7 FUNDAMENTALS OF ELECTRICITY

Part 2: Transformer operation under load

Transformers have small internal resistances due to transformer windings. This resistance will cause the transformer secondary to slightly decrease as the load current increases. Part two will demonstrate this fact.

1.

Connect the following circuit using the 25Ω resistor as shown in the diagram.

2.

Apply power to the circuit and open the switch (turn off). Measure the secondary voltage and current and record in table 3.

3.

Close the switch and measure the secondary voltage and current with the 25Ω

load connected and record in table 3.

4.

Reconfigure the load resistor to 12.5Ω by moving one resistor wire to the center banana jack. Measure the secondary voltage and current and record in table 3.

5.

Does the secondary voltage increase, decrease or stay the same as the load increases? Circle one choice below:

Increase

Decrease

Stay the same

Load

Secondary Voltage

Secondary Current

No-Load

0 mA

25Ω

12.5Ω

Table 3: Transformer under load

6.

Table 3 will show an increase in secondary current and a small decrease in secondary voltage as the load increases.

82

Part 3: Measurements taken with and without a grounded secondary.

Do not use the isolation transformer. Have your circuit inspected before applying power.

Procedure

1.

Connect the following circuit, plug it into a regular wall outlet and turn on the

lamps. Ensure that the secondary circuit is connected to a good ground.

2.

Complete the following table of voltage measurements. When taking the measurements you must choose a proper ground reference for the multimeter.

You may use the metal conduit located on the back wall of your bench.

Measurement

Voltage

Measurement

Voltage (Volts)

(Volts)

Across primary

Across secondary

One side of

One side of

primary to

secondary to ground

ground

Other side of

Other side of

primary to

secondary to ground

ground

Bulbs to ground

Bulbs to ground with

switch on

switch off

83

3. Remove the ground from the secondary circuit and repeat the measurements.

Measurement

Voltage

Measurement

Voltage (Volts)

(Volts)

Across primary

Across secondary

One side of

One side of

primary to

secondary to ground

ground

Other side of

Other side of

primary to

secondary to ground

ground

Bulbs to ground

Bulbs to ground with

switch on

switch off

84

Questions

1.

In part 3, did the lamps light up when the ground was removed?

2.

What effect did varying the primary voltage have on the Vp / Vs ratio?

3.

What circuit requirement must be present before you are able to make accurate measurements from a transformer secondary circuit to ground?

4.

The transformer has a primary winding with 680 turns. The secondary winding has 40 turns. The input voltage is 120 volts AC. Assuming an ideal transformer, what is the output voltage?

85

5.

Just before the electricity reaches a consumer it passes through a power transformer that reduces the voltage from 4kv to 240v. Which of the following must be true for this transformer?

a) The current is changed from direct current to alternating current b) The current is changed from alternating current to direct current c) There are more windings on the secondary coil than on the primary coil d) There are more windings on the primary coil than on the secondary coil 6.

A three-story building will use a hydronic heating system. This heating system is constructed of a number of zones. The main floor has 5 zones, the second floor has 3 zones and the third floor has 2 zones. Each zone requires a 24VAC valve and each valve draws 0.4A when it is open. Choose the appropriate transformer for each floor from the following list. Show your work!

5VA

40VA

10VA

50VA

15VA

65VA

20VA

85VA

25VA

100VA

30VA

125VA

a) main floor transformer ____________

b) second floor transformer ___________

c) third floor transformer _____________

d) If these transformers have a winding ratio of 8.7:1, what is the rated primary voltage?

86

Lab 12: Relays

Objectives:

 Work safely on electrical equipment

 Identify the terminals of a relay

 Verify operation of a Relay

 Use a relay to control a Fan and a Bulb

Materials:

 Multimeter with probes

 Control transformer; White Rogers

90-T40M1 40VA 24V

 Level 3 trainer

 Test leads as required

 Relay

 Fan Motor

Information:

A r e l a y i s a n e l e c t r i c a l l y c o n t r o l l e d s w i t c h . T h e s w i t c h ( o r C o n t a c t ) m o v e s w h e n t h e C o i l o f t h e r e l a y i s e n e r g i z e d o r d e – e n e r g i z e d . T o e n e r g i z e , t h e r a t e d v o l t a g e o f t h e C o i l i s t o b e s u p p l i e d t o t h e C o i l o f t h e r e l a y .

Relay Identification and Testing

1 ) I d e n t i f y t h e t w o r e l a y s o n t h e L e v e l 3 t r a i n e r ( p i c t u r e o f o n e o f t h e r e l a y s w i t h t e r m i n a l s l a b e l l e d i s o n t h e n e x t p a g e ) .

2 ) E x a m i n e a n d s t u d y o n e o f t h e r e l a y s .

3 ) W h a t i s t h e t y p e o f s w i t c h t h a t t h e C o i l o f t h e r e l a y c o n t r o l s ( t h e c o i l c o n t r o l s 2 s w i t c h e s o f t h e s a m e t y p e ) ?

4 ) W h a t i s t h e C o i l V o l t a g e R a t i n g o f t h e R e l a y ?

