Induction Motor

Induction motors produce torque based on the DIFFERENCE between the rotor speed and synchronous speed. The applied line voltage produces a rotating magnetic field whose lines of flux are cut by the squirrel cage rotor windings. When those windings cut flux lines, a current is induced that produces a rotating magnetic field. Thus you have 2 magnetic fields attracting each other. If the induction motor is sped up by an overhauling load, the difference can fall to 0 and thus no current in the ro ...

For a 3 phase induction motor, say rated 110 kW, that uses to drive a high inertia centrifugal fan. If we stop the motor, the induction motor still rotates due to load inertia. So is it harmful to run the induction motor again while it's running by load inertia?

This will lead to induction generation. Voltage will be induced in stator. Now ...

Assume that when you changed the frequency from 50 to 60 Hz that you mean the rated frequency but you then ran the motor at the same speed. By doing this you have altered the motor set up calculations that could have changed two items giving a different resistance.

The first is that the slip speed would be different that gives different equivalent rotor resistance values when referred to the stator. The equation is actual rotor resistance / slip. The other is that the skin effect f ...

The answer depends upon the type of motor. A synchronous motor can usually be driven as an alternator by simply substituting a prime mover for the mechanical load. Things are less clear for an asynchronous/induction motor since it will depend upon whether it is attached to a three-phase supply or not.

If there is a three phase source then the prime mover has to operate at greater than synchronous speed, however its output frequency will be locked to the line frequency, assuming th ...

The torque speed curves are completely different between an induction motor and slip ring motor. You need to understand the differences. If you have selected your slip ring motor to limit starting currents you can short out the slip rings and run your motor but the starting torque is reduced unless you use some features on the VFD to boost starting torque.

If you are using it as torque control on a crane then y ...

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 ...

For an AC machine (induction OR synchronous), current required for a given load is dependent on applied voltage, overall efficiency, and power factor. With power factor at unity (1.00 per unit), current is at a minimum - but there is no appreciable amount of reactive power (either leading OR lagging).

Some machine designs have a "low" power factor, but that is typically because another performance parameter is being optimized (i.e. deemed more important to the user). For example - ...

Induction motors are selected on torque requirements at a specific speed not kilowatts. The mechanical power is the rated power of the motor at the shaft not the input of the electrical energy. The difference between the two is the efficiency. The torque is the mechanical output power in kW x 9950 divided by the speed in rpm. When the induction motor starts the speed is zero and the current maximum. Power of the flux interaction causes the shaft to move slightly. This small number is divided ...

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, ...

Upon asking few friends, I came to realize that we (young ones) have inherited a system of work from experienced guys without understanding. We often walk to the machine and give it so many shots from/with a grease gun. What determine the amount of lubrication one should apply on each size of a motor? How do I work it out so I can know and measure so as to avoid too little or too much of lubrication on a motor?

The amount should accomplish flushing all old grease out and leaving fr ...

The only requirement for open loop control is to maintain proper voltage current and frequency to the motor. Motor is run in closed loop where you need controlling. For example in a 3d printer you need continuous control of motor rotation, so you need to provide feedback to know the motor position.

A pressure transducer driving a VFD driving a motor driving a pump to provide a set pressure would be a closed control loop. The motor itself is operated open loop, as part of a closed p ...

The normal frequency in Australia is 50 Hz and a client of mine runs his compressor motors up to 60 Hz which gives him greater output. But he only gets away with it because the machines are from the United States and are rated to operate at 60 Hz. Be careful because there could be a significant increase in power at the higher speeds in 60Hz.

Before you attempt to do this verify with the compressor motor manufacturer if you motor is VFD rate ...

For correct rotation, the phase sequence has to be correct (L1 to M1, L2 to M2, and L3 to M3 - where L refers to LINE and M to MOTOR phase).

If two phases are connected incorrectly (L1 to M2 and L2 to M1, for example) - the motor will rotate opposite to the desired direction. NOTE: THIS IS NOT THE SAME AS CHANGING THE POLARITY OF A SINGLE PHASE.

If one phase has the polarity changed, then the magnetic field distribution within the motor winding becomes uneven, and the ou ...

Rewinding a motor or generator implies that the bulk of the damage that took the machine out of service occurred in either the stator or rotor winding - or both.

It can also be a decision based on planned outages - where the data trends indicate that the winding has not (yet) failed, but is losing its integrity. In this case - the decision is to install a new winding at a time that is convenient for the user, rather than when it will disrupt production.

Refurbishment im ...

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 ...

In really simple terms, there are a couple of factors.

First, the total mass of the induction motor rotor bars must be sufficient to handle the heat generated by the high currents during a transient (start) condition.

Second, the number of induction motor rotor bars (and thus slots) must be slightly different from the number of stator slots to minimize both electromagnetic and acoustic noise.

Third, the number ...

Friction losses in motors (AC or DC) are pretty simple when you get right down to it. Basically, any book on the relevant bearing type (sleeve or rolling element) will cover almost everything you need to know.

Induction motors (both squirrel cage and wound rotor) are pretty easy, as they're essentially cylindrical. This means the "friction" developed from moving the rotor through a medium (typically air) is minimal. As the rotor geometry deviates from a true cylinder toward a "fan" ...

All AC machines (synchronous and induction) have a window of opportunity for reclosing. The basic resolution for reclosure (also known as "bus transfer") is either "fast" or "slow".

Typically, "fast" reclosure efforts occur within a few cycles (think less than 8, and often less than 6). In terms of actual time elapsed, it will certainly be faster than 0.12 seconds. If you're attempting this on a synchronous machine, it MUST have brushless excitation, since it can only be done with ...

Over speeding the motor with a variable frequency drive will depend a lot on the existing load on the motor. If it is a fan load and you are currently at 100% of NP rating, you have no head room left to play with. If you have a roll spinning lightly loaded you will have much more head room. In some applications we run 240vac motors on a 480vac VFD and over speed from 60 Hz rated to 90 Hz- and develop 1.5 x the HP at rated torque. If you have a 480vac ...

There are two things to consider when looking at increasing the shaft speed of a motor (by whatever method).

First: mechanical. Can integrity of rotor design be maintained? (Typical max safe speed is NP * 1.25 which does NOT mean it operate at that speed, but only that it will not come apart and cause injury.) Will bearing and/or lubrication handle the increased speed? (There's going to be more heat.) How about vibration? (Typically, faster speeds require smoother operation to prev ...