Key takeaways
- A drive busbar links the rectifier, the DC link and the inverter, so it carries a pulsed current as well as a continuous one.
- Low inductance in the DC link protects the switching devices and reduces the need for extra clamping.
- A compact layout saves cabinet space but makes clearance, cooling and service access harder.
- The termination and the screen decide the EMC result as much as the conductor shape does.
Where the busbar sits in a drive
A variable speed drive has three power stages: a rectifier that turns AC into DC, a DC link that stores energy in a capacitor bank, and an inverter that turns DC back into a controlled AC output. The busbar connects those stages. It carries the rectifier output to the capacitors, the capacitor current to the inverter and the inverter output to the motor terminals.
The current in each section is different. The rectifier output has a ripple at the supply frequency and its harmonics. The DC link carries a current that is partly continuous and partly pulsed at the switching frequency. The inverter output is a switched waveform with a high rate of change. A busbar that is sized only for the motor current misses the pulsed component that heats the DC link.
The layout also has to fit inside a cabinet that may hold several drives, a filter, a braking unit and control electronics. Space is tight, and the busbar is often the part that has to thread between the other components. That makes the mechanical route a first order design input, not a detail left to the drafter.
Low inductance in the DC link
The DC link is the loop between the capacitor bank and the switching devices. When a device switches, the current in that loop changes quickly, and the loop inductance produces a voltage overshoot. A low inductance path keeps the overshoot inside the device rating and reduces the stress on the capacitor.
The geometry that gives low inductance is the same as in any switching assembly: a short, wide loop with the outgoing and return conductors close together. A laminated stack with a thin dielectric between the positive and negative layers is the usual solution. Separate bars can work if they are routed close and parallel, but the spacing is then set by the installer rather than by the part.
The capacitor placement matters as much as the bar. A capacitor bank that is spread along the cabinet has a longer path to the devices, and the inductance rises with the length. Grouping the capacitors near the devices and using a wide landing gives a shorter loop and a more even current share.
The loop is the circuit
Draw the commutation loop on the layout before you draw the bar. If the loop crosses the cabinet, the bar cannot fix it.
Compact layouts and the clearance trade-off
A compact drive is a selling point, but the busbar has to keep clearance to the enclosure, the other phases, the control board and the door. The tightest point is often not the bar itself but the bolt head, the washer or the cable lug. A layout that passes on the bench can fail when the door is fitted or when a lug is added.
The phase spacing also affects the inductance and the fault force. Closer phases give a lower inductance but a higher force during a fault. The support and the bar have to take that force, and the clearance has to stay sufficient after the bar moves. A support that is designed only for the weight is not enough.
The service access is part of the layout. A joint that cannot be reached with a torque wrench cannot be checked, and a capacitor that cannot be removed without dismantling the busbar makes maintenance slow. The design should leave a path for the tool and for the replacement part.
- Check clearance at the bolt head, washer and lug, not only at the bar.
- Keep the phase spacing consistent along the run.
- Support the bar against fault force, not just weight.
- Leave wrench access at every joint that needs a torque check.
- Allow the capacitor bank to be serviced without removing the whole stack.
Cooling inside a crowded cabinet
A drive cabinet is a closed space with several heat sources. The busbar adds resistance loss, the joints add contact loss and the devices add the largest share. The air inside the cabinet is often warmer than the room, and the airflow may be directed at the heat sink rather than at the bars.
The busbar can be cooled by the cabinet air, by a cold plate or by conduction into the enclosure. A cold plate gives a direct path but needs a flat mounting face and a thermal interface. Cabinet air is simpler but depends on the airflow reaching the bar and the joints. A bar that is buried behind a capacitor bank may run hotter than a calculation assumes.
The joint is the part to watch. A joint with a high contact resistance runs hot even when the bar is cool, and the heat can spread into the insulation or the control board. A thermal image of the cabinet under load shows the hot joint and the effect of the airflow. It is more useful than a single temperature reading on the bar.
Laminated stack or separate bars
The choice between a laminated stack and separate bars is a balance of inductance, space, cost and change speed. A laminate gives the lowest loop area and the most repeatable build, and it can be tested as one part. Separate bars are easier to change and cheaper at low volume, but the spacing depends on the assembly and the loop is usually larger.
