Bypass Contactors: when and how should they be used?

Bypass contactors bridge out a soft starter’s SCRs when the motor is running at full speed. This eliminates heat dissipation from the SCRs during run state.

Some soft starters include built-in bypass contactors, others require an external bypass contactor.

 

Bypass contactors:
- allow soft starters to be installed in sealed enclosures
- eliminate the cost of forced-air cabinet ventilation
- save energy by eliminating SCR losses during run

Bypass contactors should be AC1 rated for the motor FLC. The AC1 rating is adequate because the bypass contactor does not carry start current or switch fault current.

Cable Length: What is the maximum allowable length of cable run between a soft starter and the motor?

The maximum distance between the starter and motor is determined by the voltage drop and the cable capacitance.

Voltage drop at the motor terminals must not exceed the limit specified in local electrical regulations when the motor is running fully loaded. Cabling should be sized accordingly.

Cable capacitance can be a factor for cable runs that are longer than 500 metres. Consult the soft starter manufacturer for advice - you will need to provide details about mains voltage, mains frequency and the soft starter model.

Cable Selection: how do I select cable when installing a soft starter?

Cable selection criteria vary according to the nature of the circuit and the location of the soft starter within the circuit.

1. Supply Cable Rating
> nominal fuse/MCCB rating
> motor FLC x 1.2

2. 6 Wire (Inside Delta) Motor Circuit Cable Rating
> motor FLC x 0.7

Note: Cable current ratings may need to be derated to account for installation factors (grouping, ambient temperature, single or parallel cabling etc). Always follow the manufacturer's instructions.

Inside Delta Connection: what is it and why would I use it?

Inside delta connection (also called six-wire connection) places the soft starter SCRs in series with each motor winding. This means that the soft starter carries only phase current, not line current. This allows the soft starter to control a motor of larger than normal full load current.

When using an inside delta connection, a main contactor or shunt trip MCCB must also be used to disconnect the motor and soft starter from the supply in the event of a trip.

Inside delta connection:
- simplifies replacement of star/delta starters because the existing wiring can be used.
- may reduce installation cost. Soft starter cost will be reduced but there are additional cabling and main contactor costs. The cost equation must be considered on an individual basis.

Only motors that allow each end of all three motor windings to be connected separately can be controlled using the inside delta connection method.

Not all soft starters can be connected in inside delta.

Main Contactors: when and how should they be used?

Soft starters can be installed with or without a main contactor.

A main contactor:
- may be required to meet local electrical regulations.
- provides physical isolation when the starter is not in use and in the event of a soft starter trip.

Even in the off state SCRs do not offer a high degree of isolation due to leakage through the SCR and protection networks.

- protects the soft starter SCRs from severe overvoltage situations (eg lightning strikes).

SCRs are most susceptible to overvoltage damage when in the off state. A main contactor disconnects the SCRs from the supply when the motor is not running, preventing possible damage.

Main contactors should be AC3 rated for the motor FLC.

Power Factor Correction: can it be used with soft starters?

Individual power factor correction capacitors can be used with soft starters, provided that they are installed on the input side of the soft starter and switched in using a dedicated contactor when the motor is running at full speed. The contactor should be AC6 rated for the motor full load current.

Connecting power factor correction capacitors to the output of a soft starter will cause equipment failure due to severe overvoltage. This overvoltage is created by resonance between the inductance of the motor and the power factor capacitance.

PFC capacitors can be sized using the following formula:

kVA (Cap) = √3 x Vline x 0.8 x motor no load current

Type 1 Circuit Protection: How do I achieve Type 1 protection of a soft starter?

Type 1 protection requires that, in the event of a short circuit on the output of a soft starter, the fault must be cleared without risk of injury to personnel. There is no requirement that the soft starter must remain operational after the fault.

Type 1 protection is provided by HRC fuses or a MCCB that form part of the motor branch circuit.

As a minimum, the protection method must be able to sustain the required motor start current.

Maximum fuse ratings for Type 1 motor protection are specified in UL and IEC standards.

	Rating (% Motor FLC)
Fuse (non-time delayed)	300%
Fuse (time delayed)	175%

Type 2 Circuit Protection: How do I achieve Type 2 protection of a soft starter?

Type 2 protection requires that in the event of a short circuit on the output of a soft starter the fault must be cleared without risk of injury to personnel or damage to the soft starter.

Type 2 protection is achieved by using semiconductor fuses. These fuses must be able to carry motor start current and have a total clearing I2t < the I2t of the soft starter SCRs.

Semiconductor fuses for Type 2 circuit protection are additional to HRC fuses or MCCBs that form part of the motor branch circuit protection.

Refer to the soft starter's Product Guide for semiconductor fuse recommendations.