Electrical Machines - Questions and Answers
Information courtesy of ALSTOM.

GENERAL INFORMATION

THERMISTORS

Thermistors are miniature semi-conductors (aspirin size or smaller), which are introduced into the stator winding overhang of an induction motor.

Protection devices range from simple mass produced thermal relays, to expensive precision instruments, whichever is determined by economics, comparing the cost of production with the cost of the motor, or loss of production due to a failure.

Fig 86 Thermistor being inserted in the stator winding overhang.

Cases of failure are categorised as follows:

1. overload*

2. stalling*

3. excessive accelerating times*

4. frequent starting, inching or plug reversing*

5. single phasing*

6. overvoltage or undervoltage*

7. unbalanced supply voltage*

8. high ambient temperature*

9. blocked ventilation*

10. incorrect enclosure

11. ingress of moisture

12. bearing failure - misalignment - mechanical overload - contamination.

NOTE: Not all of the above reflect an increase in line current; most, however, are detected by an increase in temperature.

* Thermistors offer protection against items (a) to (j) inclusive.

The advantages of thermistors - particularly those having a positive temperature coefficient of resistance - for the thermal protection of windings, stem from two particular characteristics, as follows:

RESISTANCE CHARACTERISTICS

The resistance of the thermistor is relatively low at normal temperatures and remains nearly constant up to a critical value, selected for the class of winding insulation that is to be protected. At this point, even a slight increase in temperature will cause the resistance of the thermistor to rise to a value up to several hundred times its normal value when cold. This sudden increase of resistance is easily interpreted into a positive switching action.

The positive characteristic, i.e. resistance increase with temperature, achieves inherent safety; an open circuit of a thermistor will have the same effect as an unsafe temperature.

Fig 87 Typical temperature-resistance curve for thermistor.

PHYSICAL CHARACTERISTICS

The small size of the thermistor enables it to be installed in intimate contact with the stator windings, and its low thermal inertia gives rapid and accurate response to winding temperature changes. Thermistors, being robust solid-state devices, are inserted between the conductors of the end windings during winding (see fig. 86); subsequent varnish impregnation and baking makes them an integral mass with the winding.

Three-phase motors usually have one thermistor fitted per phase, the thermistors being connected in series with each other and also to a control unit. When actuated by a sudden change in the resistance of the thermistor circuit at the critical temperature, the control unit will disconnect the main supply through the main contactor or operate a warning device. After cooling by approximately 5°C the winding can be re-energised either manually or automatically. This means that the motor can be returned to service with the minimum of delay, as shown in figures. 88 and 89.

Fig 88
Curve showing temperature-time relationship of a GEC 18,6 kW thermistor-protected motor subjected to a locked-rotor test from cold (stator winding re-energised automatically). Note the initial overshoot of temperature which the winding and insulation system must withstand.

Fig 89
Curve showing temperature-time relationship of the same motor as in fig. 88, running at 100% overload windings initially hot. Note the small differential between the 'tripping' and 'resetting' temperature.

HEATERS

Moisture attacks insulation and causes insulation failure. Breakdown may occur when the windings are energised. Insulation therefore deteriorates under conditions involving long idle periods, combined with high humidity and widely varying ambients.

Standby motors fail to operate when they are needed - motors installed twelve months before start-up, fail on commissioning.

It should also be noted that leads carrying supply to the motors also absorb moisture.

Under high humidity conditions with widely varying ambient temperature and/or where a motor is expected to suffer long idle periods, additional impregnation treatment of the winding, plus some form of anti-condensation heater is recommended. This can be achieved by either (a) low voltage heating or (b) fitting heaters.

a) LOW VOLTAGE HEATING

This involves connecting a single phase, low voltage (5 to 7,5 per cent of motor terminal voltage) across two of the three leads of the 3 phase motor, while the motor is inoperative.

This method is very effective, but necessitates a low voltage transformer. It obviates the problem of failed heaters, going undetected, plus the down-time associated with replacing a heater inside the motor.

b) INTERNALLY FITTED HEATERS

The standard heater, fitted on 80 to 315 sizes, consists of a glass fibre tape with the heating element integrally woven into it. This tape is then fitted inside a silicon impregnated glass fibre sleeve.

The tape heater is then wrapped around the stator winding overhang, braced and completely impregnated with the stator winding.

Two leads are brought out to the inside of the main terminal box, unless otherwise requested, to provide a separate heater terminal box.

Heater supply 220/250 volts, single phase. Tape type by surface heating

Fig 90 Anti-condensation heater

'Starting A' - This covers direct-on-line starting based on 7 x motor full load current for 10 seconds.
'Starting B' - This covers assisted starting based on 3,5 x motor full load current for 20 seconds.

NOTE: Contact GEC Controls (Pty) Ltd., for recommended fuse ratings to protect motors with starting conditions different from those mentioned above. Full load currents given are the average of figures supplied by various South African, British and Continental motor manufacturers.

Power Factor Correction

cos r₁ = Initial power factor 
cos r₂ = Required power factor

To correct the power factor from cos r₁ to cos r₂

Capacitor kVAr required = input kW x (tan r₁ - tan r₂) -- Equation 1

The table below gives values of (tan r₁ - tan r₂) for power factors from 0,5 upwards and it is only necessary to multiply the kW input by the factor given to obtain the required capacitor kVAr.

NOTE:

1. If the capacitor kVAr exceeds 90 per cent of the no-load kVA, then the capacitor must be connected at the supply side of the motor starter and incorporate its own isolating switch. This is to avoid the possibility of the capacitor (if connected across the motor terminals) causing high voltage and regenerative braking, which can cause terminal flashover and shaft failure.

Fig 92

If the capacitor kVAr is below 90 per cent of the no load kVA~ the capacitor can be connected directly across the motor terminals.

2. Input kW are computed as follows:

Output kW

Efficiency per unit

Example:

A 90kw motor has a full load power factor of 0,79 and an efficiency at full load of 93 per cent. Power factor to be corrected to 0,9.

Input kW =	Output kW	=	90	= 96.77 kW
	Efficiency per unit		0.93
From selection table, multiplying factor = 0.29
kVAr =	96.77 x 0.29 = 28.06

CAPACITOR kVAr REQUIRED PER UNIT kW INPUT FOR POWER FACTOR CORRECTION

NOISE LEVELS

No load sound pressure and sound power levels for TEFC 3 phase, 50Hz motors.

Sound Pressure Reference is 2 x 10^-5 N/m²
Sound Power Reference is 10^-12 watts
Sound Pressure measured at 3 metre reference radius.
For 1 metre radius, at mean sound pressure add 9.5 dB

Synchronous Speed: 3000 r/min.

Synchronous Speed: 1500 r/min

Synchronous Speed: 1 000 r/min.

Synchronous Speed: 750 r/min.

SLIDE RAILS

Fig 93

Slide rails Dimensions in millimetres

COUPLING AND PULLEY BORES
MACHINING DETAILS

Note: Setscrew must be fitted over keyway in pulley. Couplings and pulleys sent to our works for fitting must be stamped with the GEC ref. No.

Fig 94

* 2 POLE MOTOR