Starter Solenoid and Power Contacts Diagnostics

Josef Pošta, Roman Pavlíček, Tomáš Hladík

Czech University of Agriculture, Prague, Czech Republic

Availability of the compulsion engine machines depends on the starter reliability. Starter failures are often caused by gradual damage of its particular parts, this process can be monitored by means of diagnostics. In this paper the results of the starter solenoid and power contacts diagnostics verification are summarized.

Key words: starter, voltage waveform, starter solenoid, power contacts

Starter failures sources are either mechanical or electrical. Wear, seizure or fractures of moving parts come under mechanical sources of failures, short or interrupted circuit and contact resistance increase come under the latter, [2].

In the most frequently used starter design the pull-in solenoid is used for engaging the pinion. The pull-in solenoid is suitable for low armature travel and high pull-in force. The solenoid armature moves the engaging lever, which engages the pinion into the mesh with ring gear of the flywheel. After the pinion is entirely engaged the total accumulator voltage is applied to the starting electromotor, [3]. Fig. 1 shows the electrical scheme of the frequently used starter design.

Fig. 1: Electrical scheme of the starter [4]
1 – pull-in solenoid
2 – electromotor
11,12 – solenoid hold-in and pull-in winding,
13 – power contact,
30 – power contact terminal,
50 – solenoid terminal

1. Materials and Methods

1.1 Pull-in solenoid

The draw force of the solenoid [4] has the form (1).

(1)

where F is the draw force (N)

k is the armature length (m)

d is the armature diameter (m)

m₀ is the vacuum permeability (H.m^-1)

N is the solenoid winding number (1)

i is the instantaneous current (A)

l is the armature travel (m)

According to (1) it is obvious how the draw force is determined. The solenoid winding number N can change due to turn-to-turn winding fault, the drop of the instantaneous current i can emerge due to increased contact resistance. The draw force drops significantly in both mentioned cases. Considering the starter reliability, the starter end-of-life is reached when the draw force drops insofar it is unable to equal the passive resistance and the force of the solenoid armature pull-back spring.

Thus the solenoid armature draw force can be used as a diagnostic signal, which provides sufficient information on the character of the above mentioned factors and their variations in time. However, the draw force value is not measurable without disassembling. For practical use the draw force is thus not a suitable diagnostic signal.

The draw force value can be indicated indirectly by the solenoid armature pull-in time, or also by the time of engaging the pinion. The solenoid armature pull-in time can be obtained from uniformly accelerated motion formula, Newton‘s Second Law and equation (1). The substitution and rearrangement gives:

(2)

where t is the solenoid armature pull-in time (s)

m is the armature and other parts mass (kg)

c_s is constant for given starter

According to equation (2) the change of the solenoid winding number N (caused by turn-to-turn winding fault) and the drop of the instantaneous current i (caused by increased contact resistance) extends the solenoid armature pull-in time as a consequence of the armature draw force drop. Thus the solenoid armature pull-in time can be used as a diagnostic signal which provides sufficient information on the character of the above mentioned factors and their variations in time. The solenoid armature pull-in time can be easily measured without disassembling the starter by analyzing the terminal 50 voltage waveform using oscilloscope.

Fig. 2 Terminal 50 voltage waveform [4]
1 – full voltage connected to terminal 50
2 –solenoid armature entirely pulled-in, power contacts connected
3 – starter electromotor working

Full voltage is connected to terminal 50 (Fig. 2 – 1), then the solenoid armature is entirely pulled-in (Fig. 2 – 2) and the power contact connects full voltage to the starter electromotor. The starter motor then spins the combustion engine and the voltage fluctuates due to varying mechanical resistance of the cranked combustion engine. The time delay between points 1 and 2 is represents the armature pull-in time. If the solenoid’s N value decreases due to turn-to-turn winding fault or the instantaneous current i decreases due to increased contact resistance of the solenoid circuit, the pull-in time extends. Mentioned starter failures can be detected by comparing the voltage waveforms of faultless and tested starters.

1.2 Starter Power Contacts

During the combustion engine cranking process the starter electromotor circuit is carrying current of hundreds A. If the contact resistance of the power contacts increases, the losses increase and the starter output drops. The starter end-of-life is reached when the output is not sufficient for minimal cranking RPM of the combustion engine.

The contact resistance of the starter power contact is thus very important for reliable cranking.

The contact resistanceis not easy to measure directly, [1].

Indirect measurement of the contact resistance is possible by measuring the power contact voltage drop. Effective values of the measured voltage are not suitable because of varying circumstances of the circuit; instantaneous values have to be measured instead [5].

Two-channel oscilloscope can be used for the mentioned measurement. The oscilloscope enables easy comparison of the instantaneous values of the voltage on input (30) and output (30a, Fig. 1) power contact terminals when the starter is working.

Fig. 3 Power contact terminals voltage waveform [4]
1 – full voltage connected
2 – solenoid armature entirely pulled-in, power contacts connected
3 – starter electromotor working
30 – terminal 30 voltage
30a – terminal 30a voltage
DU – power contact voltage drop

Full voltage is connected to terminal 50 (Fig. 3 – 1), then the solenoid armature is entirely pulled-in and the power contacts are connected (Fig. 3 – 2). The starter motor then spins the combustion engine and the voltage fluctuates due to varying mechanical resistance of the cranked combustion engine. The voltage difference between terminals 30 and 30a is determined by contact resistance of the starter power contact (DU indicates measured contact resistance).

