Speed control in asyncronous wound rotor 2

[maxipaz]

Does anybody knows if exist some solid-state device that replace the large multi-stage bank resistor that is normally used in wound rotors in order to start and control the speed?
Thanks in advance

 

[jbartos]

Suggestion: Visit

http://www.thomasregister.com

and type Starters: Motor under Product or Service, which will return 133 companies to approach to

 

[DickDV]

You have to be a little careful with wound rotor motors, maxipaz. They are not really variable speed but rather, variable torque. The more resistance in the rotor circuit, the lower the torque.

If your load increases torque as speed increases, then you can control speed.

As for solid state rotor controllers, I wouldn't be much help on that. Every such motor I've encountered has had its rotor resistance fixed at the minimum value and an inverter added for speed control.

 

[jraef]

You can't directly replace the resistors with anything solid state. You can replace the entire controller with a VFD (provided you know what you are doing). You can also do the starting with a fixed resistor and a solid state soft starter, but this will not change the speed (actually torque as DickDV said). Are you really using it to vary speed or just as a soft start method?

"Venditori de oleum-vipera non vigere excordis populi"

 

[maxipaz]

Thanks for the advices that you jbartos, dickdv and jraef, have sent.
I think the one that have better understood what I'm looking for is jraef.
What I want to do is to replace the resistor bank for some solid state device that simulate exactly its behavior.
I've never seen such a device.
Have you?

 

[maxipaz]

What I want to do is as follows:
I have a known load to move.
With the rotor slip rings short circuited I get a certain speed called "n1".
If I add resistance using a resistance bank, then I will get another speed called "n2" where n2<n1.
My problem is that this practice is not an energy effective solution because of the energy wasting in resistance bank.
What I'm looking for is some solid state device that use less energy in order to get the same effect as the resistance bank.

 

[jbartos]

Comment on the previous posting: The energy dissipation in the rotor circuit during starting is inevitable and part of the motor operational/functional principle. The energy from the rotor circuit may potentially be stored in some energy storage device.

 

[wschel]

Speed of wound rotor motors is usually controlled by resistor banks, liquid reostats or energy recovery systems. I believe that you are looking for a solidstate energy recovery system that converts the rotor current and injects it back into the power system. A number of these system were developed 10 to 15 years ago but I don't believe any of them worked well. Since the development of PWM VFDs, most people have either shorted out the rotors and used a VFD for speed control or replaced the motor with a SCIM.

 

[jraef]

wschel is right, but if you insist on pursing this further, the only one that I am aware of still doing it is Flomatcher in Oregon USA. They bought out the old Marathon Electric energy recovery product.

http://www.flomatch.com/

The problem is that the cost is fairly close to that of a standard off-the-shelf VFD, hence the tendancy for users to just go with something less unique.

&quot;Venditori de oleum-vipera non vigere excordis populi&quot;

 

[jbartos]

Comment: To consider the energy recovery from the wound rotor induction motor rotor, the motor should have sufficiently large HP rating so that the saved energy pays for the energy recovery hardware. Incidentally, what size of the motor is at a question in the original posting?

 

[maxipaz]

We are talking about 7 motors with 600 HP. They are identical

 

[Kiribanda]

Hi maxipaz,

It seems to me that this is going to be a wrong application.

Wound rotor induction motors are used where you need to reduce the starting current or/and where you need to have a higher starting torque. As you are also aware, the motor is started with the full resistor in the rotor circuit and then it is gradually reduced to "ZERO" when the motor attains its no-load speed and then it is loaded to its full load speed.

As such the rotor resistor is never and should not be used to "control the speed" and should never be kept in rotor circuit which will result considerable heat loss.

If you regulate the speed then at that speed it may NOT develop enough torque to get the desired flow.

Therefore my understanding is using a wound rotor motor as a variable speed drive is a wrong application.

Thanks!

Kiribanda

 

[jraef]

Kiribanda,
You are right about how it works, but your statement about it not being suitable for speed control flies in the face of almost 100 years of people doing that exact application successfully!

Flomatcher is named that because that is exactly what they do; use wound rotor motors and liquid rheostats to control the speed of pump motors in order to match the flow to the application requirements. And Flomatcher did not invent this idea nor have an exclusive on it. Before the advent of Inverter drives, this was the premier "high tech" method of controlling motor speed. Ski lifts are another major application world-wide that used WRIM controls as speed control. Technically you are right that it does not directly control speed as we understand it now, i.e. frequency control. However if you can modulate output torque into a given load, without burning up the induction motor, you will in EFFECT be controlling speed. That is of course exactly what a WRIM and controller do.


&quot;Venditori de oleum-vipera non vigere excordis populi&quot;

 

[edison123]

Speed control of wound rotor motors by varying rotor external resistance has been in use for many decades. It was one of the earliest forms of speeed control used. Though it wasteful of energy, successful attempts have been made to reduce secondary losses by means of complicated slip power recovery systems. In the "modern" world of VFD's, this method will/has already become extinct.

 

[cbarn24050]

Hi Minipaz, these devices used to be made for the hoist industry, basicaly its a variation on a soft starter with tacho feedback. You still need rotor resistors however. Not many are made these days as slipring motors have mostly been replaced with normal motor/vfd setups.

 

[rbulsara]

Just a thinking out loud:

Why not consider starting the motor on the existing resistors, cut out all the resistors to bring the motor to full speed and then use the VFD on stator to control the speed?

You will be relacing the existing main contactor/controller with VFD and some messing around with rotor controls.

Risky method (or one needing a good rotor analaysis) would be to short out the resistor completely and even start the motor on VFD. Success of this will depend how much starting torue is required and produced.