ST AN1448 Application note

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AN1448

APPLICATION NOTE

HOW TO REDUCE 3r d HARM ONI CS WI TH

ST6200C MOTOR CONTROL SOFTWARE

by Microcontroller Division Application Team

INTRODUCTION

Universal motors are widely used in home appl iances such as vacuum cleaner s, washing machines, power tools and food processors. However, universal motors produce a strong 3rd harmonic current. When the TRIAC conduction is not in full wave mode, the motor current contains high amplitude 3rd harmonics which may not comply wi th the 3rd harmoni cs li mits set by the IEC61000-3-2 standard. In practice, it is very difficult to comply with the harmonics standard when driving universal motors with a power of more than 1200W .

This application note presents an innovative, cost-saving solution for suppressing 3rd harmonic current in the power line. The control principle is based on modulation of the phase angle delay times and is easily implemented in software using a low cost ST6200C microcontroller. The output motor current waveform is modulated so as to suppress specified harmonic components. T he efficiency of this method has been proven on a 1500W vacuum cleaner under various load conditions. The m easureme nt of h armonic component s and m otor power was done with a digital power meter (WT1030). The results show m uch better harmonic performance than symmetrical phase control methods.

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1 IEC61000-3-2 STANDARD

Harmonic disturbances in the power line have increased with spread of electronic equipment into homes and of fices. M any hou seho ld applianc es and similar el ectrical eq uipme nt with or without electronic control devices, are liable to introduce disturbances, especially harmonics of the supply frequency, into the systems to which they are connected. The harmonics do not represent any real or active power, but nevertheless they lead to substantial copper or iron losses. The IEC61000-3-2 standard deals w ith the l imitati on of harmonic currents i njected i nto the public supply system.

1.1 SCOPE

The IEC61000-3-2 standard is applicable to all electrical and electronic equipment with an input current up to 16A and intended to be connected to public low- voltage distr ibution systems with a nominal 50Hz or 60Hz frequency and of the following types:

- nominal voltages up to 240V, single-phase, two or three wire;

- nominal voltages up to 415V, three-phase, three or four wire;

1.2 OBJECTIVE

The objective of this standard i s to give th e general requirements and the m aximum perm issible valu es of the ha rmonic comp onen ts of th e inp ut curr ent wh ich ma y be produce d by a piece of equipment tested individually under specified conditi ons. It also gives a practical method for testing a prototype.

1.3 MEASUREMENT CIRCUIT FOR SINGLE PHASE EQUIPMENT

The equipment under test is supplied in series with shunts Z

M (see Figure 1.) or current trans-

formers from a source with the same nominal voltage and frequency as the rated voltage and frequency of the equipment under test.

The measurement circuit for single phase equipment is show in Figure 1..

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Figure 1. Measurement circuit for single phase equipment

Legend:

S = Supply source M = Measurement equipment A = Equipment under test

M = Input impedance of measurement equipment

s = Internal impedance of the supply source

n = Harmonic component of order n of the line current

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1.4 GENERAL RE QUIREMENTS AND LIMITS FOR EQUIPMENT

For Class A equipment (including universal mot ors), the har monics of the input current s hall not exceed the absolute values given in Table 1. The li mits are given as absolute values which are independent of the equipment input power.

Table 1. Limit of harmonic currents for Class A equipment

Odd harmonics

Even harmonic s

Harmonic order

3 5 7

9 11 13

≤n≤39

≤n≤40

Maximum permissib le harmon ic

current(A)

2.30

1.14

0.77

0.40

0.33

0.21

0.15* 15 / n

1.08

0.43

0.30

0.23 * 8 / n

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0000 0000 0000

0000000000000000000 0000000000000000000 0000000000000000000 0000000000000000000 0000000000000000000 0000000000000000000

2 CONTROL PRINCIPLE

2.1 SYMMETRICAL PHASE CONTROL M ETHOD

In AC power system app lications, the amoun t of power del ivered to the syste m load can be varied by controlling the phase delay time thr ough a TRIAC. The delay time is referred to the zero voltage crossing on the power line. This method is widely used in uni versal motor control applications. The motor speed can be easily controlled by changing the firing angle. Power delivered is equal to the integration of the power fr om fire angl e to pi e ( 180 degrees) . The T RIAC will conduct power aft er the firing angle a nd turn off powe r at the zero cro ssing point. The system block diagram and waveform are shown in Figure 2..

Figure 2. Topology of universal motor symmetrical phase angle control method

Umot

ST6200C

Imotor

TRIAC

A universal motor operating at reduced speed (i.e firing angle at around 90 degree) produces very high level of odd harmonics. In experiments with a 1500kW vacuum cleaner at 230V/ 50Hz with the same load, harmonic currents were found to be outside the limits when running at reduced power levels from 470W to 1200W. Practical experience shows that 3rd harmonic currents are determined not only by the motor current amplitude, but also by the conduction time.

