# Amps Required From A Generator For Soft Starter Starting Motor

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We are using a Siemens 3RW4447-6BC34 (SIRIUS 3RW44) soft starter that has failed to start a 230 kw Siemens motor (380 V delta 60 Hz,415 A, 240 KW, cosphi: 0.89 and rpm 1786. IP55). The power source is a generator, 3 phase 380/220 V, 60Hz, RPM : 1800, Amps : 1140, KVA : 750 ,Stand by Kilowatt: 600. The soft start switches the motor off after the ramp up, just as there is a switch over to full pump running.

Siemens engineers claim that we require more than 1140 Amp for startup on their soft starter! Can anybody provide a calculation as to the voltage and amps required using a softstart?

We have a 750 KVA Doosen generator connected to a Marelli power unit as the power source. The motor is connected to a pump, that when off can be turned freely by hand and is brand new.

Commissioning of this 3RW44 has failed despite 3 visits from Siemens to calibrate the soft starter 3RW44 with the Siemens motor. We have had this unresolved problem going for 5 months now. We are a new shrimp farm in Saudi Arabia  and with no pump there is no farm.

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Hello Zaidevans

Welcome to the forum.

The minimum start current is determined both by the driven load and by the motor.

The driven load (pump) determines the torque curve against speed that the motor must develop to accelerate the pump to full speed.

The design of the motor, or more particularly the rotor of the motor, determines the current required to develop that torque.

With high efficiency motors, it is not uncommon to require as much as 450% current to accelerate the motor to full speed, so I would expect the start current to be up around 1800A.

This is a function of the driven load and the motor, not necessarily the soft starter.

If the generator is a) self excited and uses a single phase peak reading AVR, then it would probably have trouble starting this load. If the generator uses three phase averaging AVR and a permanent magnet generator excitation, then the Alternator could start this motor. The other problem is the engine.

Your comment that things let go as the motor approaches full speed, suggests to me that there is an issue with the engine not having sufficient overload capacity to start the pump.

The alternator delivers KVA and the engine delivers KW.

The KW is maximum as the motor approaches full speed. I suspect that as the motor approaches full speed, the load on the engine becomes too high and the engine begins to slow dropping the frequency below the soft starter trip frequency.

The start current can sometimes be reduced by closing a valve on the output of the pump, increasing the head during start.

A slow opening valve could be a solution.

Best regards,

Mark.

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Dear Mark

Thank you for the reply. I am getting the AVR specification from the genertor supplier. The pump is the axial flow type. I know that stopping the flow drops the Amps on a centrifugal pump, but does the same pronciple apply to an axial flow pump?

Hello Zaidevans

Welcome to the forum.

The minimum start current is determined both by the driven load and by the motor.

The driven load (pump) determines the torque curve against speed that the motor must develop to accelerate the pump to full speed.

The design of the motor, or more particularly the rotor of the motor, determines the current required to develop that torque.

With high efficiency motors, it is not uncommon to require as much as 450% current to accelerate the motor to full speed, so I would expect the start current to be up around 1800A.

This is a function of the driven load and the motor, not necessarily the soft starter.

If the generator is a) self excited and uses a single phase peak reading AVR, then it would probably have trouble starting this load. If the generator uses three phase averaging AVR and a permanent magnet generator excitation, then the Alternator could start this motor. The other problem is the engine.

Your comment that things let go as the motor approaches full speed, suggests to me that there is an issue with the engine not having sufficient overload capacity to start the pump.

The alternator delivers KVA and the engine delivers KW.

The KW is maximum as the motor approaches full speed. I suspect that as the motor approaches full speed, the load on the engine becomes too high and the engine begins to slow dropping the frequency below the soft starter trip frequency.

The start current can sometimes be reduced by closing a valve on the output of the pump, increasing the head during start.

A slow opening valve could be a solution.

Best regards,

Mark.

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An axial flow pump is usually a "propeller" type pump and the flow will reduce as the head increases. They are usually a high flow low head type pump, so yes, I would expect that you can reduce the start torque requirement by reducing the flow.

Best regards,

Mark.

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An axial flow pump is usually a "propeller" type pump and the flow will reduce as the head increases. They are usually a high flow low head type pump, so yes, I would expect that you can reduce the start torque requirement by reducing the flow.

Best regards,

Mark.

Mark,

Radial flow pumps - (Often simply referred to as centrifugal pumps)

The fluid enters along the axial plane, is accelerated by the impeller and exits at right angles to the shaft (radially).

Radial flow pumps operate at higher pressures and lower flow rates than axial and mixed flow pumps.

Axial flow pumps

Axial flow pumps differ from radial flow in that the fluid enters and exits along the same direction parallel to the rotating shaft. The fluid is not accelerated but is instead "lifted" by the action of the impeller. They may be likened to a propeller spinning in a length of tube.

Axial flow pumps operate at much lower pressures and higher flow rates than radial flow pumps.

ERGO......

The Axial Flow pump does NOT mimic the Centrifugal pump characteristics.

The Axial Flow pump has an increasing Head and BHP as the Flow is decreased.

BHP is maximum at/near shut-off ...

For a characteristic curve, I refer you to Figure 7 of the below link.

Kind regards,

jOmega

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Hi JO

Yes the characteristics do depend very much on the actual design.

A submersible pump is a form of axial flow pump and it definitely reduces power demand as the outlet volve is closed OFF in most situations.

You need to look at the curves of the actual pump being employed.

A turbine pump is an axial flow pump that will commonly reduce, whereas a mono pump (moving cavity pump) is also an axial flow that will definitely not reduce with increasing head.

Best regards,

Mark.

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