Motor Starters

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A motor starter is basically a contactor with an overload block added to it. The overload block consists of a heater and overload contacts. The overload block provides protection against overcurrent for the motor that's connected to the motor starter. This means that any of the contactors shown in previous figures could be used as motor starters by adding an overload block to them. Electrical parts dealers may stock the contactor as an individual unit for use in circuits where the load is non-inductive. The overload block is stocked as a separate part and it can be added to the contactor in the field to allow the contactor to become a motor starter. The parts dealers also stock motor starters that have the overload block added during the manufacturing process so that the motor starter can be used without modification. Ill. 1 shows a typical motor starter, Ill. 1 shows a cut-away diagram of a motor starter, and Ill. 3 shows an electrical diagram of a motor starter.



Keyhole Mounting Slot
Above: Ill. 1: A typical motor starter. The motor starters are rated in size from 00-9 by NEMA.

Cut-away picture of a motor starter.
Above: Ill. 2: Cut-away picture of a motor starter.

Figs. 1 and 2 will help one identify all of the main parts of the motor starter. Notice that all of the parts except the overload block look exactly like the parts of a contactor because the motor starter is actually a contactor with the overload block added at the factory or in the field. Also notice the parts in the cut-away diagram as they appear in the electrical diagram. The terminals for incoming power are identified as L1, L2, and L3 in the electrical diagram, and they are identified at the top of the motor starter in the picture as line-side power terminals. The terminals where the motor is connected are identified as terminals T1, T2, and T3 in the electrical diagram, and they are identified as load-side terminals at the bottom of the motor starter. The location of the auxiliary contacts is shown in the picture on the left side of the motor starter just o the left of the terminals for the coil. The terminals for the overload contacts a e shown at the bottom left side of the motor starter.

The electrical diagram shows the three-phase voltage is connected to the motor starter at the terminals marked L1, L2, and L3. These terminals are connected in series with the main contacts. The overload block consists of two basic parts: the heaters and the overload contacts. The heater assembly is connected in series with each set of main contacts. The motor is connected to the motor starter at terminals T1, T2, and T3. The coil is connected in series with the overload contacts. If the motor draws excessive current, it will pass through the heater assembly and build up heat. When the heat reaches a critical point, it will cause the overload contacts to open and interrupt current flow to the coil, which will cause the main contacts to open and stop current flow to the motor. This provides protection for the motor against over-current.

Ill. 3 shows an electrical diagram of the motor starter coil connected to a stop-start circuit. This circuit shows how the auxiliary contacts are connected in parallel with the start push button so that it will seal in the circuit when the push button is depressed momentarily. In Ill. 4 the auxiliary contacts appear to be connected in series with the coil, but the auxiliary contacts only use terminal 3 to make the parallel connection to the coil. This can be seen more easily in the diagram of the start-stop circuit.

Stop push button & start push button connected to motor starter coil. Notice that the overload contacts are connected to series with the coil, & the auxiliary contacts are connected in parallel with the start push button.
Above: Ill. 3: Stop push button and start push button connected to motor starter coil. Notice that the overload contacts are connected to series with the coil, and the auxiliary contacts are connected in parallel with the start push button.

Electrical diagram of a motor starter. The coil & overload contacts are also shown. Incoming power is connected to terminals L1, L2, & L3, & the motor is connected to terminals T1, T2, & T3.
Above: Ill. 4: Electrical diagram of a motor starter. The coil and overload contacts are also shown. Incoming power is connected to terminals L1, L2, and L3, and the motor is connected to terminals T1, T2, and T3.

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