TRIACFiring PWMWait time

Speed setting ωr

* ASRPI loop+ -

ωr

Interrupt withTimerspeed ωr

Speed sensor

Motor

~Vac

neutral

TRIAC

Zero Cross Detect

TRIACFiring PWMWait time

Speed setting ωr

* ASRPI loop+ -

ωr

Interrupt withTimerspeed ωr

Speed sensor

Motor

~Vac

neutral

TRIAC

Zero Cross Detect

High Performance DC Chopper Speed and Current Control of Universal Motors Using a Microcontroller

Huangsheng Xu, Kevin King, and Yashvant Jani

Renesas Technology America, Inc. 450 Holger Way, San Jose, CA 95134

Abstract—This paper introduces a high performance DC chopper speed and current control system for universal motors. Differing from existing TRIAC and DC chopper speed control schemes, the new system adds the inner current control to improve dynamic response, and the motor starts up with closed speed and current control loops. Complete control schemes have been developed and implemented in a low cost Renesas R8C/25 16-bit microcontroller. Through a series of dynamometer tests, it has been shown that the proposed DC chopper control of the universal motor with both speed and current control has good speed regulation and fast response. It also has low acoustic noise, less current harmonics, low motor temperature, more torque and high motor efficiency. The experimental results demonstrate that the developed universal motor control has great potential and practical significance for industrial applications.

Keywords – Universal motor; DC chopper control; current control; speed control

I. INTRODUCTION

The universal motor is a low-cost solution with limited performance. It is used widely in the consumer-products industry, especially for power tools and home appliances such as washers, mixers, vacuum cleaners, etc. This type of motor is called a “universal” motor because it can run on either AC or DC power[1-4].

To implement universal motor speed control, generally a triode AC switch (TRIAC) phase-angle firing control or DC chopper with a speed loop is applied. A TRIAC-based control scheme, as shown in Fig. 1, is a simple and cost-effective solution for controlling the universal motor speed by changing the phase angle. Actual speed, measured with a simple tachometer, is coupled with the desired speed to change the firing time of the TRIAC. This solution has many disadvantages, unfortunately, including high phase current and current harmonics, high motor temperature, noise, and low control frequency.

A DC chopper provides a better solution for universal motor speed control. Fig. 2 shows a typical layout for DC chopper control of a universal motor. Driving the universal motor with a PWM signal reduces current ripple and harmonics and therefore results in better performance. Compared to the TRIAC control approach, the DC chopper control method has lower acoustic noise, less current and fewer current harmonics[5-9].

Traditionally the type of DC chopper control shown in Fig. 2 provides speed control only. It does not control the

current. The speed is asynchronous, so the regulation frequency is dependent on the speed level. At low and medium speeds, the speed regulation is poor and the average error is large. The speed regulation improves at high speeds. But at Also, the speed dynamic response is somewhat slow. In other words, the control system takes a long time to get back to a steady state when load changes or external disturbances occur. Obviously, a control scheme that responds more quickly to load variations is preferable[10].

Fig. 1: Traditional TRIAC based speed control.

Fig. 2: Typical DC chopper speed control.

To better meet the requirements of universal motor control, this paper investigates and applies inner current control to the universal motor control system. The motor starts up with the closed speed and current loops. Complete control schemes, which have been developed and implemented in a low cost Renesas R8C/25 microcontroller, are used. A series of experimental tests has been conducted to evaluate and verify the system performance. The results show that the DC chopper with both speed and current control

Register Counts

PWMVoltage Duty cycle

Speed setting ωr*

ASRPI loop

+- ωr

Interrupt withTimerspeed ωr

Speed sensor

Power MOSFET

1

Motor

+Vdc

GND

Register Counts

PWMVoltage Duty cycle

Speed setting ωr*

ASRPI loop

+- ωr

Interrupt withTimerspeed ωr

Speed sensor

Power MOSFET

1

Motor

+Vdc

GND

Power MOSFET

1

Motor

+Vdc

GND

0197-2618/07/$25.00 © 2007 IEEE 701

provides good speed regulation and fast response. This proposed universal motor control system has great potential and much practical significance for industrial applications.

