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How to read a schematic


A schematic of a three phase motor starter.
The part of the schematic that contains the motor starter.
The Disconnect Switch.
The thermal heater overload.
The control transformer.
The control transformer secondary fuse.
The control transformer primary fusing.
The control circuit.
The illustrated animation.



Shown below is a schematic of a Three Phase Motor Starter.

The following pages illustrate the various sections and demonstrate the control function with an illustrated animation.

This example is a full voltage magnetite non reversing motor starter application, with a control transformer and one start/stop station.

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The components are as follows;

Three Phase Fusible Disconnect Switch.

Control Transformer.

Primary Fusing Control Transformer.

Secondary Fusing Control Circuit.

MS1 Motor Starter.

Operator Start / Stop Buttons.

Three Phase Motor.

















The Motor Starter

Shown below is a schematic of a Three Phase Motor Starter. Labeled as MS1 (Motor Starter One)

The area highlighted in yellow is the part of the schematic which contains the motor starter.




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Power Contacts

Thermal Heater Overloads

Overload Switch Location


Control Coil

Overloads Switches

Control Contact











The Disconnect

Shown below is a schematic of a Three Phase Motor Starter.

The area highlighted in yellow is the part of the schematic which contains the Three Phase Disconnect Switch and three fuses.




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The Disconnect Switch must be rated for the job.

It must have a voltage rating equal to or greater than the supply voltage and it must have a horsepower equal to or greater than the motor it is supplying. Refer to the NEC.

The Enclosure must be grounded. Refer to the NEC.

Fuses must be UL listed.

A time-delay fuse such as class RK1, J or CC is good. The low peak fuses are best.

The size should be no more then 175% of the full load current, however, certain applications require special devices and a fuse rated at 175% of full load may be too high.

Use the lowest amperage fuse possible.

Refer to the NEC.












Overload Devices.

Shown below is a schematic of a Three Phase Motor Starter. The area highlighted in yellow is the part of the schematic which contains the motor starter's three thermal heater overload devices.


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The thermal heater overload must be sized properly to ensure motor protection.

Many factors must be considered.

1) Motor Service Factor.

2) Motor Ambient Temperature.

3) Controller Ambient Temperature.

Refer to the heater table supplied with the starter.





















The Control Transformer.

Shown below is a schematic of a Three Phase Motor Starter. The area highlighted in yellow is the part of the schematic which contains the control transformer.



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The control transformer is powered by two of the three phases. Here it receives power from phases A and B.

This is a single phase transformer and lowers the voltage to a more common value useful when adding lights, timers or remote switches not rated for the higher voltages.

In some cases a control transformer is not needed. A motor starter with a coil voltage with the same rating as the supply would work just fine.

Transformers have a primary side and a secondary side.

The primary side is the higher voltage side and the secondary side is the lower voltage side.

Control transformers are rated in volt amps (VA).

The control transformer primary side voltage must match the incoming line voltage and the secondary side must match the coil voltage.

The VA rating must be greater than the VA rating of the coil.

If lights and timers are used the VA rating must be greater than the total load.

The next page will explain how to size and fuse the control transformer.







The Secondary Fuse.

Shown below is a schematic of a Three Phase Motor Starter. The area highlighted in yellow is the part of the schematic which contains the control transformer secondary fuse.



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The secondary fuse protects the control circuit and transformer from damage, including fire damage.

The VA rating of the transformer is used to properly select the fuses.

The fuse must be smaller than the VA rating of the transformer.

Calculate the fuse size by first determining the size of the transformer.

The VA rating is the math formula used in determining the amperage at a given voltage.

A secondary voltage of 120 volts from a 100VA transformer will produce .83 amps...

e.g., 100 VA 120 Volts = .83 Amps.

The VA rating is divided by the voltage.

The result is the transformer's output amperage.

The secondary fuse size for a 100VA transformer must be 8/10 Amp or smaller.

A fast acting fuse is best. Refer to the NEC.







The Primary Fusing.

Shown below is a schematic of a Three Phase Motor Starter. The area highlighted in yellow is the part of the schematic which contains the control transformer primary fusing.



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The primary fusing protects the control transformer from damage, including fire damage.

The VA rating of the transformer is used to properly select the fuses.

The fuses must be matched to the VA rating of the transformer.

The VA rating is the math formula used in determining the amperage at a given voltage.

A primary voltage of 480 volts for a 100VA transformer will draw .21 amps...

e.g., 100 VA 480 Volts = .21 Amps.

The VA rating is divided by the voltage.

The result is the transformer's input amperage.

The primary fuse size for a 100VA transformer must be .21 Amp or 150% larger.

Oversizing primary fuses is necessary due to the initial load when power is first applied (Known as inrush).

A timed delay fuse is best. Refer to the NEC.







The Control Circuit.

Shown below is a schematic of a Three Phase Motor Starter. The area highlighted in yellow is the part of the schematic which contains the control circuit.



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The control circuit begins at the control transformer which is the voltage source.

The X2 terminal is bounded to a ground making this terminal neutral (No voltage potential to ground).

Follow the transformer X2 terminal down to the starter's overload switch.

There are three overload switches and they are pre-wired internally all the way to the coil.

The transformers X1 terminal is connected to the secondary fuse from here (wire #1) it is sent to the stop button which is normally a closed contact.

From here (wire #2) runs to a normally open start button and the motor starter's MS1 contact (also a normally open contact).

From the start button and the MS1 contact, wire #3 goes to the coil of the motor starter.

When the start button is pushed the path to the coil is completed; 1) the motor starts and 2) the MS1 contact is closed.

The coil remains energized through the MS1 contact after the start button is released and

it will run until the circuit is interrupted by the stop button or an overload trip.







Three Phase Motor Starter Animation.

The illustrated, animated schematic to the right displays a Three Phase Motor Starter with start stop button control.

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The Red Lines illustrate the circuit path.

When the start button is pushed the path is completed to the coil.

When the coil is energized the MS1 contacts close and confine power to the coil until the stop button is pushed, interrupting the circuit.

Watch the illustrated animation for a demonstration of how the circuit works.