An adjustable Linear speed drive is a device that controls speed, and direction of
an AC or DC motor. Some high-performance drives are able to run in torque regulation
DC Drive Control
A basic DC drive control system generally contains
a drive controller and DC motor as shown in The controls allow the operator to start, stop, and change direction and speed of the motor by
turning potentiometers or other operator devices. These controls may be an integral part of the controller or may be remotely mounted.
The drive controller converts a 3-phase AC voltage to an adjustable DC voltage,
which is then applied to a DC motor armature.
DC Drive Control System
The DC motor converts power from the
adjustable DC voltage
source to rotating mechanical
force. Motor shaft rotation and direction are proportional to the magnitude
and polarity of
the DC voltage applied to the motor
The tachometer (feedback device) shown in Figure 1.1 converts actual speed to an electrical signal that is summed with the desired reference signal. The output
of the summing junction
provides an error signal to the controller and a speed correction is
The following are the four basic types of
DC motors and their operating characteristics:
Shunt-wound motors have the field controlled separately from
the armature winding. With constant armature voltage and constant field excitation, the shunt-wound motor offers relatively
flat speed-torque characteristics. The shunt-wound motor offers simplified control for reversing,
especially for regenerative drives
Fig.1.2- Shunt Wound graphical
The series-wound motor has the field connected in series with the armature. Although the series-
wound motor offers high starting torque, it has poor speed regulation. Series-wound motors are
generally used on low speed, very heavy loads.
Fig.1.3 – Series Wound
The compound-wound DC motor utilizes a field winding in series with the armature in addition to the
shunt field, to obtain a compromise in performance between a series and a shunt wound type motor. The compound-wound motor offers a combination
of good starting torque and speed stability.
Fig.1.3 – compound wound
The permanent magnet motor has a conventional wound armature with commutator and brushes. Permanent magnets replace the field windings. This type of motor has excellent
starting torque, with speed regulation
slightly less than that of the compound motor. Peak starting torque is
commonly limited to 150% of rated torque to avoid demagnetizing the field poles. Typically these
– permanent magnet
controlled DC drives are capable of providing rated current and torque at any
speed between zero and the base (rated)
speed of the motor. These drives use a fixed field supply and give motor characteristics as seen in Figure 1.2. The motor output horsepower is
directly proportional to speed (50% horsepower at 50% speed).
Fig.1.5 – torque-HP graph
The term constant torque describes a load type where the torque requirement is
constant over the
Horsepower at any given operating point can be calculated with the following equation:
Torque is measured in Lb-Ft
measured in RPM.
Armature and Field Controlled DC Drives
The motor is armature voltage
controlled for constant torque-variable HP operation up to base
speed. Above base speed the motor is transferred to field current control for constant HP –
reduced torque operation up to maximum speed.
– Operation above Base Speed
One characteristic of a shunt-wound DC motor is that
a reduction in rated field current at a given
armature voltage will result in an increase in speed and lower torque output
per unit of armature
– Motor Speed and load characterstics
The speed of
an AC motor is determined for the most part by two factors: The applied frequency
and the number of poles.
N = RPM
f = frequency
P = number of
Some motors such as in a typical paddle fan have the capability to switch poles in and out to
control speed. In most cases however, the number of
poles is constant and the only way to vary
the speed is to change the applied
frequency. Changing the frequency is
the primary function of an AC drive. However, one must consider that the impedance of
a motor in determined by the inductive reactance of the windings. Refer to the equation below.
XL = Inductive reactance in Ohms f = Line frequency
L = inductance
This means that if the frequency applied
to the motor is reduced, the reactance and therefore impedance of
the motor is reduced. In order to keep current under control we must lower the applied voltage to the motor as the frequency is
reduced. This is where we get the phrase “volts per
hertz”. The most common method of controlling the applied
voltage and frequency is with a
pulse width modulated
“PWM” technique. With this method, a DC voltage
is applied to the motor windings
in time controlled pulses in order to achieve current that approximates a sine wave of the desired frequency. IGBTs or Isolated Gate Bipolar
Transistors are the latest technology
and offer the ability to switch the PWM pulses very fast. This allows several thousand pulses to be applied
in one cycle of the applied motor frequency. More pulses in a given cycle result in a smoother current waveform and better motor performance.
