Motor Starter

What is a motor starter?

A motor starter is a combination of switching and protective functions that safely switches an electric motor on and off and protects it against overload and short circuits. In its classic form a motor starter consists of short-circuit protection (fuses or a motor protection circuit breaker), a contactor that switches the motor and a thermal overload relay. Modern variants combine these functions in a single compact device, sometimes with semiconductors and a network connection. Motor starters sit in almost every control cabinet and every MCC, making them the point where the PLC actually switches the motor’s power.

The key requirement for a motor starter is that it lets the motor run up without tripping unnecessarily, yet switches off in time before the windings, cables or the starter itself are damaged.

🧠 How does a motor starter work?

A motor starter performs four functions which together form the chain from supply to motor:

1. Isolation - a main switch or motor protection circuit breaker makes the motor dead for maintenance.
2. Short-circuit protection - fuses or a circuit breaker with magnetic tripping interrupt a short-circuit current within milliseconds. The overload relay does not do this.
3. Switching - a contactor closes or opens the main circuit on a control signal from push buttons, a relay or a PLC output. Both local and remote operation are possible.
4. Overload protection - a thermal or electronic relay measures the motor current and trips if it stays too high for too long. Many relays also detect phase loss and imbalance.

You set the overload relay to the rated motor current on the nameplate, not to the desired tripping current. At 1.05 times the setting current the relay must not trip; at 1.2 times it must trip within two hours. The trip class defines how long the relay tolerates a starting current of 7.2 times the setting current.

🔍 What types of motor starter are there?

The starting method determines how much current and torque the motor draws while running up. This matters most for an induction motor, which draws six to eight times its rated current when switched on directly.

Strictly speaking a variable frequency drive is not a motor starter but a drive: it falls under IEC 61800 rather than IEC 60947. In practice it plays the same role in the chain and replaces the starter in many new installations.

📐 Which standards must a motor starter meet?

The product standards come from the IEC 60947 series for low-voltage switchgear and controlgear:

• IEC 60947-4-1 - electromechanical contactors and motor starters, including motor protective switching devices. The fifth edition was published in 2023 and, for the first time, refers to cybersecurity (IEC TS 63208) and embedded software.
• IEC 60947-4-2 - semiconductor motor controllers and starters, such as the soft starter.
• Utilisation categories - you select a contactor by its current at AC-3 (starting squirrel-cage motors and switching them off while running), or AC-3e for high-efficiency motors with a higher starting current. AC-4 covers plugging and inching, with a much lower permissible current.
• Coordination type - with type 1 the starter may be damaged after a short circuit and must be repaired. With type 2 it can return to service, at most with lightly welded contacts that are easily separated. Type 2 applies only to the combinations the manufacturer has tested.

The cabinet as a whole is covered by IEC 61439, the electrical equipment of a machine by IEC 60204-1 and installations in the Netherlands by NEN 1010. The EN IEC 60947 parts are harmonised under the Low Voltage Directive, so a compliant starter may carry the CE marking.

🏭 How do you select a motor starter in practice?

A worked example for a 4-pole IE3 motor of 7.5 kW at 400 V, with a rated current of about 14 A on the nameplate:

1. Determine the starting method - for a pump on a strong supply, DOL is sufficient. The starting current is then around 85 to 115 A, but lasts only a few seconds.
2. Select the contactor by AC-3 - a contactor with an Ie of at least 15 A at AC-3 and 400 V, or AC-3e where the manufacturer states it. In catalogues this is usually the 7.5 kW frame size with an AC-3 rating of 16 to 18 A.
3. Set the overload relay - choose a relay with a setting range around 14 A, set it to the nameplate current and use class 10.
4. Select short-circuit protection from the type 2 table - combine a motor protection circuit breaker or fuse with the contactor only as the manufacturer has tested it for the prospective short-circuit current at that point.
5. Check efficiency and starting - since 1 July 2021, new three-phase motors from 0.75 to 1000 kW must reach at least IE3 under Ecodesign Regulation (EU) 2019/1781. High-efficiency motors often have a higher starting current, which affects the choice of AC-3e and the trip class.
6. Define the control interface - which signals go to the PLC (ready, running, fault), and which stop function belongs to the emergency stop?