∆∆∆∆ I

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Table 2. 1500W vacuum cleaner harmonic currents measurement at fixed load with symmetrical phase angle control method

Power Delay time Harmonic order & Harmonic Currents (A)

P (W) t (ms) 3 5 7 9

250 6.9 1.918 0.92 0.275 0.2 320 6.5 2.122 0.931 0.322 0.213 535 5.75 2.474 0.92 0.453 0.175 590 5.55 2.513 0.923 0.478 0.156 780 5.0 2.601 0.883 0.502 0.127

900 4.5 2.576 0.859 0.44 0.162 1000 4.2 2.515 0.834 0.357 0.196 1050 4.0 2.448 0.82 0.306 0.194 1090 3.75 2.406 0.806 0.27 0.188 1130 3.6 2.331 0.793 0.232 0.182 1200 3.5 2.193 0.743 0.188 0.142 1300 3.0 1.892 0.612 0.196 0.06 1400 2.25 1.386 0.33 0.2 0.116 1450 1.6 1.11 0.138 0.13 0.111 1480 0.3 0.77 0.107 0.022 0.01

Harmonic current limit (A) 2.30 1.14 0.77 0.4

In Table 2., the figures in bold are outside the harmonic current limit specifications.

2.2 ASYMMETRICAL PHASE CONTROL METHOD (KURZ PATENT)

The experiment results in Table 2. indicates that universal motors produce the maximum 3rd harmonic currents at the middle power range. This makes it possible to reduce the harmonic current with the asymmetrical phase control method.

According to Table 2., when a 1500W v acuum cleaner works at 780W, the 3rd harmonic current is outside the limit. But when it works at 1200W and 320W , the 3rd harmoni c current is within the limit. If it works at 1200W and 320W in alternate full wave cycles (2 lon g plus 2 short), the 3rd harmonic current should not be outside the limits due to phase considerations and the average power should be around 760W. The motor speed will be still quite stable due to to the rotor momentum. The experiment results in Table 3 prove this hypothesis.

It is very simple to try out this control method with a simple experiment. During the testing period, two potentiometers can be used to set two independent phase angle delay times. In this way, the motor power can be changed with different c om b inations of the two potentiometers. The motor power and 3rd harmonic current can be measured at the same time. The best harmonic performance can be achieved by finely adjusting the two phase angle delay times. From this, it is easy to build a look up table (t1 and t2 corresponding to the motor power).

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The current wave form is shown in Table 3. Channel 1 is the AC mains voltage waveform, channel 4 is the motor current waveform.

Figure 3. . Motor current waveform of asymmetrical phase control method

Based on the simple experiment with two potentiometers, the two phase angle delay time could be optimized at different motor power rates. From this, the look up table for the two phase angle delay times can be created . With this innovative solution, the 3rd harmonic currents at middle power range have been reduced dramatically without any additional cost.

Table 3 gives the harmonic currents test resul ts with the same universal motor under the same

test conditions.

Table 3. . 1500W vacuum cleaner harmonic currents measurement at fixed load with asymmetrical phase angle control method

Power Delay time Harmonic order & Harmonic Currents (A)

P (Watt) t

535W 6.8 5.25 2.13 0.536 0.18 0.059 580W 6.8 5.0 2.033 0.38 0.151 0.104 780W 6.6 4.1 1.745 0.165 0.164 0.21 940W 6.0 3.4 1.557 0.18 0.252 0.134

1000W 6.0 2.9 1.241 0.327 0.29 0.083 1050W 5.55 2.05 0.86 0.388 0.207 0.146 1100W 5.25 2.05 1.01 0.342 0.261 0.106 1120W 4.75 2.05 1.267 0.25 0.316 0.015

Harmonic current limit (A) 2.30 1.14 0.77 0.4

1 (ms) t2 (ms)3579

(Kurz patent)

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This method has already been patented by KURZ. The patent num ber is DE19705907A1 (German Patent) and EP0859452A1 (European Patent). With the KURZ patented method, the motor current is modulated by two long half waves and two short half waves. In short, this method can be called the “two long plus two short” method.

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3 CONCLUS IONS

3.1 BUILT-IN ST62 FEATUR ES YIELD THE LOWEST TOTAL SYSTEM COST

The use of MCU-driven motor control s ystem in home appl iances i s becoming more and more popular with the availability of flexible, reliable and low cost microcontrollers.

The STMicroelectronics ST62 microcontroller family is ideal for hom e applicances because it offers many powerful on-chip peripherals and resources, a wide range of operating voltages, built-in ruggedn ess and excelle nt noise imm unity to enab le designers to ac hieve the lowes t possible system cost.

3.2 A SOFTWARE APPROACH PROVIDES A VERY CO ST EFFECTIVE SOLU TION

The design pre sented pr oposes a cost effecti ve soluti on for reduc ing the 3rd harmoni c currents for v acuum cleane r app lication s. The resul ts given by the experi ments prove th e effi ciency of this method.

The source code of the software (filename vacuum.st6) is supplied with the UMC01EVAL board. Please contac t your local STMicroelectronics sales office for information about ordering this kit.

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