I took the fuel injectors off of a wrecked Volvo 850 GLT 1996. I was testing them to see if they still worked by simply supplying voltage to them, and they do. I noticed that when I increase or decrease the voltage, the pulsing sound coming from them doesn't seem to change. Are fuel injectors designed to provide a constant flow of fuel no matter what the voltage? And the ECU just controls how long voltage is supplied to them? What I mean is, is the injector like a DC motor, speed increases with voltage, or is the "speed" constant?
The injector has a plunger/pintle which is controlled by an electromagnet. The humming you heard is from that. The plunger/pintle has a spring which pushes back against the electromagnet so when the power is shut off, the plunger will close and stop the flow of fuel. This causes the hum. Here is an example of how an injector is built:
When the power is turned on, the electromagnet attracts the (in this picture it's labeled "magnet") plunger/pintle which opens the flow. The computer energizes the electromagnet for as long as needed, according to the fuel map to achieve the amount of fuel output. An electromagnet can energize with a small amount of current, but will be stronger with more. Fuel output is a factor of three different things. Fuel pressure, size of the injector (how much it can flow for a period of time, usually at a given pressure), and the length of time an injector is left open.
When testing old injectors, while testing the electromagnet is part of that, the bigger part is flow, flow consistency across all injectors, and checking for leakage. This is usually done on a fuel injector flow bench. This is specialized equipment.