A starter that keeps tripping during run-up is usually under-sized in trip class or set to the wrong current. Never set the relay above the motor current to work around the problem.

🌐 How do smart motor starters communicate with the control system?

Classic starters only have hard wiring to the IO of a PLC: a control command and a few auxiliary contacts for feedback. Smart or communicating starters sit on a network:

• Fieldbus and industrial Ethernet - via fieldbuses such as Profibus or networks such as ProfiNET and IP over Ethernet.
• Lower levels - IO-Link and AS-Interface connect starters with minimal wiring to a master or gateway.
• Data - motor current per phase, thermal load, number of switching cycles, operating hours and the cause of the last trip.

This data feeds logging, monitoring and condition monitoring. A rising current at constant load can point to bearing wear or a blockage, and the number of switching cycles predicts when the contactor needs replacing. The starter thus becomes a source for predictive maintenance.

🔐 Which safety and security risks are involved?

• Functional safety - an emergency stop or safe stop switches the motor off via contactors with mirror contacts, monitored by a safety relay. Whether that chain is reliable enough is assessed with ISO 13849-1 or IEC 62061. An ordinary motor starter is not a safety component in itself.
• Unexpected restart - an automatic reset after an overload must not let the motor start unexpectedly. Prefer a manual reset for that reason.
• Network access - a communicating starter accepts start, stop and reset commands over the network. Place starters in a protected zone according to IEC 62443, restrict who may write and log changes to parameters such as the setting current.
• Legislation - the Machinery Regulation (EU) 2023/1230, which applies from 20 January 2027, requires control systems to withstand malicious interference. Networked starters therefore fall within the machine’s safety and security design as well.

❓ Frequently asked questions

What is the difference between a motor starter and a contactor?

A contactor is only the switching element that closes and opens the motor’s main circuit. A motor starter is the complete combination of contactor, overload protection and usually short-circuit protection. A contactor without an overload relay therefore does not protect a motor.

Which trip class does a motor starter need?

Most motor starters for pumps, fans and conveyors use trip class 10, which trips within 4 to 10 seconds at 7.2 times the setting current. Motors with a heavy or long run-up need class 20 or 30. The trip class must match the starting time the motor needs with its load.

What does type 2 coordination mean for a motor starter?

Type 2 coordination under IEC 60947-4-1 means that a motor starter can return to service after a short circuit without replacing parts. Only lightly welded contacts that are easily separated are permitted. Type 2 applies only to combinations of short-circuit protection, contactor and relay that the manufacturer has tested.

When should you choose a soft starter instead of a direct-on-line starter?

A soft starter makes sense when the starting current of a direct-on-line starter puts too much load on the supply or when the mechanical shock at start-up causes damage. Typical examples are pumps prone to water hammer and conveyors carrying loose products. If you also want to control speed during operation, a variable frequency drive is the better choice.

Can a PLC reset a motor starter remotely?

A PLC can reset a communicating motor starter remotely if the starter supports it and the function is enabled. After an overload trip the cause should be clear first, and the reset must not start the motor unexpectedly. Restrict who or what may issue a reset over the network for that reason.

Where is a motor starter installed?

A motor starter is usually installed in a control cabinet or as a withdrawable unit in an MCC. Decentralised starters with a higher degree of protection are also mounted directly next to the motor on the machine. In all cases the requirements for the cabinet or machine apply, such as IEC 61439 and IEC 60204-1.

📌 In summary

A motor starter combines short-circuit protection, a contactor and an overload relay to safely start, stop and protect an electric motor. The choice depends on the starting method, utilisation category AC-3, the trip class and the coordination type under IEC 60947-4-1. Smart starters also deliver current and diagnostic data to the control system, which enables predictive maintenance but also calls for sound network security.