What if I Short Slip Ring of IM

Yes you can short the slip rings. Do it on the rotor, not after the brushes or you will have problems eventually. That may entail that the rotor be re-balanced due to the shorting method.

Also use a very good vector drive that can be programmed for all of the motor parameters, not one that relies upon autotuning alone.

You must contact the crane manufacturer. Crane accidents are nothing to laugh at and as much as many of the posters here like to claim to be "experts" none of them know as much about your crane as the manufacturer.

From the description you gave, it sound like you have a 3 phase asynchronous motor that uses resistors in the rotor circuit to control speed, and you would like to change the control scheme from resistors to a VFD perhaps.

You can short the rings at the motor. I work on a very similar project involving a crane on a dam. We simply shorted the wiring at the motor as compared to shorting the slip rings on the rotor itself. This saves from having to remove the motor for any re-balancing of the rotor. The installation has preformed trouble free for several years.

However, as Guest suggested, the crane would have to be re-certified after such a modification.

Anvesh,

If resistance box is problem then better replace it with stainless steel resistance box instead of replacing motor or shorting slip rings. Slip ring motors are provided to smoothen starting motion of the crane. If you replace it with sq.cage motor it will give jerky starts to the crane.Slip ring motors are expensive. Why dont you contact crane expert from your area?. He will inspect the crane and give you proper advice.

Suresh Sharma

He said he is going to replace it with a sq. cage motor AND A VFD. There will be no "jerky starts" (unless you program the VFD as such, a pointless exercise).

Anvesh,

The slip rings connect the rotor to the "secondary resistors" which, by adding resistance to the rotor's windings will reduce the motor speed. Typically you start the motor at low speed and then change settings on the speed control as you want it to run faster. At the final (high speed) setting the slip rings have no external resistance (other than the wiring and contacts), so they are effectively shorted.

I can think of four problems with this fairly simple, low-technology, and bulky speed control system: 1) the speed is changed in discrete steps instead of continuously; 2) the resistors get hot and can or will burn out; 3) the contacts in the drum switch which selects the speed can or will burn out; 4) the brushes and commutator wear out or get burned out.

The first is a characteristic of this type of control and probably will be OK in most cases. This type of starter and controller is robust and will last a long time, but needs maintenance and inspection regularly.

The rest are inspection and maintenance issues. Perhaps replacing or repairing the components will be the best fix. If the resistors were sized properly they will get very hot but be within their range of safe application. Prior replacements with resistors of the proper resistance but too low of a current or power rating will have saved money then but resulted in many more service calls and failures later.

As others have posted here, as well as in an earlier thread on the same topic, crane motors and cranes in general need caution when making significant changes such as you suggest; with factory consultation, inspection, and certification being very desirable. If you do not have some means of speed control, you will have a crane which is hard to control well and burns out its main contactor and possibly its motor sooner (because of full-voltage starting and higher inrush currents), in addition to putting undesirable jerking or shock loads on the cables, etc.

Perhaps your problem is a service technician with poor information or inadequate knowledge/skills in this area.

--JMM

As others have already stated, You CAN do this. One thing noone else has supplied though is that if this is for your hoist, you need to add an encoder to the mix. The reason that you would add an encoder to the hoist motor is for feedback to the inverter. This way the inverter can tell if the motor has lost its grip and is freefalling. This allows the inverter to set the cranes brakes. Also, depending on where you are, most countries require you to have an encoder on the hoist motor with an inverter if you are indeed using an overhead crane.

Just as a general rule for clarification, Wound Rotor Induction Motors were typically used on crane travel drives, not the hoists. The caution expressed above is valid if, by chance, this is the hoist drive. Then for sure you MUST use a full closed loop vector drive with an encoder feedback. Better yet, hire an experienced expert.

But if it is for the travel drive motor, there is little danger involved and VFDs are used in that application all the time without issue.

As clarification, I used to inspect overhead cranes. There are a LOT of the older AC cranes that use a 5 step wound rotor motor. P&H and Milwaukee in particular had models like this. Usually this was only for cranes that were about 20+ Tons in rating, but they do exist.

And I would like to add a whole hearted yes to fitting VFD's to a travel motor with Open loop (no encoder). I have also fitted VFD's to travel motors before. Depending on how much the customer wanted to spend (usually as little as possible) I have even just shorted the windings from the wiring to a wound rotor motor. Of course if you do that you still have slip ring wear to worry about, but it does work. If you wish to short the bus bars at the slip ring it is usally best to send the motor to a motor rebuild shop and have them put shorting bars on the sliprings and balance the motor. If the motor is unbalanced you can tear it up in no time. If the motor is small it might even be more cost effective to purchase an inverter duty motor of the correct horsepower and fit it.