Series-Wound DC Motor (Series-Wound)
A DC motor with its field winding in series with the armature. Torque at start is very high; off load the speed runs away and polarity will not reverse it.
A series-wound DC motor is the type of DC motor whose field winding is connected in series with the armature. Because the same current feeds both the field and the armature it produces very high torque at starting, but against that the speed can rise dangerously when the load comes off. You meet it in starter motors, winches and some lifting mechanisms; it does not behave the way you expect with a standard DC driver.
Why the torque is this high
The torque grows roughly with the square of the current; that is why it excels at lifting a heavy load, but the current climbs fast as well. The current drawn at the moment of starting can typically reach 5–8 times the rated value, and this pulse lasts 0.5–3 seconds depending on the load. How many times over the starting current will be varies with the inertia and the friction of the load; do not make assumptions without measuring.
Sizing the driver
- The choice is not made on the average current: It is made on the starting pulse. If the peak current window of the driver does not cover that pulse the protection trips or the power stage is strained; read the distinction between continuous current and peak current from the board data.
- The length of the window matters as much as the value: The 45 A peak of the KS250 is for a window of 5 seconds, the 40 A maximum of the PT500 for a window of 15 seconds. On heavy loads that start slowly this difference in time becomes more decisive than the peak value.
- Running away off load: If the load comes off completely the field weakens and the speed can rise uncontrolled. Motors of this type are generally run coupled directly to the load; transmissions where the load can suddenly come off, such as a broken belt or a released clutch, are not suitable.
Why reversing is different
Reversing the supply polarity turns the field current and the armature current round together, so the direction does not change. To reverse, the terminals of only one of the field or the armature connections must be swapped. For this reason a standard H-bridge output does not give the expected direction behaviour on a series-wound motor; reversing with a relay or contactor group is done with a separate arrangement that cross-connects the field terminals. In that arrangement electrical and mechanical interlocking between the two directions is compulsory; two contactors pulling in at the same time means a short circuit.
Where it is not suitable
It is a poor choice for applications that want speed control: at the same PWM duty the speed shifts noticeably as the load changes, because the field changes with the current too. It is not preferred in mechanisms that have to hold their position either. For work of that kind a permanent magnet motor, whose field is fixed, is both more predictable and far simpler on the driver side.
A common mistake: trying it off load on the bench
Running a new motor on the bench without coupling it to the load and watching it is harmless on other types; on a series-wound motor it is not. With no load the speed climbs fast and on some bodies it goes as far as mechanical damage. If you are going to try it, turn the motor at least through a friction load or through its own transmission, and run it briefly while watching the current.
What to do
Verify the type of the motor from the number of terminals in the connection box: on a series-wound motor the field and armature terminals come out separately, while on a permanent magnet motor there are only two terminals. If reversing is needed, build the arrangement not on an H-bridge but on an interlocked contactor group, and size it after measuring the starting current with the real load.
The context in which this term is used: DC Motor Driver Selection Guide