Anyone used an Inverter with a Colchester Student

Did you get a larger capacity phase converter from direct drives? I am in a similar position in that I have 2 2 speed motors - 400v 3 phase and am looking at DirectDrives solution - I think you would need the 7.5 hp version to start the motor. The lathes are a Hardinge KL1 and a Boley 5lz. Before I go ahead It would be good to see if anyone else has got their solution to work - and, if there are any special connection / safety requirements.

You should only need an inverter rated at the motor hp - they all have reasonable overload characteristics, and if you do a ramped start (even over half a second), you will avoid a surge situation completely. I don't know if I mentioned it before without reading back through the thread, but there is also a bit of an issue with speed switching. You really need some sort of interlock to prevent you changing the motor speed using the switch whilst it's running; the back emf going from high to low has been known to knock out inverters almost instantly.

The purpose of a FUSE in an electrical power distribution network is to protect the wire conductors i.e. to prevent fires, not to protect the appliance. That is why you should fit 3 or 5 amp fuses in plugs with thin wires such as used for phone chargers and 13 amp fuses ONLY to plugs with thicker wire, such as for the kettle. The same logic applies to the incoming power to your house or workshop. The cross section area of the house wiring determines the fuse size and not what you connect to it.

Not quite sure what a fuse has to do with this, and anyway the situation regarding fuses is a good deal more complicated than that, especially when it comes to allowing for surge currents. It's certainly true that a fuse won't protect the inverter - that can blow itself to pieces way before a fuse would even think about blowing...

Edited By Steve Garnett on 02/05/2012 21:32:12

You can always use your 230V 1ph to 230V 3ph on either winding of the 415V 3ph motor, with the 'base frequency', set down to 29Hz. This as long as the Inverter Drive Current rating matches or exceeds the motor full load Current.

This works well in speed control as far as the 29Hz speed, then the motor torque is reduced towards its 50Hz speed. This is because a 400V Inverter Drive follows the same Volts x Hz curve as far as 29Hz, on its way to 50Hz - so has the same performance as far as 29Hz.

This is a low cost solution, if you only need the 0RPM to 29Hz speeds at full torque.

I don't know if this is relevant to the problem under discussion but many moons ago I bought a Colchester Student from my old college lecturer who had bought it from the college with a view to using it with a phase converter. The problem he encountered was that the Student doesn't have a clutch and so all the power was drawn off in one go when the starting handle was engaged, which tripped the converter.

I've already tried the VSD route in similar circumstances albeit on a Mk 2 Bantam. I took the original two speed motor off and replaced it with a single phase one, but didn't like the crash start stop, so changed it for a 2 HP 4 pole motor with a VSD to match. I couldn't put the original motor back  IIRC because it was a 400 volt motor and didn't have the terminals for the necessary connections for 230 volts. I never imagined (as I have since seen here) diving into the motor and finding the star point connection.

The VSD is configured to give soft start and DC braking to stop; you can't do this if the VSD is supplying other stuff as well as the lathe main motor.

Even though the original motor was 3HP my 2HP works well, except in high speed top gear (2000rpm) when it will only start if the gearbox oil is warm. So 2HP is marginal, but for the limitations of the supply to my shed I'm happy. I run the low range speeds with the inverter giving 50 Hz, and the high range at 100 Hz. I'd love to know if someone has tried running the low range at 25 HZ and the high at 50 Hz because this way the motor should be more efficient and might be man enough to pull the top 2000RPM speed, but it's a laborious experiment to try. The beauty of the frequency based speed change is that no switching of the motor connections to the inverter is needed, and as has been said above this is a no-no.

Looking at the commentary above about RCD's the concept of current balance isn't really the place to start. The internals of an RCD measure the vector sum of the current passing through the device, provided all of the out-flow current and all of the return current is passed through the sensor the total current taking account of direction and magnitude will always be zero. With a single phase device this explanation reduces to the one above about the current in the two wires being balanced, but with a three phase RCD the sum of the currents in all four wires (including the neutral if it is used) always sums to zero. If there is a bypass path around the sensor inside the RCD for the current - supply or return - then the vector sum of the currents is no longer zero, and out pops the trip. So the three phase load doesn't have to be balanced.  "Balanced" means that the magnitude of the current drawn on all three phases is the same.

