# Electro Mechanical System

 Date 16.12.2020 Size 458.83 Kb.

## High Speed Operation

• Each coil of the Stepper motor is switched through a transistor based circuit
• L and R represent the inductance and resistance of the coil
• Current pulse in winding is assumed to be ideal
• Practically it is not ideal due to the time constant To = L/R of the coil

## High Speed Operation

• The performance of the stepper motor is effected
• Due to the time constant initial current i1 rises slowly
• Initial torque developed is low
• Rotor moves slowly
• When transistor is switched off
• Current i2 continues to circulate through diode

## High Speed Operation

• Effective duration of pulse is increased to Tp+3T0
• Switching time of coil is prolonged
• Shortest possible pulse can be achieved if the current is switched on and immediately switched off when it reaches its maximum value
• Duration of shortest pulse current I is 6To sec
• Time constant of stepper motor coils is in between 1 ms (min) to 8 ms (max)

## High Speed Operation

• Therefore duration of one step can not be less than 6 ms (6To = 6 x 1 ms = 6 ms)
• Assuming the lowest time constant of 1 ms
• Max. stepping rate is 1000/6 or 166 sps
• 166 sps is considered to be slow
• To achieve higher stepping rate the coil time constant has to be reduced
• The stepper motor coil itself can not be changed
• So how can the Time constant be decreased?

## Modifying Time Constant

• Time constant can be decreased by adding external resistance and raising supply voltage so same rated current I flows
• 1000 sps become feasible
• What is the disadvantage of raising the supply voltage?
• The power supply becomes more expensive
• Considerable power is wasted in external resistor

## Bi-level Drives

• Bi-level drives allow fast switching times without using external resistors
• The switching circuit is modified
• Instead of a single transistor switch Q1
• Two transistor based switches Q1 and Q2 are used
• Switch Q1 is initially closed
• Q2 is closed to initiate pulse
• Time constant is L/R = 8 ms

## Bi-level Drives

• When Q2 is closed the switching circuit becomes a simpler circuit as shown
• Current reaches 200 A in 8 ms
• Current rises @ 200 A/8 ms = 25000 A/s
• Rated current of 10 A is reached in 0.4 ms
• Switch Q1 opens at 0.4 ms

## Bi-level Drives

• The circuit after opening of switch Q1 becomes as shown
• Current of 10 A is maintained
• The current remains at 10 A until the step pulse is to be terminated
• To terminate pulse after 5 ms Q2 is opened

## Bi-level Drives

• The circuit after opening of switch Q2 becomes as shown
• 57 V source drives current against i
• Time constant remains 8 ms
• Current decreases @ 57/60 x 25000 = 23750 A/s
• Current reaches 0 A at 10/23750 = 0.42 ms
• Q1 is closed when current reaches 0 A