5 replies to this topic

[anu_rags]

Dear All,

I have a customer whose slip ring motor is presently been run through a Slip Ring Starter (Oil immersed). I want to install VFD on this motor. I understand that we can short the rotor bars and convert the slip ring motor to almost like an AC induction motor. But the customer wants to retain the Slip Ring starter as bypass if incase the VFD fails.

I think that it wont be possible to use the Slip ring starter? Is it possible anyway that we can use this Slip ring starter as a Star Delta Starter or DOL starter? Pls guide.

Regards,
Anurag

[jraef]

Please explain why you feel the existing slip ring starter cannot be re-used. Is it defective or damaged in some way?

[anu_rags]

I have not done retrofit on any slip ring motor till now with Variable Frequency Drive. But I have the understanding that we have to short the rotor bars of slip ring motor and make its construction similar to AC induction motor, to enable it for VFD. Now if we are going to short the rotor bars then I think if we have to go back to Slip Ring Starter then again we have to physically remove the shorting at the rotor bars.

Is there any other way of doing this?

[niallnz]

anurag

The final stage of your slip ring starter applies a short circuit to the rotor slip rings. If you are determinded to retine the ability to use the slip ring starter then simply leave it in circuit but locked into its final (Shorted)position. However you will still have to maintain the slip ring brushes, and if you start to run the motor at less than rated speed the brush wear may increase (don't know for sure).

Slip ring starters are not normally installed to vary the speed of the motor, there are usually fitted to provide a reduced current and/or high torque starting method. A good correctly sized AC drive will acheive the same result, but causion must be exersized when appling an AC drive to a slip ring motor. The slip ring motor is typically old and its winding insulation may not take to kindly to the output of a drive.

[mariomaggi]

anu_rags,

you have to avoid potential causes of failures.
If you use brushes, it is possible that randomly there is a short interruption in current flowing through brushes.
This fact could damage your inverter.

Instead, I would recommend to lift brushes, and to short rotor bars in another way. In case of inverter failure, you have to remove the short-circuit connection or to saw it (you don't mentioned power, therefore I cannob be more precise).

Regards
Mario

[anu_rags]

Thanks for your feedbacks.

The motors on which Im considering to do this is of 75kw and 7.5kw. They are running pumps at booster stations.

My idea was also to short the rotor bars and remove the short circuit incase of VFD failure. But as Niallnz says that winding of the slip ring motor is weak for inverter output. Will it be good to install Output Choke on the output of the VFD to reduce the dv/dt spike?

Regards,
Anurag