Motor Starter Circuit Diagram

It doesn't hurt as much. 24V hurts even less.

in most cases its not 110V in 1 phase, instead its 55V and 55V like that.which i seen in many mcc panels.this is surely for safety reason.

For control 110V end tapped is used. So it is 110V to earth.

Centre tapped gives to many problems in the event of an earth fault.

Only ever use 55-0-55V for hand tools. Never use for a control circuit.

The Tx is earthed at the centre point so the max electric shock is 55V.

If in a control circuit you had a fault to earth mid way along the circuit the contactor coil might easily stay in at 55V. (unlikely to pull in) The fault would cause one control fuse to blow but not the other i.e. you could use your emergency stops etc.

silverfox: where do/did you find 55-0-55 volts. what country du live in? perry

I have dealt with 24v/110/240/415v control ccts. I think apart from safety and hurting factor its is more of convenience and availability of spares and price.The 415 volt control cct provides more protection, when one of the main phases is broken the control circuit is also interrupted at least 2 out of 3 times.

It is not recommended to use 415 V control supply as the operator will be exposed to higher voltage which is unsafe. Secondly, the control supply being tapped from two phases, the control cicuit will be subjected to higher short circuit current. Hence it is common practice to feed the control circuit through secondary of the control transformer, the primary winding being fed at 415 V.

There are many advantages for having a different voltage for the control circuit and hopefully I can list most of them.

By having a separate voltage for control, fault finding is not a higher risk level as it would be if using 415volts. This is because 415 v is usually provided by 2 phases, and anyone who has had their hand in a panel tracing UNMARKED control cables/wires with a control voltage of 110v, knows that you do it with the knowledge that you might get a shock but it won't be life threatening.

Commissioning & Fault finding...... new panel and you want to make sure it works.. by having 110 volts control you can connect up to any old 110v supply and if there are any faults it won't severely damage the panel or components.

Isolation.... by having two separate voltages you have isolated high power side from control side, as neither voltages will meet and if one side fails, for example, the high power side supplying a motor, then it is easily identified as the control side works.. so it has to be the power side.

Fault current..... if you have your control circuits connected to the main supply, then any fault in the control side has the potential to receive the FULL fault current, and that can be devastating in a small confined area like a control panel. Not only that if you have your hands in there, then you too are at risk of getting the same high level of fault current.

So in order to protect equipment and people, an isolating transformer supplying a low control voltage with a very low fault current that will power the control circuits

Hi

Some of the early motor circuits I worked on used 415V for the control circuit - straight between 2 phases. The control circuit was straight off the motor fuses. Yes it worked but think safety and the possible fault current and potential for a bang at the control station.

Working from 230V is still dangerous and takes the risk of electrocution into the field.

110V from a control transformer is a good safe way forward.

Never use 55-0-55V for your control circuit.

24V tends to have problems with volt drop especially on larger motor starter contactor coils.

110 volt control supply is obtained from 415v/ 110v control transformer which is installed in control panel. The above 415v supply is taken from two lines or phases of normal power supply. The advantages are :

Due to low voltage(110v), electric shock hazards are drastically reduced.

Since contorl supply is taken from secondary side of control transformer(other point is earthed) is easier to find fault in control circuit. Also due high percentage impedance of control transformer, fault current is substantially reduced and less prone to electric flash hazards.

Since it is a convention, easy availibility of spares like contactor coils etc.

Manindra.