resistance starters 1

[panelman]

Guys

I have a couple of 30 year old 8 pole 50hp slip ring motors with resistance starters.

I know I can short out the brushes and run them off a softstart but does anyone know if there is likely to be enough meat in the sliprings to let me drill & tap some copper braid to them so I can get rid of the brushes all together?

Obviously I'm looking for guidance rather than guaranteed solutions ie, is it even worth investigating?

 

[edison123]

You could try to terminate all the 3 phase leads into one slipring thereby avoiding the brushes.

But whether a 30 year old winding stand up to soft start is another question.

 

[panelman]

Thanks for that, I haven’t even looked at the rotor to see how the leads are connected to the rings.

I wouldn’t have thought a nice gentle soft start would stress anything more than the resistance starter does

 

[waross]

A small point, I think that your motor is a wound rotor motor with a wound rotor starter.
A primary resistance starter is something else.
A wound rotor motor has unique characteristics compared to conventional induction motors. The slip rings and resistance change the impedance and the characteristics of the rotor. A wound rotor motor can be designed to run at high slip frequencies (low speed or even locked rotor) without overheating the motor.
The expense of wound rotor motors and controllers was only justified for starting extremely high inertia loads. The other major application was for hoisting and similar applications where the ability to run for indefinite periods at reduced speeds was needed.
Wound rotors were occasionally used as torque motors.
In your instance, you have not told us the application.
However, I would not recommend a soft starter.
I would consider the following steps;
1> Rebuild the existing starter. For the cost of a soft starter you should be able to restore the wound rotor starter to original condition and continue to use it for years.
2> Is your application still a high inertia load or has the motor been put to other uses with easier starting duty?
If you really want a soft start, consider changing the motor to a conventional wound rotor induction motor when the soft starter is installed.
3> How is your budget? If you have an extremely high inertia load, consider changing to a VFD controller and a VFD rated motor.
It's been a lot of years but I remember wound rotor motors as very low maintainance machines. In a mill where the brushes on the DC machines needed service once a week, the wound rotor drives typically were checked several times a year.
Without knowing your application, I would recommend rebuilding the existing equipment.
I would be nervous applying a "chopped" waveform to an old wound rotor motor.
I answer to your comment on a "nice gentle soft start".
The wound rotor motor will develope high torque at low current with the resistance in the circuit. With the slip rings shorted, it is a relatively low slip motor and will require much more line current to develop the same torque. Starting on a soft start may be equivalent mechanically but it is not equivalent electrically. The effective current and motor heating will be higher with the soft start on the same starting profile.
respectfully
ps
The slip rings have enough "meat". They carry full running secondary current to the brushes during normal operation.
war

 

[DickDV]

I would not be too concerned about starting this motor with shorted slip rings and an SCR soft starter. This is commonly done and electrical problems are rare. One thing I have seen is, when shorting the slip rings, the motor rotor has resistance which is really too low making the motor too tight. The solution was to leave a little resistance between the rings.

Keep in mind however that the ability to soft start is entirely a function of breakaway torque of the load. If your load breaks away easily (a large flywheel on good bearings, for example) a very soft start is possible. On the other hand, if your load resembles a belt conveyor outdoors in an ice storm, soft starting is clearly not going to be possible due to the need to "crack" loose all of that ice.

 

[panelman]

it's a pump load so nice and easy to get going

 

[Marke]

Hello panelman

Shorting the rings out is ery commonly done with VFDs, but if you short out the rings and use a reduced voltage starter such as soft starter, you may have problems.

I would definitely try it using the brush gear before you do any permanent modifications.

1) A reduced voltage starter will reduce the voltage applid to the motor during start. This will reduce the current by the ratio of the voltage reduction and the torque by the square of the voltage reducion.

2) The Locked Rotor characteristics are essentially a function of the rotor resistance and reactance.

3) When the rotor of a wound rotor motor is shorted, it will commonly exhibit a locked rotor current in excess of 1200% and a locked rotor torque of less than 60%.

4) If we have an effective LRC of say 1200% and a LRT of say 80%, then at a reduced voltage start at 300% current, we would have a start torque of 80/4/4 = 5% which will not start anything. To achieve 40% torque which we will probably need to get a pump to speed, we would need to have a start current of 1200/rt3 = 700% current.

While there certainly are cases where the rotor has been shorted and the motor has worked with a soft starter, the majority of cases require some rotor resistance in order to develop sufficient torque to start a slip ring motor and load. - Typically, use the final stage of rotor resistance as used on the existing wound rotor starter.
- be careful!!
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jraef]

I agree with Marke, do NOT short the slip rings if you are going to use a soft starter. That is commonly done on VFDs, but it can be very problematic on soft starters. Leave one step of resistance in, then short it out after the soft starter is done.

Another problem not mentioned by Marke but alluded to by waross, is that not all WR motors were designed to have the amount of current that will flow in the rotor when starting with the rings shorted. In other words, they need to have resistance during start to avoid damaging the rotor windings.

JRaef.com
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[aolalde]

I agree with the opinions above. Use a VFD if you want to get rid of the brushes and external resistance bank, if so, short circuit the rotor terminals before the rings connection, not the rings. "Leaving some resistance" could be very hard to achieve, figure out the rotor amperes and calculate the heat dissipated Ri^2 on those resistances.

 

[LionelHutz]

I'll throw in with the others here. It's not a good idea. I have seen it done and it works but it's far from ideal.

A VFD controls frequency so it basically always operates the motor at rated speed. Since you short the slip rings of a wound rotor motor to run it at full speed shorting them when using a VFD will work well.

Jraef recommended one step of resistance with a soft-starter. I will confirm that this works very well. You can easily start conveyers and ball mills using this method so a pump is a no brainer.

If you just must do it, buy a heavy duty rated soft-starter. The motor will require a lot of current to start and it will eventually blow-up a light duty soft-starter during starting. Probably a heavy duty unit rated for a motor about 1.5 times the one you actually have would be about right.