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8 comments
Associate

Join Date: Jun 2009
Posts: 49

VFDs and Slip-Ring Induction Motors

08/12/2009 12:26 AM

Hello !!

Can a VFD be used with a Slip Ring Induction Motors for Energy Saving?

If No , then what modifications need to be done.

Can a Soft starter be used with a Slip Ring Induction Motor?

Kindly guide.

Thanks

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Commentator

Join Date: May 2009
Location: Centurion, South Africa
Posts: 87
Good Answers: 5
#1

Re: VFD and Slip Ring Induction Motors

08/12/2009 2:06 AM

Yes to operate with a VFD, but you will have to short out the rotor circuit on the motor. The motor will then operate as a standard squirrel cage motor driven by a vfd.

I will not recommend driving it with a soft starter. Chances are that the insulation of the motor will not be able to withstand the stresses placed on the winding by the soft starter during start up. These stresses will be more severe if the load requires a high starting torque motor.

Remember a slipring motor was initially installed in the application, because of the higher starting torque required (slipring motor = higher starting torque with lower starting current compared to a squirrel cage motor). With a soft starter the starting torque reduces with the reduction in voltage during start up (see other threads in this forum for this explanation). The motor driven by a soft starter might not accelerate the load, resulting in a trip during start up.

Driving the motor with a vfd (by doing the correct sizing of the vfd), you will have higher starting torque with lower currents (same as operating the motor as a standard slipring motor). With the lower starting current you will not have any additional stresses on the motor winding.

The only problem which may arrise by supplying the motor with a vfd is the insulation system of the motor. Depending on the age of the motor, the winding insulation of the motor might not withstand the spikes generated by the vfd. This will lead to a premature winding insulation failure on the motor. I suggest you contact the manufacturer of the motor, to establish if the insulation system of the motor will be able to withstand the supply generated by the vfd.

If it is a new generation motor chances are that the insulation system will be ok, because most manufacturers uses insulation suitable for vfd applicatio as a standard.

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Power-User

Join Date: Jul 2008
Location: UK
Posts: 205
Good Answers: 50
#5
In reply to #1

Re: VFD and Slip Ring Induction Motors

08/13/2009 6:48 PM

I agree with your comments re control via VFD - essentially good control and no problems in my experience. The soft start may have the effect of prolonging stall currents at the varying voltages when starting slowly causing local heating which may damage the windings, the VFD offers 'at speed' control, efficiency and current draw with good torque.

If the motor is relatively close to the VFD then the voltage will appear as normal, so if within 10m of cable I don't think you need worry unduly re insulation breakdown under PWM. If any doubt with a 20-50m run, fit a standard motor choke and low PWM carrier frequency (4kHz or less) should be more than sufficient. The 'spikes' only cause problems when 'reflected' by the motor coinciding with incoming pulses causing volt doubling which can 'puncture' the insulation.

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Guru
Engineering Fields - Mechanical Engineering - New Member

Join Date: May 2008
Location: CHENNAI, TAMIL NADU, INDIA.
Posts: 1217
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#7
In reply to #1

Re: VFD and Slip Ring Induction Motors

02/20/2012 12:28 PM

Dear Mr. Aldego,

You have nicely covered the subject.

I want to supplement to your points. In addition to the shorting the slip-Rings, the following is to be done.

1.Provide additional COOLING ARRANGEMENT for the MOTOR - WHILE RUNNIG ON LOW SPEED, ORIGINAL FAN PROVIDED TO WORK AS SLIP-RING MOTOR WILL NOT BE SUFFICIENT TO PROVIDE THE REQUIRED COOLING.

2.Provide INSULATION to the NON-DRIVE END BEARING.

3.PROVIDE Additional EARTHING to the motor.

4.If the distance is more than 10 Metres for the motor from the panel - provide Hormonic Filters/Chokes - to control the ill-effects of Hormonics.

5.Ensure that the motor is NOT started against Load.

THANKS.

DHAYANANDHAN.S

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Guru

Join Date: Mar 2007
Posts: 2546
Good Answers: 105
#2

Re: VFD and Slip Ring Induction Motors

08/12/2009 2:15 AM
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Guru

Join Date: Dec 2007
Location: Mumbai, India
Posts: 1681
Good Answers: 21
#3

Re: VFDs and Slip-Ring Induction Motors

08/13/2009 8:49 AM

Normally for the EOT Cranes when slip ring motors are used with resistance the speed is controlled through master controller. motor starts with slow motion and picks up speed slowly to avoid jerks. There is no need for VFD drives.

Suresh Sharma.

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Anonymous Poster
#4
In reply to #3

Re: VFDs and Slip-Ring Induction Motors

08/13/2009 9:26 AM

I've seen those master controllers, they have a built-in VFD.

Yahlasit

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Associate

Join Date: Jun 2009
Posts: 49
#6

Re: VFDs and Slip-Ring Induction Motors

08/31/2009 6:56 AM

Thank you gentlemen for your expert comments.

I appreciate,

Regards

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Guru
United States - Member - New Member Engineering Fields - Power Engineering - New Member

Join Date: Sep 2006
Location: California, USA, where the Godless live next door to God.
Posts: 3726
Good Answers: 545
#8

Re: VFDs and Slip-Ring Induction Motors

02/20/2012 6:04 PM

"Can a VFD be used with a Slip Ring Induction Motors for Energy Saving?"

That actually depends upon the application, not just whether or not a VFD is present. If it is a variable torque (quadratic) load, then yes, using a VFD over some other means of flow reduction will save energy. If it is a mechanically constant torque load, basically anything OTHER than a variable torque load, then no, there will not be any enegy savings. Common misconception.

If it is a fixed speed application (as evidenced by your query on using a soft starter), then adding a VFD will actually INCREASE your energy consumption because of losses in the VFD system.

"Can a Soft starter be used with a Slip Ring Induction Motor?"

Yes you can successfully use a soft starter (again, IF it is fixed speed), as long as you leave one step of resistance in the circuit. There are on-line calculators available that will provide you with the right amount of resistance to have in there. But what you cannot do is to short out the rotor and start it with a soft sgtarter, the current to torque ratio will be too low. You leave in one step of resistance during soft starter acceleration, then when the soft starter goes into bypass, you also short the rotor. It's done all the time.

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Users who posted comments:

Aldego (1); Anonymous Poster (1); dhayanandhan (1); JRaef (1); MalcolmK (1); Rooney (1); sb (1); suresh sharma (1)

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