# Calculating Motor Starting Time With Softstarter

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Hi guys.

First posting for me!

I have an application where I require to know the starting time for a pump that we propose to use a softstarter on. I know that you do not use a softstarter to control the starting time of a motor but the application does not warrant the cost nor functionality of a VSD.

The information I have available is:

Motor torque details - locked rotor, breakdown and FLT

LRC and FLC

I have tried several ways to calculate the acceleration torque but I have had varied results and no way to verify them.

Can anyone help?

Cheers

Eddie Munster

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Hello Eddie

Welcome to the forum.

You need to do several steps to calculate the acceleration time.

First, determine you start current profile that you will use with the soft starter.

Second, from that start current profile, plot the motor torque against speed. (reduced current torque)

Third, on the same graph, plot the load torque against speed.

The acceleration torque at any speed, is the difference between the motor torque and the load torque at that speed.

Next, break the speed range into small steps, say 10%. Assume the average acceleration torque for each step and apply that to the total inertia, and you can calculate the ime taken to accelerate through that step. Repeat this for all steps and add the results together. The greater the number of steps, the better the accuracy.

Hope this make sense.

Best regards,

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Hello Eddie

Welcome to the forum.

You need to do several steps to calculate the acceleration time.

First, determine you start current profile that you will use with the soft starter.

Second, from that start current profile, plot the motor torque against speed. (reduced current torque)

Third, on the same graph, plot the load torque against speed.

The acceleration torque at any speed, is the difference between the motor torque and the load torque at that speed.

Next, break the speed range into small steps, say 10%. Assume the average acceleration torque for each step and apply that to the total inertia, and you can calculate the ime taken to accelerate through that step. Repeat this for all steps and add the results together. The greater the number of steps, the better the accuracy.

Hope this make sense.

Best regards,

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Thanks Mark.

I suspected it would be something like this but it is not as easy as it sounds. How do I work out the reduced voltage torque-speed curve for the softstarter? I don't how to work out the current speed profile. I understand the relationships between voltage, current and torque during starting but I do not know how to relate all this to speed.

Does the current start at the point proportional to the initial voltage or from the current limit?

I assume it starts at, say, 0.3^2*LRC and then rises to the current limit before decreasing as the motor accelerates.

I have the motor torque curve at full voltage and the current-speed curve.

A little more assistance....Pleeeeaaaase.

Thanks and regards,

Eddie.

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Hello Eddie

At any speed, you can calculate the torque output from the full voltage torque and curent and your required start current.

You could easily set the start current to a constant value, say 400%, and then calculate the start torque for all speeds from zero to full speed.

As you reduce the voltage applied to the motor, the current reduces proportionally, and the torque reduces with the square of the current. i.e. half voltage will result in half current and a quarter torque.

Best regards,

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The ability to control the acceleration time of the motor (via a soft starter) depends on the type of soft starter you are using and your level of expertise / commissioning experience.

With a closed loop controlled soft starter and a current ramp start profile, it is possible to accelerate a motor to rated speed within +/- 10% of the set ramp time, if of-coure the ramp time is realistic for example 5 to 15 seconds.

Regards,

GGOSS

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