Sizing VFD for Slip Ring Motor

Sizing for saving - RISI

I think you will find that link is actually about replacing a slip ring motor with a standard squirrel cage motor and VFD.

While that is not what the OP wanted, it is most likely the best way to go.

If converting a slip ring motor to VFD, you would first need to short the rotor bars, and this would best be done so that the brushes are made redundant, otherwise they will still need to be maintained.

You would also want to be a bit careful of the condition of the motor as VFDs can impose pretty harsh voltage spikes on an old motor's insulation. Possibly a dV/dT filter would reduce the risk here.

As for sizing, I seem to recall that converting a slip ring motor to VFD operation will result in a loss of around 25% starting torque. Perhaps others can confirm or deny this.

I have 129 Kw slip ring motor.is it ok to select 132 Kw VFD drive?

NO

I have 129 Kw slip ring motor.is it ok to select 132 Kw VFD drive?

I suggest you ask the VFD manufacturer or supplier.

Sizing for saving - RISI

NO, NO Read the advice given, it is not just sizing a VFD, collect the information suggested and get a reputable VFD supplier to advice you and also do the installation for you and the programming.

Shorting the rotor will cause problems starting due to low torque.

You can get VSD's designed for slipring current control.

I suggest you contact the VFD manufacturers of your choice and solicit their input.

If you want to ensure best case performance I suggest soliciting input from at least three (3) separate/different VFD field application service groups to obtain good solid information for comparison and decision making.

Some wound rotor motors utilize shorting of the windings during the starting cycle to provide enough torque for the application without overloading the motor.

If the motor you are using has this type of capability then it may be possible to permanently short the windings and remove the slip rings to lower maintenance cost.

If the existing motor and control does not utilize rotor winding shorting during the start cycle then you may have to replace the wound rotor motor with a high efficient squirrel cage motor.

If you are unsure about what type of motor and/or motor control is present, I suggest you contact the original motor manufacturer and get their input and advice.

Repeating the same question over and over in the face of reasonable answers is rude. You are essentially saying "You must be too stupid to understand what I am asking so I will say it again and again until you do." You have the answers, now act upon it accordingly.

And because apparently it is YOU that is having a difficult time understanding, here it is again. There is more to this than simply sizing the VFD. Find a supplier you trust and ask THEM what they recommend. Some VFDs can handle this, some cannot. My company makes 4 different VFD product lines. Two of them would have no issues, one would need to be seriously over sized, one of them would not work at all.

a few the questions..

Why do you need a VFD, because when you change the impedance connected to the rotor circuit on the slip ring motor, the speed/current and speed/torque curves can be altered of the motor you already have? YES? So why change to a VFD now?

Why do you insist to keeping the original motor? The advice you have asked for and got is sound and practical. Change the motor!

Is the slip ring motor rated for VFD operation? I somehow doubt it!

Perhaps TS is going to use the slipring motor as a double fed induction generator...

perhaps.. but I doubt it!

Dear Mr. Lalinda,

To use the existing motor for VFD application you have to check or arrange the following.

1. One bearing of the motor on coupling side or the the other side, you should have INSULATED BEARING or BEARING INSULATION, to avoid shaft current circulation.

2. Cooling Fan of the motor - when speed reduces the air blown will be reduced and hence a seperate and independent blower is required to keep the motor winding/body temp. with in limit.

3. A dedicated Earthing OF 2.5 TIMES NORMAL TO BE PROVIDED FOR THE VFD which means no other equiment should be connected to this Earthing.

Perhaps you have decided to use the existing slip ring motor for which you have to short the rotor to make it as a squirrel cage rotor.

Pl. note the starting torque will be less than the normal slip ring motor and hence take care of starting.

DHAYANANDHAN.S

<...We...> pick up the phone and <...we...> speak to the VFD manufacturer, who, on being given the motor details, will provide <...we...> with a quotation to supply the right one.

Next!

Friends,

In the 1990's, when I worked for a water and wastewater utility we had a number of wound rotor motors (slip ring motors) that were in sewage lift stations with "Flowmatcher" type of controls (slip rings connected to copper bus bars that sat in a tank, in which the level of the salt solution was continuously controlled by the level of sewage in the wet well). They worked well, but the maintenance of all the components was very time consuming. We switched them all to VFD controls as follows:

1. All VFD's were chosen at a typical 50% oversize (to allow operation at 50-deg C with no loss of expected life).
2. All slip rings were shorted together and the brushes and brush holders removed.
3. Line and load reactors were always used (typically 2 or 3%), to protect the motors from dv/dt spikes.
4. Motor lead lengths were typically 50-feet (16-meters) or less, but never over twice this distance.
   
5. Grounding of the rotor shaft through brushes or rings was not done (no one in our area was aware of this topic then). Now, I would use them as a firm rule.

We had no problems. You don't tell us what the application is, so we can only guess. The advice from others is valid--ask the manufacturer to help you in selecting the proper VFD (size and style).

--John M.

Flowmatcher is still around!

I visited them a few years ago, I don't think there was anyone in that company under 60 years old, it was like a retirement home for electrical engineers. They only sell 3 or 4 systems per year now, but I guess it's enough to keep them in business.