Wound rotor control design reference 2

[skrab]

I need to design a control circuit and specify the components required for 5-step magnetic type starting of a wound rotor crane motor. The application is 380V 5HP 1500RPM. Could someone suggest a reference for calculations/conventions of wound rotor motor starting?

 

[skrab]

This is Skrab again. If anyone has ANY information regarding wound rotor control designs I would really appreciate it. Thanks in advance.

 

[electricuwe]

Do you talk about a control ciruit using some magnetic operated contactors used to switch resistors in the rotor circuit ?

If this is your intention I guess I will have some useful information for you, but I have to translate it from a old engineering handbook written in German.

 

[skrab]

Thank you very much, Electricuwe. I am indeed speaking of using magnetic contactors to switch out the starting resistors. Any information you could share would be greatly appreciated.

 

[llangford]

If you can find a local EASA member and someone in their shop that is familiar with wound-rotor motors ask about this subject. It is in the EASA Technical Manual. You may be able to purchase the article direct from EASA. Good Luck.

 

[electricuwe]

I have found some information on that topic, but the main problem is that the most important things are described in diagrams.

Was llangfords post helpful for you ?

Otherwise post your email address, so that I can fax you the diagrams.

The design of the circuit starts with drawing the heyland diagram (also called osanna diagram) for your motor with shorted rotor.Then the digram showing torque as a function of speed has to be derived. Do you know how to perform these steps ? I guess otherwise the information I can send you will not be very helpful.

 

[Marke]

I would be interested in seeing this also.
Mark Empson. empson@lmphotonics.com
Mark Empson

http://www.lmphotonics.com

 

[skrab]

Yes, Yes all information is helpful. My email address is:
joeskrab@stratos.net

thanks to all who comment.

 

[electricuwe]

Before I can send you the diagrams It will take some time. I have to scan them first (and I do not have a scanner at home), but may be there is an alternative approch which is less exact, but which may work for skrabs application:

If the torque/speed relation is approximated as linear, which is a good approximation for torques up nominal torque you can calculate the resistors as follows:

synchronous speed: Ns=2*pi*f/p
nominal speed: Nn from nameplate
rotor resistance: Rs from measured value between two terminals divided by sqrt(3)

The additional resistance Ra necessary to obtain the speed desired (at nominal torque) Nd can be calculated by the following formula:

Ra = (Nn-Nd)/Ns*Rs (for speeds lower than Ns)

For equal steps in speed calculate Ra for Nd =0,2*Nn and connect 4 resistors with the value Ra in series for each phase of the rotor (resistor strings in star connection) and connect contactors to short this resistor string at different points.

shorted at the rotor: Nn
operating with Ra:: 0,8*Nn
operating with 2*Ra: 0,6*Nn
operating with 3*Ra: 0,4*Nn
operating with 4*Ra: 0,2*Nn

The total power rating of the resistors should be 80% of the power rating of the motor. (Calculation is similar to operating an DC-motor with series resistor in the armature circuit)

The speeds given above are for lifting operation. For lowering the load the two stator connections have to be exchanged (as usual for three phase motors). The speeds will be:

shorted at the rotor: Ns+(Ns-Nn)
with Ra: Ns+(Ns-Nn)+0,2*Nn
and so on

Usually the faster speed ranges are blocked for lowering.

 

[skrab]

Thank you again, electricuwe. I have 2 more questions.
#1 Does anyone have any idea how to approach the timing of the acceleration? When we provide gradual acceleration to our cranes using VFD's we typically ramp up 0 to full speed in about 4-5 seconds. Would it be acceptable to design the timing circuit to short out 1 resistor each second? Should the timing be controlled via solid state timers or some other method.

#2 As far as the sizing of the resistors, it seems to me that while it might be easiest to spec 4 equal sized resistors each one rated 20% of the power of the motor, I might be able to save panel room by using different wattage resistors. What I am trying to say is: The first resistor that will be shorted out will always operate with the other 3 resistors in series + the rotor, therefore this current (& power rating) will be much less than the amount of current flowing thru the fourth resistor when only it and the rotor are operating. Am I missing something?

Thanks again to all who reply.

 

[electricuwe]

Hello Skrab, hello Mark

sorry that I havn't sent the information as suggested. I have some problems with my email-system at the moment.

But I'd like to answer skrabs question:

First I guess it is important to distinguish between speed regulation and starting. Judging from skrabs last post I assume that the resistors shall just be designed for starting.

1. Beside considering local safety regulations I guess an approach whith fixed timing based on the accelleration time for maximum load would be sufficient.

2. If you are just intrested in starting your assumption is correct. Further in this case the resistors need only be rated for intermittend operation.

What about shorting the rotor terminals and using the motor with an VFD-inverter ? To my mind this option would be a more cost effective retrofit for an existing wound rotor motor, if design effort and manufacturing is considered (at least if you are used to VFDs).

 

[skrab]

More application info: Believe it or not this is a new application. If it was up to me I would definitely go with a VFD. Time, cost and the ability for field adjustments all point to VFD. Unfortunately, the customer is mandating the wound rotor design. They believe the on-site maintenance people will be more comfortable with the older "simpler" (customer's term) wound rotor design.

They have asked for 5-step controlled acceleration. (Slow ramp-up of starting) also they would like 2-speed operation. The end-user will have a pushbutton pendant that will have one contact that will ramp up to full speed using timed drop-out of all the resistors (as I mentioned I'm thinking approx 4-5 seconds), plus another pushbutton contact that will ramp up to 1/3 of full rpm for a slow speed operation. These resistors must be capable of operating for the 30 min duty cycle (if the user constantly operates in the slow speed).

Thanks again for your reply.

 

[electricuwe]

skrab,

maybe your customer has had bad experience with VSD systems. To my mind for the power rating given the only thing maintainance would have to do in case of a failure is to replace the failed inverter with a new one of the same type.

But I can understand that your customer might be afraid of using VFDs. I know of cases where errors in applying (small!) VSDs correctly caused cost in the range of several 100000 $.

If you are not willing to convince your customer that a VFD is the best solutution you might search the following website for more information on the classic solution:

http://www.postglover.com