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APPLICATION NOTE
Improved Universal Motor Drive
JM. BOURGEOIS, JM. CHARRETON, P. RAULT
INTRODUCTION
Universal motors are mostly operated in AC current mode and are controlled by means of
TRIACS. This widespread solution leads to a cheap electronic controller board but has some
drawbacks. In particular the high peak to peak current gives poor motor efficiency and the
consequential high brush temperature leads to limited motor lifetime.
When operating in DC mode, significant improvements are obtained. The RMS and peak to
peak current of the motor are smaller, reducing Iron losses and brush temperature.
Operating in DC mode enables shrinking of the motor size, and increasing the motor lifetime.
Furthermore, magnetic constriction of motor core and torque ripple decrease; the 100Hz noise
is further reduced.
This technical note presents three solutions for motor operation in AC and DC mode, based
on phase control. Motor current and motor efficiency are compared in AC and DC mode.
Component selection is proposed for each case, enabling the design of a cost effective
solution.
AN422 / 02,94
IMPROVED UNIVERSAL MOTOR DRIVE
1 CIRCUIT TOPOLOGIES
Three different topologies for control of universal motors are shown in figures 1a, 1b, 1c:
- a conventional AC drive using a TRIAC
- a DC drive using a TRIAC and a rectifier bridge
- a DC drive using an IGBT and a rectifier bridge
Each is controlled with a low cost microcontroller, ST6. The difference between the control
software concerns the output signal of the microcontroller, which is either adapted to TRIACs
or to IGBTs.
A and B topologies are operated with a "SNUBBERLESS" TRIAC. Topology C is operated
with a slow IGBT, taking advantage of its low drop voltage.
2/10
IMPROVED UNIVERSAL MOTOR DRIVE
Figure 1a. Universal motor drive topologies - Topology A: AC mode
V
Vgate
Mains
M
t
Vmains
Figure 1b. Universal motor drive topologies - Topology B: DC mode
Ls
D
D
V
Vgate
Mains
t
M
D
Vmains
D
Figure 1c. Universal motor drive topologies - Topology C: DC mode
M
D
Mains
Vmains
V
t
Vgate
3/10
IMPROVED UNIVERSAL MOTOR DRIVE
2 MOTOR CURRENT AND THERMAL BEHAVIOUR
The following figures show two examples of motor current waveforms with the same operating
and load conditions, i.e. low speed and high torque. Figure 2 corresponds to the AC topology
where the motor current is the same as the TRIAC's current. So, motor voltage and motor
current are AC and the motor current has no DC component. Figure 3 corresponds to the DC
topology where the motor current is freewheeling through diodes D while the power switch is
off. The motor current has a DC component reducing its RMS and its peak to peak current.
Due to this fact, magnetic and copper losses are reduced thus improving motor efficiency.
Figure 2. Motor current with AC topology (Mains Voltage = 200V / Div; 2ms /Div)
Figure 3. Motor current with DC topologies (Motor Current = 5A / Div)
4/10
IMPROVED UNIVERSAL MOTOR DRIVE
2.1 MOTOR CURRENT
In this test the Universal motor has to deliver high torque at low speed. The following table
compares RMS, DC and peak to peak current in the motor depending on the topology. The
measurements are carried out with the same load, the conduction angle being adjusted to
deliver the same torque and speed.
TOPOLOGY 2.1
TOPOLOGY 2.2 & 2.3
AC mode with TRIAC
DC mode with TRIAC or IGBT
5.5A
5A
0
4.6A
26A
6A
RMS motor current
DC motor current
peak to peak motor current
In DC mode, major improvements are obtained thanks to the smaller RMS and peak to peak
current:
- Reduced brush current and brush temperature, resulting in increased motor lifetime.
- Reduced current ripple and torque ripple of the motor resulting in reduced 100Hz noise of
the motor.
- Reduced Iron and Copper losses as shown in the table below:
2.2 THERMAL BEHAVIOUR
TOPOLOGY 2.1
TOPOLOGY 2.2 & 2.3
AC mode with TRIAC DC mode with TRIAC or IGBT
Temperature rise K°
Copper Stator
73
50
Iron Stator
46
29
Rotor
77
47
3 PROPOSED SOLUTIONS
Today, low cost microcontrollers are the most advantageous solution for a flexible and specific
motor drive control circuit. Moreover, the software approach maintains confidentiality of the
application. Thus, the three circuits described in this paper take advantage of the ST6
microcontroller family. These microcontrollers have an 8-bit architecture, enabling 8-bit and
1-bit data manipulation and a low power consumption.
A large choice of peripheral interface functions are available such as LCD driver, digital
watchdog timer, internal comparator device, A/D converter and pulse counter. These enable
the easy implementation of features such as motor speed or torque control, sensor
monitoring, display control and soft start as well as protection.
5/10
IMPROVED UNIVERSAL MOTOR DRIVE
3.1 AC DRIVE WITH TRIAC
Figure 4 shows an AC drive for Universal motor suitable for a large range of applications. The
power switch is a "SNUBBERLESS" TRIAC BTA 12-600BW with a maximum specified gate
trigger current of 50mA at 25°C. This TRIAC is pulse driven; A small signal transistor
interfaces the microcontroller and the "SNUBBERLESS" TRIAC. Although no snubber is
needed in nominal operations, a small filter RfCf should be implemented if spurious triggering
is prohibited in case of mains pertubations.
There are three different user interfaces: a touch control, push button or a potentiometer. In
this example four operating modes are defined in hardware, selected by the interpretation of
the user interface by the microcontroller.
Zero voltage detection across the mains is used for synchronization.
