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- The objective of this chapter is to discuss various techniques for the control of electric machines. Basic techniques for speed and torque control will be illustrated with typical configurations of drive electronics that are used to implement the control algorithms. - The discussion of this chapter is limited to steady-state operation. The steady-state picture presented here is quite adequate for a wide variety of electricmachine applications.- In the discussion of torque control for synchronous and induction machines, the techniques of field-oriented or vector control are introduced and the analogy is made with torque control in dc motors. This material is somewhat more sophisticated mathematically than the speed-control discussion and requires application of the dq0 transformations.

Control of dc motors

Speed control

  • The three most common speed-control methods for dc motors are adjustment of the flux, usually by means of field-current control, adjustment of the resistance associated with the armature circuit, and adjustment of the armature terminal voltage.

Field-Current Control: In part because it involves control at a relatively low power level, field-current control is frequently used to control the speed of a dc motor with separately excited or shunt field windings. The equivalent circuit for a separately excited dc machine is in Fig. 10.1. The shunt field current can be adjusted by means of a variable resistance in series with the shunt field. Alternatively, the field current can be supplied by power-electronic circuits which can be used to rapidly change the field current in response to a wide variety of control signals.

  • Figure 10.2a shows a switching scheme for pulse-width modulation of the field voltage. It consists of a rectifier which rectifies the ac input voltage, a dc-link capacitor which filters the rectified voltage, producing a dc voltage V dc size 12{V rSub { size 8{ ital "dc"} } } {} , and a pulse-width modulator.
  • In this system, because only a unidirectional field current is required, the pulsewidth modulator consists of a single switch and a free-wheeling diode. Assuming both the switch and diode to be ideal, the average voltage across the field winding will be equal to

V f = DV dc size 12{V rSub { size 8{f} } = ital "DV" rSub { size 8{ ital "dc"} } } {} (10.1)

where D is the duty cycle of the switching waveform; i.e., D is the fraction of time that the switch S is on. Figure 10.2b shows the resultant field current.

  • In the steady-state the average voltage across the inductor must equal zero, the average field current I f size 12{I rSub { size 8{f} } } {} will thus be equal to

I f = V f R f = D V dc R f size 12{I rSub { size 8{f} } = { {V rSub { size 8{f} } } over {R rSub { size 8{f} } } } =D left [ { {V rSub { size 8{ ital "dc"} } } over {R rSub { size 8{f} } } } right ]} {} (10.2)

Figure 10.1 Equivalent circuit for a separately excited dc motor.

Figure 10.2 (a) Pulse-width modulation system for a dc-machine field winding.

(b) Field-current waveform.

  • Thus, the field current can be controlled simply by controlling the duty cycle of the pulse-width modulator. If the field-winding time constant L f / R f size 12{L rSub { size 8{f} } /R rSub { size 8{f} } } {} is long compared to the switching time, the ripple current Δi f size 12{Δi rSub { size 8{f} } } {} will be small compared to the average current I f size 12{I rSub { size 8{f} } } {} .
  • To examine the effect of field-current control, let us begin with the case of a dc motor driving a load of constant torque T load size 12{T rSub { size 8{ ital "load"} } } {} . The generated voltage:

Questions & Answers

can someone help me with some logarithmic and exponential equations.
Jeffrey Reply
sure. what is your question?
ninjadapaul
20/(×-6^2)
Salomon
okay, so you have 6 raised to the power of 2. what is that part of your answer
ninjadapaul
I don't understand what the A with approx sign and the boxed x mean
ninjadapaul
it think it's written 20/(X-6)^2 so it's 20 divided by X-6 squared
Salomon
I'm not sure why it wrote it the other way
Salomon
I got X =-6
Salomon
ok. so take the square root of both sides, now you have plus or minus the square root of 20= x-6
ninjadapaul
oops. ignore that.
ninjadapaul
so you not have an equal sign anywhere in the original equation?
ninjadapaul
Commplementary angles
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a perfect square v²+2v+_
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algebra 2 Inequalities:If equation 2 = 0 it is an open set?
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or infinite solutions?
Kim
The answer is neither. The function, 2 = 0 cannot exist. Hence, the function is undefined.
Al
y=10×
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if |A| not equal to 0 and order of A is n prove that adj (adj A = |A|
Nancy Reply
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ramon Reply
Kristine 2*2*2=8
Bridget Reply
Differences Between Laspeyres and Paasche Indices
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No. 7x -4y is simplified from 4x + (3y + 3x) -7y
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is it 3×y ?
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J, combine like terms 7x-4y
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Need to simplify the expresin. 3/7 (x+y)-1/7 (x-1)=
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. After 3 months on a diet, Lisa had lost 12% of her original weight. She lost 21 pounds. What was Lisa's original weight?
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I start with an easy one. carbon nanotubes woven into a long filament like a string
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AMJAD
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Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
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AMJAD
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AMJAD
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Stotaw
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
Azam
anybody can imagine what will be happen after 100 years from now in nano tech world
Prasenjit
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
Azam
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Prasenjit
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
Damian
silver nanoparticles could handle the job?
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Azam
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Prasenjit Reply
At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
Ali Reply
the Beer law works very well for dilute solutions but fails for very high concentrations. why?
bamidele Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
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Source:  OpenStax, Electrical machines. OpenStax CNX. Jul 29, 2009 Download for free at http://cnx.org/content/col10767/1.1
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