Belt conveyor softstart

[mdlafferty]

Need to start a belt conveyor with 2 motors. Each 300 HP 4160 volt, 39 fla, 248 lra, NEMA design B, 120% locked rotor torque. Motors start and run together.

Plan to use 2 softstarts to limit starting current. Big problem is electric utility that allows only 1645 kVA inrush (for both motors combined).

Will use fluid coupling with "unloaded motor start" feature so that motor gets up to speed with minimal load. Once motor is up to speed the fluid coupling will engage very slowly to ramp up the conveyor speed.

I need to be sure that motors will start and stay within utility inrush requirement of 1645 kVA.

1645 kVA / 2 motors = 825 kVA inrush per motor

Starting at 2787 volts (67% voltage) I believe will reduce the starting current to 67% (166 amps) and the torque to 45% of locked rotor torque. This results in starting kVA of 2787x166x1.73=800 kVA.

Is this a sound approach? Are my voltage/current/torque assumptions okay? Any comments are much appreciated. Thanks. Mark

[marke]

Hello mdlafferty

 

I would suggest that you use delay fill fluid couplings and start the two motors in sequence rather than together. This way, you can have a higher start torque per motor, it will get up to speed quicker and you will have less chance of the fluid coupling preventing the motor from getting to full speed by applying torque in excess of the start torque developed. I would be happier to see a start torque of 60% or better because the start torque will sag at somewhere between 50% and 70% full speed. If you use a current limited or current ramp soft start with a reasonably fast ramp rate, you could use the full voltage indicator from one starter to enable the second, preventing any powwibility of both starting at the same time. If the two motors are positioned at diverent points on the belt (common) I would recommend starting the front motor first.

I would suggest that you need to set up the delay fill fluid couplings to have a maximum torque coupling in the order of 160% if you are going to start a loaded conveyor.

The major disadvantage of this approach is that the fluid couplings have a full load slip of up to 5%. This is loss power and is dissipated as heat. As you lengthen the fill time, you increase the start time for the conveyor and the heat dissipated in the couplings.

 

Best regards,

[mdlafferty]

Thanks for your reply.

[Anderson]

Hello Marke

Could you tell me if the two motors are positioned at the same points on the belt,that is ok to use one soft starter to drive the two motors and start at the same time?The conveyor started will be unloaded.In this case,is it necessary to use delay fill fluid couplings?

Thank you.

Best regards,

Anderson

[marke]

Hello Anderson

 

If both motors are identical, then you can operate them in parallel. This does not just mean the same size, the Locked Rotor Torque, Locked Rotor Current and the slip of the motors must be exactly the same, or one motor will doo all the work. Both motor MUST have exactly the same rated speed at full load.

 

Best regards,

[jraef]

And I'll emphasize exact, as in not only the same manufacturers and model, but possibly even the same production run. I have seen a situation where 2 motors from the same mfr. were purchased months apart and the slip was different by 2 RPM, which made one motor bear all of the load and trip.

[Anderson]

Hello marke and jraef, thank you very much for your helpness.

I have another question about this. If two motors with two soft starter seperately are used to drive a belt conveyor, then is it necessary to use the delay fluid couplings in this belt? The belt conveyor will be started unloaded.

Thanks

Best Regards,

 

[marke]

Hello Anderson

 

If the belt is always started unloaded, then there is no need for the fluid couplings.

The fluid couplings are a means of allowing the motor to operate at full speed while the belt is starting this cuases the motor to operate at high efficiency rather than the low efficiency that normally occurs during start.

With the delay fill, the motor is able to start very quickly, reducing the overload time during start, but the fluid couplings absorb the energy during start rather than the motor.

Fluid couplings typically have a 3 - 5% loss during run.

 

If you start the conveyor loaded, it can require up to 180% torque to get it going. This is where a fluid coupling will work with a bad motor whereas for a good result with a soft starter, you would need to ensure that you had a motor with a high start torque.

 

Best regards,

[Anderson]

Thanks Marke.

Best regards,

 

[Anderson]

Hello, Marke:

From your experience of the MV soft starter, can you tell me the name of the material handlling project which the MV soft starter have been used? And,can you believe it is reliable to use the MV soft starter in the belt conveyors?

Thanks.

Best regard.

Anderson