II. DESRIPTION AND ANALYSIS OF A UNIVERSAL

MOTOR CONTROL SYSTEM

A universal motor consists of three main parts: (1) a stator with field windings to generate magnetic flux; (2) a rotor with armature windings supplied with current via the carbon brushes to generate magnetic flux; and (3) brushes for mechanical linking of the power supply and rotor windings. If the universal motor is connected to a DC power supply, it behaves like a DC motor with a direction.

The equivalent circuit of a proposed universal motor with speed and current control is shown in Fig. 3.

fR

aR

aL

fi ai

e

fv

av

MOSFET

PWM

MCUnesascontrollerchopperDC

Re

DC

Fig.3: Equivalent circuit of universal motor with DC chopper controller.

The model of the proposed universal motor control can be described mathematically as:

(1)

af ii = (2)

aike ω1−= (3)

Where e is the motor back EMF; 1k is the motor constant dependent on the motor characteristics.

Because the universal motor is a serial excitation motor, the motor torque is proportional to the square of the motor current.

212 aLe ikkJpTT −=++= ωω (4)

The overall control system is composed of a speed proportional-integral (PI) regulator, a current PI controller, a speed measurement sensor, and a pulse width modulation (PWM) generator, as shown in Fig. 4. The system has two control loops: an inner current loop and an outer speed loop.

Both the speed and current controller use a typical PI control-loop format. The speed PI loop outputs the reference current required for the proper torque. The current PI control loop generates the voltage necessary to maintain the torque.

Whenever a reference speed r∗ω is given, the control

system automatically compares that reference speed to the motor’s actual speed rω , which is directly measured from the speed sensor. Based on the equation for the motor’s mechanical motion, the speed error rω∆ is the torque profile. Therefore, the output of the speed controller can be considered as the torque or the current reference value ∗i . An adjustment is made so that the motor speed rω follows the given reference value r

∗ω , and the motor achieves a steady state quickly. The current PI controller regulates the motor’s real current i and the reference current ∗i , and tracks the reference current ∗i and load in time. The output of the current controller is the PWM duty ratio, which directly controls the MOSFET ON or OFF states.

The current control loop is designed to be much faster than the speed control, so that the proposed DC chopper control system has a fast dynamic response to load changes and system variations. For this reason the addition of current control is crucial to the application.

Fig.4: Block diagram of proposed speed and current DC chopper control.

III. MICROCONTROLLER CONFIGURATION AND

IMPLEMENTATION

The proposed speed and current DC chopper for universal motor control has been implemented using a low-cost, low-pin-count 16-bit Renesas R8C/25 microcontroller to regulate both speed and current in the system.

The R8C/25 operates at a 20 MHz clock speed and has 16KB of on-chip flash memory and 1KB of RAM. It also provides useful on-chip peripherals including an I2C-bus interface (IIC)/chip-select clock synchronous serial interface (SSU), 10-bit A/D converter (ADC), output-compare timer, voltage-detection circuit, power-on reset circuit, high/low-speed on-chip oscillator, sub-clock generation circuit, and LIN module hardware. The R8C/25-based microcontroller configuration is shown in Fig. 5.

Register Counts

PWM

VDuty cycle

Speed setting ωr

* CurrentPI loop+ -

ωr

Interrupt withTimerspeed ωr

Speed sensorPower

MOSFET

1

Motor

+Vdc

GND

SpeedPI loop

Current measurement

+

-

ADCShuntOrDCCT

RefCurrent

Register Counts

PWM

VDuty cycle

Speed setting ωr

* CurrentPI loop+ -

ωr

Interrupt withTimerspeed ωr

Speed sensorPower

MOSFET

1

Motor

+Vdc

GND

SpeedPI loop

Current measurement

+

-

ADCShuntOrDCCT

RefCurrent

+−

+=

a

f

aa

ff

a

f

ii

pLRkpLR

vv

ω1

0

702

Four interrupts in the control scheme are designed to ensure time-critical performance. Motor speed is measured by means of a sensor interrupt and timer RB. This speed is calculated as the weighted average over 32 samples. Motor current is sampled every carrier frequency using an ADC interrupt and averaged over two measurements. The PWMs are generated by the timer RD PWM interrupt to provide voltage to the motor.

