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View Full Version : How to size a DC choke?



Peter.
05-04-2013, 11:51 AM
I have a Reliance 3HP DC motor powered by a Parker SSD drive for the armature and a phase-angle device & bridge rectifier for the field. Field uses about an amp @115v and the armature up to 12a @ up to 230V, though currently it's capped at 190vdc.

Powering the drive makes the field winding hum slightly and I asked the Parker tech about the feasability of putting a capacitor across the windings to get rid of the hum, it's not really intrusive but gets quite a bit louder when there's a load on it.

The tech put the question to someone else his end and then came back and said I could but I would be better off putting a choke on it to add some inductance and change the form factor.

The manual states:


The controller is designed for armature current form factor of 1.5 or less. It is recommended
that a armature choke be fitted where a formfactor of less than 1.5 current cannot be
guaranteed.

Now these are alien terms to me so I don't know where to go by way of sizing the choke required. From research it's my understanding that a choke is simply some turns of insulated wire around a ferrite ring - so do I just get the correct-sized ferrite ring and simply pass the armature wires through it for a number of turns? If so - how do I go about selecting the proper size ring and the best number of turns?

MaxHeadRoom
05-04-2013, 12:17 PM
IMO, You would put the inductance in series with the Field to use it as a filter.
Better, I would look at providing a fixed capacitor smoothed DC for the field, cap smoothed rectified 120ac will take you a bit higher, but you could add a series resistor. to end up with the correct field current.
A choke is usually used in the armature circuit for impedance match for low impedance motors.
The reply they gave you mentions the armature not the field?
Max.

chipmaker4130
05-04-2013, 12:38 PM
All I have to offer is that when I was trying to silence a 2HP Leeson DC motor (20A max @ 180VDC) the only thing I could find that I could afford was the transformer from an old 20A battery charger. I used the secondary as the series choke and shorted the primary. It works very well, but its heavy!

MaxHeadRoom
05-04-2013, 12:43 PM
BTW, NEVER put a choke across the field winding if the drive is fitted with field-loss detection otherwise it defeats the field loss circuit.
The armature choke will not affect the field, if that is what you are trying to silence.
Max.

Peter.
05-04-2013, 12:47 PM
IMO, You would put the inductance in series with the Field to use it as a filter.
Better, I would look at providing a fixed capacitor smoothed DC for the field, cap smoothed rectified 120ac will take you a bit higher, but you could add a series resistor. to end up with the correct field current.
A choke is usually used in the armature circuit for impedance match for low impedance motors.
The reply they gave you mentions the armature not the field?
Max.

My query was two-fold, whetether it would harm the drive if I used capacitors to smooth out the field winding hum and whether I could raise the DC voltage on the armature using a capacitor, also smoothing out the hum on that. His reply wasn't specific to either and I didn't press him because he had to go and take advice himself.

My understanding is that the thyristor drive switches on and off at different angles on the phase waveform giving a variable-DC output.

Forrest Addy
05-04-2013, 12:51 PM
I assume you refer to a full wave SCR or PWM electronic drive. A half wve drives like a router SCR speed control are noisy as hell.

You dont want a filter in the armature circuit. Ideally you want the motor RPM to be constant regardless of load. A DC motor's RPM drops a percentage in rough proportion to load. An electronic drive uses EMF feedback from the armature to adjust its respose to load. If drive and motor are matched and tweaked to optimize performance the motor's speed regulation can be held to a small percent over a wide RPM ranfe.

Chokes and capacitors in the armature circuit masks feedback signals preventing the control from responding to motor load. You either have to live with a little hum from the motor, change out the drive to PRM technology (these may squeal depending on carrier frequency,) change out to a polyphase drive, or abandon electronic drive entirely and go to a variac and full wave bridge rectifier with an L/C flter. The variac bridge solution results in a full load speed droop of 7% Vs 2% with an optimized SCR drive. Regardless there will be some motor noise. It depends on your 10% budget and the motor noise you can tolerate.

If you can hear hum from the field you can insert a suitable filter capacitor without compormizing the motor performance. Filter in the armature corcuit of an electronic drive carries a speed regulation penalty.

