Iacdrive|Automation Solution

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.

Variable frequency drive key functions

Soft Starter, Auto Transformer, Electrolyte, series resistance – wound rotor- etc,). The starting factor of VFD drive is usually 1 up to 1.2 with respect to the rated load current while for Direct On line about 5-6.

Moreover and as you know the variable frequency drive can control the speed of the AC motors in accordance to the formula N=120f/P rpm
where f = the supply frequency and P = number of the Poles.
According to this formula, Motor Speed can be changed either by changing/control the frequency or by changing the number of Poles of the Motor by which step changed in the RPM will be given, while the former gives continuous variable speed as per application demand.

However, as per newly developed power Semi Conductor IGCT based on PWM VFD became the most smart, effective and efficient control device in Industries since is associated also with protective and monitoring means.

From my experience, I know that variable frequency drive plays around with the frequency which the motor operates. It starts at low speed and varies the frequency to attain maximum speed. This reduces the high starting torque usually experienced when motors are started on DOL, Star/Delta etc. When you are driving delicate materials through your conveyors or pumping liquid through pipes etc., VFD plays a useful role. It reduces hammering in pipes usually experienced when using DOL. In large hotel application, variable frequency drive could be used with pressure switches to regulate water flow and reduce hammering when guests are showing. The volume of water required will determine the speed at which the motor runs through VFD control. However, very large KW motors at high voltage level are usually started DOL due to the cost of ac drive but that is when one has enough (power) capacity otherwise it will impact on other users in the network.

Transformer tap changer

Q:
We are frequently changing tap position of Unit station transformer due to voltage problem. What are the impacts on transformer life and is there any solution to minimize this?

A:
Having more tap changing per week is not bad, but it wears out the tap changer faster and does require more maintenance. We set our bandwidth at 1.5 volts, 0.75 up and 0.75 down, with a minimum timer of 30 seconds (voltage has to be out of bandwidth for more than 30 seconds for tap changer to move). Voltage for the OLTC controller is based on a 120V base. This normally worked well for our city loads, but perhaps your loads vary even more. I have used a bandwidth of 2 volts maximum with good success to keep the OLTC from tapping more than I liked (250 taps per week, and naturally if your loads swing more than what we had then your taps per week are going to be higher). The 250 count per week maximum is just a goal we set to try and maximize the life of our tap changers and minimize our maintenance. Looking at your timer and bandwidth may help reduce the taps per week. When the tap count per week jumps up suddenly you can suspect the controller might be bad. One more thing, I never use the X setting, just the R. I would draw the voltage “curve” versus the current and figure out my maximum voltage based on the maximum current. This worked well for me for my 23 years of utility work (again, these are city loads, base power factor during the summer was 85%). The power factor would be higher in the winter and lower in the summer (summer at 85% and winter was over 95% because in the winter we had no air conditioning loads). That is why I did not use the X setting (one setting year round).

Since it appears that you are talking about OLTC, then 250 taps per week is the maximum level that is reasonable in my opinion for a transformer serving varying loads, such as a city. I worked for electric utilities in the US for 23 years and looked at load tap changing counts every week for over 450 MW of transformers (15 MVA to 46 MVA all serving city loads). This count is the top end we would allow. The average count was in the 125-150 range per week (summer loads, with wide varying loads each day, winter loads caused less tapping per week). Oil does not degrade rapidly in the OLTC (that is operating properly) even with a maximum of 250 counts per week, but we would take oil samples every year of the OLTC and the transformer to keep tabs on their overall health. If the oil in the OLTC does degrade rapidly, then there is a good chance that the alignment of the taps is improper and arcing may be occurring during the tap changing.

OLTC has little or no effect on the life of the transformer. Also, there are two separate oil compartments, one for the OLTC and one for the transformer.

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.

Add filters to frequency inverter to eliminate harmful

The high frequency edges of switched waveforms can cause capacitively coupled currents to flow from windings to frame, returning through the bearings, and these can accelerate corrosion in the bearings, causing early failure. Small filters on the motor leads allowing these currents to return locally to ground will avoid this.

The best way, though, is to use filters which can eliminate sharp transitions and leave only (like +/-10% ripple) fundamental frequency (motor’s RPM at given point) of the motor drives. However if somebody can handle 40 – 50kHz of the switching frequency the filter’s size shrinks dramatically and it is not too expensive anymore. Again, the problem is in ability to handle 100 (or so) kWs and 50kHz together.

Why BLDC motors are noiseless compare to Induction motor?

If referring to the acoustic noise generated at or around the PWM frequency of the PWM frequency. There are more laminations in an induction machine. This may account for some of the difference.

I also don’t know what the relative power difference is between the BLDC and the induction machine. If it’s about a 5Hp BLDC and a 100Hp induction machine, then you can bet that the PWM frequency of the BLDC is likely above the audible range and the PWM frequency of the induction inverter is well within the audible range.

These are just a few reasons that you may find subtle differences between the two. There are many factors and more information is needed to really help understand your specific situation.
I also believe there are simply sophomoric and unprofessional answers. My statement is based on the general rule that there is greater surface area between laminations of squirrel cage induction machine then there are in BLDC machines. Of course, if you want to state that you have a thin lamination on a long stack length design for a BLDC then there may be an argument that such a motor design when compared to a typical induction machine of the same power has a similar surface-to-surface lamination area. It is these laminations moving due to eddy currents at the PWM frequency that causes the audible noise.

BLDC can come with very small inductance which requires a higher PWM frequency, if you compare both them with controller that may cause different.
If you build 2 motor using exact mechanical shapes and electrical parameter they should be very close. You can build 2 induction machines from 2 different vendors to same electrical spec and they will not sound the same.