Lecture 38Electro Mechanical System1  In star-stop fashion, there is upper limit to a stepping rate  For too fast pulse rate, motor is unable to follow & will loose steps  For synchronism, the rotor must settle before advancing  Interval between two steps is at lease 6 ms, so the stepping rate is limited to a 1000/6 = 167 steps per second (sps) Start-Stop stepping rate  We also know that higher the load and greater the inertia, the lower will be the allowable number of steps per second  The start-stop stepping mode is also called start without error mode

Lecture 38Electro Mechanical System2  The stepper motor can be made to run at a uniform speed, without starting and stopping at any every step. When the motor runs this way it is said to be slewing.  Since the motor runs at a uniform speed the inertia effect is absent.  For a given stepping rate the motor can carry a greater load torque when it is slewing. Slew speed

Lecture 38Electro Mechanical System3  When a stepper motor is carrying a load, it cannot suddenly go from zero to a stepping rate of 5000 sps.  In the same way, a motor that is slewing at 5000 sps cannot brought to a dead stop in one step.  The motor must be accelerated gradually and it must be decelerated gradually.  The instantaneous position of the motor must correspond to the number of pulses.  Process whereby motor accelerates/decelerate is called ramping  The ramp is generated by the power supply that drives the stepper motor. Types of Stepper Motor There are three main types of stepper motors  Variable reluctance stepper motor  Permanent magnet stepper motor  Hybrid stepper motor Ramping

Lecture 38Electro Mechanical System4  The stepper motor we studied is called variable reluctance stepper motor  Structure can be modified to obtain small angular steps of 1.8 o instead of 60 o  Circular rotor can be used milling out slots around its periphery Variable reluctance stepper motor  The teeth created thereby constitutes salient poles of the rotor  Stator can have four, five or even eight poles  steps of 18 o, 15 o, 7.5 o, 5 o, and 1.8 o are quite common

Lecture 38Electro Mechanical System5  Similar to variable reluctance stepper motor except that motor has permanent N and S poles  Due to permanent magnets the rotor remains lined up with the last pair of stator poles excited by the driver  The rotor develops detent torque which keeps it in place without any current Permanent magnet stepper motor  Coils A1, A2 are connected in series, as are B1,B2  Coils B are excited the rotor will move 30 o  If coils B produce N & S poles rotor will move ccw

Lecture 38Electro Mechanical System6  It has two identical soft iron armature mounted on same shaft  Diagram shows a 5-pole armature driven by a 4-pole stator  Permanent magnet PM is sandwiched between armatures  It produces a unidirectional magnetic field. All poles on armature 1 are N and those on armature 2 are S  Stator coils A1, A2 are in series, and so are B1, B2  If we excite coil B rotor will rotate 18 o, lining up with pole B Hybrid stepper motor

Lecture 38Electro Mechanical System7  It should be noted that no of poles on stator of a stepper motor are never equal to the number of poles of rotor  This feature is totally different from any other motor  It is this difference that enables the motor to move in steps as they do Stepper motor