Motor Thermal Protection
GENERAL
Overheating is the most common cause of electrical motor failure. Adequate protection against overheating is necessary for all electrical motors, and adequate protection against motor overloading (which causes overheating and mechanical strain) is required by the
National Electrical Code - Art. 430, Part C.
Motor protection against overloading can be done indirectly, controlling the cause - i.e.
high running current for a prolonged period of time. or directly controlling the effect - high motor stator temperature, The second type of protection is provided for almost all
Flygt pumps by inserting temperature sensors into the motor stator windings.
THERMAL SWITCHES
Flygt pump motors, with a few exceptions, are provided with two (2) or three (3) thermal switches embedded in the stator windings. The exceptions are:
- some big pumps and hydroturbines;
- warm liquid pumps (high temperature thermal switches are optional).
DESCRIPTION
Thermal switches installed into the pump motor stator windings are bimetallic, normally closed contact type. The device consists of a strip made of two dissimilar metals. The two metals have different temperature coefficient of expansion, so the heat causes the strip to bend. At a preset temperature the switch contact will open. As the stator cools down, the metallic strip returns to its original shape and the switch will reclose its contact.
For different pumps/motors. Flygt uses a few different thermal switches, rated for different openclose temperatures. (See Technical Data.)
INSTALLATION
There are usually two (2) or three (3) switches inserted into the stator end turns during its production. The thermal switches are wired in series and connected by two (2) leads to the pump terminal board, (Terminals T1 and T2.) For small pumps, such as the 3085, the thermal switch connection to the cable is made with wire nuts. The closed/open status of the thermal switches is supervised by a relay or other device and the opening of one of the thermal switches shall trip the pump branch circuit breaker, or deenergize the motor starter, interrupting the power to the pump motor. The thermal switches in the stator will reset automatically, as described above, but to restart the motor a manual reset might be required, depending on the type of operation. If a leakage detector is also installed in the pump in conjunction with a
Mini CAS supervision relay, the thermal switches are wired in series with the leakage sensor. (See Mini CAS manual-wiring diagram.)
TECHNICAL DATA:
Rated Voltage: 250 VAC
Maximum Operating Voltage: 600 VAC
Rated Current: 6.3 Amps @ 250 VAC, PF 0.60
Thermal Class F (+155°C) insulation
CSA/UL Approved
Operating Temperature: Opens at 110°C through 155°C, depending on pump type. Check the pump manual for the specific thermal switch temperature rating.
3/09

Motor Thermal Protection
TESTING
The thermal switches have normally closed contacts and are wired in series. For a cold motor, between the leads connecting the thermal switch, the resistance measured with a regular ohm meter should be under 1 ohm. If the thermal switch resistance is measured at the end of the cable, cable leads’ resistance should be considered in series with the switches when reading the meter. An open circuit or high ohms’ reading for the cold motor indicates a defective thermal switch or a loose connection.
PTC THERMISTORS
Older Flygt pumps and some FM-Approved, explosion-proof pumps can use PTC (positive temperature coefficient) thermistors for stator over temperature protection, instead of thermal switches.
DESCRIPTION
A PTC thermistor increases its internal resistance when temperature is increased. The thermistors used in above-mentioned pumps are non-linear type, which means that for a preset temperature. thermistor resistance will suddenly begin to increase rapidly. (See attached Resistance/Temperature curve.)
Non-Linear PTC Thermistor Curve
100
50
3K
2K
1K
700
500
250
Point of activation
Switch-over
Point
20
˚ F 68 104 140 176 201 248 284
˚ C 20 40 60 80 94 120 140
Temperature
INSTALLATION
Stator thermistor circuits use three thermistors connected in series. The same type of thermistors are also used for measuring the bearing temperature. For temperature supervision, the thermistors are wired in the control panel to special thermistor relays - one for stator temperature and one for bearing temperature. The relay will react to the sudden change in thermistor temperature and trip the pump circuit breaker or de-energize pump motor starter.
TECHNICAL DATA
Stator temperature PTC thermistors - three (3) thermistors connected in series. Total resistance at 25°C (77°F) - 150-300 ohms not to exceed 750 ohms.
Bearing temperature PTC thermistor - one (1) each - Resistance 50 - 100 ohms at 25°C (77°F) - not to exceed 250 ohms.

