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Medium frequency power supply maintain
Furnace of choose
Reference of furnace lining
Furnace of choose
Matching between furnace capacity, power and frequency
1. Melting speed of certain capacity furnace is determined by its power, the bigger the power, the faster the melting speed. However, there is an economic choice for you, the usually matched power of the common metal within a certain capacity and time as follows:
Metal          density (KW/ton/hour)
Iron             500
Steel           600
Brass          350
Aluminum    600.
 
If customers choose 5 tons furnace for melting steel and melting speed is 1 hour, 3000KW needs to be matched. Home and abroad, max power density of MF furnace as follows:
Metal          density (KW/ton/hour)
Iron            1000
Steel          1200
Brass          ~500
Aluminum    1000.
 
Melted metal in the furnace will generate induction current that heats the metal when the furnace is given a certain power, at the same time, because of the effect of electromagnetism, liquid state metal movement will be created once the metal melts.
The stirring movement begins from the centre of the molten bath and then moves towards the coils at ends. Because the metal is restrained by the bottom and wall of the furnace, its final movement is always up and camel peak is formed on the top of the furnace pond.
   Stirring force is direct ratio to input power and inverse ratio to square root of frequency. Stirring force of mains frequency crucible induction furnace is bigger than that of medium frequency crucible induction furnace. Power density of medium frequency crucible induction furnace is bigger than that of mains frequency crucible induction furnace. Under the condition of the same power, the size of medium frequency crucible induction furnace is smaller than that of mains frequency crucible induction furnace. In order to limit stirring force, unit capacity input power should be limited at a certain frequency. Max input power Pg can be found (in the below drawing) according to the different frequency and capacity.
The most convenient way to show stirring force is proportion between the height of camel and diameter of molten bath. If camel peak height serves as one unit, the diameter of molten bath as ten units. So
H/D=1/10=0.1
We know from experience that the proper stirring forces as follows:
 
Metal        ratio of H/D
Iron       0.125-0.2
steel       0.07-0.125
Brass       0.07-0.15
Aluminum  0.035-0.5
Stronger stirring force is needed for iron to make carbon element equally divided. Generally, in the final step of melting, carbon and silicon elements are put into it. Because the two elements are lighter than others, stronger stirring force is needed. Direct mutually relationship exists between stirring intension and reduction percentage of carbon and silicon. Large parts of carbon and silicon will form slag if stirring intension is not enough. As for iron, reduction rate can reach 95% and silicon 78%, however, if stirring intension is not enough, that of iron will lower than 80% and silicon 60%.
Lower stirring intensity is needed in the process of melting steel. Because steel generally is irrigated with higher temperature and therefore easy to be oxidized, the higher the stirring intension, the worse the degree of steel exposure to the air and meanwhile, furnace lining will be badly destroyed if the stirring intensity is over high.
Similarly, oxidation also exists in the process of melting copper and therefore, lower stirring intension is proposed:
As for melting aluminum, a wide range of stirring intensity is required. Melted aluminum liquid is easy to be oxidized and take place chemical combination with hydrogen as stirring intensity grows high. Therefore, lower stirring intensity is suggested when melting aluminum; however, it is necessary for alloy and certain form of furnace charging to stir.
2. Choice for frequency
Economical and operation function is the priority of choice for current frequency, and economical refers to charge for electricity and furnace lining. Considerations can be taken from the following aspects:
1). Electricity efficiency:
    Furnace with various capacities should be equipped with proper frequency in order to acquire the max electricity efficiency. From the theoretical analysis we know that the electricity efficiency of furnace is max when the proportion between crucible diameter and current permeating depth is approximate 10 hours.
2)  Stir:
    Proper stirring makes metal liquid element equal and strong stirring makes furnace rapidly worn and torn and even worse, makes metal liquid mixed with slag and creates air hole. Furnace lining life will be very short if mains frequency crucible induction furnace is used for melting steel; melting needs commentate metal, such as cast iron and melting scraps, by making use of stirring to make element and temperature equal and accelerate the speed of melting scraps, however, melting non-ferrous metal, such as copper, aluminum, and so on, improper to make stronger stirring, otherwise, will accelerates the speed of burning and oxidizing the metal.
3) Investment for equipment:
The price of medium frequency power supply grows as frequency rises.
3, Choice of crucible inner diameter of furnace capacity and crucible wall thickness.
The below table shows the relationship between crucible inner diameter of MF induction furnace and crucible wall thickness.
 
Table 3-1 relationship between furnace capacity and crucible wall thickness (melting pond diameter)

Capacity (t)
 <0.5
0.5~3
>3
Crucible wall thickness(㎝)
(0.25~0.2)
(0.2~0.15)
(0.15~0.1)

Table 3-2 crucible inner diameter of different furnace capacity and crucible wall thickness.

Capacity (t)
20
10
5
3
1.5
D2(㎝)
141
113
90
73
61
12(㎝)
17
15
13
12
11.5

4、Common- used rated power matching (iron and steel) and frequency is recommended according to our experience and common usage:
Capacity    power range   frequency  
0.5 tons      250-310 KW    1 KHZ
     0.75 tons     350-570 KW     1 KHZ
     1 tons       500—750KW    1 KHZ
     1.5 tons      750-1000 KW     0.5-1 KHZ
     2 tons       1000—1500KW   0.5 KHZ
     3 tons       1500—2000KW   0.5 KHZ
     5 tons       2500—3000 KW    0.5 KHZ
     8 tons       4000—4500 KW   0.3-0.5 KHZ
     10 tons       5000—6000 KW    0.2-0.4 KHZ
     15 tons          7500-8000 KW   0.2-0.3 KHZ

  The min power of Iron holding furnace is recommended as follows:
  
 Capacity    power range   frequency   
2 tons       350 KW      1 KHZ
  3/4 tons      400KW       0.5-1 KHZ
  5/6 tons       600KW       0.5 KHZ
  8 tons       750KW       0.5 KHZ
  10 tons      1000KW      0.2-0.5 KHZ
  12 tons       1250KW      0.2-0.4 KHZ
  15 tons       1500KW      0.2-0.3 KHZ
  20 tons      2000KW      0.2 KHZ
  Power of holding furnace is determined by min power needed by sintering furnace    lining
 
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