Nov 21, 2011

Difference Between PMDC and Brushless Servo Motor

In our earlier two articles we have discussed about the Servo and Motion basics and also the advantages of Servo Motors. Today in this post we will see the two different type of Servo Motor and the differences between them. PMDC (Permanent Magnet DC) Servo Motor and Brush less Servo motor are two different types of servo motor which differ in their construction and the commutation method.

PMDC Servo Motor Simple Explanation/Construction and Working :-

PMDC Motor consists of Stator which has Permanent magnet i.e stator field is produced by permanent magnets. The stator field is stationery. The rotor field is produced by passing current through the rotor and commutator assembly. When current is supplied, a field is generated inside rotor and it rotates to align itself with the stator field but the commutator switches the rotor field and thus the rotor field never catches up the stator field. The speed of the rotor will be dependent on the Rotor field which in turn is dependent on the voltage supplied to it. The stator field is produced by permanent magnets and is constant at all speeds.PMDC Servo motor has high starting torque and acceleration torque. Also it has smaller frame size and less weight as compared to the brush less servo motor. Below is the diagram of the construction of PMDC servo motor.

AC Brush less servo Motor Construction and Simple Working explanation :-

In AC brush less servo motor the method of commutation is different as compared to PMDC Servo Motor. The power is supplied to the stator windings and thus a field is setup. The rotor field is set up by the permanent magnet. The rotor field rotates itself in order to align with the stator field but as soon as it is nearer to align , the polarity changes at stator side and thus the field and then again rotor keeps on rotating itself to align with the stator field. Thus with the electronic commutation the power is switched from winding to winding and rotor rotation is achieved. AC brush less motor have higher speed capabilities and lower inertia , so suitable for faster and accurate positioning applications.
See the below diagram for the complete constructional view of the AC Brush less Servo motor.

Till now in our three artciles we have discussed the basics and different types of Servo and Motion. In further articles we will try to discuss advanced information on Servo and Motion. If you have any specific query related to Servo's , you can write to us in the comment section.
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