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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, that make slipping 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 the torque generation in same direction, the current flow must be reversed when the rotor north pole passes the stator south pole.
For this the slipping 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.
Real DC motors have more than two windings and commutator segments, for generation of a more constant torque.
Picture below shows a 5 segment design (HC685LG).
This is an exploded view of JE PMDC motor size #200:
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).
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.
The stall torque for the favor rotation will be less, but the 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.
JE offers 3 basic versions of magnets:
- Motor code P: Plastics (Rubber) magnet, glued to housing.
- Motor code H: Dry or wet pressed magnet, segments held by springs or glued. 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).
JE adds a soft iron sheet around the motor housing, for many motors. It reduces the magnetic circuit resistance and improves performance.
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
- lower costs
- no ball bearing possible
EMI noise suppression
Brushed DC motors generate EMI (ElectroMagnetic Interference) noise, whenever the brushes change from one to another commutator segment.
The higher the supply voltage, speed and current - the higher the noise emission. The emission also depends on number of segments (i.e. voltage drop between neighbouring segments).
The noise is emitted by two ways:
- Conduction along power supply wires
- Radiation through the air
The noise has to be limited according to directives and standards. For that, JE can make tests and implement noise suppression means. Preferably, for these tests the whole customer appliance should be used.
Additional to means such as twisting and shielding of wires, there is a choice of several suppression components:
Capacitor can be inserted inside #300, but not inside #200. 2 Capacitors + 2 chokes can be inserted inside #600 (metal end cap version).
|U = I • R + U
Ui ... voltage induced in windings (back-EMF)
Ui = k • ω (ω … angular speed, ω = 2 • π • n)
Torque generation is: T = k • I
Factor k depends on motor design features (number of winding turns, permanent magnet strength, air gap distance, rotor diamter, rotor length).
It is fixed with the design, but varies due to motor manufacturing tolerances.
There is also an influence of temperature (k gets lower at rising temperature, due to lower magnet strength).
Basic equation of DC motor is:
ω = U/k – R/k2 • T
It says, that speed is highest at no load condition and decreases with rising load torque.
Losses and Efficiency
No-load losses PO:
Friction losses at bearings and at brushes, Hysteresis and Eddy current losses.As these losses must be covered by a no-load torque and hence no-load current – there are also no-load current losses in the winding: IO2 x R.
At rising load torque, the current and thus winding current losses increase.
Winding current is maximum when rotor is stalled: Ploss max = U2 / R
At point of max.efficiency there is best ratio Pmech / Pel = Pmech /(U • I).
Efficiency at this point is about η = (1 - √ IO/Istall)2
At point of max.mechanical output power, the speed is half of no-load speed.
Output power Pmech max is little less than U2 / 4 R.
DC motor lifetime is limited by wear of brushes. It depends on
- Speed (commutator surface speed)
- Current load (average current and peaks, start/stop frequency)
- Brush configuration, brush material (see above) and commutator design
Typical lifetime of JE PMDC motors ranges from 300 hours (small motors) to about 2000 hours (larger motor sizes, cage brushes, continuous operation of rather low output power).
In serial production, the motors have a manufacturing tolerance.
Tolerance of no load speed is specified by +-10%, but is lower usually.
Rated operating point
Rated operating point (rated torque) is defined for continuous duty cycle operation, usually.But motor design can be also made for low duty cycle operation or short time operation at rated point, especially for high speed motors.
Rated point is located at point of max.efficiency or between point of max.efficiency and point of max output power.
Lower duty cycle operation allows higher torque load (overload).
Permissible overload is restricted by winding temperature increase.
If the motor carries high current (low voltage, thick wire design) it is also effected by commutator/brushes.
A heat sink (metal flange to metal rack or gearbox) increases loadability.
Performance at increased motor temperature
An increase of motor temperature can be caused by
- Self-warming up due to motor losses after some time of operation
- Higher ambient temperatur
There are two effects:
- Permanent magnet strength decreases – causing higher no load speed
- Winding resistance increases – causing lower stall current.
Both effects reduce stall torque value.
Performance at reduced supply voltage
The speed curve reduces proportional to the voltage. Vice versa, it increases with rising voltage (but to consider reduced lifetime or even thermal overloading of the motor).
This is valid within limits, about +-50% decrease/increase of voltage.
High Voltage DC Motors (HVDC)
HVDC motors can be connected to mains supply voltage level 120 Vac or 230 Vac. JE has special expertise to design motor components accordingly, especially the commutator assembly.
A rectifier is needed to convert AC into DC. It is located inside the
motor (if there is space available) or outside.
A changeover switch is to use for bidirectional operation.
HVDC are replacing Universal motors in many applications. Advantage is a reduction of copper material (use of per-manent magnets instead of stator field winding) and less weight.
Encoders (Sensors) can be implemented inside the motor or mounted to motor rearside.
They are needed for a precise speed control or angular position control. Typical application: Printers and scanners.
There are different options:
- Optical encoders
- Hall-effect magnetic sensors
JE offers a range of several optical encoders.
Basic principle is:
A slotted wheel or a black/transparent strip photo print is rotating with the motor shaft.
A photosensor provides a sinuswave or pulse pattern, that follows the speed of the motor.
Output signal can be analog (0...3.3Vac) or digital (0...3.3 Vdc or 0...5Vdc).
One signal or two signals phase shifted 90° (enabling detection of direction of rotation ; allows 4x resolution)
Several options for resolution. Terminology: CPR (Counts Per Revolution) and LPI (Lines Per Inch).