The hall sensor interrupt has the highest priority, and the rate varies from 240 Hz to 1200 Hz. The ADC interrupt has the second highest priority, and the rate is identical to the PWM rate because it is triggered in the PWM interrupt. The PWM interrupt is ranked third in priority and occurs at the 12 kHz carrier frequency.

Fig.5: R8C25 based universal motor control configuration.

IV. EXPERIMENTAL TEST RESULTS AND ANALYSIS

To verify the proposed speed and current DC chopper control, a series of experimental tests was conducted on the dynamometer. The dynamometer setup consists of the following: a universal motor, brake (with max torque of 3.25 Nm and max speed of 15,000 rpm) and Magtrol DSP6001 dynamometer controller. The entire speed range with the maximum load was evaluated.

Fig. 6 shows the motor startup and the speed ramp-up from 0 rpm to around 134,000 rpm. The motor speed is shown to be stable for the entire speed range.

A speed deviation of less than 2% was required. To determine whether the proposed DC chopper control system satisfied this requirement, the system was verified using a step maximum load. Figs. 7 and 8 compare the speed and speed response of the proposed (“developed”) speed-and-current DC chopper control system with an existing speed-loop-only DC chopper control system. Both examples use the same step load at a low speed of 2,200 rpm.

Fig. 7 shows that the speed of the developed control system is stable. The system has little drop when the load is

added, and minimum overshoot when the step load is removed. Also, the speed fluctuation in this case is small, only 1-2 rpm at the steady state. The test data for this system show that the speed deviation is less than 2% at all speeds. By contrast, Fig. 8 shows that the speed of the existing control system varies more. In particular, the motor speed jumps more than 800 rpm when the load is removed. Additionally, with the existing control system the speed takes longer to recover from sudden changes in the load.

Figs. 9 and 10 give the speed and speed response at the highest speed of 13,500 rpm. Fig. 9 shows that the speed provided by the developed DC chopper control remains relatively stable even with the 32oz-in load. Also, the speed stays constant. On the other hand, the speed provided by the existing control system drops 200 rpm under the same load condition.

Universal Motor Speed1-10 and Torque by RTA DC Chopper Control

0

2000

4000

6000

8000

10000

12000

14000

1 21 41 61 81 101 121 141 161 181 201

Time (tic = 0.25 sec)

Spee

d (R

PM)

Fig.6: Motor startup and all speeds from 0 to 13,000rpm.

Universal Motor Speed and Torque by RTA DC Chopper Control

0

500

1000

1500

2000

2500

3000

3500

1 21 41 61 81

Time (tic = 0.25 sec)

Spee

d (R

PM)

0

50

100

150

200

250

300

Torq

ue (o

z-in

)

Fig.7: Speed and speed response for developed control at speed of 2,200rpm.

Timer RD Complimentary

as well as selectable

outputs

Timer RB

INT0

Hall Sensor

Up

Vp

Wp

Un

Vn

Wn

U

V

IPM f1

R8C/25 Group

16bit-PWM

Emergency ShutDown

Error Pulse

DC Brushless

Motor

AN0

AN2

AN3

AN4

P14

P15

P16

AN1

Current Sensor Input

(Display) LED3

LED2

LED1

Speed Setting Input

Temperature Input

Motor Speed Detection

703

Universal Motor Speed1 and Torque by existing Control

0

500

1000

1500

2000

2500

3000

3500

1 21 41 61 81 101 121 141

Time (tic = 0.25 sec)

Spee

d (R

PM)

0

50

100

150

200

250

300

Torq

ue (o

z-in

)

Fig.8: Speed and speed response for existing control at speed of 2,200rpm.

Universal Motor Speed10 and Torque by RTA DC Chopper Control

0

2000

4000

6000

8000

10000

12000

14000

1 21 41 61 81 101 121 141 161 181 201 221 241 261

Time (tic = 0.25 sec/sample point)

Spee

d (R

PM)

0

10

20

30

40

50

60

70

80

90

100

Torq

ue (o

z-in

)

Fig.9: Speed and speed response for developed control at speed of 13,500rpm.

