Download How to Select a motor? (Part 1) - Types of motors

How to Select a motor? (Part 1) - Types of motors
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How to Select a motor? (Part 1) - Background and types of
motors
Electric motor converts electrical energy into mechanical energy. There are many ways to
perform this conversion and many different types of electric motors. The principle common to all
is a change in the electromagnetic field causes changes in the motors electrical behavior and
these cause mechanical movement.
All electric motor consists of two parts causes the movement:
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How to Select a motor? (Part 1) - Types of motors
• Stator is the fixed part in the motor and consists of magnet and coils wrapped around/inside it. • Rotor is a moving axle that goes through the stator and magnetic forces are exerted on it
causing it to spin.
The following guide describes the types of electric motors and tips for choosing the right motor
for your robot or machine.
Electric motors are classified into two groups: direct current motors (DC) and alternating current
motors (AC). Within each such group Other types of motors. Below is a list of all types of motors
and an explanation of each one:
DC electric motor with permanent magnet and Brushes
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How to Select a motor? (Part 1) - Types of motors
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By Abnormaal [ GFDL or CC-BY-SA-3.0 ], via Wikimedia Commons
Brushless DC Electric Motor with a Permanent Magnet
These motors are called Permanent-Magnet Brushless DC Motor. The arrangement of the
magnets and the coils built in the opposite way to prevent the brushes. In Brushless motor, the
stator contains many coils that are wound one on another and the rotor is attached with a
magnet. Inside the motor is a Built-in magnetic sensor that picks up the position and the speed
of the rotating rotor.
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How to Select a motor? (Part 1) - Types of motors
Current flowing through the stator coils which cause a change in the magnetic field. Each time,
the current flows to a different coil and thus the magnetic field direction changes. The rotor with
the magnet seeks to align with the magnetic field and this causes the rotation. The rate of the
current is determined with the magnetic sensor that measures the position of the rotor. When
the rotor reaches a specific position, the electric circuit that drives the current to the stator
selects a different coil to be energized with current.
Coreless DC Electric Motor
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How to Select a motor? (Part 1) - Types of motors
These motors are called Coreless DC Motor or Ironless DC Motor. Unlike standard motors, in
this motor the core is not made of iron rotor but from the coil itself. The body of the motor is
hollow and the permanent magnet that is built on the rotor is spinning. These motors are usually
small in size.
Printed Electric DC Motor
These motors are called Printed Armature DC Motor or Pancake DC Motor. Stator is actually a
printed circuit board (PCB) that the current that flows thru him generates the magnetic field
which drives the rotor. These motors are usually very thin/flat.
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How to Select a motor? (Part 1) - Types of motors
Synchronous AC motor
These are called Synchronous AC motor. AC motors are driven using AC Current. The motor
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How to Select a motor? (Part 1) - Types of motors
consists of stator with several coils wound around it in a triangle or star poles formation and a
rotor that rotates according to the magnetic field generated by the AC Current in the stator. The
rotor is magnetized using a coil wound around it and a DC Current makes it an electromagnet.
The magnetic field in AC motors changes as the current changes its polarity. Unlike in DC
Motors which require a circuit to change each coil in the motor, AC Motor doesn’t require such
circuit.
A synchronous motor which connects to 3-phase Alternating current doesn’t require any circuit
or software to change its polarity. Each phase is 120 degrees apart from one another creating a
continuous variable magnetic field which causing the rotor to spin. When synchronous motor is
connected to a 1-phase alternating current, a capacitor must be added to the controlling circuit
to shift the phase in 90 degrees. This is actually how you get 2 poles from one voltage source.
In every given time, the poles will be in 90 degrees apart from one another and continuity in the
magnetic field is preserved.
Synchronous motor is named that way because the spinning rotor is synchronized with the
change of the current in the stator. The speed of synchronous motor depends on the frequency
of the current supplied to the motor. A change in the frequency will change the speed of the
synchronous motor. The speed of the synchronous motor can be calculated using the following
formula:
V=120 * f / n
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How to Select a motor? (Part 1) - Types of motors
when f indicates the frequency of the alternating voltage and n is the number of motor poles.
When too large load is exerted on the synchronous motor, he goes out of synchronization and
The rotor fails to follow the changing magnetic field. Consequently, the motor stop immediately
when it fails to be synchronized.
To activate synchronous motor, create a situation in which the polarity of the rotor will be
synchronized with the magnetic field of the stator. This can be achieved through a number of
ways, such as a small motor that drives the rotor to the state it enters synchronization with the
magnetic field of the stator or decrease the frequency of the currents at startup so the motor will
enter synchronization slowly.
Synchronic motor speed does not depend on the load applied to the motor. The voltage
frequency affecting the speed.
When DC voltage supplied to the stator and the rotor at the same time, the motor will lock in
place and won’t move.
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How to Select a motor? (Part 1) - Types of motors
Synchronous motors are moving in synchronous speed only. Coming out of synchronization will
stop the motor completely. Asynchronous AC Motor (Induction)
By Zureks [ CC-BY-SA-3.0 or GFDL ], via Wikimedia Commons
By user:ikaxer [ GFDL or CC-BY-SA-3.0 ], via Wikimedia Commons
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How to Select a motor? (Part 1) - Types of motors
These motors are called Asynchronous AC motor or Induction motor. AC motors driven through
AC current. The motor consists of a stator with coils wound inside it and a number of poles in a
star or triangle shape and a rotor built in a round cage shape. The rotor conductors cut thru the
magnetic field creating induction currents in the coils. These generates forces that cause the
rotor to spin.
Unlike the synchronic motor where the speed depends on the frequency of the voltage supplied
to the stator, here the speed is influenced by currents flowing in the rotor cage. For reasons of
energy conservation, the rotor speed will always be smaller than the magnetic field change
speed. The difference between the rotor speed to the rate change of the magnetic field called
slip, Hence the definition A-Synchronized motor. The motor does not move in sync with the
stator magnetic field change.
Rotor speed is computed using the formula:
nr = ns (1-S)
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How to Select a motor? (Part 1) - Types of motors
when nr is the rotor speed, ns speed of change of the magnetic field in the stator and S
represents the slip (characteristic of the motor).
//
Step Motor
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How to Select a motor? (Part 1) - Types of motors
By Wapcaplet; Teravolt [ GFDL ], from Wikimedia Commons
These motors are called Stepper Motor or Step Motor. Behavior of Step Motor recalls the
behavior of AC motors. Step motor is built of magnetizes rotor and a stator with number of poles
with coils wound around each pole. The step motor motion is characterized by steps, hence not
continues motion. Each time a current is flowing thru one of the coils in a pole, the pole is
magnetized creating a magnetic field and the rotor is moving to the appropriate position and
aligns itself with the magnetized pole. When the current is redirected to another pole, that pole
is magnetized and the rotor is moving towards it. In this method the rotor moves in steps. A
more smoothly motion can be created using micro steps, meaning engaging two poles at the
same time creating a step between the two poles. The rotor will align in the middle of the two
poles and thus creating a “half” step.
The steps are measured in angles and calculate with the following formula:
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(m*n)/360
Where m is the number of stator poles, n is the number of rotor poles. Step motor is always operates at full torque (unlike DC motor that the torque and the speed is
depending on each other). In high speed, the step motor loses the power it providing.
Unlike the simple operation of a DC motor (power supply via a simple switch), running stepper
motors require complex circuit which sends the corresponding inductors currents depending on
the speed and direction required. In addition, the operating system has to “remember" the
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How to Select a motor? (Part 1) - Types of motors
current position of the motor and direct the current to the correct coils.
One advantage of step motors is a state of "self-locking". That is, they are characterized by a
very high holding Torque, provided that a voltage is supplied to the motor.
There are three main types of stepper motors: Unipolar stepper motors, bipolar stepper motors
and hybrid stepper motors. Unipolar stepper motors have four electromagnets, are simpler to
control and but weaker. In contrast, bipolar stepper motors contain only two electromagnets are
more powerful, but require a more complex control circuit that needs to change the direction of
the current every step the motor moves.
Hybrid stepper motors is a combination of the two types of motors: Unipolar and Bipolar by
changing the usage of the wires to the motor.
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How to Select a motor? (Part 1) - Types of motors
Stepper motor moves a number of steps but it is possible to increase the steps resolution by
changing the control method. There are four methods to drive a stepper motor:
-
Normal Steps
Double Steps
Half Steps
Micro-Steps
Step Motor Drive Mode - Normal Step
For example, in a unipolar motor with 4 coils, current is flowing once per coil. In each time one
step is performed. This method is the most economical in terms of energy consumption (for a
full turn of the motor a current must flow in all 4 coils) and provides a smooth motion and
descent torque.
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How to Select a motor? (Part 1) - Types of motors
Step Number
Step Position
Coil 1
Coil 2
1
0
OFF
OFF
2
1
OFF
OFF
3
2
OFF
ON
4
3
ON
OFF
Animated illustration:
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How to Select a motor? (Part 1) - Types of motors
Step Motor Drive Mode - Double Step
In this method, current is directed to two consecutive coils each step. In this method, the energy
consumption of the motor is double (for each turn of the motor, current must flow in 8 coils) but
the torque delivered by the motor is 2 times bigger than in the normal step method.
Step Number
Step Position
Coil 1
Coil 2
Coil 3
Coil 4
1
0
ON
ON
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How to Select a motor? (Part 1) - Types of motors
2
2
OFF
ON
3
4
OFF
OFF
4
6
ON
OFF
Animated illustration:
Step Motor Drive Mode - Half Step
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How to Select a motor? (Part 1) - Types of motors
This method combines the two previous methods. Current is flowing to the one coil. Then the
two consecutive coils are infused, then the next coil is infused and so on. In this method the
power consumption if 3 times bigger than the normal step method (for full turn of the motor,
current have to flow in 12 coils) but the motor precision can be doubled (a motor with 1.8
degrees per step can deliver a resolution of 0.9 degrees per step in the half step method).
Step Number
Stpe Position
Coil 1
0.0
ON
Coil 2
Coil 3
Coil 4
1
OFF
2
0.5
ON
ON
3
1.0
OFF
ON
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How to Select a motor? (Part 1) - Types of motors
4
1.5
OFF
ON
5
2.0
OFF
OFF
6
2.5
OFF
OFF
7
3.0
OFF
OFF
8
3.5
ON
OFF
Animated illustration:
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How to Select a motor? (Part 1) - Types of motors
Step Motor Drive Mode - MicroStep
This method provides alternating current to the motor coils thereby “ignoring” the stepping
attribute of the step motor and receiving a smooth and quite motion. The current delivered to the
coils are in 90 degrees phase shift to one another (one current is flowing as a sine and the other
as cosine). The power consumption in this method is the highest among all methods and the
control circuit required to drive the motor is the most complicated but the biggest advantage is
smooth motion and higher torque.
Example for steps in a bipolar step motor:
Step Number
Step Position
Coil 1 Current
0
0.0%
Coil 2 Current
1
100%
2
1/8
19.5%
98.1%
3
1/4
38.2%
92.4%
4
3/8
55.5%
83.1%
5
1/2
70.7%
70.7%
6
5/8
83.1%
55.5%
7
3/4
92.4%
38.2%
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How to Select a motor? (Part 1) - Types of motors
8
7/8
98.1%
19.5%
9
1
100%
0.0%
RC servo motor
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By José Luis Gálvez (Digigalos) [ CC-BY-SA-2.5 ], via Wikimedia Commons
These motors are called RC servo motor. RC servo motors are basically a system that contains
a DC motor, transmission system consisting of gears and closed-loop control unit that measures
the position of the motor (usually by potentiometer) and drives it by commands sent to it. RC
servo motors are not sequential movement motors but moving in the range of 0 to 180 degrees.
There are some RC Servo motors that their movement range can range up to 210 degrees but
those are depends on different manufacturers.
These motors are usually small in size, delivers strong torque relatively to their small size, their
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power consumption is proportional the load subjected on the motor shaft, running on low
voltage, accurate and reliable thanks to the internal control circuit.
RC Servo motor driving is performed by sending digital signals to the controlling circuit of the
motor. The digital signal is a square pulse in a voltage level predefined by the control circuit.
The length of the pulse determines the angle in which the motor will turn to. There are many
ways defining the relationship between the pulse length and the angle but those depends on the
motors manufacturers but the basic principal is the same in every motor.
Example 1:
Pulse width of 1ms will drive the motor to a 0 degrees angle.
Pulse width of 1.5ms will drive the motor to a 90 degrees angle.
Pulse width of 2ms will drive the motor to a 180 degrees angle.
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How to Select a motor? (Part 1) - Types of motors
Example 2:
Pulse width of 1ms will drive the motor to a 0 degrees angle.
Pulse width of 1.25ms will drive the motor to a 90 degrees angle.
Pulse width of 1.5ms will drive the motor to a 180 degrees angle.
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Servo Motors have three wires with one wire (black) as ground, on wire (red) as input voltage
(+v) and the third wire (color can vary between different manufacturers) is the controlling signal
(squared pulses) determines the motors angle.
Industrial servo motor
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How to Select a motor? (Part 1) - Types of motors
These motors are called Industrial Servo motor. Industrial servo motors are very common in the
industry and driving systems of machines and robotics. These motors are usually built base on
AC motors, internal transmission (gearbox) and an internal encoder measuring the motors
position every moment. Unlike RC Servo motors, there are no internal control circuits in the
industrial servo motor. These motor usually require an external and complicated control unit
(Controller).
There are models of industrial servo motors with integrated temperature sensors that measure
the motors temperature when it is in motion, current sensors measuring the current in the motor
coils every moment and supplying large information regarding its mechanical and electrical
loads to the controller.
Linear Motor
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How to Select a motor? (Part 1) - Types of motors
These motors are called Linear motor. Linear motors are basically standard circular motion
motors with a transmission that transforms angular motion to linear with a use of gears, racks,
cams, etc. a very common linear motor is a carriage with a nut place on a long screw shaft. The
screw is connected to the motor shaft. When the motors spins, the screw spins too and the nut
can move back and forth on the screw.
Linear motors have a limited range of motion and called “stroke” as RC Servo motors have a
limited range of angles.
Solenoid
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Solenoid is a linear actuator that operates as linear motor (it also has a limited linear motion
range) but work son a different concept. The solenoid built of a coil and a core. When current
flows thru the coil and a magnetic field is generated, it generates a pushing force that pushes or
pulls the core. The motion of the solenoid is very short and characterized by a very high speed.
Unlike the linear motor, solenoid has only two positions: extended position and contracted
position. Simple Solenoid can’t be moved half way (more complex solenoid has double coiling
system that generates a magnetic field that holds the core in a 3 rd position – middle).
©societyofrobots
Muscle Wire / Nitinol
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How to Select a motor? (Part 1) - Types of motors
The Muscle Wire (or Nitinol) is a very compact linear actuator. It called that way because of the
similarity to the human body muscle system. This is basically a thin thread maid of a
nickel-titanium compound. When current is flowing thru that thread it reached a certain
temperature (around 75 degreed Celsius) and shrinks. The thread is completely shrank when it
reaches a temperature of 110 degreed Celsius. When the current stops and the thread is cooled
down, it expanded to its initial state. The thread can carry heavy loads (larger even 1000 times
than normal). Comparison of types of motors
Motor Type
Advantages
Disadvantages
Electric Drive
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How to Select a motor? (Part 1) - Types of motors
AC synchronous motor
Speed ​​sync without slipping
High cost
AC three-phase or single phase AC with a control circuit (capacitor)
Asynchronous AC motor (inspired)
High power
High starting torque
Sleeping and out of synchronization
Require Strating Mechanism
Single Phase AC
DC motors with brushes
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Simple speed control
Maintenance (brushes)
Low Reliability
Expensive parts (brushes)
DC or PWM
Brushless DC motor
High Reliability
Low maintenance
High efficiency
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How to Select a motor? (Part 1) - Types of motors
High cost
Requires controller for speed control
DC
Printed DC motors
Small Sized
Simple speed control
Average Price
Avarage Reliability
DC or PWM
Step Motors
Motor position greater accuracy
High holding torque
High Cost
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How to Select a motor? (Part 1) - Types of motors
Requires controller for speed control
DC
RC servo motors
High torque
Small Sized
Built-in control system
Relatively high cost
Limited motion range (angles)
PWM
Industrial servo motors
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How to Select a motor? (Part 1) - Types of motors
High torque
Built-in transmission and control circuit
High cost
Large in size
Requires complicated controller.
Combined input voltages (high voltage for the motor and a low DC voltage for encoders)
DC + AC
Linear Motor
Linear motion
High power
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How to Select a motor? (Part 1) - Types of motors
High starting torque
Requires Controller
May cause slipping and coming out of synchornization
AC
Solenoid
Linear motion
Discrete movement (open / close)
Very High Speed Motion
Small size
Short Motion range
Easily heats up with high mechanical loads
Low forces
Muscle Wire
DC
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How to Select a motor? (Part 1) - Types of motors
Thin and extremely small sized.
Simple Control implementation
Requires complicated mechanical
DC / AC mechanism
Next article: understand the technical data of motors
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How to Select a motor? (Part 1) - Types of motors
Written by Eran Cenciper (Robot-and-Machines-Design webmaster)
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