Motor Protection & Overloads

Sometimes, an electric motor begins to draw excess current while it's running. This is called an overload. The motor is at risk of overheating, and this will cause damage to the motor windings. Motor protection is therefore very important, as well as protection for the motor branch circuit and its components. For this, we use overload relays.

At Rowse Automation we stock a wide selection of motor protection solutions from our principal manufacturers ABB and Siemens. When choosing overload relays, there are several protective functions that you should take into account. These include the type of process for which the motor is being used, and the environment in which it will be working. You'll have to consider its electrical power supply parameters, such as available short-circuit current and voltage, as well as what phase type your motor is and how it's grounded.

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Damage Prevention

Motor protection is critical in preventing damage and causing internal faults. Overload relays also protect the motor against abnormal external conditions, such as when connecting the motor's electrical circuit to the power grid. They prevent any electrical disturbance from spreading back into the grid.

Overload relays provide motor protection by monitoring the current flowing through the motor's electrical circuit, and they form part of the motor starter assembly. The overload relay will trip if the current being drawn by the motor rises above a certain preset limit, measured over a specific time period from 5 to 30 seconds. The overload relay's tripping time decreases as the current increases. Relays are categorised in trip classes according to how long it takes them to open when an overload condition is reached.

The usual trip classes for motor protection are 5, 10, 20, and 30, corresponding to the number of seconds they take. Trip class 10 and 20 relays are typically used in general purpose applications; class 5 is for fast tripping motors, and 30 for loads with high inertia. When the overload relay trips, it will trigger an auxiliary contact to de-energise the contactor by interrupting the motor control circuit. This will prevent it from overheating. Overload relays are available with both manual and automatic reset options.

Choosing Overload Relays

At Rowse, we can supply motor protection and overload relays for a variety of operating situations. ABB's EF range and Siemens 3RB range, for example, are designed for circuits that measure current overloads electronically. We can also supply solid state overload relays for the same purpose, and thermal overload relays in a wide range of sizes and configurations. We offer ABB overload relays of the EF and TF type, and Siemens overload relays in the Sirius or Sirius Innovations range. Optional power ranges go from as low as 0.10-0.13A to as high as 160-630A.

Mechanical considerations for your overload relays include the motor's type and horsepower rating, how frequently it's started up, and its torque and vibration limits. Motor protection relays can be supplied for a variety of drives, including synchronous and asynchronous motors, and motors controlled by contactors or circuit-breakers.

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