AC Drives

To answer your first question, regenerative braking in variable frequency drives (VFD), the premise is that whenever a motor is rotating faster than the winding are excited the motor becomes a generator. Regenerative braking means the voltage produced is "placed somewhere else". Often the excess voltage can be absorbed by the IGBTs or the addition of a braking resistor...maybe even placed back on the AC power network. In ...

I have an interesting (yet frustrating) project I am trying to get to function accurately. My customer is a machine tool shop that refurbishes well casing for oil and water drilling. We installed a VFD driven system to move the pipes from area to area. The reason for the VFD is for the high torque start and stop and also the fact that not all the pipes are the same size and weight. The problem I am trying to overcome is when they are driven into the CNC lathes that clean up the threads on the ...

All electrical machines have some amount of dis-symmetry to them, just because they are constructed of individual parts. Sometimes this is in the steel portions (segmental laminations, or unequal airgaps, etc) and sometimes in the winding (unequal resistance paths, different connections and/or lead lengths, lead routing, etc).

As soon as there is a difference in the magnetic path (or the magnetic strength), some amount of voltage will be induced on the motor shaft. The purpose of a ...

This question is a little more complex than it seems on the surface. Let's start with understanding the reason you believe you need to provide some form of braking in the first place. Do you need to stop very quickly under either normal or emergency stop conditions? Does your process need to be able to decelerate quickly when changing from a higher speed to a lower speed? If none of these conditions exist you may not need to provide any form of braking at all. The system friction and windage ...

The variable frequency drive (VFD) has the capacity of 11kW, it's a rotogravure printing machine which is used in packaging industry (plastic web films). The data is collected from the VFD display when the motor was running:

The set speed or speed command given was 100 m/min via HMI (Touchscreen LCD)
Frequency Reference: 48.83 Hz
Output Frequency: -50.10 Hz to -50.82 Hz (Not stable, fluctu ...

Most manufacturers have separate product families for "servo drives" and "variable frequency drive" or "inverter". However, if we take one of the many VFDs that support PM motors with sensor and a PM servo drive, the control, is the same up to the speed loop. Both servo and VFD (except for DTC) need an inner vector current control loop, which means torque control, and a speed control regulator. They also need a self-tuning pr ...

I have used variable frequency drives (VFDs) from a number of manufacturers over the years (ABB, Gozuk, Control Techniques, Emerson, to name a few), and I have not had specific failures with any of them. What I did do:

1) Spent a lot of time up front to size the VFD properly for the application.

2) Spent time up front to research the VFD product specified to be used for the application - in s ...

One operational point is to leave a surge buffer at the top of the tank, based on maximum possible input flow. I would eliminate the control valve, to use the VFD, but you may need a stop valve if the discharge of the pump allows gravity flow, when the pump is stopped for Low-Low Level, for instance. A stop valve is easier to apply and more reliable than a vacuum breaker on the discharge line.

The VFD provides significant energy sa ...

For a given torque rating, the AC motor will almost always be physically LARGER than the equivalent torque rating in a DC design. This is particularly true for slower (< 450 rpm) shaft speeds. The reason for this is two-fold. First, the AC machine must run at a lower magnetic saturation to enable equivalent performance in terms of speed control (generally the windings are higher inductance than the DC version, since more turns and less amps). Second, the AC machine is often relying on an i ...

Conversion of DC drive to AC drive is complex. Most important are the changes, impact or improvement in your power system (motor sizing, power factor, power quality, harmonics, efficiency, operation/maintenance, controls, etc.).

Many of the considerations for comparing AC and DC drive systems can be covered if you have a procedure for making an economic evaluation between the two technologies. Areas of comparison include: system purchase price, spare parts costs, motor cooling fac ...

Most modern day variable frequency drives (VFD) will offer V/Hz control, sensorless Vector control and optionally closed loop control for full field orientation control method. The V/Hz control is retained for applications generally whereby a high torque speed or dynamic control is not required such as applications like centrifugal pumps and fans, or lightly loaded applications.

Although it can be argued that in modern Sensorless Vector VFDs the motor modeling and speed of the M ...

It's useful to warn engineers against taking decisions without any supporting facts and numbers, just taking for granted what salesmen say. When just connected to the grid, an induction motor will probably have more than necessary flux, because it will probably work below its nominal power. The assumption that one can exactly match the size of the motor to the application is unrealistic. The motor cannot be sized for exactly the required torque for many reasons, including the fact that sizes ...

First, what size of variable frequency drive (VFD) is being approached? When we speak about costs, remember that cost is quite different between a 10 MW VFD and a 7.5 kW VFD. Performances vary as well, in per unit basis and absolute value (kW).

There are also different types of VFDs for different types of motors, that is synchronous or induction for example. Therefore, evaluating case by case makes sense and other considerations along with the above mentioned also needs to be carr ...

The most basic difference is the power source. AC motors are powered from alternating current (AC) while DC motors are powered from direct current (DC), such as batteries, DC power supplies or an AC-to-DC power converter. DC wound field motors are constructed with brushes and a commutator, which add to the maintenance, limit the speed and usually reduce the life expectancy of brushed DC motors. AC induction motors do not use brushes; they are very rugged and have long life expectancies. The f ...

Most of the users installed variable frequency drive (VFD) near the MCC because it's easy to maintain the system but there are some issue's which may happen due to placing the VFD near the MCC like there is a kind voltage reflection issue if the length of the cable is more than the critical length which may damage the motor.

In this case we need to follow instructions given in the user's manual supplied with the variable frequency drive. Generally following are recommended by manuf ...

The rectifier switches (SCRs or IGBTs) are likely the only components that will be similar between AC drives and DC drives. There will be many differences some of which are the design of the energy storage and filtering components (inductors, capacitors), the control of the DC section and the inverter control and switching in the AC drive, the design range of the DC bus voltage (for the DC drive it needs to go to zero; for the AC drive it does not).

An AC drive does contain an int ...

Variable Frequency Drives convert AC to DC and then from the DC creates an output voltage at an adjustable frequency to run the load motor at whatever speed you want from 0 to 60 Hz. The voltage generally varies from 0 to full voltage as the speed goes from 0 to 60 Hz. They can also go above 60 Hz up to 400 Hz, but the torque falls off as the speed increases.

A VFD is a device that changes the frequency that an AC motor sees an ...

Variable frequency drive (VFD) is a whole science and even a very broad deliberation here will not give you any good picture. I was teaching VFD course for one of the manufacturers and for an introductory session I talked for 8 hours, full course - 5 days and this is still just facts. Surely, you can buy 1HP VFD hook to 1HP motor and it is going to work regulating the speed by a pot on it. However, if you buy 4MW (that is 4 mega watts) VFD to run pilo ...

What really happens with a variable frequency drive (VFD) / variable speed drive (VSD) and the actual frequency that is imposed on the electric motor terminals? For induction motors, it is never 0 Hz. For synchronous motors - it can be DC / 0 Hz - but this is only in the case of Permanent Magnet Synchronous machines, and motors with Wound Fields that have the ability to supply significant Current into the wound fields at standstill.

I am not going to discuss synchronous motors fur ...

Any non-linear load (such as a computer, a microwave oven, some refrigerators and electric stoves, or a variable frequency drive) will generate harmonics to some extent. Motors and generators themselves - because of inherent dissymmetries in the manufacture - can create some as well. The variable frequency drives that are referred to as "active front end" topologies are probably the worst, since their whole approach is to introduce ...