Primary Resistance Starters: How does soft start compare to primary resistance starting?

Compared with primary resistence starters, soft starters are more flexible and reliable.
Primary resistance starters offer limited performance because:
- start torque cannot be fine-tuned to match motor and load characteristics.
- current and torque transients occur at each voltage step.
- they are large and expensive.
- liquid resistance versions require frequent maintenance.
- start performance changes as the resistance heats up. So multiple or restart situation are not well controlled.
- unable to accommodate changing load conditions. e.g. loaded or unloaded starts.
- they cannot provide soft stop.

Auto-transformer Starters: How does soft start compare to auto-transformer starting?

Compared with auto-transformer starters, soft starters are much more flexible and provide a much smoother start.
Auto-transformer starters offer limited performance because:
- they offer only limited ability to adjust start torque to accommodate motor and load characteristics.
- there are still current and torque transients associated with steps between voltages.
- they large and expensive.
- they are especially expensive if high start frequency is required.
- unable to accommodate changing load conditions. e.g. loaded or unloaded starts.
- unable to provide soft stop.

Flying Loads: Can soft starters control an already rotating motor (flying load)?

Yes, soft starters can start motors that are already rotating.
In general, the faster the motor is still rotating, the shorter the start time will be.
If the motor is rotating in the reverse direction, it will be slowed to a standstill and then accelerate forwards.
No special wiring or soft starter setup is required.

Minimumising Start Current: What start current I can expect if I use a soft starter?

Soft starters can limit start current to any desired level. However, the minimum level of start current for a successful start depends on the motor and load.
To start successfully, the motor must produce more acceleration torque than the load requires, throughout the start.
Reducing the start current also reduces the torque produced by the motor. The start current can only be lowered to the point where the torque output remains just greater than the load torque requirement.
The likely start current can be estimated from experience, but more precise predictions require analysis of motor and load speed/torque curves.

Successful Start:

Unsuccessful Start:

Motor Thermal Capacity: What is motor thermal capacity?

A motor’s thermal capacity is the maximum time a motor can run at locked rotor current from cold. Thermal capacity is also referred to as "maximum locked rotor time" or "maximum DOL start time". This information is usually available from the motor datasheet.

Overload Protection: How is AuCom's motor thermal model different from other forms of overload protection?

AuCom's motor thermal model offers precise motor protection normally only available from high-end motor protection relays.
- Protection is based on the motor's actual thermal capacity.
- Motor temperature is continually modeled.
- The thermal model accounts for different heating and cooling rates when the motor is in different operating states (starting, running or stopped).
- Both iron and copper losses are modeled.
The accuracy of the motor thermal model means the motor can be used to its maximum potential without nuisance tripping.
Thermal overload relays are imprecise because:
- The mass of the bimetal strips in the thermal overload is fixed and cannot be altered to match motor characteristics.
- They do not account for iron loss.
- They do not allow for different cooling rates at different stages of motor operation.
- The bimetal strips are affected by their ambient temperature, which is typically different to the motor’s ambient temperature.
- Adjustment is imprecise.
Inverse time-current and I2T electronic overloads offer limited protection because:
- The trip curves do not closely match motor heating.
- Inverse time-current protection does not account for motor temperature before the overload.
- They do not typically allow for differing cooling rates at different stages of motor operation.
- They do not account for iron loss.
- Adjustment is limited.

Soft Starter Formats: Are all three phase soft starters the same?

No. There are different styles of soft starter which control the motor in different ways and offer different features.

These devices reduce torque shock at start but do not reduce start current. Also known as torque controllers, these devices must be used in conjunction with a direct on-line starter.

These devices eliminate torque transients and reduce motor start current. The uncontrolled phase has slightly higher current than the two controlled phases during motor starting. They are suitable for all but severe loads.

These devices control all three phases, providing the optimum in soft start control. Three phase control should be used for severe starting situations.

Soft Starters: What are the key benefits of soft start?

Soft start enhances motor start performance in many ways including:
- smooth acceleration without the torque transients associated with electro-mechanical reduced voltage starters.
- voltage or current is applied gradually, without the voltage and current transients associated with electro-mechanical reduced voltage starters.
- lower start currents and/or shorter start times because constant current control gives higher torque as motor speed increases.
- easy adjustment of start performance to suit the specific motor and load.
- precise control over the current limit.
- consistent performance even with frequent starts.
- reliable performance even if load characteristics vary between starts (eg loaded or unloaded starts).

In addition to superior starting performance, soft starters also provide a range of features not available from other reduced voltage starters. This features include areas such as:
- soft stop (which helps eliminate water hammer)
- braking
- motor and system protection
- metering and monitoring
- operating history and event logs
- communication network integration

The extra features built into soft starters can reduce the overall installed cost of the equipment and reduce the long-term maintenance requirement.

Star Delta Starters: How does soft start compare with star/delta starting?

Compared with star/delta starters, soft starters are much more flexible and provide a smooth start with no risk of transients.

Star/delta starters offer limited performance because:
- start torque cannot be adjusted to accommodate motor and load characteristics.
- there is an open transition between star and delta connection that results in damaging torque and current transients.
- they cannot accommodate varying load conditions (eg loaded or unloaded starts).
- they do not provide soft stop.

The main advantages of star/delta starters are:
- they may be cheaper than a soft starter.
- when used to start an extremely light load, they may limit the start current to a lower level than a soft starter. However, severe current and torque transients may still occur.