Control Techniques / Mentor II DC drives

A DC drive controls two circuits, not one. The armature sets the speed and the field sets the flux, and a machine that will not run is as likely to have lost its field as its drive. Check the field before ordering anything.

Mentor II is the Control Techniques DC drive and it has outlasted almost everything of its generation. It is still running winders, extruders, cranes, test rigs and paper and timber machinery across New Zealand.

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The model number is the current rating

Unusually direct, and useful.

The number is amps of armature current, not kilowatts. So sizing a replacement starts from the motor's armature current and its armature voltage, not from a kW figure.

Send the full designation including any suffix, because the suffix carries the quadrant arrangement and that is the next section.

Two quadrant against four quadrant

The most important distinction on any DC drive, and the one that produces the most expensive wrong order.

A two quadrant drive fitted where a four quadrant one was cannot hold back a descending load, cannot brake a winder and cannot reverse electronically. On a crane hoist or an unwinder that is not an inconvenience, it is a safety and process problem.

The reverse substitution generally works and costs more than it needed to.

So the quadrant arrangement is not optional information. Same warning as the 1Q against 4Q issue on our Siemens SIMOREG page. Tell us what the machine does with the load, and send the whole model designation.

The field is a separate circuit

The single most useful troubleshooting content on this page, because it redirects a lot of unnecessary drive orders.

A DC motor needs both an armature supply and a field supply. The armature carries the current that produces torque and the field produces the magnetic flux it works against.

A motor that runs away when it should be holding speed is a field problem, not a speed loop problem. Weakening the field raises speed, so losing field at speed is dangerous on a machine with inertia.

On larger installations the field has its own controller, such as an FXM5 field controller, mounted separately. That is a distinct part with its own fuses and its own failure modes, and it is frequently missed on a parts list.

So before ordering a drive: check the field fuses, measure the field winding, and establish whether there is a separate field controller. That sequence resolves a good proportion of Mentor II call-outs without a drive.

Semiconductor fuses are part of the drive

Not an accessory, and matched to the unit.

A thyristor converter needs high rupturing capacity semiconductor fuses to protect the bridge. Ordinary fuses do not act fast enough to save a thyristor.

If a Mentor II has failed hard, assume the fuses went with it. Tell us the drive rating and we will quote the correct fuses alongside.

A failed thyristor looks like a mechanical fault

Worth recognising, because the symptom sends people to the motor.

A DC drive fires six thyristors in sequence. If one fails, the armature current becomes uneven and the motor runs rough with high ripple.

The motor tests fine, because it is not the motor. Uneven firing is an electrical fault in the bridge or its gate drive.

And a commutator fault looks like a drive fault

The mirror image, and just as common on machinery of this age.

DC motors have brushes and a commutator, and both wear. A worn commutator, glazed or grooved surface, or brushes worn down or sticking in their holders will cause sparking, flashover and overcurrent trips.

So on a Mentor II overcurrent trip, the motor is as good a suspect as the drive. Lifting the inspection cover and looking at the brushes takes minutes and it is free.

That is worth saying plainly, because a drive is the expensive answer and brushes are the cheap one.

Speed feedback, and which type you have

Changing it is a parameter and a wiring change, and it affects how well the machine holds speed.

If there is no feedback device on the motor, the drive is on armature voltage feedback and that is correct. It holds speed less tightly, especially at low speed and under changing load.

A tachogenerator is a wear item in its own right. It has brushes too. A machine that has become unstable at low speed, or hunts, may have a failing tacho rather than a failing drive.

Tell us which feedback the machine uses. A replacement drive set for the wrong one will not control properly.

The MD29 coprocessor

The same trap as SM-Applications on the Unidrive SP, one generation earlier.

An MD29 module fits the Mentor II and runs its own program, so winding maths, tension control, line coordination and sequencing can live in the module rather than in a PLC.

So if there is an MD29 in the drive, it is the part that matters. Take it out before the old unit leaves the site, and tell us it is there so we quote the right thing.

Source code for these programs is often long lost, which is exactly why the physical module is valuable.