AC Motor Types
AC motors can be divided into two main types: induction and synchronous. Induction motors are
most common in industry. Synchronous motors are special purpose motors that do not require
any slip and operate at synchronous speed.
The induction motor is
the simplest and most rugged of
all electric motors. The induction motor is
generally classified by a NEMA design category. Before a meaningful discussion on NEMA type motors can be had, we should first look at what makes up a torque speed curve.
Anatomy of a Speed Torque Curve
Generally speaking the following can be said about a speed torque curve when starting across the
line. Starting torque is
usually around 200% even though
current is at 600%.
This is when slip is the greatest. (Starting torque is
called Blocked Rotor Torque, Locked Rotor Torque or Breakaway Torque.) Such a large inrush of
current may cause the supply voltage
to dip momentarily, affecting other equipment connected to the same lines. To prevent this, large motors will connect extra resistors to inductors in series with the stator during starting. Extra protective
devices are also required to remove the motor from the supply lines if an excessive load causes a stalled condition.
As the motor begins
to accelerate, the torque drops off, reaching a minimum value, called Pull-up
Torque, between 25-40% of synchronous speed (Point B). Pull-up Torque is
caused by harmonics that
the stator windings
being concentrated in slots. If the windings
distributed around the stator periphery, Pull-up Torque is
greatly reduced. Some motor design curves show no actual Pull-up Torque and follow the dashed line between points A and C.
As acceleration continues, rotor frequency and inductive reactance decrease. The rotor flux moves more in phase with the stator flux and torque increases. Maximum Torque (or Breakdown
Torque) is developed at point C where inductive reactance becomes equal to the rotor resistance. Beyond point C, (points D, E and F) the inductive reactance continues to drop off but rotor current
decreases at the same rate, reducing torque.
Point G is
synchronous speed and proves that if rotor and stator are at the same speed, rotor
current and torque are zero.
At running speed, the motor will operate between points F and D, depending on load. However
temporary load surges may cause the motor to slip all the way back near point C on the “knee” of the
Beyond point C, the power factor decreases faster than current increases causing torque to drop
On the linear part of the
motor curve (points C to G), rotor frequency is only
1 to 3 hertz –
almost DC. Inductive reactance is
essentially zero and rotor power factor approaches unity.
Torque and current now become directly proportional – 100% current produces 100% torque. If a
1HP motor has a nameplate current of
3.6 amps, then when it draws 3.6 amps (at proper voltage and frequency) it must be producing 100% of it’s
nameplate torque. Torque and current remain
directly proportional up to approximately 10% slip.
Notice that as motor load increases from zero (point F) to 100% (point E), the speed drops only
45-55 RPM, about 3% of
synchronous speed. This makes the squirrel cage induction motor very suitable for most constant speed applications (such as conveyors) where, in some cases, 3%
speed regulation might be acceptable. If better speed regulation is required, the squirrel cage motor may be operated from
a closed loop regulator
such as a Rockwell Automation variable
The locked rotor torque and current, breakdown torque, pull-up torque and the percent slip,
determine the classifications
for NEMA design motors. The speed-torque curve and characteristics of each design are as follows:
Design A — motors have a low resistance, low inductance rotor producing low starting torque and
high breakdown torque. The low resistance characteristic causes starting current to be high.
It is a high efficiency design; therefore, the slip is
usually 3% or less.
Fig.1.7 – Design A AC
Linear induction motors are often less
efficient than conventional rotary induction motors; the end effects and the
relatively large air gap that is often present will typically reduce the forces
produced for the same electrical power. The larger air gap also increases the
inductance of the motor which can require larger and more expensive capacitors.
