Permanent Magnet DC Motor (PMDC)
A brushed DC motor whose stator field comes from magnets. Speed varies with voltage, torque with current; open-loop PWM suffices in most applications.
A permanent magnet DC motor (PMDC) is the type of brushed DC motor that produces the stator field with permanent magnets instead of a winding. Because the field is fixed, the speed varies almost linearly with the applied voltage and the torque with the armature current. This is the type met most often in automatic door, actuator and small winch applications, and it is driven without trouble by a standard H-bridge output.
Two constants sum up its behaviour
Because there is no field current, its behaviour can be summed up by two constants: the back-EMF constant, which gives the voltage generated per unit of speed, and the torque constant, which gives the torque per ampere. That is why, when you halve the average voltage with PWM, the no-load speed roughly halves as well; the torque is preserved as long as the current stays the same.
Its practical consequences:
- Open-loop speed setting is predictable enough; in most applications no encoder is needed.
- When the rotor stops the speed goes to zero and so does the back EMF, and the current is limited only by the winding resistance; the stall current rises very high.
- While slowing down the motor behaves like a generator and pushes energy back into the supply line.
The moment that strains the driver
On a PMDC the moment that strains the driver is not the moment the motor turns but the moment it cannot. When the load jams or comes up against a mechanical limit the back EMF disappears and the current climbs to the level that the winding resistance alone allows. That is why the choice is made on the basis of current-threshold cut-out and the peak current window alongside the continuous current; the current measured off load says nothing about the choice.
The difference from the series-wound type
| Property | Permanent magnet | Series wound |
|---|---|---|
| Starting torque | Linear with the current | With the square of the current; very high |
| Reversing | Reversing the supply polarity is enough | Only the field or the armature terminals are swapped |
| If the load comes off | The speed stays limited by the voltage | The speed can rise uncontrolled |
| Speed setting with PWM | Predictable | Very dependent on the load |
A mistake often made in the field
On 12 V and 24 V systems the same motor body can be produced with different windings; always verify the rated voltage on the label. Running a 12 V motor on a 24 V supply by cutting the speed at the driver is not a safe method as regards heat and brush life: although the average voltage halves, the current ripple grows, the wear between brush and commutator speeds up, and the driver going to full duty for any reason applies the full voltage straight to the motor.
Where PMDC is not enough
Because the field is fixed there is no easy way of taking the speed above rated; the field weakening method used on wound motors is not valid here. It is not the natural choice for winch and starter type work that wants very high starting torque either. Brushes and commutator are wearing parts: in a dusty environment and in mechanisms that reverse often the maintenance interval shortens and the wear dust spoils the insulation. On top of that the brush spark produces electrical noise; if there is a sensitive receiver or sensor in the same cabinet, suppressing that noise is a separate item of work.
What to do
Match the voltage on the motor label with the supply voltage, choose the driver by the continuous current measured under load, and on a mechanism that comes up against a mechanical limit always bring current-threshold cut-out into service. Make the measurement not off load but with the real load and after the mechanism has been fully assembled.
The context in which this term is used: DC Motor Driver Selection Guide