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COGGING torque is defined as the attraction/interaction of the magnetic poles (typically on the rotor) to the steel teeth (typically on the stator lamination) within an un-energized motor.

STALL torque is the torque produced by a mechanical device whose output rotational speed is zero. It may also mean the torque load that causes the output rotational speed of a device to become zero, i.e., to cause stalling.

Basically - cogging means the motor cannot begin rotating, whi ...

In general, if you are using variable frequency drives (VFD) where the control mode can be programmed (as most of the modern VFDs), each VFD has its own control philosophy and some parameters to adjust how efficient they follow the specified control mode: Speed control mode (as follower to a speed command); or Torque control mode follower (as follower of a torque command). Basically the parameters to adjust in the VFDs are related to gains in their internal control loops.

I worked ...

Question: I have a motor of 100HP with FLC of 123A. It's load is Centrifugal Fan. We are currently using a Star Delta Starter. It's current in star mode various from 276A to 220A for 35secs. Is it safe? While changeover in delta it takes a peak of 376A and then comes normal 60A phase current. In this operation we are using a microprocessor based relay which is bypassed initially for 25 seconds. I want to know how safe is this operation and life of motor in long run. Or should ...

So called "sensorless control" of AC machines means having no mechanical (speed and/or position) sensors. However, usually this term only includes vector controlled drives, i.e. it does not comprise V/f open-loop control of Induction Motors (which is the simplest technique for driving an IM motor without a mechanical sensor).

For the induction mold, sensorless vector control is a mature technology, it has been around for a few decades. Almost all reputable drive manufacturers have ...

The rule of thumb is less than the rated voltage or the maximum capacity of the test equipment.

There are several test techniques available in literature and standards (IEEE400). All these tests have advantages and disadvantages. Sometimes the client or the utility may have specific test requirements. However selection of tests must generally be done with the following requirements in mind:

1. If it's a new installation, is it safe t ...

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 secon ...

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 ...

Higher output power of generator (hundreds of MWs) has bigger impact on the system frequencies than low power generators, because of its massive spin momentum (torque). The active (R) load acts directly on generator speed because it creates strong magnetic field in stator that is perpendicular rotor's spinning magnetic field thus forcing an opposite momentum(torque) which slows down the turbine, this is fixed with a speed governor which controls turbine speed by manipulating its spin momentum ...

The beauty of variable frequency drive (VFD) is that the maintenance is negligible if are well maintained.
Yearly maintenance:
Tighten all nuts and bolts, all terminal blocks, cable/wire connections.
Remove dust everywhere (vacuum cleaner is good). Do not try any kind of solvent.
Check all power supply for correct voltage level.
Replace / clean filters.
GoHz maintenance plan do specify which component to change and when (for example blowers every ...

Yes you can do it but need to exercise some precaution: The LV winding that was intended by design to be the secondary winding, will serves as the primary & the value of the magnetizing inrush current actually will be greater than expected. When a transformer is reverse fed, the taps move to the output side and so their operation is reversed. Taps will control the output voltage so chances of over excitation will be there. This is not serious concern till the input voltage variation is wi ...

Most UPS's are sized for the client load requirements. However in cases of smaller UPS systems won't comply with electrical regulations!
Why?
Let's consider a client needs a 30kW supply, to 2 100amp distribution boards. One 20 meters from the UPS and one 80 meters away. Each DB has some 32amp circuits up to 20m long. The distribution circuit is protected by 63amp BS60947 devices and the final circuits are 32amp BS60898. Specifically for a UPS system, what is going to define your UPS ...

There are 2 different scenarios that occur. One is a circulating current that can occur, mostly through the fact that the rotor is not perfectly formed (you can think of it as not being perfectly round) which creates a circulating current. Insulating one bearing will stop this circulation. The second scenario occurs when a motor is on a VFD (variable frequency drive) and especially a PWM VFD. This produces a charge on the rotor which wants to discharge, and the easiest path is almost always t ...

First, there is the (NO) maintenance approach. This is essentially "run until done" style. Kind of like never changing the oil in the car engine or replacing tires and expecting infinite life. If lucky, the damage that occurs is mild and repairable (at relatively high cost) - if unlucky, complete replacement of machinery required (EXTREMELY expensive proposition, since it also has to include potentially lengthy delay in acquiring new equipment and unanticipated loss-of-production)

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In a position feedback PID controller the integrator component is the sum of all the position error. You could call it the hysteresis, or history of error. Regardless, it is like a running average error and tells your control system how consistently off the mark it is in general. Typically the integrator gain is small to keep a stable system, so that you don't have a strong P or D response to the I induced response itself (the gains reacting to each other's reactions is a source of instabilit ...

Question: How can I control through the VFD Torque for a motor as I have a pump motor of 110 KW of rated current 200 A controlled by Soft Starter and during the actual operation it consumed only 65 KW with 98 A, so I want to change this motor with another one lower in KW with VFD to control at the new motor Torque, so could you help me how can I do that?

Answer: If the new motor is large enough to provide the load requirements then reducing the motor ...

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 ...

Neutral is the normal return path for current coming out of the live terminal and through the load. Ground is a safety wire that (all being well) should not be carrying any current. It is connected to the chassis etc. If some goes wrong inside the box and a live wire touches the chassis, the ground connection should prevent the metal parts going to high voltage. In this pathological case, the ground wire will carry a (possibility very large) current, at least until the fuse blows or the circu ...

UPS modules are no different to other high impedance sources such as generators, they need special consideration that some or most professional design software don't do accurately. If you're an electrical engineer you should have the ability to make suitable decisions based on experience, qualifications and information.

Do your homework, double check any digital calculations and don't expect a lot of help from manufacturers. I have tried and they don't want to assist or can't. The ...

In substation there is Line Matching Unit (LMU) where the equipment impedance and line impedance matching is done, it is connected below coupling capacitor CC, and we have studied in network theory maximum power transfer thermo where impedance matching term is used. It is used for maximum communication signal can pass to other end with reduced loss.

This is the first part of the theory which is maximum power transfer. This is basically figuring out what should be the line impedance ...

The 3rd harmonic currents are not in phase with each other, they are just at 150 Hz but still 120 degrees apart relative to their period now being 6.67 ms instead of 20 ms - i.e. the zero crossings on each phase now occur every 3.33 ms instead of every 10 ms and are spread apart as a normal 3-phase set of waveforms.

But that does not mean that the A phase @ 150 Hz is in phase with the B phase and in phase with the C phase.

Remember what a delta is doing - we generally re ...