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 a moving sidewalk at the Paris Exposition of 1900, where many others had failed to operate properly. It was applied to railway locomotives used in World War I, and was used in anti-aircraft radars in World War II. Connected to automatic anti-aircraft gun directors, the tracking motion in two dimensions had to be extremely smooth and precise. The MIT Radiation Laboratory selected Ward-Leonard to equip the famous radar SCR-584 in 1942. The Ward Leonard control system was widely used for elevators until thyristor drives became available in the 1980s, because it offered smooth speed control and consistent torque. Many Ward Leonard control systems and variations on them remain in use.^[1]

Basic concept

The key feature of the Ward Leonard control system is the ability to smoothly vary the speed of a DC motor, including reversing it, by controlling the field windings and hence the output voltage of a DC generator. As the speed of a DC motor is dictated by the supplied voltage, this gives simple speed control. The DC generator could be driven by any means, provided that also supplied a voltage source for the field windings. This 'prime mover' could be an AC motor, or it could be an internal combustion engine (its application to vehicles was patented by H.W. Leonard in 1903^[2]).

A Ward Leonard drive can be viewed as a high-power amplifier in the multi-kilowatt range, built from rotating electrical machinery. Where the 'prime mover' is electrical, a Ward Leonard drive unit consists of a motor and generator with shafts coupled together. The prime mover, 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 higher power output comes from the armature windings. (See Excitation (magnetic)#Amplifier principle for how a generator can act as an amplifier.) 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.^[3]

A flywheel may be used to reduce voltage fluctuations during sudden load changes. The Ward Leonard system with this modification is known as Ward Leonard Ilgner Control.^[4]

A more technical description

A Ward Leonard Control system with generator and motor connected directly.

The speed of a DC motor is controlled by varying the voltage fed to the generator field windings, 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 on the right shows the Ward Leonard control system, with the V_gf feeding the generator and V_mf feeding the motor.^[5]

Transfer function

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 vector
$K$ = gain
$t$ = time constant
$J$ = polar moment of inertia
$D$ = angular viscous friction
$G$ = rotational inductance constant
$s$ = Laplace operator

Eq. 1: The generator field equation

V_{g}^{f}=R_{g}^{f}I_{g}^{f}+L_{g}^{f}I_{g}^{f}

Eq. 2: The equation of electrical equilibrium in the armature circuit

-G_{g}^{f}aI_{g}^{f}W_{g}^{r}+(R_{g}^{a}+R_{m}^{a})I^{a}+(L_{g}^{a}+L_{m}^{a})I^{a}+G_{m}^{f}aI_{m}^{f}W_{m}^{r}=0

Eq. 3: Motor torque equation

-T_{L}=J_{m}W_{m}^{r}+D_{m}W_{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_{B}K_{v}/D_{m}}{\left(t_{g}^{f}s+1\right)\left(t_{m}s+{\frac {K_{m}}{D_{m}}}\right)}}

^[5]

with the constants defined as below:

K_{B}={\frac {G_{m}^{f}aV_{m}^{f}}{R_{m}^{f}(R_{g}^{a}+R_{m}^{a})}}

K_{v}={\frac {G_{g}^{f}aW_{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})

References

Citations

↑ Kulkarni, A.B. (Oct 2000). "Energy consumption analysis for geared elevator modernization: upgrade from DC Ward Leonard system to AC vector controlled drive". Conference Record of the 2000 IEEE Industry Applications Conference. 4. Institute of Electrical and Electronics Engineers. pp. 2066–2070.
↑ "Electrically propelled Vehicle", H.W. Leonard, US Patent 1121382, originally filed March, 1903
↑ Shinners, Stanley M (1998). Modern Control System Theory. Wiley and Sons. p. 202. ISBN 978-0471249061.
↑ Rajput, R.K. (2005). Basic Electrical Engineering. Laxmi Publications Pvt Limited. p. 571. ISBN 9788170081203. Retrieved 2014-06-14.
1 2 Datta, A.K. (1973). "Computerless optimal control of Ward Leonard drive system". International Journal of Systems Science. 4 (4): 671–678. doi:10.1080/00207727308920047.