MaxHeadRoom
05-04-2013, 01:02 PM
You should be able to use a capacitor on the field, if you find you rpm has dropped lower than is preferable, you would need to add a series resistor to bring the current down to what it was on the previous un-smoothed supply.
The up side is your torque will have increased!
It is common to add a series armature choke on SCR drives when low inductance motors are used, not so prevelent on PWM due to the higher switching rate.
You will not harm the drive with a capacitor on the field, assuming it is a simple bridge as mentioned in the first post.
Max.

J Tiers
05-04-2013, 01:42 PM
BOTH the field and the armature have some inductance already, they are a coil on an iron core. Generally the field has more, since there are usually more turns on it.

Torque in a DC motor is generally proportional to armature current. However, when the armature has too little inductance, as the drive tech's response implies,. the current is not well "integrated", especially at low speeds. The current tends to be "choppy", since the "on time" of the SCRs is low, and the circuit has "discontinuous conduction" (current goes to zero between pulses).

Adding armature inductance can help to smooth out the current, and make it more proportional to SCR "on" time, as it should be. Too much inductance will lead to problems, yes, but so does too little.
.
.

In the case of the field, which seems to be your concern, the inductance usually is enough to ensure more-or-less continuous conduction (current never goes to, or usually even near, zero). If the inductance is too little, the field current will vary a lot, (large "current ripple") and you may hear that as a hum.

Adding inductance can smooth it, as will adding a capacitor. Both make field current more continuous, in different ways. That raises the average field current, and the field strength, increasing the torque per ampere of armature current. (it also raises the back EMF and lowers the "base speed" of the motor).

The capacitor is possible in any drive where the field is given a constant voltage. If the drive does field weakening, a capacitor should generally not be used, and you would not want to use one on the armature in any case..

chipmaker4130
05-04-2013, 02:07 PM
The improvised choke I mentioned above in post 3 is in series with the armature. It reduced the perceived hum/buzz by 70% and had no effect on speed control. The controller is a Minarik RG500UA which has an option for tach-generator reference but I'm using armature feedback. My motor, however, is a PM type.

Peter.
05-04-2013, 02:09 PM
It's the field hum that I initially noticed but it's not intrusive with no load - I've heard louder refrigerators. When I spin up the motor and put a load on it by holding the flat-belt pulley with my hands - then it becomes much louder.

Perhaps I should be a little more clear as to how I have this rigged. The SSD drive is a thyristor drive and powers only the armature. It does have armature feedback for speed stability, also with the option of tachogenerator feedback.

The field I am using a 10A phase-angle chip and bridge rectifier with a 250k pot to regulate DC voltage from 115V to 45V for field-weakening. This all works well enough here on the floor but I thought I could maybe quieten it down a bit.

DICKEYBIRD
05-04-2013, 02:11 PM
The DC motor on my lathe conversion was very noisy (loud hum, worse under load) and the fix turned out to be switching over from a bolted-solid-to-the-frame motor to a mounting system with 4 rubber lord mounts. The belt tension of course tweaked the motor out of line a bit so I added a rubber block on the pulley end to stop excessive movement. It's whisper quiet now with no electronic wizardry needed.

Mine is a Baldor PM motor/KB speed control so this info might not apply to your wound field motor. I thought I'd pass it along anyway as I tried the KB "official" choke with no effect at all. The soft mounting absolutely killed the noise.

MaxHeadRoom
05-04-2013, 02:52 PM
OK so you do have field weakening/control, from the sound of it it is armature noise you can hear, there is generally not enough field current to cause that much of a buzz at 100Hz.
Placing a 10f - 40f capacitor across the DC out should eliminate the field as the problem, the capacitor will slow the response of the pot, but that should be inconsequential for the field.
If you use an inductor/choke in the armature circuit it has to be in series, the inductance depends a lot on the present inductance of the armature, you could experiment if you have some transformers with very low voltage - high current secondary and just the secondary winding.
Or wind on a coil on a iron bar using at least 18g wire.
Max.

J Tiers
05-04-2013, 06:53 PM
Field-caused noise is usually due to the armature reacting to the field changes, not so much direct noise from the field.

I don't know that I would suggest putting much, if any, capacitor across the field, since that stresses the field controller, which is stated to be a phase angle control similar to the thyristor armature control. it causes an inrush current to be added to the thyristor current, which is normally controlled by the inductance to not have a sharp inrush. It also can cause problems with the thyristors commutating off at a consistent point.

Peter.
05-05-2013, 06:48 AM
Well I have learned several new things from this thread. Thanks to everyone who has replied - I shall do some testing and report back.