Resistance and Motor Slip

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mbrooke said:
Does anyone have a picture and 3 line diagram of such a motor?
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Brochure is Cortina Brochure. ISK.
I'll try to show it.......
 

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Cortina Improved Static Kramer (ISK) is a highly efficient means of speed control of wound rotor motors typically in the 500kW to 10MW range. It is particularly suited to centrifugal loads such as pumps and fans. The very low losses and harmonic content is a distinct advantage over other speed control systems
 
I could draw one out in my head. Some technical details...

Look at a NEMA design diagram. Now imagine if you had a rotor that could smoothly go from design D to design B. So you get peak torque at any speed.


That’s what wound rotors do. It’s like the original soft start. This is also key...it is not per se variable SPEED. That’s a VFD. It’s variable torque. And they aren’t terribly efficient below about 500-1000 HP but very efficient above that.

There are kind of 5 “resistance” techniques. The first is use grid resistors and contactors to vary the resistance. Think of it like a manual transmission car. In fact that’s what the motor sounds like as it starts up. Very popular in the US. The second is the liquid resistor or liquid rheostat. Take a tank if salt water. Connect 3 rods to an insulated winch and raise or lower them into the salt water bath. Works like the contactors except infinitely variable. Popular in Europe and on some car shredders. Another antique one I’ve seen used a pump and valves with a lower and upper tank and varied the salt water level instead of the electrodes. The third design uses a soft start and resistor bank although so far I’ve only seen laboratory versions. ABB has advertised slip energy recovery where they back feed energy from the slip rings to the primary side of the motor with a VFD. Interesting concept but hardly seems worth the effort. Now here though is the interesting part. We have access to six coils and we can remove OR inject current into both sides. So if we use a VFD on the rotor it becomes a doubly fed motor which can achieve twice the power density of the conventional design or we can pull power as a generator, which is how wind mills use it.

Wound rotor motors have been mostly abandoned by the big manufacturers but they are still out there and ideal for certain applications. My shop typically rewinds 6-10 per year and it’s.a very profitable business. The following photo shows a few we did for one customer a couple months ago. It’s the four larger ones in the background. As wound rotor motors go they are small.

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mbrooke said:
Neat concept. How well does it work over a DC motor?
Click to expand...

As with AC motors the peak torque envelope is greatly expanded but even more so. You can overexcite a DC motor up to around 150%’of name plate torque below base speed where NEMA standard design B motors get up to around 175-200% peak torque. But we can reach the theoretical peak torque in a wound rotor motor so up to 275-300% peak torque is typical and this can be done from 0 RPM all the way up close to synchronous speed. This is without using it in doubly fed mode just using resistors. Since you can get maximum torque at zero RPM the high stall current of a squirrel cage is eliminated.

That being said it is very easy to control torque under open loop control. Speed control on the other hand is not so easy. It can be done but the result is that it becomes just as complicated as a VFD.

Wound rotors were the original variable speed motor. They were used extensively until Ward Leonard control care about in the 1930s. Ward Leonard loops use DC generators (Motor-Generator sets) as electro-mechanical amplifiers which allowed tight control over DC motors prior to power electronics. DC really took off in the 1960s and 1970s with electronics. By that time wound rotors dominated only where very rugged motors were required like logging cranes and car shredders or where soft starting with high starting torque was needed on large motors (1000-10,000 HP). DC faded out under 600 V as IGBT VFDs became reliable and was all but eliminated in the late 1990s as reliable medium voltage VFDs became available. Wound rotor motors have been relegated to the niche markets of high starting/peak torque at constant speeds (car shredders, log chippers) over 1000 HP, and where high power and limited space is needed (wind mills). I haven’t seen a variable speed application in years that isn’t VFD or DC, and DCs tend to get converted since the rebuild cost on a DC is pretty close to the conversion cost to AC VFD. The exception being small permanent magnet servo motors and some battery applications in power plants.

I’ve worked on a couple recent log chippers that try to use multiple motors load sharing and one 3000 HP chip mill that uses a synchronous motor but in practice wound rotor motors are more reliable and simpler to troubleshoot and maintain.

Another “niche” application is long mining conveyors. At startup conveyors need roughly 140% of starting torque but the high demand lasts for 20-60 seconds. It can use a VFD but at 1000+ HP the high starting torque of wound rotors plus no losses during running makes it more efficient (5-6% efficiency boost) initially plus it can use lower cost smaller motors and simple contactors for starting which is much less than a comparable VFD. The only downside is that rewind costs are almost twice as much since both the rotor and stator have coils.
 
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