2. Experimental Verification

The verification was carried out for starting system of Škoda Felicia 1,3:

· 4-cylinder combustion engine, 4-cycle, compression pressure 1,2 MPa, 124 teeth flywheel gear ring,

· Starter 12 V, 0,8 kW, type number 443 115 142 350, 9 teeth pinion, commutator diameter 35 mm, 28 commutator bars,

· Accumulator 12 V, 44 Ah, no-load voltage 12,8 V.

2.1 Verification Of The Pull-In Solenoid Diagnostics

The instantaneous value of the voltage on terminal 50 was measured using digital oscilloscope (sample rate 4,883 kHz), realized bypersonal computer oscilloscopic converting card PCX-1230. The measurement was carried out for faultless starter and for starters with increased solenoid circuit resistance, [4]. The results are summarized in Tab.1 and Fig.4.

Tab. 1 Solenoid Measurement Results

Solenoid	Circuit resistance increase	Armature pull-in time	Terminal 50 voltage
	DR (W)	t (ms)	U (V)
Faultless	0	25	11,0
Increased resistance	0,2	50	8,5
Increased resistance	0,5	75	7,0
Increased resistance	1,0	∞	6,5

Fig. 4 Terminal 50 voltage waveforms
A – faultless, B – circuit resistance increased by 0,2 W,
C – circuit resistance increased by 0,5 W, D – circuit resistance increased by 1 W

2.2 Verification Of The Power Contacts Diagnostics

The instantaneous values of the voltage on terminals 30 and 30a were measured using digital oscilloscope with dual timebase (sample rate 4,883 kHz). The measurement was carried out for faultless starter and for starter with damaged power contacts, [4]. The results are summarized in Fig. 5.

Fig. 5 Power contacts terminal voltage waveform
A – faultless, DU = 0,5 V
B – damaged contact, DU = 2,2 V

3. Discussion

The solenoid armature pull-in time can be easily obtained from oscillogram of the terminal 50 voltage waveform. This time is proportional to the distance between first two peaks of the waveform.

Besides that, the oscillogram provides the information on the voltage drop (or the voltage level after the drop). This information indicates the accumulator state or short circuit failure of the solenoid circuit.

The voltage drop DU can be easily obtained from oscillogram of the terminals 30 and 30a voltage waveforms. From this the absolute and relative losses of the cranking output can be determined. In case 5A the losses amount to 50 VA if the current is 100 A (i.e. 5% considering common 12 V starting system). In case 5B and current 100 A the losses amount to 220 VA (i.e. 22% considering common 12 V starting system). This dissipated energy turns in heat which additionally strains the power contacts.

4. Conclusion

The experiments prove that the solenoid armature pull-in time is suitable technical state indicator for solenoid diagnostics. The pull-in time can be easily obtained from the terminal 50 voltage oscillogram without disassembling the starter itself. The pull-in time sufficiently indicates the starter technical state reacting to the circuit resistance increase (possibly caused by increased contact resistance) as well as to the circuit resistance decrease (possibly caused by turn-to-turn winding fault). In both cases the solenoid armature pull-in time extends making it impossible to distinguish between these two cases, however the solenoid removal and its detailed inspection is necessary in both cases.

The experiments also prove that dual measuring of the power contact terminals voltage enables to diagnose increased contact resistance without disassembling the starter. Significant losses in the starter circuit (which often cause the whole starting system malfunction) can thus be detected.

It can be stated that described methods allow to diagnose the solenoid and power contacts technical state without disassembling the starter and consequently carry out decisions on its preventive renewal. This significantly contributes to availability of the whole machine.

5. References

1. Balog, J.: Počítačová podpora diagnostiky traktorového motora. (Habilitační práce). SPU, Nitra, 1999, 198 s.

2. Fajman, M. – Ondráček, J.: A proposal for evaluating the operating state of agricultural tractors. In: Research in Agriculturae Engineering, Volume 47, 1/2001, Prague 2001, p. 27-32, ISSN 1212-9151

3. PAL Magneton, http://www.magneton.cz

4. Pavlíček, R.: Dynamická diagnostická měření. (Disertační práce). TF ČZU, Praha, 2001, 114 s.

5. Pošta, J. – Pavlíček, R.: Sample rate selection for oscilloscopic diagnostics of alternator and starter. In: Research in Agriculturae Engineering, ISSN 1212-9151

6. Contact Address

Doc. Ing. Josef Pošta, CSc., Czech University of Agriculture, 165 21 Prague 6 – Suchdol, Czech Republic, E-mail: posta@tf.czu.cz Phone: +4202/24383266 Fax: +4202/20921361

Diagnostika zasouvacího elektromagnetu a silových kontaktů spouštěče

SOUHRN: Pohotovost strojů se spalovacím motorem do značné míry závisí na spolehlivosti spouštěče. Poruchy spouštěče jsou často působeny postupně narůstajícím poškozením jeho jednotlivých částí, které může být dobře sledováno diagnosticky. V tomto příspěvku jsou shrnuty výsledky ověření osciloskopické diagnostiky zasouvacího elektromagnetu spouštěče a silového spínače spouštěče.

Klíčová slova: spouštěč, průběh napětí, zasouvací elektromagnet, silové kontakty