Blog

The main difference between slip ring and cage types is that the slip ring rotor generally has a much lower slip at full speed than the cage rotor if it is properly shorted directly at the rings. We need to check that the variable frequency drive (VFD) slip setting can go low enough. Otherwise only general things need to be considered......mainly the insulation quality of the stator since its likely you are dealing with an older motor.

The slip-ring motor has slightly different imp ...

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 ...

IF you are connected to STIFF grid, yes. If your generator is connected to a soft grid, it's different.

By definition, if this generator can impact the Hz or V of the grid, it is a soft grid to this generator's control system. For example, two 10MW generators and no grid connection. One generator can "push" the other around up to some point. It is NOT size dependent: you can parallel a 10MW with a 10kW and to the 10kw, it is a stiff grid. To the 10MW, it is a soft grid.

...

The most correct term - according to IEEE and IEC - is ASD (Adjustable Speed Drive). This refers to the electronic equipment used to regulate the operating speed of the motor and driven equipment by controlling the frequency and voltage applied to the motor.

NOTE that other terms in common use include (but are not limited to): VSD (Variable Speed Drive), AFD (Adjustable Frequency Drive), and VFD (Variable Frequency Drive), they are referring to the same motor drives, just different ...

While switch to "self-control", all of the output parameters are controlled by the frequency converter, at the moment, all external controls are inoperative;

While switch to "external PLC control", the keyboard of the frequency converter is inoperative, all control commands are transmitted by external PLC, refer to following instru ...

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 ...

Metering current transformers are designed to be within quite accurate values up to say two x rated current, and down to <0.01 x rated. Consequently a protection relay connected to a metering current transformer will get a heavily saturated waveform during fault conditions and it won't work properly.

IEC Class P CTs for standard IDMT OC or even distance may only be 10% accuracy - it is obviously better of they are mo ...

Most generator designers use either IEC or NEMA as guidelines; the specific choice of which is applicable is often made by the end user (or at least the one purchasing the equipment). Both of these entities specify "normal" operation as falling within a set range of voltage, frequency, and a combination of both.

Normal operation occurs when ALL of the following are true.
(0.90 PU Volts < operating line voltage < 1.10 PU volts)
(0.95 PU Hertz < operating line freq ...

Variable frequency drive (VFD) will capable of varying the output speed of a motor without the need for mechanical pulleys, thus reducing the number of mechanical components and overall maintenance. But the biggest advantage that a VFD has is the ability to save the user money through its inherit nature to save energy by consuming only the power that's needed.

A VFD will convert its incoming power, fixed voltage and frequency to variable volta ...

This is a typical example of the gap between disciplines. As a process engineer / automation and control technician you're probably aware that a P&ID does not contain enough information to build a control system and as such is not sufficient to act as a 'control specification'. It should however at least has the information to extract a 'sensor and actor list' from it, containing all electrical properties of all the instruments. It might even show the control loops between sensors and act ...

In EHV/HV system where more than 1 -in coming power supplies are received from Two of more Power sources (synchronized). Supposing one source is in one direction of the station where as the other is on the opposite direction of the station. If feeding from one source fails the supply to all the feeders on that particular bus shall get affected and shall stop supplying the load. In such cases the bus tie breaker plays a vital role of feeding the load through the live remaining bus and interrup ...

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 ...

A VFD-driven general purpose motor can overheat if it is run too slowly. (Motors can get hot if they’re run slower than their rated speed.) Since most general purpose motors cool themselves with shaft-mounted fans, slow speeds mean less cooling. If the motor overheats, bearing and insulation life will be reduced. Therefore there are minimum speed requirements for all motors.

General purpose motors can be run with VFDs in many applications; however inverter-duty motors are de ...

The first thing is to define the average load you need to keep powered. Second is to determine how many days you may be without sufficient solar energy. Does your system have to be powered by battery, or will there be an emergency generator available? If only by battery, then you have to look at worse case bad weather. Offset that by some solar power, my panels provide at least 25% of their daily energy during rainy weather, up to 60% during cloudy days. Us that to calculate how much battery ...

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 ...

To cope with the effect of the power electronics and the pulse thereof, it is usual to wind stator with specially varnished wire from supplier. This is often referred as triple coated as opposed to double coated (Standard offering). Don't be confused by number of coatings terminology. The wire has multiple coats applied. The term double and triple are generic to the industry. In effect the number of pin holes per m length of wire is reduced and the insulation (Enamel coating) on the wire has ...

I've spent a good deal of time in the field recently trouble shooting motor problems and observed the following. Often the bus voltages run high. When the voltage is high the magnetizing requirement of induction motors is also higher. Often the assumption is that if the voltage is higher the current will be lower without considering motor efficiency or power factor. Motors are designed to perform best at their rated nameplate voltage. First make sure that your bus voltages are where they shou ...

That's the way motors work, at low loads you're supplying the magnetizing current for the windings/iron combination. The power factor improves as the motor does some real work as it approaches synchronous speed. A variable frequency drive (VFD) is not a device that changes the design characteristics of a motor.

Assuming it is an induction motor and the VFD is used only for starting and the motor runs DOL once it is up to full s ...