Linear Drive
A method in which the power transistor is operated in the intermediate region, like an adjustable resistor, instead of fully on or fully off, and thereby lowers the motor voltage. Since the excess voltage turns into heat, driving a motor that draws 10 A at 24 V at half speed produces about 120 W of heat.
In linear drive the transistor conducts partially, in proportion to the input signal. The motor sees a smooth DC voltage; there is no switching and therefore no switching noise. The control is simple and the current ripple is negligible.
The problem is the heat budget. The product of the voltage dropped across the transistor and the current flowing through it is directly the lost power. If you want to give the motor 12 V from a 24 V supply, the remaining 12 V is dropped across the transistor; at 10 A that means 120 W of heat. A chopper driver does the same job with a few watts of loss.
That is why linear drive has in practice been abandoned in power applications; you meet it only in special measurement setups with very low current or where noise is absolutely unacceptable. Today the standard approach in motor control is PWM-based chopper drive. The same logic applies to attempts at reducing speed with a resistor: a series resistor does not destroy the energy, it turns it into heat, and as the load changes the speed changes too.
The context in which this term is used: DC Motor Driver Selection Guide