Phase Synchronous Motor Starting Term Paper Examples

Type of paper: Term Paper

Topic: Motor, Rotor, Induction, Supply, Power, Speed, Control, Starter

Pages: 2

Words: 550

Published: 2021/02/17

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Phase Synchronous Motor Starter Circuit

Operation of three phase synchronous motor starter circuit
During starting, a three phase synchronous motor opeerates as a normal onduction motor. After three phase power is connected to the system, all the three main comtacts “M” close and the auxilliary contacts change state, the normally opena contacts close while the normally closed contacts open. The three phase current then flows through the overload heaters into the the three motor stator leads. The current therefore flows through the stator windings causing a rotating magentic field. This rotating field interacts with the induction windings in the rotor causing it to rotate accordig to Lenz’s law (Herman, 2006). At this stage, the sunchronous motor is running as an induction motor.
When the main three phase disconnect is switched on, current is induced on the lower part of the circuit through the transformer separating the stator circuitly and the rotor. The “M” coil controls the operation of the DC rotor supply. The coils is supplied power by two conatcs, a normally closed and a normally open contact. The normally closed contact is looped to the Overlod Heaters on the stator leads. When the start button is pushed to the “on” position, current flows through the coil and it activates the normally open contact to close. All the other M conatcs on the lower part of the circuit also closes. Current then flows to the Variable DC power supply whicn converts the AC to DC as required by the rotor. The On Delay Timers (TD) delay the DC from being fed directly to the rotor untill its rotating close to the sunchronous speeds. After a predetermined time lapse programmed into the TDs passes, the DC is fed to the rotor, whereby it starts rotating at the synchronous speed.
If too much curret is fed to the motor for a long time, the overload heaters will heat up and open up their conatcs thereby cutting off the stator power supply. Also they will open up the normally closed contact feeding M coil thereby cutting off the DC supply to the rotor such that power to the stator and the rotor is simulteneously disconnected. When the overload heaters have cooled down, they close their contacts and the normally closed contact coupled to the the M coil. This reconnects power supply to the rotor and the stator and the motor starts running again.

Comparision between synchronous motor and induct motor starting circuits

There are two major differences between synchronous motor and induction motor starting circuits. In induction motors, the starting circuit reduces initial speed and torque by limiting current fed to the motor. Conversely, sunchronous motor reduces starting speed and torque by delaying powering of the rotor windings. Secondly, induction motors can control initial starting torque through two methods, rearranging the stator windings into a star followed by a delta configuration (Traister, 1994). Also, induction motors control starting current and volatge through auto transformation. This method uses variable tranformer tappings to control initial voltage and current. On the other hand, synchronous motors limit the starting torque by intially running the motors as an induction motor and later converting it to a synchronous motor by supplying current to the rotor (Herman, 2006).
The role of DC power supply in synchronous motors and the difference between its role and of DC in DC motors.
The DC supply in synchronous motors creates a permanent magnetic field which interacts with a rotating field in the stator. Like poles repulse while unlike poles attract between the two fields, these attraction and repulsion forces drive the rotor at the constant synchronous speed (Herman, 2006). The main difference between the role of DC supply in synchronous motors and in DC motors is that it creates a permanent magnet in synchronous motors and an alternating magnetic field in DC motors. The rotating magnetic field requires a permanent magnet in order to produce torque while the permanent magnet in DC motors requires an alternating magnetic field for the same purpose.

Summary of DC, induction, and Synchronous motor starter circuits

Starter circuits reduce initial starting current and torque. The three motor types employ different starter circuits to perform this task. In DC motors, variable series resistors are used to reduce voltage across the armature windings and hence current. After the motor has started running, a back emf is developed in the armature and the variable resistance is reduced until the motor is running at full speed. In induction motors, starter circuits are of two types, star delta or auto transformer. Star delta reduces the starting volatge by reconfigurring motor stator windings from star the to delta. Star configuration reduces full speed torque by a factor of three (Herman, 2006). Also in induction motors, an autotransformer reduces voltage across the motor and hence the current by progressively connecting motor to different transfomer tappings. Synchronous motors as observed use On Delay Timers to delay supply of the DC to the rotor untill its running close to the synchrnous speed.


Herman, S. L. (2006). Understanding motor controls. Clifton Park, NY: Thomson Delmar Learning.
Traister, J. E. (1994). Complete handbook of electric motor controls. Lilburn, GA: Fairmont Press.

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