Time for start-up between two motors 1

[redox]

We have two motors on a conveyor belt (Each 50kW) How do you determine the time-delay between the start-up time of the two motors? (DOL-starters) I don't want to start both imediatelly (due to the high start-up current), but I also don't want the time-delay between the start-up of motor1 and motor2 to be too long. (overloading of motor1, especially when there is load on the belt)

Is 0.5 seconds too long/short?

thanks
regards
RCC

 

[podo]

That would depend on the type of motor starter and mechanical coupler that are being used. On the mechanical side, if fluid filled couplers are used, the motors start under no load, and 0.5 second should be fine.
On the electrical side, if some form of reduced voltage starters are used, (auto-transformer, SCR, VFD,etc.) they should be started together. I do not know what DOL - starter means, so maybe this last statement is invalid.
If the motors are across the line started, I would be more concerned about belt slippage or damage than overheating the first motor.
Good luck,
Podo

 

[GordS]

If your sole purpose is to keep the total current down, then you need to know the motor accelerating time which is related to the mechanics of your system and motor design. Then your time delay is equal to the time it takes the first motor's current to drop. Of course, this assumes that one motor is even able to start the conveyor on its own.

 

[aolalde]

I recommend allowing full speed of the first motor before attempting to start motor 2, these are my reasons:

1- Combined inrush currents (DOL) will increase the voltage drop of the line, reducing the accelerating torque and extending the time to accelerate.
2- The substation and lines will suffer overheating (r*i^2) due to the combined inrush currents.

Measure the inrush current of the first motor to know the time when it drops to the steady load current (3 to 5 seconds), and then start the second motor.

 

[macmil]

This is actually quite a complex question, and really can't be answered properly without full information on the conveyor, and the motors and drives. Typically double drives are provided to reduce the drive tension on the belt both at start up and during operation. Assuming the motors are in fact on separate drive pulleys (sometimes there are two motors on a single pulley ),there will be some belt stretch depending on the distance between the drive pulleys which possibly may allow a time delay between starting the two motors. However, typical conveyor design will require both motors to deliver enough torque to start the conveyor. Have you tried starting it with just one motor ?
I much prefer the suggestion of having fluid couplings, or magnetic couplings, to allow a cushioned start. Because these couplings will significantly reduce the period of maximum in rush current, it should be possible to stagger the starts without overstressing the belt. The actual delay will be a factor of the design of the belt, but I would try to keep it as short as possible. The 0.5 second that you suggest should be enough. Monitor the start sequence with recording ammeters to ensure you are getting what you want.But without the couplings, you will probably just have an additional 0.5 second of locked rotor current on the first motor, which won't help your situation.

 

[redox]

Thanks all!

The motors are not on the same pulleys. (2 pulleys on one belt) There are fluid-couplings between the motors and the gearboxes. The problem is that one motor can't take a fully loaded belt away on its own. (Without any material on the belt there is no problem) With a long delay between the motors (3seconds) the first motor eventually trip on overload.

Macmil, what will happen with two motors on one pulley? (Both with fluidcouplings) Will there be any difference?

Regards
RCC

 

[macmil]

Two motors on the one pulley will not improve things, unless there is a slippage problem with the other pulley. There is a real risk of overstressing the belt, or more probably the splice, which is probably why it was designed with two drives to begin with. It sounds as though the real problem is starting a loaded belt, and you would have to go back to the original design criterria to see how that was to be accomplished, or if the design assumptions are close to actual. A 3 second delay seems a lot with fluid couplings. Typically the in rush current only lasts a few cycles. Have you taken current readings during a start to see the inrush and starting current profile ? you will probably need a high speed trace. If starting the belt loaded is part of the operational requirements, you may want to review the whole belt design.

 

[podo]

Redox,
We have used fluid couplers with extended times before they start to transfer torque to the load. They are called double-delay couplers. They are made by all of the fluid coupler people. They will allow for a short delay between motor start-ups.
Also, 3 seconds seems like a very short period of time for an overload to react, especially with a fluid coupler drive system. You may want to look a little deeper. Does this circuit include any type of "jam" protection? This may be part of the problem.
Podo