Transistor as Switch for 1.5A dc motor
A BJT transistor is a current amplifier. The current Vbe is multiplied by a constant (hFE, which often varies from piece to piece but is large, say 500), and up to that amount of current is allowed to flow in the direction Vce (if available).

When people say it's a current amplifier what they mean, in more intuitive voltage and resistance terms, is that the resistance varies such that the current matches that value.

There is a constant(-ish) drop at Vbe of 0.6V, (and much less at Vce). Consider the circuit from the supply, through the base, and through the emitter to ground, which is controlling the rest of the circuit. You have 0.6V drop in the transistor, and the rest of the voltage is determined by the resistor. So calculate the current through the resistor using Ohm's law from the supply voltage minus the 0.6V. Multiply that by a known or nominal hFE, and that's the maximum current which can flow through Vce. The transistor is only part on, and limited to that Vce.

In the unlikely event that this isn't a homework problem you need a higher base current (easy), a darlington (easy), or a transistor with a very high hFE (hard to source, silly for an application like this). A darlington is a pair of transistors where the base current for the second comes from the first. They're often chunky and have attachments for heatsinks and you can probably find one lying around. They switch more cleanly than a single BJT, so that's what I'd use. I'd also recommend a flywheel diode across the motor.