Changing operation from 50Hz to 60Hz is obtained by changing the EPROM/ROM table
defining TRIAC conduction angle versus power level. It can be hardware programmed, or
self-adapted by software.
Figure 4. TRIAC controlled AC mode Universal motor drive
+5V
Cf
1K
2N2905
MAINS
BTA
12-600BW
Rf
VDD
19
18
PA0
PA1
13
12
PB2
PB3
11
PB4
MODE
0V
M
0V
1M
RESET
PB0
ST6210
OSCOUT
4
+5V
3x
4.7M
7
1
33
PB1
TOUCH
SENSOR
15
PUSH
BUTTON
14
NMI 5
TEST 6
VSS 20
+5V
0V
220K
OSCIN
3
8MHz
BZX55C4V7
820
1/2W
100uF
6.3V
220nF
400V
0V
22pF
All resistors
1N4148
0V
6/10
0V
22pF
POTENTIOMETER
0V
0V
1/4W unless otherwise specified
IMPROVED UNIVERSAL MOTOR DRIVE
3.2 DC DRIVE WITH TRIAC
Figure 5 shows a DC drive for a Universal motor. A diode bridge has been added to the
previous circuit in order to supply DC current to the motor. So, after the TRIAC current
crosses zero, the motor current freewheels through the diode bridge.
An inductance in series with the TRIAC is required to limit the rate of fall of the current as
defined in the TRIAC specifications (12A/msec for BTA 12-600BW requiring 3mH in series).
However, a larger value may be mandatory due to the standards limiting harmonic content of
mains current. This serial inductance should be advantageously used as a part of the EMI
filter. In normal operation, no snubber is needed. However, the small filter RfCf should be
implemented to avoid spurious triggering if large mains disturbances occur.
Figure 5. TRIAC controlled DC mode Universal motor drive
3mH
Cf
Rf
A1
1K
2N2905
BTA
A2
12-600BW
19
18
PA0
PA1
13
12
PB2
PB3
0V
100K
0V
M
220K
100K
11
10
3x
4.7M
7
RESET
PB0
ST6210
MODE
+5V
MAINS
1
VDD
33
G
PB4
PB5
OSCOUT
4
8MHz
PB1
TOUCH
SENSOR
15
PUSH
BUTTON
14
NMI 5
TEST 6
VSS 20
+5V
0V
POTENTIOMETER
220K
OSCIN
3
0V
220K
0V
22pF
22pF
+5V
22K
BZX55C4V7
820
1/2W
100uF
6.3V
220nF
400V
0V
0V
0V
1N4148
0V
7/10
IMPROVED UNIVERSAL MOTOR DRIVE
3.3 DC DRIVE WITH IGBT
Switching behaviour of IGBTs can be controlled by means of the gate drive. Controlled slow
or "soft" switching can be obtained (Figure 7), leading to reducing high frequency interference
and thus reducing filter cost.
Figure 6 shows a DC drive for Universal motor taking advantage of soft commutation with
IGBT.
The same ST6 controller with the same software as the DC drive with Triac can be used, the
only difference being the output pulse width: the signal must be held at the high state until
zero current is detected in the mains. Only a few instructions have to be changed, and zero
current detection is implemented, as shown as an example in Figure 6.
Figure 6. IGBT controlled DC Universal motor drive.
BY214-600
M
BYT08
PI600
Synchro
Vdd
15V
5V
MAINS
Vss
IGBT
GND
PA0
PA1
PA2
GND
8/10
ST6
GND
IMPROVED UNIVERSAL MOTOR DRIVE
Figure 7a. Control of IGBT Switching behaviour
MILLER effect limits dV/dt and dI/dt during turn-on and turn-off dependingon R 1- R2 and
TR1A : 1.0V; 20µS; Vg= 10V
Ic
R1
TR3A : 1.0V; 20µS; Vc= 100V
Crss
Vc
TR4A : 10.0mV; 20µS; Ic= 5A
R2
Vg
Figure 7b. Control of IGBT Switching behaviour
Vc
Inductance L limits dI/dt at turn-on
and turn-off
_ 15V - Vth at turn-on
VL = L dI/dt ~
_
~ Vth at turn-off
dI/dt ON
dI/dt OFF
Ic
t
1.0V : 2µS, Vc= 100V
Ic
R1
Vc
10.0V : 2µS, Ic= 5A
Vth
R2
Vl
L
9/10
IMPROVED UNIVERSAL MOTOR DRIVE
4 CONCLUSION
In many applications, Universal motors can be shrunk if used in DC current mode instead of
AC current mode. In addition to reducing the size and the weight of the motor, operating in
DC current mode increases motor lifetime and decreases motor noise.
This note proposes a flexible solution based on using a low cost 8-bit ST6 micro-controller.
Depending upon hardware configuration, the ST6 micro-controller provides a DC or AC
current motor drive with a large choice of user interfaces. It enables the implementation of
features such as motor speed or torque control, sensor monitoring, display control or soft start
as well as protection.
From both DC mode solutions, the IGBT solution reduces RFI and filter cost due to the IGBT
soft commutations. So, the same ST6 microcontroller provides a flexible solution for many
different universal motor drives, directly suited for each application.
References
[1] - Microcontrollers and Triacs on the 110/240V Mains
AN 392 - Ph. RABIER and L. PERIER (SGS-THOMSON Microelectronics)
[2] - Improvement in Triac Commutation
Application Note - P. RAULT (SGS-THOMSON Microelectronics)
[3] - New Triacs: is a snubber necessary?
Application Note - T. CASTAGNET (SGS-THOMSON Microelectronics)
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10/10
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