General references

The Editors (Nov 1989). "Technology for Electrical Components". Power Transmission Design: 25–27.
Ward Leonard, H. (1896). "Volts versus ohms - the speed regulation of electric motors". AIEE Trans. 13: 375–384.
Gottlieb, I.M. (1994). "Electric Motors & Control Techniques 2nd Edition". TAB Books.
Malcolm Barnes (2003). Practical Variable Speed Drives and Power Electronics. Oxford: Newnes. pp. 20–21. ISBN 978-0-7506-5808-9.

Electric motors

AC - Alternating current DC - Direct current PM - Permanent magnet SC - Self-commutated

Fundamental types	AC motor DC motor

DC motors	Homopolar Electrically-excited field or PM field brushed DC Unipolar

AC SC mechanical commutator	Repulsion Universal

AC SC electronic commutator	Brushless DC (BLDC) Switched reluctance (SRM)

AC asynchronous (induction (IM))	Shaded-pole (single-phase) Dahlander pole changing motor Squirrel-cage rotor (SCIM) Wound-rotor (WRIM) Linear induction

AC synchronous (SM)	Hysteresis Interior / Surface PM (IPMSM / SPMSM) Reluctance (SyRM) Wound-rotor (WRSM)

Special magnetic machines	Doubly-fed Linear Servomotor Stepper Traction

Non-magnetic	Electrostatic Piezoelectric Ultrasonic

Enclosure Type	Hermetic TEFC

Components and accessories	Armature Braking chopper Brush Commutator DC injection braking Field coil Rotor Slip ring Stator Winding

Motor controllers	AC/AC converter Amplidyne Adjustable-speed drive (ASD) Cycloconverter Direct torque control (DTC) Metadyne Motor controller Motor soft starter Motor starter (engine) Power inverter Thyristor drive Variable-frequency drive (VFD) Vector control Voltage controller Ward Leonard control

History, education, recreational use	Timeline of the electric motor Ball bearing motor Barlow's wheel Lynch motor Mendocino motor Mouse mill motor

Experimental, futuristic	Coilgun Railgun Superconducting machine

Related topics	Blocked-rotor test Circle diagram Closed-loop control Electromagnetism Open-circuit test Open-loop controller Power-to-weight ratio Torque and speed of a DC motor Two-phase system

People	Arago Baily Barlow Botto Cook Davenport Davidson Dolivo-Dobrovolsky Faraday Ferraris Froment Gramme Henry Jacobi Jedlik Lenz Maxwell Ørsted Park Pixii Saxton Siemens Sprague Steinmetz Stimpson Sturgeon Tesla

See also	Alternator Electric generator Inchworm motor

SI electromagnetism units	C - Capacitance (F) Q - Charge (C) G, B, Y - Conductance, susceptance, admittance (S) κ, γ, σ - Conductivity (S/m) I - Current (A) D - Electric displacement field (C/m²) E - Electric field (V/m) Φ_E - Electric flux (V·m) χ_e - Electric susceptibility U, ΔV, Δφ; E - Emf (V) L, M - Inductance (H) H - Magnetic field (A/m) strength Φ - Magnetic flux (Wb) B - Magnetic flux density (T) χ - Magnetic susceptibility μ - Permeability (H/m) ε - Permittivity (F/m) P - Power (W) R, X, Z - Resistance, reactance, impedance (Ω) ρ - Resistivity (Ω·m)

This article is issued from Wikipedia - version of the 11/15/2016. The text is available under the Creative Commons Attribution/Share Alike but additional terms may apply for the media files.

Ward Leonard control

Basic concept

A more technical description

Transfer function

See also

References