VFD's Vector and Scalar 4

[itsmoked]

Hi,
I have always wondered what I was missing out on when I ordered VFD verse a Vector drive. So the faq: faq237-1062
was great to read.

Thanks skogsgurra and jraef!

But now I have more questions!

If a scalar drive is set to 60% it doesn't really know the motor is going to run at that speed, does it? In fact it probaly won't be at exactly that speed because of slip and what-not right?

A vector drive motor can actually provide full torque at zero RPM??!? WOW! I take it you must generally provide auxillary cooling in these cases that has nothing to do with the motor turning?

Can vector drives take signals from quadrature encoders and actually be used like servos if cooling is provided to hold precise positions or do they only take frequency inputs?


If you run torque boost on a scalar drive what does the drive output that is different from "unboosted"?

Motors; Are vector driven motors different from scalar driven motors, different from standard motors? If so, what general construction difference is there?

If the motors are different what is the short/long term effect of running a standard motor in "drive" service?

Thanks

 

[Skogsgurra]

Thanks, itsmoked!

Since my sun rises about six hours before jraf's does, I will start answering your questions. Leaving the tidying-up for jraef.


Q: If a scalar drive is set to 60% it doesn't really know the motor is going to run at that speed, does it? In fact it probaly won't be at exactly that speed because of slip and what-not right?
A: Right. No, it doesnt know that. And the 60 Hz mains doesn't know either what speed a connected motor will run at. Slip being the main cause for that uncertainty. DOL and scalar drives know nothing about the motor and its speed.


Q: A vector drive motor can actually provide full torque at zero RPM??!? WOW! I take it you must generally provide auxillary cooling in these cases that has nothing to do with the motor turning?
A: Yes. There were some early mistakes where the coling aspect had been forgotten. The interesting thing is that the windings didn't burn - the bearings failed instead. The winding was getting hot, alright, but the rotor even more so. So the bearings had a hot outer race and an even hotter inner race - that led to grease turning into charcoal and very rapid bearing destruction. External cooling is a must in these applications.


Q: Can vector drives take signals from quadrature encoders and actually be used like servos if cooling is provided to hold precise positions or do they only take frequency inputs?
A: Most (I would say all) major drive manufacturers have quadrature encoder inputs and position counters either built-in or as an option. A modern vector VFD has torque loop times in the sub-ms order and can be used anywhere where a fast servo is needed. For super fast servos, a PWM drive and a PM motor is even better.


Q: If you run torque boost on a scalar drive what does the drive output that is different from "unboosted"?
A: Torque boost is active at low speeds. The reason that it is needed is that a scalar drive has an output voltage that is proportional with frequency. But, the motor needs full magnetizing current also at those low speeds. Zero Hz would produce zero V and hence no magnetizing current. It is to have the necessary magnetizing current that the boost is added. It simply provides the voltage needed to overcome the resistance in the windings so that you can have full flux even close to zero Hz.


Q: Motors; Are vector driven motors different from scalar driven motors, different from standard motors? If so, what general construction difference is there?
A: Motors for VFDs are nowadays usually "a little better" than ordinary motors. The reason is that PWM has a bad influence on voltage distribution in the windings (first turns take more voltage than the rest). PWM also causes some extra losses (HF eddy currents) in copper and iron so there is some extra heat to take care of. Bearings and grease may also be different. And, for VFDs with high output frequency, the motor's maximum speed may have been improved (bearings, balancing etc). But there is no difference between motors for scalar and vector drives. Not that I am aware of.

Q: If the motors are different what is the short/long term effect of running a standard motor in "drive" service?
A: A standard motor that is run from a PWM VFD (scalar or vector) will run hotter, the bearings may fail early (three different failure mechanisms - too much to get into here), there may be insulation problems due to uneven voltage distriburion in the windings, there may be insulation problems due to reflected voltages in long cables (about twice the DC link voltage possible).

You have really put together an interesting list of questions and I hope that my answers have been of some value to you.

Gunnar Englund

 

[jraef]

Wow, I have nothing substantial to add to that, except if you look further down on the FAQ page, I wrote (again with skogsgurra's help) another FAQ on the difference between using a Vector VFD and a Servo motor and controller.
faq237-1063

Also, be aware that there is some unscrupulous marketing going on in the AC motor world. Since there is no definitive standard as to what "Inverter Duty" really means, some manufacturers interpret it much more loosely than others. If you are concerned with making sure you get real value for the extra expense, take the time to thoroughly analyze the specifications.


"Venditori de oleum-vipera non vigere excordis populi"

 

[itsmoked]

Wow, You dun answered all my questions...for the moment
And yes they have painted in a large part of my drive ignorance.

Thanks skogsqurra for the clearer-than-the-usual-text-book answers!

Thanks too jraef. I couldn't see that other faq that was mentioned but I will look at it now that you've put the number to the cat. Probably cause more questions.. hehehe.

By the way I live with a VFD(scalar) in my living room. It takes 240Vac single phase and belts out 3 phase to a 3/4 HP centrifical pump under my aquarium. I inject air into the suction line which is shattered in the pump before being delivered back into my tank. This simulates a wave breaking. I command the VFD with a PC that ramps the pump from zero to 3600 RPM holds for a moment then back down. It does this randomly between 1 and 20 minutes between the hours of 8am and 11pm.

 

[DickDV]

Just two small additions to the above excellent posts.

A V/Hz drive has no direct feedback regarding motor speed or slip but there is an indirect feedback in the motor current. Most V/Hz or scalar drives use the motor current to activate "slip compensation" which is simple a small increase in output frequency as the motor amps increase.

Slip compensation can usually offset about half of motor slip. Using more slip comp than that often results in speed instability.

At least in North America, it is most unusual for AC drives of any type to have positioning front ends. It is true that high performance vector drives coupled with low rotor inertia motors can approach the dynamic performance of servos, but positioning is generally left to the servo folks. An exception would be a high performance vector drive slaving off a positioning controller which can work very well.

 

[itsmoked]

Thanks DickDV for that added info.