No DC bus capacitors, so no reforming

A genuine difference from every AC drive page on this site, and worth knowing if you hold spares.

Mentor II is a thyristor converter, not an inverter. It rectifies the mains to a controlled DC output and has no large DC link capacitor bank.

So a Mentor II is a much better long term shelf spare than an AC drive. It sits in a dry store for years and comes out ready to work.

That is a real advantage for a plant that wants to secure its position on an obsolete platform, and it is one of the reasons we recommend buying the spare while supply exists.

Why plants keep DC rather than converting

Worth laying out, because it is the honest comparison.

A DC machine gives full torque at zero speed and holds it, which is why these machines were built this way. On a winder, an extruder or a hoist that characteristic is the point.

So keeping the Mentor II running is frequently far cheaper than converting, and the drive is the least expensive component in the whole arrangement.

Mentor MP is the successor if you do want to stay DC and move forward. That is a smaller step than going to AC, and it keeps the motor.

Before ordering a replacement

What to send us

The full model designation. The number is the armature current and the suffix carries the arrangement.

Whether it is two quadrant or four quadrant, or what the machine does with the load if you are not sure.

The motor armature voltage and current, and the field voltage and current. Off the motor nameplate.

Whether there is a separate field controller.

The speed feedback type. Armature voltage, tacho or encoder.

Whether there is an MD29 module fitted.

Whether you need the semiconductor fuses.

A photo of the drive and the motor nameplate.

What we can supply

Drives. Mentor II across the current ratings, in two quadrant and four quadrant arrangements. New old stock, surplus and tested used.

Field controllers. Including FXM5 units for larger installations.

Modules. MD29 coprocessors and communication options.

Fuses. Semiconductor fuses matched to the drive rating.

Parts. Cooling fans on the larger frames, and control boards where they are available separately.

The successor. Mentor MP, if you want to stay DC and move generation.

A sensible Mentor II spares holding

Brushes and fuses are the two nobody holds and everybody needs. Between them they account for a large share of Mentor II downtime and neither costs much.

We buy Mentor II

Mentor II holds real value, because plants running DC machinery cannot buy new and converting to AC means a new motor.

Keep MD29 modules in the drive, keep field controllers with the drive they served, and keep unblown semiconductor fuses. All three add value. See Sell To Us.

Common questions

Can you get a part that is not listed on your site?
Usually. The site shows what we hold, not what we can source.

Is M105 a kilowatt rating?
No. It is 105 A of armature current. Sizing starts from the motor nameplate.

My motor will not run and the drive shows a fault.
Check the field circuit and its fuses before anything else. A lost field stops the motor and the drive is often fine.

The motor runs rough.
Suspect a thyristor or the firing circuit. The motor will test fine because it is not the motor.

It keeps tripping on overcurrent.
Look at the brushes and the commutator first. That is free and it is frequently the answer.

Should I convert to AC?
Sometimes. It means a new motor and mechanical work, so on a machine with years left in it a spare drive is usually far cheaper. We can quote both.

Do stored Mentor II drives go off?
Much less than AC drives. There is no DC link capacitor bank to reform, so they store well.

Do you supply used equipment?
Yes. On Mentor II it is usually the only option, and units are tested before they ship.

What is the lead time?
Ex stock items ship the next working day within New Zealand. We give you a firm lead time with the quote.

Do you quote in NZD?
Yes, in NZD including GST for New Zealand customers.

Related

Control Techniques overview and Unidrive SP, if the machine is AC.

For DC drives from other brands see Siemens SIMOREG DC MASTER and ABB DCS.

Send us your model numbers

Send the motor nameplate as well as the drive. On a DC machine the field circuit is half the answer, and it is the half people leave out. Quotes in NZD including GST.

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Industech is an independent supplier of new, surplus and used industrial automation equipment. We are not an authorised distributor of, approved by, sanctioned by, or affiliated with Nidec Control Techniques or Emerson. Control Techniques, Mentor and Mentor MP are trademarks of Nidec Control Techniques Ltd. All trademarks, brand names and model numbers are the property of their respective owners and are used here for identification purposes only.