Captive Transformer start over VFD/Soft starter 1

[NickParker]

When and where is the captive transformer method of starting is preferred over VFD/Soft starter?
My understanding is that,
1. A captive transformer start is typically used where only a single motor needs to be accelerated.
2. Unless speed regulation by VFD is required for process purposes, it is worthwhile to analyze if direct on line start of an electric motor, or a start through a captive transformer, would satisfy the project requirements. It is the simplest solution with the lowest total installed cost.

 

[waross]

The choice of starting method depends on both the utility requirements and the inertia of the load to be accelerated.
Many years ago, long before VFDs, I worked in a mill with the following starting schemes.
We were not subject to utility restrictions.
1. Several low inertia loads.

60 HP, 100 HP, 150 hp, 200 HP, 250 HP.
All started DOL.​

2. 300 HP high inertia load, compensator start (Manual auto-transformer start)
3. 200 HP Very high inertia load, wound rotor start.

It is the simplest solution with the lowest total installed cost.

That would be series impedance starting.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[LionelHutz]

What exactly is a captive transformer starter? Do you mean an autotransformer starter?

My terminology equates a captive transformer with a dedicated transformer, typically a step-down from a higher utility voltage. How you start the motor has nothing to do with it having a dedicated transformer or not. You could very easily use a VFD or soft-starter between the transformer and the motor.

It is the simplest solution with the lowest total installed cost.

I very highly doubt it.

 

[NickParker]

By captive transformer, I meant a motor with dedicated transformer, see the one line diagram picture below,

 

[waross]

Some would call that dedicated transformers, Nick.
That may be more an economic decision than a starting decision.
If the captive/dedicated transformers are sized at the motor capacity, they will act as series impedances during starting.
As a rough estimation, the transformer may reduce the starting surge from 6X to possibly 5x or 4x.
The actual voltage reduction will be described by a number somewhere between the percent regulation and the percent impedance voltage.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[dpc]

What are the project requirements? Why not a full voltage start - even less expensive and more reliable. If you are trying to reduce system voltage drop during starting, the captive transformer could be a solution, depending on the voltage drop requirements. A soft starter would probably be less expensive than a transformer and more efficient overall, assuming a bypass contactor. If the issue is reducing starting torque on the driven equipment, then that is a different analysis. Any project specification that requires all motors over a certain size to have reduced voltage starting is highly suspect to begin with.

 

[LionelHutz]

That one-line diagram doesn't show the motor starter or any kind of protection. I doubt the operations would be happy throwing a switch closed to start those motors.

It's rather silly to put in a 1:1 transformer that size, so the transformer is likely a step-down from the 11kV bus to a lower motor voltage. This means it's not an option to use a captive transformer or a VFD, soft-starter, ATL since the motor voltage doesn't match the bus voltage. You need both, the step-down transformer and a way to actually start the motor.

 

[jraef]

I don’t see a starting method there, I just see a dedicated transformer. In that schematic, the “start” method appears to just be the switch ahead of the transformer. With no changeover scheme, this is just an Across-the-Line (DOL) starting method. Starting large motors in this way is going to be really difficult on the mechanical components in the system. I wouldn’t do it.

The only time I have seen this as reasonably valid is when the motors are started infrequently, such as irrigation pumps started in early summer and run non-stop until turned off when rains start in winter.

Auto transformer starters are now generally expensive than reduced voltage solid state soft starters in Low Voltage (600V and below) because of volume; few people are using RVAT starters any longer. In Medium Voltage, RVAT starters are roughly at price equity with RVSS, but RVSS starters have added benefits worth taking advantage of.

VFDs at these sizes would be difficult to justify unless speed control is part of the process. But if there are any flow control methods being employed, the potential energy savings from using VFDs can usually pay for themselves in a short time.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[bacon4life]

Nick-The criteria you provided align with my experience.