The inverter may have other ideas, as most won't tolerate gross imbalance in the load applied to them, it gives 'em indigestion .

Pedantically, an Earth Leakage Circuit Breaker is a different animal, and for these purposes is obsolete. However the industry tends to use the two terms interchangeably, but what you need is an RCD (Residual Current Device).

And thanks to Nicholas Farr for picking up the point about a 110 volts site transformer is really a two phase 55 volts source, with the centre tap connected to earth. The concept here is that (according to historical editions of the wiring regs) 55 volts is (nearly) a safe touch voltage. As I remember it, that isn't to say that you can't get a shock off it, and in unfavourable conditions I guess it could be lethal, but it is low enough not to cause involuntary movement. There is still a contradiction in the 17th edition in that 24 volts is deemed to be "Safety extra low voltage" but 55 volts is acceptable on a construction site because the safe touch voltage is 50 volts - as used for calculating earth path impedance criteria.

My recollection is that provided you don't apply enough volts to the skin to break down the skin resistance you would be very unlucky to get enough current to pass though the heart to cause fibrillation starting from a 50 (55!) volt source. The same is not true of cows, IIRC, which is why the regs have special requirements for farm buildings.

To return to the original post, I guess the answer is that the original two speed motor is unsuitable (without a lot of modification) for running on an inverter, the choices are either to go single phase (clonky!) or a single speed motor and corresponding VSD (more money, but a very nice machine results).

HTH Simon

Edited By Simon Williams 3 on 27/04/2016 19:58:30

Edited By Simon Williams 3 on 27/04/2016 20:09:56

Simon, there is no reason why a two speed 400V motor should not be used with an Inverter drive. Many of our Customers do this. This if you can accept the motor run to 2/3 of its normal speed at full torque, then reducing in torque progressively to its 50Hz speed and beyond. A 400V Inverter drive will follow the same VxF curve and therefore perform in exactly the same way as far as 29Hz x 230V on its way to 400V x 50Hz. You will only need to rate the 230V Inverter drive to match or exceed the 400V motor full load Current and not the kW.

I have seen the two speeds switched over while the Inverter Drive was running without a trip occurring, but it is impossible to say if this is ok for all applications. Switching while running is not recommended.

DC braking is untidy. Best to use an Inverter drive that has a brake switch built in and fit an external braking resistor. The is much more control, better braking and less stress on the motor.

The RCD problem is not clear. When an Inverter Drive trips an RCD, it is usually on power up and not in steady state conditions. This means the EMC caps are not charged and the instant of switch on could be at any point in the Supply Voltage waveform.

Single speed motors and Inverter Drives are not always expensive when purchased over the Internet.

Colin - thank you, and I'm very intrigued by your "part speed 400 volt" motor approach, this looks like a very elegant answer to the star/delta problem, and I'm looking forward to trying it out.

"Single speed motors and inverter drives are not always expensive" - I've got caught by initially buying a (Oriental) motor that was plainly not VSD compatible, it's torque just fell off a cliff as I went above or below 50Hz, and buying another motor (Weg) cured the problem. For all I got both motors at good prices it was still a bit of an expensive lesson to learn! Should have asked the experts...

The RCD thing was initially about would an RCD on the supply side of an inverter trip if there was a fault downstream of the electronics. I can't imagine it would. The business about whether the drive would trip the supply RCD was a different thing. Certainly I've got that problem here, and have sidestepped it by feeding my shed from its own source. SWMBO put her foot down with a firm hand when Eastenders went off three nights in a row. AFAIK that's about the initial charge current into the transfer capacitors.

As for DC braking, I've come from an industrial drives environment where the HSE wanted to see DC braking on unguarded or poorly protected rotating spindles such as drilling machines, lathe chucks and the like. For my conversion of my Bantam lathe I found that to give an acceptable stop time with the difference in inertia between a 10 inch 4j chuck at 1000 revs or a 5 inch 3j the braking resistor didn't really do the job.

I've got several small sub kilowatt VSD's which use DC injection to stop the spindle (vert' mill and a pillar drill) - these work a treat but of course the inertia is so much less. For the purposes of this thread the point of the discussion was "don't disconnect the motor from its drive while it's rotating".