Universal Motor Speed10 and Torque by existing Control

0

2000

4000

6000

8000

10000

12000

14000

1 21 41 61 81 101 121 141

Time (tic = 0.25 sec)

Spee

d (R

PM)

0

10

20

30

40

50

60

70

80

90

100

Torq

ue (o

z-in

)

Fig.10: Speed and speed response for existing control at speed of 13,500rpm

Fig. 11 shows the speed-torque curves for the proposed control and existing control systems under the same speed and load. The blue curve is for the developed control system and the pink curve is for the existing control system. It is apparent that the developed speed-and-current DC chopper control system generates more torque—nearly 20% more—across the entire speed range.

Speed_torque Comparison for RTA Control and Production Control

0

200

400

600

800

1000

1200

1400

1600

0 2000 4000 6000 8000 10000 12000 14000

speed

torq

ue

Fig.11: Speed and torque comparison between developed DC chopper control and existing control.

Figs. 12 and 13 show the motor phase currents generated by the proposed DC chopper control system without and with the load. The motor current shows little variation even under the maximum load condition. This results in fewer current harmonics and, thus, less EMI.

Fig.12: Motor current by proposed DC chopper control without load.

704

Fig.13: Motor current by proposed DC chopper control with maximum load.

V. CONCLUSIONS

A DC chopper-based system for speed and current control of universal motors has been developed successfully. The system can deliver maximum torque in the entire speed range, from 2,000 rpm to 13,000 rpm. Compared to traditional TRIAC systems and systems that use DC chopper control with a speed loop only, the newly developed speed-and-current DC chopper control system offers significantly improved performance.

It provides the known mechanical advantages of a DC chopper over a TRIAC system—including more efficient motor operation (low power usage, low temperature operation), elimination of audible “dimmer switch” noise, and overall reduced acoustic noise. Moreover, tests show that the DC chopper speed and current control system offers the following benefits:

• better speed regulation; • speed and current loop combination for faster

response to load changes and external disturbances; • more torque available to the motor (about 20% more

estimated).

These experimental results suggest that the proposed universal motor control has great practical significance for industrial applications.

REFERENCES

[1] H. Toliyat, G. Kliman, Handbook of Electric Motors, 2nd edition, Marcel Dekker, 2003.

[2] P. Vas, Sensorless Vector and Direct Torque Control, Oxford Science Publications, New York, 1998.

[3] D. Novotny and T.A. Lipo, Vector Control and Dynamics of AC Drives, Oxford University Press, New York, 1996.

[4] W. Leonhard, Control of Electric Drives, Springer-Verlag, New York, 1996.

[5] J. Cros, P. Viarouge, Y. Chalifour, and J. Figueroa, “A new structure of universal motor using soft magnetic composite”, IEEE Proc. IAS – Thirty-Sixth annual meeting, 2001, vol. 1, page(s): 75-82.

[6] C. Ozturk, B. Gok, H. Acikgoz, and A. Balikicioglu, “ Noise emission of universal motor drives”, IEEE industry applications magazine, vol. 4, issue 4, 1998, page(s): 49-66.

[7] H. Bodur, A. Bakan, and M. Sarul, “Universal motor speed control with current controlled PWM AC chopper using a microcontroller”, IEEE Proc. industrial technology 2000, vol. 1, page(s): 394-398.

[8] A. S. Zein El Din, M. El-Shebiny, and M. Khater, “Microcomputer- controlled universal motor”, IEEE Proc. Induratial electronics, 1996, Vol. 2, page(s): 653-658.

[9] A. Di Gerlando, R. Perini, and G. Rapi, “Equivalent circuit for the performance analysis of universal motors”, IEEE Trans. on Energy Conversion, vol.19, issue 1, March 2004, page(s): 18-27.

[10] R. N. Lea, and Y. Jani, “Design and performance comparison of fuzzy logic based tracking controllers”, IEEE Proc. industrial fuzzy control and intelligent systems, and NASA joint tech., Dec. 1994, page(s): 340-344.

705