Brushed DC Motor Fundamentals - Microchip Technology

1 2010 Microchip Technology 1AN905 INTRODUCTIONB rushed DC motors are widely used in applicationsranging from toys to push-button adjustable car DC (BDC) motors are inexpensive, easy todrive, and are readily available in all sizes and application note will discuss how a BDC motorworks, how to drive a BDC Motor , and how a drivecircuit can be interfaced to a PIC OF OPERATIONThe construction of a simple BDC Motor is shown inFigure 1. All BDC motors are made of the same basiccomponents: a stator, rotor, brushes and a following paragraphs will explain each componentin greater stator generates a stationary magnetic field thatsurrounds the rotor.

2 This field is generated by eitherpermanent magnets or electromagnetic windings. Thedifferent types of BDC motors are distinguished by theconstruction of the stator or the way the electromag-netic windings are connected to the power source.(See Types of Stepping Motors for the different BDCmotor types).RotorThe rotor, also called the armature, is made up of oneor more windings. When these windings are energizedthey produce a magnetic field. The magnetic poles ofthis rotor field will be attracted to the opposite polesgenerated by the stator, causing the rotor to turn. Asthe Motor turns, the windings are constantly beingenergized in a different sequence so that the magneticpoles generated by the rotor do not overrun the polesgenerated in the stator.

3 This switching of the field in therotor windings is called 1:SIMPLE TWO-POLE Brushed DC MOTORA uthor:Reston ConditMicrochip Technology MagnetSOUTHNORTHA xleor CoilBrushed DC Motor FundamentalsAN905DS00905B-page 2 2010 Microchip Technology and CommutatorUnlike other electric Motor types ( , brushless DC,AC induction), BDC motors do not require a controllerto switch current in the Motor windings. Instead, thecommutation of the windings of a BDC Motor is donemechanically. A segmented copper sleeve, called acommutator, resides on the axle of a BDC Motor . As themotor turns, carbon brushes slide over the commutator,coming in contact with different segments of thecommutator.

4 The segments are attached to differentrotor windings, therefore, a dynamic magnetic field isgenerated inside the Motor when a voltage is appliedacross the brushes of the Motor . It is important to notethat the brushes and commutator are the parts of aBDC Motor that are most prone to wear because theyare sliding past each OF STEPPING MOTORSAs mentioned earlier, the way the stationary magneticfield is produced in the stator differentiates the varioustypes of BDC motors. This section will discuss thedifferent types of BDC motors and the advantages/disadvantages of MagnetPermanent Magnet Brushed DC (PMDC) motors arethe most common BDC motors found in the motors use permanent magnets to produce thestator field.

5 PMDC motors are generally used in appli-cations involving fractional horsepower because it ismore cost effective to use permanent magnets thanwound stators. The drawback of PMDC motors is thatthe magnets lose their magnetic properties over PMDC motors have windings built into them toprevent this from happening. The performance curve(voltage vs. speed), is very linear for PMDC draw also varies linearly with torque. Thesemotors respond to changes in voltage very quicklybecause the stator field is always 2:PERMANENT MAGNET DC MOTORSS hunt-WoundShunt-wound Brushed DC (SHWDC) motors have thefield coil in parallel (shunt) with the armature.

6 Thecurrent in the field coil and the armature are indepen-dent of one another. As a result, these motors haveexcellent speed control. SHWDC motors are typicallyused in applications that require five or more horse-power. Loss of magnetism is not an issue in SHWDC motors so they are generally more robust than 3:SHUNT-WOUND DC MOTORSS eries-WoundSeries-wound Brushed DC (SWDC) motors have thefield coil in series with the armature. These motors areideally suited for high-torque applications because thecurrent in both the stator and armature increases underload. A drawback to SWDC motors is that they do nothave precise speed control like PMDC and SHWDC motors 4:SERIES-WOUND DC MOTORSA rmatureBrushPermanentMagnet PolesDCVoltageSupplyArmatureBrushShuntFi eldDCVoltageSupplyArmatureBrushSeriesFie ldDCVoltageSupply 2010 Microchip Technology 3AN905 Compound-WoundCompound Wound (CWDC) motors are a combinationof shunt-wound and series-wound motors.

7 As shown inFigure 5, CWDC motors employ both a series and ashunt field. The performance of a CWDC Motor is acombination of SWDC and SHWDC motors. CWDC motors have higher torque than a SHWDC Motor whileoffering better speed control than a SWDC 5:COMPOUND-WOUND DC MOTORSBASIC DRIVE CIRCUITSD rive circuits are used in applications where a control-ler of some kind is being used and speed control isrequired. The purpose of a drive circuit is to give thecontroller a way to vary the current in the windings ofthe BDC Motor . The drive circuits discussed in thissection allow the controller to pulse-width modulate thevoltage supplied to a BDC Motor .

8 In terms of powerconsumption, this method of speed control is a far moreefficient way to vary the speed of a BDC motorcompared to traditional analog control analog control required the addition of aninefficient variable resistance in series with the motors are driven in a variety of ways. In somecases the Motor only needs to spin in one 6 and Figure 7 show circuits for driving a BDCmotor in one direction. The first is a low-side drive andthe second is a high-side drive. The advantage to usingthe low-side drive is that a FET driver is not typicallyneeded.

9 A FET driver is used the TTL signal driving a MOSFET to thepotential level of the supply voltage, enough current to drive the MOSFET(1), provide level shifting in that in each circuit there is a diode across themotor. This diode is there to prevent Back Electromag-netic Flux (BEMF) voltage from harming the is generated when the Motor is spinning. Whenthe MOSFET is turned off, the winding in the Motor isstill charged at this point and will produce reversecurrent flow. D1 must be rated appropriately so that itwill dissipate this 6:LOW-SIDE BDC Motor DRIVE CIRCUITFIGURE 7:HIGH-SIDE BDC Motor DRIVE CIRCUITR esistors R1 and R2 in Figure 6 and Figure 7 are impor-tant to the operation of each circuit.

10 R1 protects themicrocontroller from current spikes while R2 ensuresthat Q1 is turned off when the input pin is :The second point typically does not applyto most PIC microcontroller applicationsbecause PIC MCU I/O pins can source ControllerD1 BDCM otorR1R2To ControllerVCCVCCAN905DS00905B-page 4 2010 Microchip Technology control of a BDC Motor requires a circuitcalled an H-bridge. The H-bridge, named for itsschematic appearance, is able to move current in eitherdirection through the Motor winding. To understandthis, the H-bridge must be broken into its two sides, orhalf-bridges.