Is this actuator really 12VDC? Could it be 24V?

Running a lower voltage will cause the motor to run slower. The current draw of a motor depends on the resistance of the windings AND the interaction between the windings and internal magnets when the motor is running... When it gets up to speed this interaction will resist a great deal of current.
As such, the 'start-up' current for ANY motor will be high. This can also be called a 'stall current'. This initial spike in current is always high, and can often cause damage if directly connected to less capable drivers or it can blow fast-acting fuses.
Lowing the voltage will lower the current draw, but the stall / startup current will still be high! (Although not as high as if you had used 24V ... )
Using a 24V source on a 12V motor can cause damage to it... remember the startup/stall current is high because the only things resisting the current is the inductance in the windings to help mitigate the immediate in-rush, then the only thing left is the resistance of the coils until the motor begins to move. You know what happens when too much current goes into something... it heats up... and the wire used in the windings can be quite thin depending on the design, so the insulation on the wires melts, or the wires themselves do... This usually leads to part of the coils shorting together which in effect lowers the resistance and inductance of the coil and lets even more current through! Very bad things happen.
As long as you can manage the heat and current, you can often apply more voltage than a motor is rated for.

 

The effect that limits the current in a DC motor is the generated back EMF, which with no load almost reaches the applied voltage, obviously if the generated voltage was equal there would be no current (torque).
When a load is applied the motor, rpm is reduced and hence increasing the voltage difference between applied and the generated increasing the current and hence torque.
The effect of higher voltage is higher RPM which may, or may not be detrimental to the motor, depending on construction and the degree of load.
If you have an RPM rating of the motor, it is easy to determine the rated voltage by measuring rpm at a given voltage.
M.

 

Double posting does not speed up any reply.

 

Manufacturer, part number, and data sheet do.

ak

 

Resistance in-between the power source and the actuator will cause this sluggish behaviour.
The more current passes through the wire, the more voltage gets lost in it. You can compensate by using thicker wire.
Additionally, connector and joints can introduce additional resistance in the line which can also cause this behaviour.

I would suggest providing specs on the motor/actuator, this will greatly help.
If you have a multi-meter, you can measure the voltage on the power supply to ensure it stays at a stable level. Then measure the voltage before and after the extension cord, you may find that the voltage is less on the far side of the extension cord. Just because it's the same gauge means nothing...
Thicker gauges are required for longer distances, better conductors are also required... if you are using a cheap conductor that is weathered or perhaps corroded, it should be replaced as soon as you are able.

 

Please confirm 'where' you tested with Test 2.
The extension cable that was used caused sluggish behaviour, correct? If you measure the 'actuator' side of the extension cable, that will indicate if there is a voltage drop. These measurements should be taken while the actuator is running.

Test 3 was done correctly, I find it odd that the actuator power was reduced so much.

 

My first suggestion would be to try the actuator with a fully charged 12v car battery.
The power supply voltage may sag when the load comes on, current will increase, and supply voltage may not recover.
A car battery can deliver a very high load without the voltage sagging by very much.

You need to measure the voltage at both the supply and the actuator whilst under load.

In Test 3 did you use the multimeter's leads?
This would have put two extra lengths of wire into the circuit, thus increasing the circuit resistance, increasing voltage drop, and slowing the actuator down.

The manufacturer's specs would have been done in a lab under perfect conditions with an oversized power supply connected directly to the actuator.

 

The manufacturers maximum current rating is usually the maximum continuous current, IOW, if you limit the current to 4.8amps with a continuous stalled load, you would not damage the motor, the peak load current is much higher and should only be experienced for a very brief period, otherwise damage may occur.
One of the factors for maximum current will depend on the armature resistance and this should be measured with a current test at stall, a simple resistance check is usually not accurate.
Often too high a current shows an overload condition.
M.

 

This is what I had expected. Glad to see the measurements agree.
The lower voltage under load means that there is resistance in the additional wire and/or connectors that you are using. This higher the current draw of the actuator, the more the voltage will decrease on the far end near the actuator. The solution here would be to ensure you have proper connections, and better extension wire. (Thicker gauge helps, but I'm not sure the quality of the wire... You can measure the resistance of the wire. I'd expect it to be at least about 0.2Ω if the actuator pull up to 8Amps. Lower is always better here.
Poke a little with the resistance measurement of the wire you are using and the connections. You may find a particular length of wire or the connection used is causing the voltage drop. I'd be willing to blame to the 16g wire at the moment.