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slip ring motors and vfd's


charles ray

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Hello charles ray.

Welcome to the forums.

Yes, a slip ring motor can be used with a vfd, and because when under VFD control, the slip ring motor will never be operated under high slip, you just short out the rotor circuit. i.e. dispose of the secondary starter circuit completely. You can short out at the out put of the brushes, but you still need to maintain the brushes, or you can short out on the rotor itself and remove the brushes. You can not leave the rotor circuit open circuit though.

The only thing to be careful of is, you may have problems if you use a fancy sensorless vector drive due to motor characteristics that are different from the norm.

 

Best regards,

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I don't think that modern drives with "fancy sensorless vector" would be the concern of problems...if you choose the right one:P Most modern drives with SLV have an autotune function. This will calculate the motor parameters and store them in the EEPROM. The more sophisticated drives actually continue to calculate whilst the motor is running to compensate for errors made during the initial autotune. The only tricky part is following the manufacturers instructions...which can be a major project in itself!

 

charles ray;

The other trap is to ensure that the current delivering capability (overload) of the drive is sufficient for the motor you are connecting to it. For this you will need the motor curves.

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Mr TheDog,

 

Assuming you agree with marke's comment above;

 

"....and because when under VFD control, the slip ring motor will never be operated under high slip...."

 

....and I do, how can overload capacity become a trap?

 

VSD manufacturers only have to provide overload capacity to allow for the mis-setting of a VSD (ie insufficient accel time), or for the use of otherwise undersized motors, resulting in 'slip' and overload currents.

 

Applications requiring in excess of motor FLA (assuming the motor is sized correctly) are surely rare. In fact, I can't think of one, and all commisioned jobs I have been involved in have never required an 'overload capacity'.

 

Perhaps your experience is different?

 

Cheers!

 

BigMax;b;

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BigMax;

 

A WRM is generally used because of the high starting torque that can be achieved (up to 250% FLT). This is the ONLY reason I suggested looking at the drive overload capability. If it is an exsiting application (as this one sounds like), a WRM may have been used because of high starting torque requirement with little running torque. A good example of this type of load would be a loaded rock crusher. The drive must be capable of delivering the current demmanded by the motor/load.

Do you agree?

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Mr TheDog,

 

Yep, good point! Seems that driving WRMs with VSDs under some circumstances may require some caution.

 

It all sounds a bit 'messy' to me though. I like everything to be simple;b;

 

"If I don't understand it, it must be BS"

Quote by unknown author.

 

Cheers!

 

BigMax

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Yes, that is a good point, however with modern drives, it is quite practical to get 120 - 150% acceleration torque up to rated speed, and if you look at a typical high torque start using a slip ring motor, it has a limited number of stages. Plot the torque of each stage and then overlay them all and you find the the torque is varying from a figure, up to the maximum of perhaps 250% then dropping back down till the next stage takes over etc, so a plot of torque against speed has peaks and troughs. The greater the number of steps, the lower the ripple on the speed torque curve, and this essentially elevates the lower torque levels (in the troughs). So it isn't necessary to target 250% torque, probably more like 150%. The biggest problem can possibly be the initial breakaway in the case of a loaded rock crusher.

Best regards

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