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It seems like you may be describing an eddy-current clutch or coupling, also called a magnetic coupling or drive. An EC clutch has an input rotor, and output rotor with a variable DC magnetic field produced either by a stator or by a coil in one of the rotors energized through slip-rings. They...

The image posted is simply the diagram that defines four quadrant operation. Q1 is forward motoring, Q2 is forward braking, Q3 is reverse motoring and Q4 is reverse braking. As Gunnar said, four quadrant has been implemented with various types of motors mated with various types of control...

jraef: "...most of those people regret it later or (ironically) complain that VFDs are more trouble than they are worth...." Some complain vociferously to the VFD manufacturer and others that they have been sold a piece of junk, rip out the VFDs, and buy a competitors product which they quietly...

For a VFD with a rated input current of 200 amps or so, 8 amps of unbalanced input current with an unloaded drive is not likely anything to be concerned about. What are the drive input currents when the motor is running closer to full speed / full load? CJC http://www.vfdriveinfo.com

I think that can be accomplished with a VFD. You will probably need a vector drive with encoder feedback. There may be a manufacturer that would provide the motor with encoder and the VFD as a package. That would be the best way to buy it even if the supplier doesn't actually manufacture both...

The transformer needs to be designed for the highest V/Hz that it is going to see. For a fan or centrifugal pump load, that could be the rated V/Hz of the motor. A constant torque load will need voltage boost at low frequencies even if it only operates at low frequencies when the motor starts...

It is true, a larger motor will accelerate the load faster. However, if the load inertia is small, it is possible that the increased rotor inertia of the larger motor will offset the increased torque. CJC http://www.vfdriveinfo.com

"So is the current more related to the slip being greater during faster ramps, or the motor trying to accelerate quicker and producing more torque? Or are these two aspects one in the same? " The two are pretty much two sides of the same coin. If you set the ramp and solve F = ma for F, that...

I agree with the others who have said that the motors should not be less reliable the VFDs. If you have had some experience to the contrary, you probably need to do the things that assure that the motors will be as reliable as they can be. 1. Purchase motors that are advertised by the...

The displacement power factor of a PWM inverter is 0.95 or higher. That is the inherent chracteristic of the input rectifer circuit. The DC bus capacitors supply the magnetizing current for the motor. The total power factor including the effect of harmonic distortion is as you have stated it...

In V/Hz mode, the drive will have a boost adjustment to manually boost the V/Hz below some some frequency. That usually involves a breakpoint in the voltage vs frequency line at some frequency and an adjustable voltage at zero frequency intercept point. The breakpoint frequency may or may not be...

The current does not need to rise to compensate for the reduced V/Hz across the magetizing part of the equivalent circuit, only the V/Hz needs to rise. The problem is that it is difficult to get the V/Hz high enough for the worst case starting condition without it being too high under some other...

About that last sentence. I should have said that the adjustments are probably OK if the current doesn't exceed the rated current on a continuous basis. The motor can still overheat if it operates at too low a speed for too long without appropriate cooling provisions. CJC...

The V/Hz boost is needed because the resistance of the windings becomes more significant in comparison to the inductive impedance at lower frequencies. The motor needs the magnetizing V/Hz to remain constant. The magnetizing voltage is what is left over after you subtract the IR drop. Even if...

We made a trade. The faster switching devices made PWM drives a reasonable design option. Going to PWM simplified and reduced the cost of the power circuits. The added complexity in the control circuits was hidden away inside large scale ICs and then microprocessors. The more complex control...

For anyone with access: Boglietti, A.; Cavagnino, A.; Knight, A.M.; Zhan, Y., "Factors Affecting Losses in Induction Motors with Non-Sinusoidal Supply," Industry Applications Conference, 2007. 42nd IAS Annual Meeting. Conference Record of the 2007 IEEE , vol., no., pp.1193-1199, 23-27 Sept. 2007

At least one major manufacturer advertizes 98% efficiency at rated power. Since there are both fixed and variable losses, efficiency is less at less than base speed and rated torque, but losses are less also. That would be for only the VFD. The VFD causes a small increase in motor losses...

Both Trans-Coil and MTE seem to be recommending dv/dt filters for 1000 ft. The list prices are $527 for the former and $702 for the latter in a NEMA 1 enclosure. The net selling prices are likely competitive with each other, so the net selling price is likely to be somewhat less than $500. Just...

The VFD is started at less than 2 Hz. At that frequency, the synchronous speed is only about 50 RPM (4 pole motor). If the normal slip is 50 RPM, the motor is at normal slip at zero speed. Since the motor is at normal slip, it can produce rated torque at the normal full load current. The...

Here is a technical marketing publication that includes a power vs. speed plot indicating the range of motors and controllers that may be available. Vacon AC Drives For High Speed Applications http://www.vacon.com/File.aspx?id=466971&ext=pdf&routing=396771&name=AN00017B I found some Siemens...