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A guide to the four basic functions of a motor starter

How to control an AC motor safely?

Safety is always the top priority in any industrial environment.

Factories and process plants are high-risk workplaces, facing the real danger of catastrophic events such as fires and explosions.

 Safely controlling the high levels of electrical current that flow through the multiple AC motors that power today’s industrial facilities is central to plant safety.

In any installation with electric motors, different kinds of faults can occur.  These include: short circuits between phases of the power supply, overvoltage of the power supply and overloading of the motor leading to a power surge.

The effects of such faults range from temporary shutdowns and destruction of the motor and its starter components, through to electrical fires.

To avoid such damage – or to at least limit its effects - every motor should be protected from:

  • short circuits: by fuses, magnetic circuit breakers, etc.
  • overloads: by thermal or electronic overload relays, multi-function relays, etc.

In a motor starter, these protection elements are combined with a switch disconnector and a control device. To ensure that they perform their functions correctly, they should be coordinated.

Below is a quick guide to the four basic functions of a motor starter, followed by an explainer on the importance of making sure they are all coordinated to work together correctly.

 

1 - Disconnection and breaking

 Any motor starter must be able to be disconnected from the mains and isolated to prevent a restart. The ability to switch off the power and isolate allows maintenance and repair work on the motor, driven equipment or its starter components to be carried out safely.

In its most basic form, this can be provided by a switch disconnect at the top of the circuit.

However, manufacturers offer numerous devices that can perform this function. The switch disconnector and short-circuit protection functions (below) are often combined in a single device, such as a fused switch disconnector.

2 - Short-circuit protection

 The challenge with turning off electricity is that it likes to flow. It will keep flowing and can flow through air – this can be seen in the home when you see a little flash as you turn a light off at night. 

 In a domestic environment, the normal load current is only a few amps but under a short circuit fault, this can be as high as a few thousand amps.  The MCBs in your distribution board are capable of clearing this fault current.

 In an industrial environment everything increases in scale.  The normal load current can be a 1000 amps and the prospective fault current with a short circuit fault can be over 100,000 amps.  These levels of energy need the correct equipment to prevent damaging explosions and fire.

 Short circuit protection devices are selected appropriate to the prospective fault current they may be required to clear.  They detect the short circuit and then disconnect the supply in a safe manner.  The function is provided by circuit breaker or fuses.

 3 - Overload protection

Overloads are caused by the motor drawing on more power than it is designed to use, invariably because it is being asked to work harder than it should: for example, a conveyor belt moving heavier than normal items, or a pump with a blockage.

Overload protection detects excess currents from the overload and opens the circuit to prevent the motor from overheating and burning out.   

A complication is that motors draw high currents when starting.  The device needs to allow short time overloads which the motor is designed to withstand but trip if the overload continues.

This protection is provided by electromechanical or electronic overload relays, combined with a breaking device such as a circuit-breaker or contactor. It can also be incorporated into electronic starters or variable speed drives.

4 - Control

This is the closing and opening of an electrical circuit under load, and is most commonly carried out by a contactor – first invented by Telemecanique (part of Schneider Electric) in 1924.

The contactor has main poles that do the switching.  These poles are opened and closed by energising an electromagnet called a coil.  The coil is typically designed for either ac or dc voltage and has a nominal control voltage. 

Over or under voltage to the coil can have devastating consequences to the contactor.  The failure mode usually results in a burnt out coil that just turns the contactor off but it can fail with the contactor jammed closed.  The new Tesys D Green contactors have coils which can accept ac or dc and have a wide range of tolerance on the control voltage.

Coordination is critical

The four different functions of a motor starter must work, or coordinate, together properly.

A single device known as a starter-controller or Control and Protective Switching device (CPS), such as Tesys U, can be used to deliver all four functions.

Other components can incorporate more than one function in a single device and then combinations of two or three devices can be used.  As an example a magnetic circuit breaker such as the GV2L is a short circuit protection device and a switch disconnect.  Combined with a separate overload and a contactor, it can deliver all four functions with only 3 components.

To help system designers select these components of a motor starter, all major manufacturers of motor starters publish combination tables for their equipment in their catalogues.

People who install motor starters should ensure that there is genuine coordination between these components.

Schneider test these combinations and publish coordination tables of devices that will work together correctly.

If you mix and match manufacturers, the combination is unlikely to have been tested and may not perform correctly under a fault condition.

If combinations are not correctly coordinated, the result could be catastrophic.  As an example, under a short circuit condition, the circuit breaker may well clear the fault but the energy it has let through in the process may have caused the contactor to explode or catch fire. 

Type 1 coordinated devices will ensure the fault is contained within the motor starter components.  The components may need replacing but the operator is protected from harm and the panel is protected from any other damage.

Type 2 coordinated devices are an improvement on type 1 in that you can get the plant up and running again.  You may have to remove a tack weld on the contactor but the components will be serviceable.

This is why coordination is critical and, if you’re unsure, an all-in-one solution like TeSys U may be your best bet.  The TeSys U provides “total coordination” – no risk of damage, no risk of contact weld, just maintenance free continuity of service.

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I'm the TeSys product marketing manager at Schneider, based in Coventry, UK. Responsible for Motor Control and Protection which includes contactors, circuit breakers and overload relays.