Ask three people for a "DC servo motor" and you will get three different parts. One means a small brushed motor with a gearbox and an encoder. One means a brushless motor running on a DC bus. One means a servo motor with an AC drive that happens to be fed from a DC link.
All three get ordered under the same words, and that is how the sizing goes wrong before anyone measures anything. The motor arrives, the driver does not match it, and the torque figure on the datasheet turns out to belong to one of the other two.
This is how we sort out which one you actually need, and the numbers that decide the size.
Three different parts share the name
A brushed DC servo motor is the one most people picture: permanent magnets, a commutator, two wires. Feed it voltage and it turns, reverse the polarity and it turns the other way. Add an encoder and a controller that reads it and it becomes a servo. These are cheap, simple and fine for light axes and positioning jobs.
A brushless DC servo motor has no commutator and needs three-phase drive electronics instead. Same idea, more torque per volume, longer life, more expensive. If your control cabinet already has a servo drive, this is what is on the other end of it.
And then there is the case where what you have is an AC servo motor on a DC bus - the drive takes DC in and generates the three-phase waveform itself. The motor is not a DC motor at all, but the label on the machine says DC because that is what feeds the cabinet.
It matters which one you have because the sizing rules are not identical and the drivers are definitely not interchangeable. If you are ordering a replacement, the drive model number tells us more than the motor label does.
Continuous torque is the number that sizes the motor
Datasheets lead with peak torque because it is the bigger number. Peak torque is what the motor can deliver for a second or two before it overheats. Continuous torque is what it can hold indefinitely, and it is the only figure that matters for an axis that runs all day.
Sizing on peak torque is the most common mistake in this whole category. A motor rated 3 N·m peak and 1 N·m continuous will happily move a load it cannot hold for a full shift, and the failure shows up as a motor that runs hot and slowly loses position rather than one that stops dead.
| What to record | Why it decides the motor |
|---|---|
| Continuous torque at the axis | Sets the motor size. Include the friction and the cutting force, not just the moving mass |
| Peak torque and how long it lasts | Acceleration only. If the peak lasts longer than a second, size on it instead |
| Top speed at the axis | Rules out high-torque low-speed motors once you add the reduction ratio |
| Encoder resolution needed | Follows from your positioning tolerance, not from the motor |
| Duty cycle | A motor that is fine on an intermittent axis is not fine on a continuous one |
We size brushless servo motors in N·m for exactly this reason. Horsepower and watts are useful for comparing motors of the same type, and close to useless when you are trying to work out whether one will hold a gantry against a cutter.
12 V, 24 V, or 48 V
On small brushed motors the supply voltage is often fixed by whatever the machine already has - 12 V on a vehicle or a small bench tool, 24 V on industrial controls, 48 V when there is room for it. Changing it later means changing the power supply and usually the driver too.
Voltage does not set torque. It sets how fast the motor can reach its rated current, which is why a 24 V brushed motor feels noticeably stronger at speed than the same motor on 12 V. On a screw-driven axis that difference shows up as usable rapid speed rather than as holding force.
If you are choosing rather than replacing, 24 V is the sensible default: enough headroom for speed, still inside the range where drivers and power supplies are cheap and available. Go to 48 V when the axis needs to move quickly and the payload is light.
The encoder is what makes it a servo
A DC motor with no feedback is a motor, not a servo. What turns it into one is the encoder, and there are two things to get right about it: resolution and type.
Resolution needs to come from the positioning tolerance you actually need, working backwards through the reduction ratio. A common mistake is to buy the highest count encoder available and then discover the controller cannot read it at the axis speed - the pulse rate runs out of input bandwidth somewhere around the rapid moves.
Type is the quieter problem. Incremental encoders tell the controller how far it has moved, so the machine has to home on every power-up. Absolute encoders know where they are, which matters on a machine that restarts mid-job or has an axis you cannot easily re-home.
Two numbers that prevent most mismatches.
The drive's continuous output current, and the motor's continuous current at the duty cycle you run. If the motor wants more than the drive can give continuously, the axis will hold position on the bench and lose it under load - and no parameter will fix it.
Matching the motor to its driver
The motor and the driver have to agree on three things: current rating, voltage, and feedback format. Two out of three is not enough, and the failure mode of a two-out-of-three match is intermittent rather than obvious, which makes it expensive to diagnose on a machine that is already in production.
Current is the one people skip. A driver rated below the motor's continuous current will current-limit during cutting and let the axis drift; a driver rated far above it will not protect the motor from a stall. Match continuous to continuous, then check that the peak figures are in the same league.
Feedback format is the second. Incremental and absolute encoders are not interchangeable at the driver, and neither are the different serial protocols used by absolute types. If you are pairing a new motor with an existing drive, send us the drive model as well as the motor details - that is usually where the answer is.
What to send us
Four things get you a firm recommendation: the continuous torque and speed you need at the axis, the reduction ratio if there is a gearbox, the supply voltage in the cabinet, and the positioning tolerance. Add the drive model if you are replacing a motor rather than building a new axis.
We keep servo motors and matching drivers as pairs, with the current and feedback settings made for the motor each one ships with. The 24 V DC servo motors cover most small positioning axes, and the 12 V range suits low-voltage machinery and retrofit work. Send the axis details and we will size it before anything leaves the factory.
If you are not sure which of the three types you need, send a photo of the motor and the drive it is wired to. The drive model usually settles it in one look.
We supply DC servo motors, drivers and matched pairs from a factory that builds both ends - send the details through here.
