slip ring motor

With portal cranes, unless one has a mechanical shaft linkage, the motors on the two sides are required to be kept in electrical synchronism, to prevent the portal crane from jamming by running askew. This is usually achieved by running the two side slipring motors in power selsyn modes.

This can be the only consideration when modifying the drives.

In selection and application of motors one cannot only go by KW or HP requirement.There are other requirements like the starting torque to over come the inertia and accelerate the load,Braking , dissipation of energy during braking ,No of starts /stops per hour,etc.In this case travel motor has to have the capability over come the inertia and move the load.This also requires frequent starts and stops.The starting definitely will need a high starting torque and hence any form of assisted torque for same rating may not be sufficient.In case of hoist it will involve much more frequent starts and stops .In slip ring motor the the external rotor resistance helps obtaining the high starting torque and also helps in dissipating the energy generated during braking.So one to one replacement based on KW alone may not be correct.You can consider VFD.ABB has some ac crane control systems.Probably with their /any one of the drive manufacturers help entire system including the motor may be replaced.

Why you want to change?. Actualy slip ring motors are best suited for crane duty application. Have old motors lived up their life cycle?. or they need regular maintenance?.

thanks for your comment. the reason to change is the crane may be 25-30 years old. the resistor bank need frequent maint. I thought of modifiying by adding VFDs & remove resistor bank. There is one more advantages of VFD is smooth start & stop.

even with the same motors , VFD can eleminate the res. bank. will do for me.

You cannot short slip ring and use it as cage motor as the rotor is not designed for this particularly thermal withstand time under hot and cold conditions will be lower than a cage rotor.

dsathya; i have done the same as your thinking, in the past 20 years i have shorted slip rings & installed VFD drives, their are still operating with no problems perry

dsathya,

I don't see any reasons why you cannot make the change.

I worked for a while with a water/waste-water utility. We converted a number of slip-ring (wound rotor) motors to VFD speed control. The rings were shorted together by having a pair of bars brazed across them (this was to maintain balance). There were no observed problems with overheating of the motors or other problems (other than one motor on which the brazing had to be redone). With a slip-ring or wound-rotor type motor, to operate it at full speed, the external resistor bank is shorted to zero resistance; they are designed to operate indefinitely under this condition.

When you install the VFD's, consider using a VFD sized for the next larger size of motor, particularly if the VFD will be located in an area with elevated ambient temperature. Also install load reactors (3% or 5%) between the drive and the motor. Finally, you may need to add a resistor bank to the drive's DC bus if the deceleration under load is regenerating too much energy back into the DC bus. Unlike the resistor bank with a slip-ring motor, this one does not need a variable resistance; it is simply a load-shedding device to prevent over-voltage on the DC bus. There are terminals on the drive for this external resistor connection.

If, as mentioned by one person, you have two motors being run simultaneously to move the crane, you can install one drive for both motors, with separate overload relays. The drive manufacturers and their applications engineers can help you with the details of this.

Regards--JMM

Normally drives on cranes need higher starting torques as most of the time they are starting with 'load' on. Shorting of slip rings may not allow the heat generated at the starting time to be brought out of the motor for dissipation in the external resistances. this will overheat the rotor , more so in hot climates like in India. A shorted slip ring motor will work fine with low starting loads such as in pumps.

Can any body help, in case the motor need to be repalced by crane duty brushless async. motor, then what would be the equilvalent KW of the motor ?

Motor is 45 KW , 415 V, 3 ph , 60 HZ slip ring motor . No. of resitor stage is 5.

We will need some more details.You propose to use a cage induction motor.Duty cycle, No of starts / stops per hour,CDF.Particularly one has to be careful when replacing a slip ring motor by a cage rotor where the entire heat has to be dissipated in the rotor as against the slip ring where it can be done conveniently in the external resistance.Also the starting torque and Pull out torque matching is essential.

pls go through ABB software,which is available on net