However, linear induction motors can avoid the need
for gearboxes and similar drivetrains, and these have their own losses; and
working knowledge of the importance of the goodness factor can minimise
the effects of the larger air gap. In any case power use is not always the most
important consideration. For example, in many cases linear induction motors
have far fewer moving parts, and have very low maintenance. Also, using linear
induction motors instead of rotating motors with rotary-to-linear transmissions
in Motion Control systems, enables higher bandwidth and accuracy of
the Control systems because rotary-to-linear
transmissions introduce backlash, static friction and/or mechanical compliance
in the control system.
Because of these properties, linear motors are often used in
Maglev propulsion, as in the Japanese Liminio Magnetic levitating train
line near Nagoya. The world’s first commercial automated maglev system was
low speed Maglev that ran from the airport terminal of Birmingham
international airport to the nearby station between 1984–1995. The
length of the track was 600 metres (2,000 ft), and trains “flew”
at an altitude of 15 millimeters (0.59 in), levitated by electromagnets,
and propelled with linear induction motors. It was in operation for nearly
eleven years, but problems with the electronic systems made it unreliable
in its later years. One of the original cars is now on display
at Peterborough, together with the RTV31 hover train vehicle.
However, linear motors have been used
independently of magnetic levitation, Tokyo is an example of an automated
system that utilizes LIM propulsion; the longest Transit system system
employing such technology is Vancouver sky train, with approximately 60 km
(37 mi) of track compatible with Innovia Metro trains.
Linear induction motor technology is also
used in some Roller coster At present it is still impractical on street
running Trams, although this, in theory, could be done by burying it in a
Outside of public transportation, vertical
linear motors have been proposed as lifting mechanisms in deep Mines, and
the use of linear motors is growing in Motion control applications. They are
also often used on sliding doors, such as those of low floor trams such as the
citadis and the eurotram
Dual axis linear motors also exist. These
specialized devices have been used to provide direct X-Y motion
for precision laser cutting of cloth and sheet metal, automated Drafting,
and cable forming. Also, linear induction motors with a cylindrical secondary
have been used to provide simultaneous linear and rotating motion for mounting
electronic devices on printed circuit boards.
Most linear motors in use are LIM (linear
induction motors) or LSM (linear synchronous motors). Linear DC motors are not
used as it includes more cost and linear SRM suffers from poor thrust. So for
long run in traction LIM is mostly preferred and for short run LSM is mostly
Linear induction motors have also been used
for launching aircraft, the Westinghouse Electropult system in 1945 was an
early example and the (EMALS) was due to be delivered in 2010.
Linear induction motors are also used in
looms, magnetic levitation enable bobbins to float between the fibers without
direct contact.A motor rated for 60hz operation may be run at higher frequencies when powered by Rockwell
Automation AC Drive. The top speed depends
upon the voltage
limits of the motor and it’s mechanical balancing. 230V and 460V motors normally employ insulation
rated for as much as1600V,
so the voltage limit is not
usually a problem. An average 2 pole industrial motor can safely exceed its base speed by 25%. Many manufacturers balance their 3 pole and 4 pole rotors to the same speed – 25% over the 2 pole base speed. A 4-pole motor may therefore operate up to
125% over base speed before reaching its balance limit. A 60hz 4-pole motor might run up to
135hz, whereas a 60hz 2-pole motor would reach its balance limit at 75hz. Both motors would run
at the same RPM. Always contact your motor manufacturer if you plan to operate at these speeds.
Constant Voltage Operation
What happens to the volts per hertz ratio above rated frequency? If output frequency is increased to 120hz with 100% voltage
applied to the motor; the Volts per Hertz of
the drive is no longer
7.6 but rather 3.83. The same Volts per Hertz ratio results when a line started motor is
60hz with only 50% voltage applied
(for reduced voltage
starting). As might be expected the effect
on torque is the same. Recall that torque varies as the square of the
K 1 xE
As such, maximum torque at 120hz is only
25% of the maximum torque at 60hz.
If AC drive output frequency is reduced from
120hz to 90hz at a constant voltage,
the Volts per Hertz ration improves from
3.83 to 5.1 V/Hz. This is
the same as providing 66% voltage at 60hz to a line-started motor. Torque will be 0.662 or 44% of
the full voltage
torque at 60hz. Below illustrates the peak torque curve for constant voltage operation from base speed to 4 times base