Spice Model Source Voltage Ramp Source Cables unshielded
Spice Model Source Voltage (Ramp) Source Cables (unshielded artificially long) Motor Supply Shielded Cables Motor (fron LC to motor) Conductor Shield ground Long source cable to eliminate any reflections from source during timeframe of simulation Pulse hits here (Nsource) at 5 sec, which is same time that plotting starts These resistors establish whether or not shield is grounded: • Set to 1 n. Ohm for shield grounded • Set to 1 g. Ohm for shield ungrounded 1
Shield grounded both ends. Shield remains at 0 volts. Upper TL acts like simple transmission line throughout. Voltage at termination is 2*2/(2+1) = 4/3, as expected. V(n 3) = 4/3 2
Shield grounded at motor only. When the Ttee: is hit, the voltage on each TL reduces since parallel lines have lower impedance (1/2). Each TL on output of Tee transmits 2*1/2/(1/2+1) = 2/3. Voltage from conductor to ground is sum i. e 2*2/3 = 4/3. Upon hitting shorted termination 2/3 is reflected (inverted) and 2/3 remains. . . that 2/3 hitting motor impedance rises by 4/3 as before to give 8/9 at motor (node N 3). This voltage is 2/3 lower than it was when shielded both ends. . . this is a result of assumption that all TL impedances are 1 (ratio 2/3 would remain the same regardless of Z 0_motor) “Tee” Acts like 2 separate TL’s in this area V(n 3) = 8/9 3
Shield grounded at supply end only. Results in lower voltage than shielded at both ends for this particular choice of Zmotor. Higher Zmotor=> grounding at supply end only shows not much benefit over grounding at both ends V(n 3) = 1 4
Dimensionleess Variables and Notes • Dimensionless Variables – Choose 3 “base” quantities: Vmax, Trise, Z 0 (choose Z 0 of one particular cable, others will be ratio to that one) – Express everything else as dimensionless ratio of the base quantities • Vpu = V/Vmax… Vmax_pu = 1 • Tpu = T/Trise…. Trise_pu = 1 – For 0. 1 usec actual rise time (base), and 3 E 8 m/sec actual speed, Tpu=1 sec corresponds to actual distance of 30 m • Zpu = Z / Z 0…. Z 0_pu = 1 • Cpu = C*Z 0 / Trise = Tau / Trise where Tau is time constant of a C*Z 0 circuit. – i. e. Cpu is a time constant expressed as multiple of rise time. – In the following slides, all quantities are pu => the pu post-script is dropped. • Other Notes – Chose values for simplicity of calc, not to represent a particular situation – Chose all Z’s the same, even though there are likely differences – Notably source TL length chosen to make source impedance irrelevant. Should look at reflections from source – Neglected motor capacitance which is included in parallel with Z 0 m in other models 5
- Slides: 5