4 replies to this topic

[boby]

we have a 90 hp slipring motor of stator full load amps -119, rotor amps of-83. pls give suggissitions can we regulate the speed of this motor over wide range by using vfd

[mariomaggi]

Dear boby,
are you sure that stator windings are OK for PWM supply? In this case, you can make a short circuit on rings, or you can add 3 fixed resistors.
What kind of application do you have in mind? Suggestions could be different, depending on application.
Regards
Mario

[marke]

Hello Boby

Welcome to the forum.

Provided that the slip ring motor is in good condition, you can use it with a VFD. All you need to do is to short out the rotor at the slip rings.

Using a VFD with an old motor can lead to problems with the stator insulation. The stresses on the insulation with a VFD are higher than ont the normal supply, and if the insulation is poor, then you could have a problem.

Best regards,

[jOmega]

Boby,

There are hundreds, if not thousands of wound rotor motors operating today with VFDs as their source of power and control.

In applying a VFD to a legacy wound rotor motor, insulation is the major concern.

First, the insulation was never intended to be sourced from a VFD using fast switching IGBTs or even power darlingtons in the inverter bridge. Thus, the insulation. even when new, would have trouble surviving the dV/dt (dU/dt) induced high voltage pulses with steep wavefronts.

Second, add to this consideration the fact that in legacy motors, the insulation has degraded with time and temperature, and is more succeptible to dV/dt (dU/dt) induced insulation failure.

But that has not kept the industry from applying VFDs to wound rotor motors.

Here's a procedure that can remove the risk from the application of a VFD to a wound rotor motor.

1. Using a DC impulse (surge) tester, evaluate the ability of the windings insulation to withstand dV/dt (dU/dt) stress
2. if stress test shows weak insulation, connect a dV/dt (dU/dt) filter or sine-wave filter at output of VFD
3. if insulation stress test is not possible to obtain, connect dV/dt (dU/dt) or sine-wave filter at output of VFD

Click on below underlined link for an informative tutorial on DC Impulse (surge) testing of motor insulation.
In particular, see pages 7 thru 11.

Insulation Stress Testing

Not everyone has an instrument such as the Baker Instruments AWA 2.2 in their compliment of test equipment. Perhaps you can locate a motor repair shop or facility that has such an instrument or a similar type portable insturment that could be brought to your facility and field test your motor. The cost would be well worth it.

If you are unable to locate such service in your region, then I would suggest that you proceed directly to step 3. above.

If you are able to locate such service, and the stress test shows that the motor insulation is weak, then certainly, step 2. above must be followed.

Your choice.

For information on dV/dt (dU/dt) filters, click on the below links.
(These links are offered for information only and are not necessarily an endorsment thereof.)

dV/dt Filters from MTE Corp.

Same as above : available thru Westek Electronics Pty Ltd

Westek also represents a line if Schaffner filters that are suitable for the use described herein.

Schaffner Output Filters


Since I don't know where you're located, in case you're in the USA or Canada geography, I offer this additional link for consideration.

KLC & KMG Filters from a company know as TCI

Hope this helps, and come on back if you have further questions about the above.

Kind regards,
jΩ

[boby]

thank u sir now i am very clear abt this.... thank u very much

Boby,

There are hundreds, if not thousands of wound rotor motors operating today with VFDs as their source of power and control.

In applying a VFD to a legacy wound rotor motor, insulation is the major concern.

First, the insulation was never intended to be sourced from a VFD using fast switching IGBTs or even power darlingtons in the inverter bridge. Thus, the insulation. even when new, would have trouble surviving the dV/dt (dU/dt) induced high voltage pulses with steep wavefronts.

Second, add to this consideration the fact that in legacy motors, the insulation has degraded with time and temperature, and is more succeptible to dV/dt (dU/dt) induced insulation failure.

But that has not kept the industry from applying VFDs to wound rotor motors.

Here's a procedure that can remove the risk from the application of a VFD to a wound rotor motor.

1. Using a DC impulse (surge) tester, evaluate the ability of the windings insulation to withstand dV/dt (dU/dt) stress
2. if stress test shows weak insulation, connect a dV/dt (dU/dt) filter or sine-wave filter at output of VFD
3. if insulation stress test is not possible to obtain, connect dV/dt (dU/dt) or sine-wave filter at output of VFD

Click on below underlined link for an informative tutorial on DC Impulse (surge) testing of motor insulation.
In particular, see pages 7 thru 11.

Insulation Stress Testing

Not everyone has an instrument such as the Baker Instruments AWA 2.2 in their compliment of test equipment. Perhaps you can locate a motor repair shop or facility that has such an instrument or a similar type portable insturment that could be brought to your facility and field test your motor. The cost would be well worth it.

If you are unable to locate such service in your region, then I would suggest that you proceed directly to step 3. above.

If you are able to locate such service, and the stress test shows that the motor insulation is weak, then certainly, step 2. above must be followed.

Your choice.

For information on dV/dt (dU/dt) filters, click on the below links.
(These links are offered for information only and are not necessarily an endorsment thereof.)

dV/dt Filters from MTE Corp.

Same as above : available thru Westek Electronics Pty Ltd

Westek also represents a line if Schaffner filters that are suitable for the use described herein.

Schaffner Output Filters
Since I don't know where you're located, in case you're in the USA or Canada geography, I offer this additional link for consideration.

KLC & KMG Filters from a company know as TCI

Hope this helps, and come on back if you have further questions about the above.

Kind regards,
jΩ