Converting wound rotor motors to squirrel cage

[LGAPCI]

Hi all,

Came across this scenario a few weeks back. Customer's plan was to convert some old wound rotor motors on liquid rheostats to a squirrel cage mimic on soft starters. So we short out the windings in the motors terminal box and hook them up to a soft starter. Hey presto! we start them and they just sit there humming....So I do a little research online and discover a few bits and pieces suggesting this isn't a good idea due to the fact wound rotors have inherently low torque for a motor when they aren't tuned with their rotor resistors (shorted) and the fact we are applying a lower voltage through the soft starter lead to a really low starting torque. Now my question is: is it purely the low torque preventing the motors from going? As by chance one of our converted motors didn't have a load so a brave electrician gave it a pre-spin before I ran the soft starter and it went like normal...then we stopped it and ran it again and it ran once more and finally we ran it another time to be sure and it went back to humming. So is there other factors at play here like alignment of slots within the motor. If you understood this and have an idea I would love to hear?
Thanks,
Luke

 

[davidbeach]

People far more knowledgeable than I with this particular branch of machine engineering will certainly reply in the near future, but until then I can assure you that you've taken an apple and tried to squeeze orange juice out of it. Wound rotor machines and squirrel cage machines are two different beasts and can not be be readily interchanged. You didn't even try to interchange them, you just tried to make a wound rotor machine pretend it was a squirrel cage machine.

 

[waross]

What size are these motors??

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LGAPCI]

75kW

 

[edison123]

Squirrel cage rotors must meet certain stator/rotor slot numbers requirements for effective starting. The wound rotor does not need those requirements due to the external resistance starting. In all probability, your 'converted' cage rotor is not meeting the first requirement.

That said, a VFD is a better option than a soft starter in such cases of conversion.

Muthu

http://www.edison.co.in

 

[LGAPCI]

Yeah so that's the bit that gets me, so having resistors on the rotor effect more than just the starting torque or is simply that with such a large starting torque the WR motor overcomes this stator/rotor slot arrangement?

 

[edison123]

Yes, adding resistance in the rotor circuit increases the torque significantly (without corresponding increase in stator current), which will overcome these slot configuration issues.

Muthu

http://www.edison.co.in

 

[LionelHutz]

Without knowing the load it's hard to judge what will happen, but generally speaking what you did is a very bad idea. Wound rotor motors typically aren't installed because the load is easy to start except for loads where the motor is used for speed control, and you're not requiring any speed control so that type of load is likely out.

 

[zlatkodo]

Very often, wound rotor motor will not be able to start as a squirrel-cage motor, even in no-load conditions, because of low torque.

[URL unfurl="true"]http://winding.wix.com/design[/url]

 

[waross]

Years ago there was a company marketing a solid state control scheme for wound rotor motors. They rectified the rotor current and then inverted it back to AC and fed it back into the grid. You had good torque for starting and speed control without heat energy being lost in a resistor bank.
Has your customer considered a resistor bank and contactors to replace the liquid rheostats? 75 kW is well within the range of resistor starters for wound rotor motors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

I should have posted this before, but the soft-starter will work great with a small starting resistor step shorted out once the motor is running. Size the resistor to give around 10% slip.

 

[LGAPCI]

Interesting, so basically if by luck or be pre spinning the motor the slots are aligned the motor will start fine. If they are not well aligned the starting will be that poor that even unloaded it cannot get itself moving. Essentially creates deadspots.

 

[LGAPCI]

On that last comment, yeah in the end the customer put the rheostats back on and started the motors with one of the lower resistance steps and removed it once running. This was just a quick fix with the intention to actually install proper squirrel cage down the line.

 

[LionelHutz]

Well, even if the motor has an OK rotor/stator slot combination to start, you are basically creating a motor with a very low locked-rotor torque combined with a very high inrush current. So, for example, the motor which previously started using <2x rated current when using resistors could now draw 15x rated current and only produce 1/4 of the torque it was producing while using the resistors. Then, the lower torque isn't enough to even turn the load and it fails to start. Also, with the higher starting currents required you could exceed the thermal capacity of the motor.

 

[jraef]

That's the issue, low torque P.U. of current. The motor essentially goes into saturation without creating enough torque to begin acceleration. The current is almost all reactive. By moving the rotor you just got lucky by finding a pole alignment that created a little bit more torque. But in starting that way, you might have eventually damaged the insulation on the stator because the acceleration time at high current would be very long. Leaving a step of resistance in the circuit during starting is the accepted norm for using a soft starter on a WRIM. One of our long time members, MarkE, has a web page describing this issue in detail.

http://www.lmphotonics.com/slipring.htm

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington