1. ME 440: Numerically Controlled Machine Tools
Electric Motors and Drive Systems
Assistant Prof. Melik Dölen
Department of Mechanical Engineering
Middle East Technical University

2. Outline – Motors Motors in CNC Technology Classification of Motors
Stepper Motors
DC Motors
Brushless DC Motors
Induction Motors
Fundamentals of Motor Drives
DC Motor
Stepper Motor
Chapter 11
ME 440

3. Electrical Motors
In most CNC machine tool applications, electrical motors are extensively used as actuators:
Axis motion
Spindle motion
Four motor systems are common alternatives in machine tool designs:
Stepper motors: Simple applications (e.g. desktop manufacturing tools)
DC motors: Earlier CNC machine tools and specialized machine tools
Brushless DC motors: Principle axis drives for contemporary CNC machine tools
AC (Induction) motors: High-power spindle drives.
Chapter 11
ME 440

4. Stepper Motors Stepper Motors
Permanent Magnet
Relies on rotor magnets
Variable Reluctance
Relies on rotor saliency
Hybrid Motors
Relies on both rotor saliency and magnets
Each pulse moves rotor by a discrete angle (i.e. “step angle”).
Counting pulses tells how far motor has turned without actually measuring (no feedback!).
Chapter 11
ME 440

5. Advantages / Disadvantages
Low cost
Simple and rugged
Very reliable
Maintenance free
No sensors needed
Widely accepted in industry
Resonance effects are dominant
Rough performance at low speed
Open-loop operation
Consume power even at no load
Chapter 11
ME 440

6. (Simplified) Full-Step Operation
Rotor of a PM stepper motor consists of a permanent magnet:
Stator has a number of windings.
Just as the rotor aligns with one of the stator poles, the second phase is energized.
The two phases alternate on and off to create motion.
There are four steps.
Chapter 11
ME 440

7. (Simplified) Half-Step Operation
Chapter 11
ME 440

8. Half-Step Operation (Cont’d)
Commutation sequence has eight steps instead of four.
The main difference is that the second phase is turned on before the first one is turned off.
Sometimes, both phases are energized at the same time.
During the half-steps, the rotor is held in between the two full-step positions.
A half-step motor has twice the resolution of a full-step motor.
Very popular due to this reason.
Chapter 11
ME 440

9. Winding Connections Unipolar motor: Bipolar motor:
Bipolar (4-wire):
Unipolar (5-wire):
Unipolar motor:
Current flows through a coil only in one direction.
Bipolar motor:
Current flowing through a winding changes direction during the operation.
Unipolar (6-wire):
Chapter 11
ME 440

10. Actual Stepper Motor*
The stator of a real motor constitutes more coils (typically 8).
These individual coils are interconnected to form only two windings:
one connects coils A, C, E, and G:
A and C have S-polarity
E and G have N-polarity
one connects coils B, D, F, and H:
B and D have S-polarity
F and H have N-polarity
Chapter 11
ME 440
[*] Courtesy of Microchip.

11. PM Stepper-Motor Animations*
Full-step:
Half-step:
Chapter 11
ME 440
[*] Courtesy of Motorola, Inc.

12. VR Full-Step Motor Rotor and stator saliency Unequal number of poles
Stator current effectively pulls rotor pole in line with stator pole.
Chapter 11
ME 440

13. VR Half-Step Motor
Possible to move rotor by half steps by exciting two windings equally.
Finer steps (a.k.a. “micro-steps”) are possible by exciting two windings unequally.
Chapter 11
ME 440

14. Conventional DC Motor
The stator of a DC motor is composed of two or more permanent magnet pole pieces.
The rotor is composed of windings which are connected to a mechanical commutator. In this case the rotor has three pole pairs.
The opposite polarities of the energized winding and the stator magnet attract and the rotor will rotate until it is aligned with the stator.
Just as the rotor reaches alignment, the brushes move across the commutator contacts and energize the next winding.
A spark shows when the brushes switch to the next winding.
Courtesy of Motorola, Inc.
Chapter 11
ME 440

15. Brushless DC Motor
A brushless DC motor (BLDC) has a rotor with permanent magnets and a stator with windings.
It is essentially a DC motor turned inside out. The brushes and commutator have been eliminated and the windings are connected to the control electronics.
The control electronics replace the function of the commutator and energize the proper winding.
he energized stator winding leads the rotor magnet, and switches just as the rotor aligns with the stator.
BLDC motors are potentially cleaner, faster, more efficient, less noisy and more reliable.
Chapter 11
ME 440

16. AC (Induction) Motor
Motor is essentially driven like an AC synchronous motor by applying sinusoidal current to motor windings.
The drive needs to generate 3 currents that are in the correct spatial relationship to each other at every rotor position.
High-resolution optical encoder is needed to control the commutation accurately.
Very smooth low speed rotation.
Negligible torque ripple.
Chapter 11
ME 440

17. Operating Modes of DC Motor
In motor mode, the machine drives the “load” and needs energy from the supply.
In generator mode, the “load-side” drives the machine and it generates power.
Chapter 11
ME 440

18. “Forward Motor” Control
Electronically-controlled (unidirectional) switch is turned on/off rapidly.
Pulse width modulation
Desired (average) voltage at the terminals of DC motor is obtained via controlling switching times:
where Tp is PWM period
(constant) and Td/Tp = d
is called duty cycle.
Chapter 11
ME 440

19. Forward Motor Control (Cont’d)
Mode 1:
When S1 is turned off, ia flowing through the motor cannot be cut off immediately.
It must flow somewhere!
The “clamp” diode allows current flow in Mode 2:
La drives a decaying current.
If D1 isn’t in place, a very large voltage will build up across S1 and blow it up.
Mode 2:
Chapter 11
ME 440

20. Four-Quadrant Motor Control
“H” bridge is used to operate the motor in four quadrants.
Driver is composed of two half-bridges.
Switches in a half-bridge cannot turned at the same time.
causes short-circuit.
If one of the switches is turned, the other must be off.
Chapter 11
ME 440

21. Forward Motor To go forward,
Mode 1:
Mode 2:
To go forward,
S3 is fully turned on;
PWM and ~PWM (inverted PWM) signals are applied to S2 and S1 respectively.
Unidirectional switch S1 can carry current only in the indicated direction.
Chapter 11
ME 440

22. Reverse Motor To go backward, S1 is fully turned on;
Mode 1:
Mode 2:
To go backward,
S1 is fully turned on;
PWM and ~PWM signals are applied to S4 and S3 respectively.
Chapter 11
ME 440

23. DC Motor Drivers Commercial Motor Drivers
Include all bells and whistles!
Custom Solutions (high-power)
Switches: Power MOSFETs, IGBT
Needs gate drivers and signal isolation barriers.
Bridge ICs (upto a few-hundred Watts)
LMD 18200
L298
For driving small DC motors,
L293D
ULN 2003A
Chapter 11
ME 440

24. Electromagnetic Relays
Relays are electromagnets connected to mechanical switches.
When the electromagnets are energized, the switches are pulled into contact.
Hence, the corresponding circuit is powered up.
Relays allow the control of high-power devices.
Small power is sufficient to energize electromagnets in relays.
Suitable for on/off control of slow devices:
Pump (AC/DC) motors, solenoids
Heaters, lamps, etc.
If compared to solid-state switches, relays are more susceptible to malfunction.
Chapter 11
ME 440

25. Simple On/Off Control
Chapter 11
ME 440

26. Drives for VR Stepper Motors
Currents do not need to reverse.
Circuit uses incomplete switch poles that can pass current only one direction through motor phase.
Chapter 11
ME 440