Best rgds,

Simon

Simon, The 29Hz cheat works well and we (The Inverter Drive Supermarket) have supplied many Inverter Drives for home use machine tools and vehicle lifts. By not expensive, I mean a 2HP Inverter Drive will usually control a 3HP motor to 2/3 speed as at the 400V Amps, the motor is working to 2/3 Power output.

The RCD should trip, if there is a fault down stream of the electronics. Inside the small Inveters AC mains is fed through a rectifier to the DC link. DC is chopped up usually at 3kHz or 4kHz by the power transistors and pulse width modulated to reform AC at variable frequency and variable Voltage. If there is an earth fault, there is always a path to earth from the Main supply.

What was the value of the brake resistor vs the minimum resistance for the Inverter Drive. It must have been too high a resistance, if the braking was less than motoring torque.

DC Injection braking is ok for low intertia loads to be infrequently stopped, as the braking energy is dumped into the rotor of the motor. Better braking is with a DBR correctly sized, as it is under control of the deceleration ramp in the Inverter Drive. We have supplied fully regenerative Inverter Drives for Lathes, but they were large and must have been stopping very frequently to make the cost of a 'regen' Inverter Drive worth while.

Colin - thanks for the correction over the RCD - yes if there is a path to earth on the load side of the inverter it must upset the balance in the rcd, so it trips. Only if the inverter output was truly isolated from the supply would this not be true, and of course that's not the case. I've just been reading the Telemecanique Application notes for the Altivar range and they agree. Of course.

You've got me thinking now about the braking of my drive. Re-reading the user manual for the drive in question (Ativar 18) I realise that it can do both DC injection and braking resistor, but at this moment my brain is refusing to co-operate in understanding what I've set up. It was a long time ago and the user manual is a bit vague. Or maybe it's me!

I have got a braking resistor connected, and my scribbles in the margin say it should be 68 ohms/36 watts, though it's not entirely clear why except that this appears to be the value taken from TME's catalogue of the proprietary item. But that's irrelevant, because on looking closely I realise that the resistor fitted is actually 6R8. So I'm not too sure if this is doing anything! Checking the resistor confirms this value.

If I'm trying to stop a big inertia, I get an overvoltage error on the display of the drive, which trips out and then the load just spins down. You must be right though, if the drive will accelerate the load it can stop it, but it doesn't. I guess that could be a number of reasons, not least I've cooked the braking circuit. I'll try it with a 68 ohm resistor as soon as I can get one.

I'd imagined that the drive went into current limit on acceleration so it just extended the ramp time, whereas on braking the drive actually aborts. I'm wondering now if the braking resistor is actually doing anything useful! If I measure voltage across the braking resistor during deceleration what sort of voltage am I expecting and for how long?

Said drive is a fairly ancient TME Altivar drive, type ATV18U41M2 (2.2 Kw), motor is 3 ph 4 pole 5.2A FLC 1.1 Kw (oops - I said earlier it was 2 HP, apologies, no wonder it doesn't like the top speed!) connected for 230 v in delta.

Motor rating plate says Hydrovane on it, but that's what I scrapped and kept the motor, I'm fairly sure it's actually Leroy Somer.

Rgds Simon

Simon, It sounds as if you have lost the brake switch in the Inverter Drive. There should be a figure in the product manual for the minimum resistance for the external braking resistor. If a resistor is fitted lower than this value, the braking Current will be too high and the brake switch (brake chopper) will be damaged and left open circuit.

A Bosch drive at this rating from our web site calls for a braking resistor between 65 and 70 Ohms. This means 65 Ohms is the value where maximum Current passes through the switch for its rating and 70 Ohms is the point were maximum braking is still available.

The 6R8 resistor is only 6.8 Ohms and may well have taken your brake switch out.

The DC bus runs at around 300V, the brake chopper threshold will be around 350V to 380V and the overvolttage trip 400V or more? Not sure if the actual figures for an Altivar.

We keep large numbers of brake resistors and we are happy to supply one to you, but it will not provide you with any braking, if the brake switch is open circuit.

Hi im new to the site i see a fellow trying to get a VFD sorted for a colchester student 6" i also own a c.v.a milling machine, Going back to VFD what was the outcome of the type of VFD and did he get it working ?.

gary aka greeves 246