5 ) W h a t i s t h e r e s i s t a n c e o f t h e C o i l o f t h e r e l a y ( t h e c o i l i s c o n n e c t e d b e t w e e n t h e b r o w n t e r m i n a l s 1 & 0 ) ?

87

7

6

8

1

4

0

3

2

6 ) T h e C o i l i s n o t e n e r g i z e d . I n t h i s s t a t e , w h a t d o y o u e x p e c t t h e r e s i s t a n c e t o b e b e t w e e n t e r m i n a l s 7 & 8 ( s t u d y t h e d i a g r a m o n t h e r e l a y ) ?

7 ) M e a s u r e t h e r e s i s t a n c e b e t w e e n t e r m i n a l s 7 & 8 .

8 ) T h e C o i l i s n o t e n e r g i z e d . I n t h i s s t a t e , w h a t d o y o u e x p e c t t h e r e s i s t a n c e t o b e b e t w e e n t e r m i n a l s 6 & 8 ?

9 ) M e a s u r e t h e r e s i s t a n c e b e t w e e n t e r m i n a l s 6 & 8 .

88

1 0 ) I d e n t i f y t e r m i n a l s 6 , 7 & 8 . W h i c h o f t h e s e t h r e e t e r m i n a l s i s C o m m o n , w h i c h i s N o r m a l l y C l o s e d a n d w h i c h i s N o r m a l l y O p e n ?

1 1 ) W h a t w o u l d t h e r e s i s t a n c e b e t w e e n t e r m i n a l s 6 & 8 b e i f t h e c o i l o f t h e r e l a y w a s e n e r g i z e d ?

Power supply check

1 2 ) T u r n o f f t h e p o w e r s w i t c h o n t h e l e f t s i d e o f t h e t r a i n e r .

1 3 ) R e m o v e a n y w i r e s t h a t m a y h a v e b e e n l e f t b e t w e e n a n y c o n n e c t i n g t e r m i n a l s o f t h e t r a i n e r .

1 4 ) P l u g a p o w e r c o r d b e t w e e n t h e t r a i n e r a n d a 1 2 0 V w a l l r e c e p t a c l e .

1 5 ) T u r n o n t h e p o w e r s w i t c h o n t h e s i d e o f t h e t r a i n e r . Y o u s h o u l d s e e a g r e e n i n d i c a t o r l i g h t t u r n o n a t t h e t o p l e f t c o r n e r o f t h e t r a i n e r .

I f y o u d o n o t s e e t h e g r e e n p o w e r i n d i c a t o r t u r n o n , a s k f o r a s s i s t a n c e .

1 6 ) T u r n o f f t h e p o w e r s w i t c h o f t h e t r a i n e r .

Setting up the Transformer

I t i s u n s a f e t o t o u c h l i v e e l e c t r i c a l c o n n e c t i o n s , b e c a r e f u l w h e n m a k i n g v o l t a g e m e a s u r e m e n t s .

T h e b l a c k a n d w h i t e w i r e s o f t h e t r a n s f o r m e r a r e c o n n e c t e d t o t h e p r i m a r y c o i l . T h e s e w i r e s w i l l b e c o n n e c t e d t o 1 2 0 V p o w e r .

T h e y e l l o w a n d b l u e w i r e s o f t h e t r a n s f o r m e r a r e c o n n e c t e d t o t h e s e c o n d a r y c o i l . T h e s e w i r e s w i l l s u p p l y 2 4 V t o t h e c o i l o f t h e r e l a y t h r o u g h t h e T h e r m o s t a t .

89

1 7 ) C o n n e c t a s h o r t b l a c k w i r e b e t w e e n o n e o f t h e b l a c k p o w e r t e r m i n a l s a n d t h e b l a c k t e r m i n a l o f t h e t r a n s f o r m e r p r i m a r y .

1 8 ) C o n n e c t a s h o r t w h i t e w i r e b e t w e e n o n e o f t h e w h i t e p o w e r t e r m i n a l s a n d t h e w h i t e t e r m i n a l o f t h e t r a n s f o r m e r p r i m a r y .

1 9 ) T u r n o n t h e p o w e r s w i t c h o n t h e l e f t s i d e o f t h e t r a i n e r .

2 0 ) M e a s u r e t h e v o l t a g e b e t w e e n t h e t r a n s f o r m e r p r i m a r y t e r m i n a l s a n d r e c o r d o n t h e d i a g r a m b e l o w .

2 1 ) M e a s u r e t h e v o l t a g e b e t w e e n t h e t r a n s f o r m e r s e c o n d a r y t e r m i n a l s a n d r e c o r d i n t h e d i a g r a m b e l o w .

2 2 ) T u r n o f f t h e p o w e r s w i t c h .

2 3 ) L e a v e t h e t r a n s f o r m e r p r i m a r y t e r m i n a l s c o n n e c t e d t o 1 2 0 V

p o w e r t e r m i n a l s .

