Autotransformer Starter vs. Soft Starter

mspicka

Member
Location
Colorado
Occupation
Owner
I am confused by Mike's definition on autotransformer starter.

An autotransformer seems to be a starter with a single-winding transformer attached to it. While a soft starter could be the same but the voltage is ramped up to the desired voltage slowly.

His definition says... A Starter equipped with an autotransformer that serves to reduce the voltage applied to the motor terminals during starting.

If I look online they appear to be two different items.

Thanks!
 

Macbeth

Member
Location
Livonia NY
Softstarters come in 4 flavors
1. Autotransformer
2. Resistive
3. Delta/Wye
4. Semiconductor

They all reduce the voltage in different ways to accomplish a slow start of the motor. However the semiconductor variation commonly has a ramp times verses two steps.
 

paulengr

Senior Member
Functionally there are significant differences in torque control. Wye-delta is the simplest but offers only a fixed voltage reduction and requires a 6 or 12 lead motor. Auto transformer offers more possible settings via the tap. Resistor starting is hardly ever used but just as flexible. Electronic soft starting can control speed or torque smoothly and avoids a sudden torque or current spike as contactors engage.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Solid State Soft Starters also add the advantage of being able to Soft Stop, which can be useful for pumps and some conveyor applications.

As a general rule, the cost picture is that Wye-Delta is cheapest (but more prone to getting misused), followed by Solid State, then Primary Resistor / Reactor, and Autotransformer is generally the most expensive and largest / heaviest. As a general rule I always use Solid State now, the risks with Wye Delta are not worth it for the small difference in cost.

The only advantage to an Autotransformer starter is that it adds the transformer advantage when it comes to Line Current vs Motor Current. In all of the other types, the Line Current and Motor Current are the same. But in an Autotransformer starter, the Line Current is lower by the transformer ratio. So if you are starting on an extremely weak line or small generator, an RVAT starter may work better.
 

retirede

Senior Member
Location
Illinois
Solid State Soft Starters also add the advantage of being able to Soft Stop, which can be useful for pumps and some conveyor applications.

As a general rule, the cost picture is that Wye-Delta is cheapest (but more prone to getting misused), followed by Solid State, then Primary Resistor / Reactor, and Autotransformer is generally the most expensive and largest / heaviest. As a general rule I always use Solid State now, the risks with Wye Delta are not worth it for the small difference in cost.

The only advantage to an Autotransformer starter is that it adds the transformer advantage when it comes to Line Current vs Motor Current. In all of the other types, the Line Current and Motor Current are the same. But in an Autotransformer starter, the Line Current is lower by the transformer ratio. So if you are starting on an extremely weak line or small generator, an RVAT starter may work better.

Great summary - I’d add that using wye-delta requires a motor with all winding connections brought out to the terminal box. Most standard dual voltage motors are only 9 lead which won’t work.
 

paulengr

Senior Member
Solid State Soft Starters also add the advantage of being able to Soft Stop, which can be useful for pumps and some conveyor applications.

As a general rule, the cost picture is that Wye-Delta is cheapest (but more prone to getting misused), followed by Solid State, then Primary Resistor / Reactor, and Autotransformer is generally the most expensive and largest / heaviest. As a general rule I always use Solid State now, the risks with Wye Delta are not worth it for the small difference in cost.

The only advantage to an Autotransformer starter is that it adds the transformer advantage when it comes to Line Current vs Motor Current. In all of the other types, the Line Current and Motor Current are the same. But in an Autotransformer starter, the Line Current is lower by the transformer ratio. So if you are starting on an extremely weak line or small generator, an RVAT starter may work better.

“Soft stop” as you describe means stopping slower than coast to stop. Soft starters can also easily do DC injection braking. The torque you get from it is amazing but it also counts as a “start” thermally but can apply up to 200% of FLA which since it’s rectified is equivalent to roughly 400-600% And it can finish the job by triggering an external mechanical brake.

Also many soft starters contain cycloconverter functions. This means that it can act as a primitive type of VFD. It is limited to under 30 Hz. That’s great if all you need is a “slow roll” or say machine positioning such as for maintenance.

With wye delta you need 3 or 4 contactors, at least one additional control/timer relay, aux contacts, and a lot of wiring. It may also include resistors or inductors for closed transition. So initially it looks cheaper in terms of raw materials but it quickly adds up in terms of labor costs. Plus it is locked at 58% voltage. That’s too low (33% of torque) for many applications which demand something closer to 50-80% which results in over sizing the motor to defeat what is fundamentally a wye-delta limitation. So from personal experience I’d agree the materials-only cost is lower but the fully loaded cost is the same as the electronic soft start most of the time assuming 33% torque is acceptable.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
The biggest problem I find with Wye-Delta starting is that people who have to service the motor later often don’t understand it and hook something up wrong, them something blows.
 

paulengr

Senior Member
The biggest problem I find with Wye-Delta starting is that people who have to service the motor later often don’t understand it and hook something up wrong, them something blows.

Yes, and as a motor shop service contractor I see no problems at all with that!! It’s practically free money, for motor shops.

It also baffles them when one contactor is worn out, or when the timer relay which is often pneumatic stops working.

Guess I shouldn’t sell electronic soft starts but I like to think I’m selling reliability.
 
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