~Selecting the correctly designed resonant test set for typically complex testing applications requires a great deal of expertise and a technically experienced application team.  The information below is a general guide for the decisions that will be faced during the selection of an RTS that will meet one's testing requirements.

Resonant  test systems (RTS) have been used worldwide for over half a century for the testing of dominantly capacitive test objects. You may be wondering what is the theoretical concept behind resonance or how does it work.  Below is a basic description behind resonance.
Resonance is achieved when XL (RTS) = XC(test object). When resonance is achieved the supply of the RTS only needs to supply the losses and resistive components of the test set and test object.

Variable Inductance Resonant (VL) Test System:
(Typically for laboratory, OEM, routine Variable inductance (L) resonant systems use a special single winding transformer with a mechanically variable gap drive called an HV reactor powered by single phase, L-N, step-up transformer called the Excitation Transformer.
For XL=XC or 2πFL = 1/(2πFC) , F=Fixed @ 60Hz, C=Capacitance of test object, L=Variable reactor

Variable Frequency Resonant (VF) Test System:
Variable Frequency (F) resonant systems use a special single winding transformer with a fixed inductance called an HV reactor powered by single phase, L-N, step-up transformer called the Excitation Transformer.
For XL=XC or 2πFL = 1/(2πFC) , F=variable (generally 25-400Hz), C=Capacitance of test object, L=Fixed Reactor

Onsite or Laboratory
Many RTS designed will function both as an onsite or lab type system. However variable frequency sets only are available with a cylinder type reactor. This means that clearance needs to be accounted for all around the reactor. Many lower power systems have been especially designed for mobile testing using variable inductance which are mounted in "Sprinter" vans, cargo trucks, skid mounted, or shipping containers. These types of systems take advantage of a tank type reactor with a HV bushing . 
Higher power, higher voltage applications typically use VF systems because VF reactors are smaller and lighter than their VL counterparts. In addition VF reactors can be easily stacked (cascade) or paralleled for a wide variety of voltage, current, and tuning range set-up configurations.