Iacdrive|Automation Solution

Motor line starting and ramp starting with VFD

Variable frequency drives are important power electronic devices. When we start an electric motor, we are increasing from 0 speed to full operating speed. A VFD ensures that the motor accelerates (increases its speed) to its full speed in a smooth manner, without causing much irregularities. In other words, VFDs make the motor accelerate uniformly.
VFDs are also easy to install and use. VFD drives are not only for starting motors (like the normal starters), but for easy speed control as well.

The difference between line starting a motor and ramp starting the motor with a variable frequency drive is that the motor/load does not pull the 6-7 times rated current of the motor, because the motor winding are not saturated with the full EMF produced to get the motor to synchronous speed it is ramped to it. If you are not trying to control the motors speed from process control then a soft start will serve the same purpose. The VFD drive main purpose is to control the V/F of the motor.

You will have to adjust the ramp time on the VFD or soft starter to over the force required to turn whatever the motor is turning, this can be accomplished with both devices. Soft starter is less expensive than variable frequency drive, thus it has limitations.

“Hissing sound” in SF 6 Gas insulated HV Switch Gear

This could be internal corona discharge. The switchgear should be de-energized and closely examined. That means pump out the SF6 and take it apart. Examine all insulating components.

Especially if the sound can be localized to portions of the switchgear which do not have bushings for connection to overhead lines. Even if the sound is in the area of air bushings, deenergizing will allow more in-depth inspection and addressing any sharp edges or cracked insulators, etc.

Take this pieces of equipment out of service immediately, perform a “hi-pot” or high potential test on the various elements of the switchgear and attempt to locate the area that is “leaking” to ground (or between phases). Inspect closely for indications of tracking on insulators from corona discharge and replace any compromised components. After component replacement, installation of new SF6 gas and other repairs, re-run the Hi-Pot test to confirm that the switchgear is able to withstand voltages at least 50% greater than the name-plate rating. Of course all of this advice is worthless if the unit has already failed.

Remember that a Hi-pot is actually a destructive test. It challenges the insulation to the point of breakdown. Check the vendor recommendations before you Hi-pot equipment that has been in-service.

Can a VFD reduces motor starting kick?

At zero speed the motor requires torque which is flux (voltage) and current (mostly reactive). Only a little bit of active current to compensate for the motor power losses.
Only the power losses need to be drawn from the grid at that time, which means a very small amount of current. It may produce 200% current on the motor and pull only 10% current from the grid.

Of course, as the motor is accelerating, the motor will require kW and the current pulled from the grid will increase accordingly, as the active power consumed by the motor is increasing.

Regarding the kick of torque on the motor, it is controlled by the maximum current ramp limit or through the speed reference as the ramp rate defines the current and the derivative of that rate is the current rate. For this reason, many large machines will be started using an S-Curve speed reference where the S part will adjust the torque (current) rate to avoid stressing the mechanical components, especially if there is mechanical backlash in the gears.

Actually the starting method depends on the type of motor itself SR or SQ type the voltage supply, the motor capacity and motor function, for the MV Motor a liquid or oil starter was the best solution used before.

In case the operation process required a change in the equipment speed the variable frequency drive (Air or water-cooling) based on the drive capacity is the optimum and reliable solution.

Definitely it reduces starting kick of the motor. Actually, the degree of starting kick of a motor is depending upon the starting speed of the motor. If you start your motor at low speed you will have a low starting kick but if started at high speed, you will have high starting kick. This is generally the condition for low and high kw motors. One factors of varying the speed of motors is by varying the frequency of the motors (from the formula N=120f/p) and VFD drive is use to vary the frequency, thus varying the speed of the motors. But if used for starting only, this is expensive as there is more cheaper way like using the Soft Starter or use a WRIM/Slip-ring motors with LRH/Resistor Starters or other. Normally, variable frequency drive is use on operation with speed reduction/varying requirements at required number of time or continuously.

Motor power cable – bigger or smaller?

When a choosing a power cable for a motor, we prefer using one larger diameter cable than two smaller diameter cables in parallel, although it would cost less to do so. Why?

1. Conductors/Cables/Feeders in parallel connection generally are not recommended unless there is no option, therefore it can be adopted under the following conditions:
i. Cables are of the same material and cross section area.
ii. Are of the same route and length.
iii. The sum of the current carring capacity of the parallel circuits after applying all necessary applicable correction factors should be greater than the nominal regulated current of the protective device.
iv. The current carrying capacity (before derating) shall be not less than 300A (according to the local authority/Service provider requirement/regulation).
v. Capability of addressing the Thermal & electrodynamics constraints in proper way.

2. Some designs call for parallel connection so as to:
i. Overcome the voltage drop.
ii. Avoid the difficulties of installing big size cables (bending, pulling) due corridor limitation,etc.
iii. Meet the Power demand.
iv. Mitigate the cost (Costwise).

3. For electrical Motors, two connections are normally required. One from MDB to Motor CP and other from CP to the Motor.
By virtue of the requirement of Delta/star starter, two cables are required (Mandatory) between CP & motor (one will be dead just after changing to delta connection).
While the connection from the MDB to CP will be one, sized according to the Motor rating.

However, Parallel connection of Feeders need an expert engineer(s) to meet the requirement since Short Circuit fault protection for parallel circuits require further evaluation from the Engineer that the impact of the short circuit current within the parallel section will have severe fault due to fault current path that can occur in addition subtransient contribution of the downstream system.

