Drive and Motor Basics

Drive Fundamentals
Drive/Motor Basics
Revision 1.0
Drive and Motor Basics
An adjustable 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 mode.
DC Drives
DC Drive Control System
A basic DC drive control system generally contains a drive controller and DC motor as shown in
Figure 1.1.
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.
Figure 1.1
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 made.
DC Motors
The following are the four basic types of DC motors and their operating characteristics:
Shunt Wound
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.
Series Wound
The series-wound motor has the field connected in series with the armature. Although the serieswound motor offers high starting torque, it has poor speed regulation. Series-wound motors are
generally used on low speed, very heavy loads.
Compound 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
Permanent Magnet
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
are low horsepower.
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Armature voltage 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).
Figure 1.2
Constant Torque
The term constant torque describes a load type where the torque requirement is constant over the
speed range.
Horsepower at any given operating point can be calculated with the following equation:
HP =
Torque × Speed
Torque is measured in Lb-Ft
Speed is measured in RPM.
Constant Horsepower
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. (Refer to Figure 1.3)
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Figure 1.3
Constant Torque and Horsepower
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
current (see Figure 1.3). This concept can be seen in Figure 1.4
Figure 1.4
Motor Speed and Load Characteristics
AC Drives
The speed of an AC motor is determined for the most part by two factors: The applied frequency
and the number of poles.
f = frequency
P = number of poles
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.
X L = 2p fL
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
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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 also 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.
D 150%
1725 1750 1795
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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 result from the stator windings being concentrated in slots. If the windings are uniformly
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
also 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 curve.
Beyond point C, the power factor decreases faster than current increases causing torque to drop
off. 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
frequency drive.
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.
Design A
AC Induction Motor
Design B — motors have a higher impedance rotor producing a slightly higher starting torque and
lower current draw. For this reason, design B motors are a general-purpose type motor and
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account for the largest share of induction motors sold. The slip of a Design B motor is
approximately 3-5% or less.
Design B
AC Induction Motor
Design C — motors uses a two-cage rotor design, high resistance for starting low resistance for
running. This creates a high starting torque with a normal starting current and low slip. During
starting, most of the current flows in the low inductance outer bars. As the rotor slip decreases,
current flows more in the inner low resistance bars.
The Design C motor is usually used where breakaway loads are high at starting, but are normally
run at rated full load, and are not subject to high overload demands after running speed has been
reached. The slip of the Design C motor is 5% or less.
Design C
AC Induction Motor
Design D — motors have the highest resistance rotor creating high slip, high starting torque and
low starting current. Because of the high amount of slip, the speed varies dramatically with load.
The slip of this type motor is approximately 5 to 8%. This high slip characteristic relates to a low
efficiency design and a motor that runs hot.
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Design D
AC Induction Motor
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Synchronous Motors
Synchronous motors operate at synchronism with the line frequency and maintain a constant
speed regardless of load without sophisticated electronic control. The two most common types of
synchronous motors are reluctance and permanent magnet. The synchronous motor typically
provides up to a maximum of 140% of rated torque. These designs start like an induction motor
but quickly accelerate from approximately 90% sync speed to synchronous speed. When
operated from an ac drive they require boost voltage to produce the required torque to
synchronize quickly after power application.
% Speed
Also available in high horsepower motors is the separately excited synchronous motor. This
design requires a Load Commutated Inverter (LCI) which is not presently available from AllenBradley.
Wound Rotor
Some large motors may have a “Wound Rotor”. This allows the motor characteristics to be altered
by adding resistors in series with the rotor. This can effective lets the user define the motor torque
curve as Nema A, B, C, or D. More resistance means higher slip and higher starting torque across
the line while using a low value of series resistance results in lower slip and greater efficiency.
Often the resistors will be present for start up then jumped out while running.
In a case where a wound rotor motor is fed by an ac drive, the wound rotor connections should be
permanently jumpered (no series resistance added).
30% W
10% W
2% W
% Speed
Operation above Base Speed
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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 as
1600V, 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 operated at
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 applied voltage:
T = K 1 xE 2
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 0.66 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
For 60hz Motor: 60
Since the voltage, in reality, is not changing above base speed, it is more appropriate to define
torque in terms of frequency change instead of voltage change. It can be stated then that torque
above base speed drops as the square of the frequency – doubling the frequency, quarters the
available torque. Applied frequency and synchronous speed are equivalent, so going one step
further; torque may be defined in terms of speed. In the constant voltage range then, motor torque
drops off as the inverse of synchronous speed squared, or 1/N . This is shown in the curves
Many machine applications are constant horsepower in their load characteristics. As speed
increases, the torque drops off as the inverse of speed, or 1/N. The torque drop-off is not as
severe as the motor’s peak torque, 1/N . Below compare peak torque to rated torque.
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Torque Above Base Speed
Peak Torque
% Torque
Rated Torque
Base Speed
Frequency (HZ)
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