tandem motors 1

[littleriver]

I want to direct couple two 500hp 480v induction motors to a grinding mill,one motor at each end of the shaft.I would like to use two soft starts to start the motors at the same time.Mechanically we are capable of handling the hp.I would like to know if this is possible without using fluid couplings or vfds to account for slip and speed control.The
two motors are rated at the same rpm.

 

[jraef]

Assuming you realized your error in posting in the "other topics" forum and moved your question here (as it should be), I am re-posting my answer here and will red flag the cross post.

There are several issues here.

First off; will it work? Yes, it is done all the time. The critical thing here is that the motors must be exactly the same, as is make, model, slip %, everything. If not, one motor will drag the other.

Second; Can I use 2 soft starters? Yes, but there are additional complications. While I have made it work when someone before me has designed it with separate soft starters starting simultaneously, that is NOT the best approach. It is virtually impossible to achieve uniform current limit or ramp settings between them, so again, you end up with one motor dragging the other anyway. If this is the case, there was no point in using the second soft starter. Let me explain. If one motor is capable of accelerating the load by itself, as is usually the case with grinders by the way, you can use a soft starter on one motor, then bring the second motor on across-the-line after the load is at full speed. You will get only a brief (as in 1 or 2 cycles) inrush to magnetize the windings, but no appreciable starting current surge. I do this all the time with large rock crushers, works great and costs a lot less.

If one motor is incapable of accelerating the load by itself, then your 2 soft starter method is at risk to not work anyway. Whichever motor is allowed to have even a slight amount of voltage more than the other will attempt to bear the entire load and trip on overload. Sometimes the soft starter outputs can be close enough so that motor #2 can contribute some, but my experience is that is rare and likely a chance occurrence rather than an accuracy issue. So if you need more than 500HP to accelerate the load to full speed, use 1 soft starter big enough to start both motors. Then all you need is separate overload relays for each motor and you have a complete system.

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[LionelHutz]

Two soft-starters should work fine. Just make sure they are current controlled and that the current ramp settings are programmed identically. In other words "pot" controlled starters are no good - you need digital keypad settings and RMS metering. Then, apply the start command to both at the same time. The starters need to be interlocked so that if one trips it stops the other one. We've done this before without any issues.

However, I would get a quote for starters both ways - two individual units and one for both. I expect the individual starters will be cheaper though.

The load sharing is something you should take-up with the motor manufacturer.

Honestly, I don't buy the theory that a slight inbalance between the starters would cause one motor to bear all the load. Im somewhat guessing but it's probably closer to the square of the difference. ie, 5% voltage (or current) difference between the units would be 25% sharing difference - one motor sees 63% of the load and the other sees 37% of the load. It's probably not even that much. At any rate, two good current controlled starters should easily beat 5% current difference and have good sharing.

 

[aolalde]

Assuming the 1000 BHP load is not imposed until the system has started. You can start with one motor at full voltage and then after it reaches full speed connect the second motor, at full voltage too, that way it will have a very short time current peak.
The two motor's design must be as close as possible to share the load. But if they are not identical the one with lower slip will take more load. The line currents will give you an idea of the load sharing. If the total load is less than 1000 HP you could have some margin to play without overloading out of allowed limits one motor (see NEMA MG1 standard).

 

[mc5w]

If you want tp use variable frequency drives instead of soft starters and then transfer to bypass contactors, the 2 drives need to be to be ordered with torque sharing capability where the 2 drives communicate with each other. However, that is a very expensive solution and if your load is hard to accelerate the 2 drives need to rated and programmed to deliver 200% of motor full load current and would need external motor overload relay because the electronic relays in the drives are programmed for 200% of full load current.

An interesting alternative is if a 100 HP or 200 HP motor can accelerate the load before load is thrown on it, you could belt that smaller motor to the larger motors, soft start the small motor, energize the big motors one at a time, and then deenergize the small motor. This is a rather old fashioned idea from the days when design A motors had absurdly high locked rotor current and soft starters just did not exist. In some cases the smaller pony motor had a torque converter.

I have also seen some rather new industrial machinery that does use a torque converter instead of a soft start and this method is frequently used for amusement park rides. Since torque converters can be purchased with locking clutches this is a very efficient alternative to soft starters. However, your environment might be too abrasive or corrosive for this.

My my brief experience with crushers is that I would rather be dealing with 4 smaller crushers. If something goes down you only lose 1/4 of production capacity. Also, rock crushers require enough in the way of daily maintenance that a welder needs to have a permanently connected ground cable.

Mike Cole

 

[LionelHutz]

In my last paragraph i was talking about the motor start with soft-starters. I left that part out. Once full speed is reached the starters are applying full-voltage and the motors still have to share themselves.

 

[macmil]

Another possible way to achieve your intended result is to use a magnetic coupling between each motor and the drive shaft. The motors could be arranged to start with an appropriate time lag. There would be the sdditional benefit of reducing vibration which might be transferred from the mill back to through to the motors. Also, the motors would not have to be absolutely identical, as the couplings would allow the motors to run at slightly different speeds.