Power Transformer power losses

Power losses of ferromagnetic core depend from voltage and frequency. In case where is no-load secondary winding, power transformer has a power losses in primary winding (active and reactive power losses) which are very small, due to low current of primary winding (less than 1% of rated current) and power losses of ferromagnetic core (active and reactive power losses) which are the highest in case of rated voltage between ends of primary winding…

Of course, we can give voltage between the ends of primary winding of power transformer (voltage who is higher from rated voltage), but we need include some limits before that:

1. if we increase voltage in the primary winding of power transformer (voltage who is higher from rated voltage), we need to set down frequency, otherwise ferromagnetic core of power transformer will come in area of saturation, where are losses to high, which has a consequence warming of ferromagnetic core of power transformer and finally, has a consequence own damage,

2. if we increase voltage in the primary winding of power transformer (voltage who is higher from rated voltage), also intensity of magnetic field and magnetic induction will rise until “knee point voltage”: after that point, we can’t anymore increase magnetic induction, because ferromagnetic core is in area of saturation…

In that case, current of primary winding of power transformer is just limited by impedance of primary winding… By other side, in aspect of magnetising current, active component of this current is limited by resistance of ferromagnetic core, while is reactive component of this current limited by reactance of ferromagnetic core.

There is a finite amount of energy or power that can be handled by various ferromagnetic materials used for core material. Current increases greatly with relatively small voltage increases when you are over the knee of the magnetization curve characterized by the hysteresis loop. Nickel/steel mix materials saturate at lower flux densities than silicon steel materials. 50ni/50fe materials saturate at about 12kG; 80Ni/20Fe as low as 6kG. Vanadium Permendur material saturates at levels as high as 22kGauss- Nano-crystallines- 12.5kG (type), Ferrites -typically over 4kG at room, decreasing as temperature rises. What causes saturation?: Exceeding material limits.

Overcurrent protection of generators

Overcurrent protection uses as back-up protection for protection generators from faults between two windings of stator (two phases of stator). Setting of overcurrent protection depends from two settings: current setting of relay protection and time setting of relay protection.

Current setting of relay protection represents minimal value of current under which relay protection will send signal to breaker to act and this value is higher from value of rated current in generator (higher from maximum allowed value of current in generator).

Time setting of relay protection represents time after that relay need to send signal to breaker to break fault. Of course, when we talk about time setting of relay protection, we need to have on mind time delay. Time delay represents time during other protections need to act before overcurrent protection acts in case where is overcurrent back-up protection for protection of generator.

Then there is voltage restrained time overcurrent protection (ANSI 51V) which is commonly applied on generators. The pickup setting of these relays reduces (becomes more sensitive) when the applied voltage reduces. It is supposed to aid in sensing faults that are electrically close to the generator terminals as there is insufficient fault impedance to maintain the voltage at the generator. It is especially useful in tripping out faults that have persisted long enough for the generator fault decrement curve to get to the portion where the synchronous reactance is the characteristic impedance. When this happens the fault current will be at the same levels as normal load currents and increased sensitivity is needed.

Transformer Saturation

Bushing insulation testing

In bushing insulation test there are three major current elements which any of those could affect the test result. These current elements are Capacitive current

Motor connection

Many years ago I had an experience of 4nos 37kW fin-fan motors wrongly connected at site to a star. After running for almost 1 year, the operators reported these motors were very warm and felt unusual. We removed one of them to the workshop and opened for inspection. All windings were OK but the rotor lamination surface had turned to light blue colour which showed a sign of abnormal heating.

I asked different experts in the industries for advices. From the advices, we suspected the motor could be designed for a delta connection even though the nameplate indicated a Star connection for 415V. We contacted the motor manufacturer by quoting the motor serial no. The manufacturer confirmed that the motors were designed for delta connection at 415V. The manufacturer apologized for the error in nameplate and gave us a free spare motor.

One clear sign that could lead us to believe that the motor was in a wrong star connection instead of delta was, for a 2 or 4-pole motor the no load running current should be more or less around 30% of FLC. When we tested run the motor in the workshop, the no load current was less than 15%xFLC.

After the rectification of all the 4 motors to delta connection, we had no complaint anymore. It was a good lesson out of this solved problem.

Torque ripple information from low resolution speed signal

Q:
I am trying to develop a controller for switched reluctance motor which minimizes torque ripple. My design is acquiring torque ripple information from speed signal. In simulation a high pass filter for speed gives me good ripple information. But in experiments I am using a 500 PPR optical absolute encoder to get the position and then calculate the speed using microcontroller (dspace) capture module. But the filtered speed signal does not provide much ripple information. Can you suggest any method to extract ripple information from low resolution speed signal.

A:
1. In simulation, do you consider motor inertia? Inertia filters out torque ripple’s impact on speed, resulting in a smooth speed signal. 2. Generally speaking, a low resolution position sensor produces speed signal of more noise, especially at low speed. I would expect more noise out of your high pass filter.

An encoder generally does not specify an accuracy for the A to A! channel or B to B! channel or it is so broad a spec that it is useless. If you have the ability to trigger a clock on A and B to determine the period between A and B channels the difference between successive reads will give you a good indication of your ripple.

In some cases of motor – encoder installations the mechanical alignment of the encoder to the exact center of motor shaft can cause misalignment noise to occur in the resolved speed signal. In theory the ripple signal could provide useful information however in practice there are too many other influences. Even the shaftless encoder mounting has some of these difficulties.