Chopper Drive
The driver type that sets the average armature voltage by rapidly chopping a fixed supply and applying it to the motor as pulses; its losses are low.
A chopper drive is the type of driver that sets the average armature voltage by rapidly interrupting a fixed supply voltage and applying it to the motor as pulses. Because the switch is either fully on or fully off its losses are low; because the voltage can be raised starting from zero, the start is naturally soft. Electronic DC motor driver modules work predominantly on this principle.
How the average voltage is set
The name chopper comes from its chopping a continuous voltage along the time axis and so changing its average. As the width of the applied pulses grows, the average voltage the motor sees rises and the speed increases. The motor winding behaves like a coil and filters these pulses into a relatively smooth current; the motor sees the average, not the pulses one by one. Why this method is superior to dropping the voltage has to do directly with efficiency. How far the current falls between pulses depends on the inductance of the motor and on the switching frequency; if the inductance is small or the frequency low, the current ripple grows and the motor does the same job while heating up more.
Where the efficiency advantage comes from
While the switch is off no current passes through it, and while it is on almost no voltage is left across it. In both states the product of voltage and current is small, so the loss occurs only in the channel resistance and during the transitions. If you tried to make the same drop in speed with a series resistor or with linear drive, the voltage dropped would turn straight into heat; burning half the energy of a motor running at half speed in a resistor is both inefficient and creates a cooling problem.
Starting behaviour and the ramp
The second natural benefit of the chopper arrangement is at starting. Because the voltage can be raised starting from zero the current pulse stays limited; that is why a separate soft starter is generally not needed for DC drivers. On modules whose ramp time can be set, this behaviour is opened up to user control: on the PT500 the RMP trimmer sets the ramp and the HIZ trimmer sets the target speed. How far the ramp should be set depends on the inertia of the load and on the backlash in the mechanical chain.
The trade-off in choosing the frequency
| Frequency | Gain | Price |
|---|---|---|
| Low | Switching loss and heat fall | Audible hum, current ripple increases |
| High | The motor grows quieter, the current is smoother | Switching loss and heating increase |
The right point varies with the inductance of the motor and with the noise tolerance of the application; how the frequency is chosen cannot be reduced to a single number.
What the chopper does not solve
- It gives no guarantee of constant speed. Without feedback the speed falls when the load rises; the chopper holds only the average voltage constant, not the speed.
- Torque can fall at low speed. How reducing the speed affects the torque depends on the character of the load; in lifting applications this distinction is critical.
- Cooling weakens. On motors cooled by their own fan, low speed means the cooling drops as well; running slowly for a long time heats the winding.
- It produces noise. Fast switching is the source of the interference that spreads along the motor cable; that is why the rules on twisting and separating cables apply.
What you have to do in practice is this: set the ramp long enough not to strain the load at the start and short enough not to spoil the work rate, balance the frequency between heat and noise, then run the system under load and check the driver temperature by hand. If speed stability is critical, do not look for the answer in the chopper settings; what is needed is to add feedback or to move to a module with feedback.
The context in which this term is used: DC Motor Driver Selection Guide