90

Energize the Coil

Ste

p 4

Ste

p 2

St

ep

3

2 4 ) P r o v i d e 2 4 V t o t h e C o i l o f t h e r e l a y t h r o u g h t h e t h e r m o s t a t .

T h e t h e r m o s t a t w i l l a c t a s a s w i t c h t o e n e r g i z e a n d d e – e n e r g i z e t h e C o i l :

2 5 ) E n s u r e t h a t t h e F a n S w i t c h i s ‘ O f f ’ o r s e t t o ‘ A u t o ’ .

2 6 ) C o n n e c t t h e Y e l l o w t e r m i n a l o f t h e s e c o n d a r y o f t h e T r a n s f o r m e r t o t h e l o w e r R e d t e r m i n a l o f t h e T h e r m o s t a t ( R c ) a s s h o w n i n t h e p i c t u r e o n t h i s p a g e .

2 7 ) C o n n e c t t h e G r e e n F a n t e r m i n a l ( G ) o f t h e T h e r m o s t a t t o t h e l o w e r B r o w n t e r m i n a l ‘ 0 ’ o f t h e r e l a y ( B r o w n t e r m i n a l s s u p p l y v o l t a g e t o t h e C o i l o f t h e r e l a y ) .

2 8 ) C o n n e c t t h e B l u e t e r m i n a l o f t h e s e c o n d a r y o f t h e T r a n s f o r m e r t o t h e u p p e r B r o w n t e r m i n a l ‘ 1 ’ o f t h e r e l a y .

2 9 ) T u r n o n t h e 1 2 0 V p o w e r .

3 0 ) T u r n t h e F a n S w i t c h t o ‘ O n ’ .

3 1 ) T h e C o i l o f t h e r e l a y s h o u l d n o w b e e n e r g i z e d . M e a s u r e t h e r e s i s t a n c e b e t w e e n t e r m i n a l s 6 & 8 o f t h e C o n t a c t o f t h e r e l a y .

D o e s i t m a t c h y o u a n s w e r t o q u e s t i o n # 1 1 o n p a g e # 3 ?

91

3 2 ) T u r n o f f t h e F a n s w i t c h o r t o ‘ A u t o ’ .

3 3 ) T u r n o f f t h e 1 2 0 V p o w e r s w i t c h .

C o n n e c t t h e F a n

T o R e d F a n W i r e ( S t e p 1 1 )

T o B l a c k F a n W i r e ( S t e p 1 2 )

7

8

6

T e r m i n a l 8 t o B l a c k P o w e r T e r m i n a l ( S t e p 1 0 )

92

T o Y e l l o w F a n W i r e

T o G r e e n F a n W i r e ( S t e p 1 4 )

( S t e p 1 3 )

3 4 ) C o n n e c t a B l a c k T e r m i n a l ( L i v e ) o f t h e P o w e r T e r m i n a l t o t h e B l u e T e r m i n a l # 8 o f t h e R e l a y .

3 5 ) C o n n e c t t h e R e d T e r m i n a l # 7 ( N o r m a l l y C l o s e d ) o f t h e R e l a y t o t h e R e d L o w S p e e d w i r e o f t h e f a n .

3 6 ) C o n n e c t t h e Y e l l o w T e r m i n a l # 6 ( N o r m a l l y O p e n ) o f t h e R e l a y t o t h e B l a c k H i g h S p e e d w i r e o f t h e f a n .

3 7 ) C o n n e c t t h e W h i t e t e r m i n a l ( N e u t r a l ) o f t h e P o w e r T e r m i n a l t o t h e Y e l l o w N e u t r a l w i r e o f t h e f a n .

3 8 ) C o n n e c t t h e G r e e n , g r o u n d , t e r m i n a l f r o m t h e p o w e r t e r m i n a l s t o t h e G r e e n , g r o u n d , w i r e o f t h e f a n .

3 9 ) T u r n o n t h e 1 2 0 V P o w e r S w i t c h .

4 0 ) I s t h e F a n r u n n i n g o n l o w s p e e d ?

4 1 ) T u r n t h e F a n S w i t c h t o ‘ O n ’ .

4 2 ) I s t h e F a n n o w r u n n i n g o n H i g h S p e e d ?

4 3 ) T u r n o f f t h e 1 2 0 V P o w e r S w i t c h .

4 4 ) T u r n t h e F a n S w i t c h o f f o r t o ‘ A u t o ’ .

4 5 ) D i s c o n n e c t a l l t h e w i r e s .

4 6 ) R e t u r n e q u i p m e n t , a n s w e r t h e f o l l o w i n g q u e s t i o n s .

93

Questions

1. What is the resistance between the C and NC contacts when the coil of a relay is not energized?

2. What is the resistance between the C and NC contacts when the coil of a relay is energized?

3. What value of voltage should be supplied to the coil of a relay to energize it?

a. 120V.

b. Whatever the contact rating is.

c. Whatever the coil voltage rating is.

d. 12V.

4. If a 12V AC relay coil is connected to 12V DC would you expect the coil to draw more or less current than if it was connected to an AC supply?

5. Record the coil and list of contact ratings for the relays used in this lab.

94