DC Motors: Basic Configuration

Permanent magnet DC brushed motors (PMDC motors) consist of permanent magnets, located in the stator, and windings, located in the rotor.

The ends of the winding coils are connected to commutator segments, which have a sliding contact with the stationary brushes. Brushes are connected to DC voltage supply across motor terminals.

Change of direction of rotation can be achieved by reversal of voltage polarity.

The current flow through the coils creates magnetic poles in the rotor, that interact wthe permanent magnet poles.

In order to keep torque generation in same direction, the current flow must be reversed when the rotor north pole passes the stator south pole.

For this the sliding contacts are segmented. This segmented slip ring is called commutator. Left picture shows angular position just before commutation of rotor winding current, right picture after it.

 

This is a schmetic only, real DC motors have more than two windings and commutator segments, for generation of a more constant torque.

Picture left shows 5 segment design (motor model HC685LG).

		Permanet magnets are fixed by a spring (model NF213G)

 

See below an exploded view of JE PMDC motor (model NF2):

  

Commutator segments are made from copper. This motor above has 3 segments.

Brushes are made from precious metal (metal finger leaf brush) or carbon (graphite brush).

Precious metal brush features :

• Used for low voltage, continuous operation.
• Lower contact resistance and thus voltage drop than graphite brushes.
• Less electromagnic noise generation than graphite brushes
• Designed as “Finger leaf metal brushes” – brush is splitted into several thin fingers, providing better contact to commutator segments.

 

Graphite brush features:

• Used for high power , high speed, frequent starting, high lifetime, high voltage.
• Designed as carbon leaf brush or cage brush (for especial high lifetime).

Left: carbon leaf brushes Right: finger leaf brushes		Cage brushes:

 

Brush offset / Direction of rotation

Brushes can be shifted by some small angle in reference to the permanent magnet (brush offset).

That can favor one direction of rotation, but the other direction has higher sparkling and worse performance.

Stall torque for the favor rotation will be less, but max. efficiency point will be higher.

JE has both options, to provide offset and no offset (zero oriented) according to the customer application.

Existing JE motor codes are designed for an offset or not, must be checked if to use them for new application.

Permanent magnets

JE offers 3 basic versions of magnets:

• Motor code P: Plastics (Rubber) magnet
• Motor code H: Dry or wet pressed magnet , stronger than P
• Motor code Q: Rare earth magnet, for very high torque and efficiency.

 

Rotor iron lamination stack

The windings are located in slots, around the rotor silicon steel. For reduction of eddy current losses, the rotor steel is made of sheets, with insulation layer between (lamination stack).

Keeper ring

Keeper ring

JE can add a soft iron sheet around the motor housing.

It reduces the magnetic circuit resistance and improves performance.

End cap JE offers two versions of end-cap: metal (plastics brush holder inserted) better heat sink for bushing (at high speed) ball bearing can be pressed in EMI noise shielded better plastics lower costs no ball bearing possible