The table below sets out the decision for a drive DC link. The right answer depends on the volume, the switching speed and the space available, not on a general preference for one construction.
| Question | Laminated stack | Separate bars |
|---|---|---|
| Loop inductance | Low and repeatable | Higher and dependent on assembly |
| Space | Compact, fixed stack height | Needs clearance between bars |
| Build time | One part to fit | Several bars and joints |
| Change speed | New laminate for a layout change | Rebend or re-drill a bar |
| Tooling cost | Higher entry cost | Lower entry cost |
| Test | Can be tested as one unit | Test after assembly |
| Best volume | Medium to high | Low to medium |
| Cooling | Uniform, but layers are enclosed | Access to each bar |
Many drives use a laminate for the DC link and separate bars for the output. The two constructions can sit in the same cabinet.
Terminations, screens and EMC
A drive is an EMC source. The switched output has a high rate of change, and the cable to the motor can radiate. The busbar is part of the path. A screen or a return conductor that runs close to the output can reduce the loop area and the emitted field. An earth connection that is long or inductive can make the screen ineffective.
The termination should bond the screen to the enclosure over a wide area, not through a single wire. A cable gland that makes a 360 degree bond is better than a drain wire. The busbar and the enclosure should be bonded at the same point so the fault current has a defined path.
The control board should be kept away from the high dv/dt conductors, or screened from them. A ribbon cable that runs across the busbar can pick up noise even if the busbar is well designed. The layout should separate the power and control zones, and the screen should be bonded at both ends where the design allows.
Testing the assembly
A drive busbar should be checked for continuity, insulation and dimensions, and the assembly should be checked for the loop inductance or the overshoot at the specified current. A double pulse test on a sample gives a direct measure of the switching behaviour. A thermal test under load shows the joint temperature and the effect of the cabinet airflow.
For production, the routine checks can be continuity, insulation, torque marks and a visual check. The switching and thermal tests belong on the first article and on any change to the laminate, the capacitor bank or the device package. A change to the layout can move the loop and the overshoot, so it should trigger a new check.
What to send for a useful quote
Send the drive rating, the switching frequency, the DC link voltage and the capacitor layout. Add the device package, the cabinet drawing and the cooling method. State the ambient inside the cabinet, the fault level and the EMC requirement.
If the drive is part of a family, send the range of ratings. A busbar that is designed for the largest frame may be oversized for the smallest, and a family that shares one stack can save tooling if the current path allows it.
- Drive rating and switching frequency
- DC link voltage and capacitor layout
- Device package and pad geometry
- Cabinet drawing and cooling method
- Ambient inside the cabinet
- Fault level and protection
- EMC and earth requirement
Frequently asked questions
Why does a drive need a low inductance busbar?
The DC link loop carries a current that changes quickly at every switch. A low inductance path reduces the voltage overshoot across the device, which protects it and reduces the extra clamping or snubber needed. The loop area between the capacitor bank and the devices is the main variable.
Can I use separate bars instead of a laminated busbar in a drive?
Yes, especially at low volume or when the layout may change. Separate bars are easier to modify, but the loop is usually larger and the spacing depends on the assembly. A laminate gives a lower and more repeatable inductance and a more compact build.
How do I cool a busbar inside a drive cabinet?
Use the cabinet airflow, a cold plate or conduction into the enclosure. The joint is usually the hottest point, so a thermal image under load is the practical check. A bar that is buried behind other components may not see the airflow that a calculation assumes.
What causes a hot joint in a drive busbar?
A hot joint can come from a rough or dirty contact face, the wrong torque, a washer that has lost its load or a joint that is undersized for the current. Thermal cycling and vibration can loosen it over time. A torque check and a millivolt drop check find it.
Sources and further reading
- IEC 61800-5-1, Adjustable speed electrical power drive systems, safety requirements
- IEC 60664-1, Insulation co-ordination for equipment within low-voltage supply systems
- Qumek drive busbar and cabinet layout reviews
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
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Send the rating, switching frequency, capacitor layout and cabinet drawing. The engineering desk will review the DC link loop, cooling and terminations.
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