LAB REPORT
Part 2: Safe work with large inductances
1. One of the main hazards of using inductances is electrical energy stored in the magnetic field. If a circuit containing an inductance is interrupted then the stored energy can release through air (forming an arc). This can destroy a switch contact, burn fingers, create other damage. In the circuit of Figure 1, show how to mitigate the risk (pay attention to polarity!).
We give the elements in this circuit some simple values for the purpose of better description and explanation. Say V = 5V, R= 1 Ω and L = 1H. Assume inductor is initially uncharged.
FIGURE 1
From Figure 1:
At t0, switch is opened. VL= L (di/dt) .
L = 1H di=(Ifinal – Iinitial) = (0 – 5) dt= 0.02s, say it takes this amount of time to open the switch.
Hence, VL = (1) (-5) (0.02) VL = -250 V
The electrical potential difference is so great that the air between the contacts of the switch might ionise to provide a conductive path for the current. Consequently this phenomenon might damage the contacts of the switch over time. (We assume that the switch used in this circuit is rated at only 100 Vdc).
FIGURE 3
From Figure 3:
A diode in series with a resistor is connected across the inductor to resolve the arcing phenomena across the switch.
At t 0, when the switch is opened, the diode is forward bias hence allowing current to its branch. The current, I in this circuit is then given by: (VL - VF ) /R , where VF is the forward voltage drop of the diode.
The stored energy in the inductor is dissipated as heat in the resistor. Consequently, there will be no more arcing or ‘sparks’ across the contacts of the switch.
2. Current that is needed for coil saturation experiment is of the order of 5A. This large current, if allowed to flow for a long time, would heat resistors and other elements of a circuit. This can cause skin burns and damage the electric
1. One of the main hazards of using inductances is electrical energy stored in the magnetic field. If a circuit containing an inductance is interrupted then the stored energy can release through air (forming an arc). This can destroy a switch contact, burn fingers, create other damage. In the circuit of Figure 1, show how to mitigate the risk (pay attention to polarity!).
We give the elements in this circuit some simple values for the purpose of better description and explanation. Say V = 5V, R= 1 Ω and L = 1H. Assume inductor is initially uncharged.
FIGURE 1
From Figure 1:
At t0, switch is opened. VL= L (di/dt) .
L = 1H di=(Ifinal – Iinitial) = (0 – 5) dt= 0.02s, say it takes this amount of time to open the switch.
Hence, VL = (1) (-5) (0.02) VL = -250 V
The electrical potential difference is so great that the air between the contacts of the switch might ionise to provide a conductive path for the current. Consequently this phenomenon might damage the contacts of the switch over time. (We assume that the switch used in this circuit is rated at only 100 Vdc).
FIGURE 3
From Figure 3:
A diode in series with a resistor is connected across the inductor to resolve the arcing phenomena across the switch.
At t 0, when the switch is opened, the diode is forward bias hence allowing current to its branch. The current, I in this circuit is then given by: (VL - VF ) /R , where VF is the forward voltage drop of the diode.
The stored energy in the inductor is dissipated as heat in the resistor. Consequently, there will be no more arcing or ‘sparks’ across the contacts of the switch.
2. Current that is needed for coil saturation experiment is of the order of 5A. This large current, if allowed to flow for a long time, would heat resistors and other elements of a circuit. This can cause skin burns and damage the electric