Motor Thermal Protection
TESTING
CAUTION: MAXIMUM TEST VOLTAGE FOR PTC THERMISTORS IS 2.5 VOLTS D.C.
Application of more than 2.5 volts will destroy the thermistors. At room temperature, PTC thermistor resistance should be within the above limits.
PT 100 SENSOR FOR BEARING TEMPERATURE SUPERVISION
DESCRIPTION
PT 100 sensors are Resistance Temperature Detectors (RTD) made of platinum. The resistance of platinum is linear with temperature over a wide range and they are thus capable of a great temperature accuracy.
“100” refers to the sensor resistance at 0°C (32°F), which is 100 ohms. Used together with the Flygt
MAS 711 pump monitoring system, the PT 100 sensor provides an accurate indication and permanent monitoring of the bearing temperature.
Linear PT 100 Lower Bearing Sensor Curve
160
150
140
130
120
110
100
˚ F 68 104 140 176 212 248 284
˚ C 20 40 60 80 100 120 140
Temperature

Motor Thermal Protection
INSTALLATION
One PT 100 sensor is installed into the bearing housing of the big pumps and hydroturbines.
Standard connection to the MAS 711 is made with two (2) wires. As the wires’ resistance adds to sensor resistance, a long cable run from the pump terminal board to the supervision relay will change the temperature reading. Example: At 90°C (194°F), PT 100 resistance is 134.7 ohms. A two-wire connection with AWG #14 cable, 1,000 ft. Long, will add: 2 x 3.07 x 1.0 = 6.14 ohms. Total resistance is now 140.84 ohms, which corresponds to 105°C temperature for PT 100 itself. If the MAS 711 unit is set for l00°C, the pump will be tripped even if the bearing temperature is still acceptable.
To avoid having this happen, the MAS 711 must be programmed with a resistance offset to compensate for lead resistance.
If the PT100 has three leads, connect red lead(s) to one measuring terminal point and blue lead(s) to the other measuring terminal point. Terminal points can be found in the MAS 711 Installation Manual.
TECHNICAL DATA
Maximum measuring voltage: 2.5V

°C
-55
-50
-45
-40
-35
-30
-25
-90
-85
-80
-75
-70
-65
-60
-20
-15
-10
-5
0
5
10
15
-170
-165
-160
-155
-150
-145
-140
-135
-130
-125
-120
-115
-110
-105
-100
-95
-200
-195
-190
-185
-180
-175
Motor Thermal Protection
Ω °C Ω °C Ω °C
92.16
94.12
96.09
98.04
100.00
101.95
103.90
105.85
78.32
80.31
82.29
84.27
86.25
88.22
90.19
64.30
66.31
68.33
70.33
72.33
74.33
76.33
18.52
20.68
22.83
24.97
27.10
29.22
31.33
33.44
35.54
37.64
39.72
41.80
43.88
45.94
48.00
50.06
52.11
54.15
56.19
58.23
60.26
62.28
165
170
175
180
185
190
195
130
135
140
145
150
155
160
200
205
210
215
220
225
230
235
75
80
85
90
50
55
60
65
70
95
100
105
110
115
120
125
20
25
30
35
40
45
149.83
151.71
153.58
155.46
157.33
159.19
161.05
162.91
164.77
166.63
168.48
170.33
172.17
174.02
107.79
109.73
111.67
113.61
115.54
117.47
119.40
121.32
123.24
125.16
127.08
128.99
130.90
132.80
134.71
136.61
138.51
140.40
142.29
144.18
146.07
147.95
175.86
177.69
179.53
181.36
183.19
185.01
186.84
188.66
385
390
395
400
405
410
415
350
355
360
365
370
375
380
420
425
430
435
440
445
450
455
270
275
280
285
290
295
300
305
310
315
320
325
330
335
340
345
240
245
250
255
260
265
229.72
231.47
233.21
234.96
236.70
238.44
240.18
241.91
243.64
245.37
247.09
248.81
250.53
252.25
190.47
192.29
194.10
195.91
197.71
199.51
201.31
203.11
204.90
206.70
208.48
210.27
212.05
213.83
215.61
217.38
219.15
220.92
222.69
224.45
226.21
227.96
253.96
255.67
257.38
259.08
260.78
262.48
264.18
265.87
605
610
615
620
625
630
635
640
645
650
655
660
570
575
580
585
590
595
600
490
495
500
505
510
515
520
525
530
535
540
545
550
555
560
565
460
465
470
475
480
485
CAUTION: MAXIMUM TEST VOLTAGE FOR PT 100 SENSOR IS 2.5 VDC.
Ω
304.01
305.63
307.25
308.87
310.49
312.10
313.71
315.31
316.92
318.52
320.12
321.71
323.30
324.89
326.48
328.06
329.64
331.22
332.79
267.56
269.25
270.93
272.61
274.29
275.97
277.64
279.31
280.98
282.64
284.30
285.96
287.62
289.27
290.92
292.56
294.21
295.85
297.49
299.12
300.75
302.38