Motor Starting

[bob14]

Hi,

 

We are replacing a relatively lower inertia load by a higher one and still keeping the same wound motor and starter.The rotoric starter consists of multiple steps of shorting resistors.

I am worried that the motor would still be capable of starting the load.I have got the moment inertia of the new load but as the slip ring motor is very old, built in the 70's, I don't have any data on the starting torque and locked rotor current.

The motor is a 236 kW 400 V 50 Hz one. My questions are as follows:

 

1. What typical value I could use for the averaged starting torque ?

2. What is the maximum starting time that the rotor would tolerate ?

 

Thanks.

 

Bob

[marke]

Hello Bob

 

One of the advantages of the slip ring motor, is that the start characteristics are controlled by the starter rather than the motor. That includes the high power dissipation during start.

With a standard cage motor, the rotor dissipates the full speed kinetic energy of the driven load, but with a slip ring motor, this power is dissipated in the starter instead of the rotor.

 

The start torque developed is a function of the resistance applied to the rotor and typically can be in the order of 200%.

 

The major problem that you may have, is the maximum power dissipation of the slip ring starter. You may need to boost this up.

If you are using metalic resistors, then perhaps some forced air cooling may be the answer.

 

Best regards,

Mark.

[bob14]

Hi Mark,

 

Thanks for your very pertinent comments.I am going to continue on this thread and we are now planning to run two identical motors in tandem to drive this high inertia load.I am looking for possible negative effects of starting the two motors, identical wound motors,in parallel that is switching the two motors on one go.

Thanks for your comments.

Bob

 

 

[rotomoulder]

Hi Mark,

 

Thanks for your very pertinent comments.I am going to continue on this thread and we are now planning to run two identical motors in tandem to drive this high inertia load.I am looking for possible negative effects of starting the two motors, identical wound motors,in parallel that is switching the two motors on one go.

Thanks for your comments.

Bob

 

Bob. Please understand that I am in no way an expert. However I can tell you that some years ago I tried to use 2 identical motors and gearboxes to turn a very heavy object. The motors and gearboxes were at opposite ends of the machine. So far so good as the motors and G boxes were identical except this is not a perfect world and one,the gearboxes could not be reverse driven ( a point I later regretted) and perhaps more importantly and as far as I can tell, no 2 S cage motors are the same..

We checked the direction of rotation and then switched on using one soft starter. Within a few seconds the unit overloaded and stopped. Thinking it was the SS I fitted one DOL to both motors with the same result. I tried all that I could but could not correct the problem. Finally I gave up and fitted one large motor at one end and all was well.

Not to be beaten I tried again but this time, by chance I used 2 G boxes, both of which could be reverse drive and all was well.

I am sure that somebody on this forum can explain why it did not work in the first set up as it something to do with the fact that all motors are a little bit different.

 

Regards

Rotomoulder