output of pwm/ speed controller with input from voltage stabilizer
Without load, a PWM speed controller will typically read higher voltage due to parasitic capacitances in the circuit. Here is a simplified schematic of a unidirectional PWM motor speed control:-







simulate this circuit &ndash; Schematic created using CircuitLab

C1 and C2 represent the parasitic capacitances inside D1 and M1. 

In operation, M1 acts as a switch which connects Motor- to ground when turned ON (PWM high), and is open circuit when OFF (PWM low). When M1 is ON the parasitic capacitance is rapidly charged to the supply voltage. However when M1 is OFF the parasitic capacitance can only discharge through the meter, which it does slowly due to the high meter resistance. This 'fills in the gaps' between PWM ON periods and causes the average voltage to be higher than expected.

As well as parasitic capacitance the FET will also have some leakage, which may cause the meter to read some voltage even at 0% PWM.

When you attach a load with much lower resistance (eg. a motor or light bulb) the parasitic capacitance can discharge rapidly and leakage current produces very little voltage across the load, so the average voltage is closer to what you would expect. 

Also since the motor is drawing relatively high current your power supply may be putting out a lower voltage. A '12V' unregulated power supply typically produces as much as 17V when unloaded because the peak voltage of rectified AC is 1.4 times the rms voltage. As current draw approaches the power supply's rated value the voltage drops due to losses in the transformer, rectifier diodes and filter capacitor. This may explain why you see up to 17V unloaded, but only 15V with the motor connected.