The Transformer Upgrade is used to upgrade a machine to the next higher tier of input power. Upon placement in an upgrade slot on any IndustrialCraft machine, it will increase that machine's input tolerance up to the next tier of EU power.
Be sure you apply the Transformer Upgrade(s) to your machines before connecting a higher tier power input to them. Connecting the power before you apply the upgrade will result in the machine's explosion.
Also, in SMP a player can fiddle with machine upgrades despite zone protection. The result is a cascade of exploding machines.
One upgrade enables a tier 1 LV (32 EU/t) machine to accept a maximum current of 128 EU/t (MV). Two upgrades will allow the same machine to accept a maximum current of HV (512 EU/t). Finally, three upgrades will allow a simple Macerator to accept Extreme Voltage current, a whopping 2048 EU/t. Putting in a fourth upgrade will not allow a machine to accept higher than 2048 EU/t, such as from a Nuclear Reactor. It will blow up regardless of the upgrades, making four upgrades in one machine completely useless.
Transformer Upgrades do not alter the speed at which machines run.
Machines which support usage of the Transformer Upgrade include:
Since the Induction Furnace cannot accept upgrades, it will only ever be able to accept MV (128 EU/t). However, if you wish to have all of your advanced machines accept power from the same cable, put one Transformer Upgrade into each of your machines and have the input at MV.