, and .65 or worse on the other two. The meter that monitors the overall bus reads a combined power factor for the tr ...

We know the equation kva= kw/power factor. When power factor reduces, kva increases. The substation authorities have to pay a penalty if demand indicator (kva) increases over an integrating period. How the maximum demand (kva) is being calculated in substations. Are there any ways to avoid load shedding of critical loads in substations if the maximum demand indicator goes over a maximum value when power factor is reduced?

In a Tra ...

Situation
I have recently encountered a fairly unusual installation where a YNyn0 transformer has been used to supply a low voltage pump installation incorporating a large VFD.
The transformer was initially chosen to offset harmonic distortion with an adjacent Dyn11 supplying an identical VFD.
The transformer has been installed with the primary neutral connection floating in an effort to prevent zero sequence currents from causing nuisance tripping at the su ...

Situation: Here is my current problem and was wondering if anyone had experience of the following:
Application - 260Kw submersible motors/pumps must be running up to full speed within 5-6 seconds of startup.

My proposal had always been to use a soft start until the pump manufacturer has stated that in order to provide sufficient cooling to the stator windings (using the sea water), it must be at full speed in 5-6 seconds.

I have been in touch with ...