Ward Leonard control

Ward Leonard Control, also known as the Ward Leonard Drive System, was a widely used DC motor speed control system introduced by Harry Ward Leonard in 1891. In early 1900s, the control system of Ward Leonard was adopted by the U.S Navy and also used in passenger lift of large mines. It also provided a solution to moving sidewalk at the Paris Exposition of 1900, which many others had failed to operate properly.Fact|date=November 2007 Until the 1980s, when the Ward Leonard control system started to be replaced by other systems, primarily thyristor controllers, it was widely used for elevators because of it offered smooth speed control and consistent torque. Many Ward Leonard control systems and variations on them remain in use. [cite conference | first = A.B. | last = Kulkarni | authorlink = | coauthors = | title = Energy consumption analysis for geared elevator modernization: upgrade from DC Ward Leonard system to AC vector controlled drive | booktitle = Conference Record of the 2000 IEEE Industry Applications Conference | pages = vol.4, pp.2066-2070 | publisher = Institute of Electrical and Electronics Engineers | date= Oct 2000 | location = | url = | doi = | id = | accessdate = ]

Basic concept

A Ward Leonard drive is a high-power amplifier in the multi-kilowatt range, built from rotating electrical machinery. A Ward Leonard drive unit consists of a motor and generator with shafts coupled together. The motor, which turns at a constant speed, may be AC or DC powered. The generator is a DC generator, with field windings and armature windings. The input to the amplifier is applied to the field windings, and the output comes from the armature windings. The amplifier output is usually connected to a second motor, which moves the load, such as an elevator. With this arrangement, small changes in current applied to the input, and thus the generator field, result in large changes in the output, allowing smooth speed control.

A more technical description

The speed of motor is controlled by varying the voltage fed from the generator, V_gf, which varies the output voltage of the generator. The varied output voltage will change the voltage of the motor, since they are connected directly through the armature. Consequently changing the V_gf will control the speed of the motor. The picture of the right shows the Ward Leonard control system, with the V_gf feeding the generator and V_mf feeding the motor. [cite journal | first = A.K. | last = Datta | authorlink = | coauthors = | title = Computerless optimal control of Ward Leonard drive system | booktitle = International Journal of Systems Science | pages = vol.4, pp.671–678 | publisher = | date= 1973| location = | url = | doi = | id = | accessdate = ]

Mathematical approach

Among many ways of defining the characteristic of a system, obtaining a transfer characteristic is one of the most commonly used methods. Below are the steps to obtain the transfer function, eq 4.

Before going into the equations, first conventions should be set up, which will follow the convention Datta used. The first subscripts 'g' and 'm' each represents generator and motor. The superscripts 'f', 'r',and 'a', correspond to field, rotor, and armature.

W_i = plant state vertorK = gaint = time constantJ = polar moment of inertiaD = angular viscous frictionG = rotational inductance constants = laplace operator

eq 1: The generator field equation V_g^f = R_g^fI_g^f + L_g^fI_g^f

eq 2: The equation of electrical equilibrium in the armature circuit -G_g^faI_g^fW_g^r + (R_g^a + R_m^a) I^a + (L_g^a + L_m^a) I^a + G_m^faI_m^fW_m^r = 0

eq 3: Motor torque equation -T_L = J_mW_m^r+D_mW_m^r

With total impedance, L_g^a + L_m^a, neglected, the transfer function can be obtained by solving eq 3 T_L = 0.

eq 4: Transfer function: $frac\{W\_m^r(S)\}\{V\_g^f(S)\}\; =\; cfrac\{K\_BK\_v/D\_m\}\{(t\_g^fs+1)left\; [t\_ms+cfrac\{K\_m\}\{D\_m\}\; ight]\; \}$ [cite journal | first = A.K. | last = Datta | authorlink = | coauthors = | title = Computerless optimal control of Ward Leonard drive system | booktitle = International Journal of Systems Science | pages = vol.4, pp. 671–678 | publisher = | date= 1973| location = | url = | doi = | id = | accessdate = ]

with the constants defined as below.

: $K\_B\; =\; frac\{G\_m^faV\_m^f\}\{R\_m^f(R\_g^a+R\_m^a)\}$

: $K\_v\; =\; frac\{G\_g^faW\_g^r\}\{R\_g^f\}$

: $t\_m\; =\; frac\{J\_m\}\{D\_m\}$

: $t\_g^f\; =\; frac\{L\_g^f\}\{R\_g^f\}$

: $K\_m\; =\; D\_m+K\_B^2(R\_g^a\; +\; R\_m^a)$

ee also

* Adjustable-speed drive
* Amplidyne
* Brushed DC electric motor
* Electric motor
* Electronic speed control
* Harry Ward Leonard
* Metadyne
* Motor controller
* Motor-generator

References

;Citations

;General references
*cite web | last = Leonard | first = H. Ward | title = Descendants Of William Ward of Sudbury Born Abt 1603, and Other Related Families| work = | publisher = rootsweb.com | date = 2006 | url = http://worldconnect.genealogy.rootsweb.com/cgi-bin/igm.cgi?op=GET&db=rckline-wards&id=I14813 | accessdate = 2006-08-08
*cite journal | last = The Editors | first = | authorlink = | coauthors = | title = Technology for Electrical Components | journal = Power Transmission Design | volume = | issue = | pages = 25–27 | publisher = | date= Nov 1989 | url = | doi = | id = | accessdate =
*cite journal | last = Gottlieb | first = I.M. | authorlink = | coauthors = | title = Electric Motors & Control Techniques 2nd Edition | journal = | volume = | issue = | pages = | publisher = TAB Books | date= 1994 | url = | doi = | id = | accessdate =
*cite book | url = http://books.google.com/books?id=8mQewRR_3q4C&pg=RA1-PA20&lpg=RA1-PA20&dq=%22ward+leonard%22+drive&source=web&ots=TcpJmCH4mV&sig=VasW_IFD1-M-oFNZwt2wNxQSSes#PRA1-PA20,M1
pages = 20–21 | title = Practical Variable Speed Drives and Power Electronics | author = Malcolm Barnes | year = 2003 | isbn = 978-0750658089