3-Phase Motor PTC

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3-Phase Motor PTC

This topic has 10 replies, 6 voices, and was last updated 12 June 2014 at 07:13 by frank brown.

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1 June 2014 at 14:30 #31749
Neil Wyatt
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@neilwyatt
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1 June 2014 at 14:30 #154059
Neil Wyatt
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@neilwyatt
My inverter and motor are both set up for monitoring of motor temperature, but neither manual completely explains it

I'd quite like to enable monitoring of the winding temperature.

I have a Tec motor and in the control box two wires are brought out to a choc block. The manual hints (but does not confirm) that the motor may have a PT100 sensor.

Their website suggests this is actually 3 sensors.

It's about 20 degrees in here and the 324 ohm reading is about right for three PT100 sensors in series.

All well and good.

After running for about 5 minutes, at a couple of hundred rpm, the reading is down to 287 ohms…

I've sent a query to TEC, but does anyone have an idea what's going on.

Neil
1 June 2014 at 16:38 #154077
Thor 🇳🇴
Participant
@thor
Hi Neil,

looking forward to hear what you find out. I too have e TEC motor with the two temp. sensor wires and an inverter, but don't know how to connect the two.

Thor
1 June 2014 at 16:44 #154078
John Stevenson 1
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@johnstevenson1
Could find out but our TEC agent has just gone on holiday for two weeks and I'm off to China for 3 weeks so not going to be straight away.
1 June 2014 at 19:33 #154097
Neil Wyatt
Moderator
@neilwyatt
Connecting is 'easy':

Thermistor between 11 (0V) and C1 (current input)

1K resistor between C1 and 13 (+10V)

There is a diagram on P 2.14 of the Cub manual. C1 is the input to an A2D and uses a 250R resistor to 11 turn the current into a voltage. Wired as shown the 250R is in parallel with the PTC.

Activation is not explained well, all the book says (on P.5-9) is program H28 to 1 to activate the PTC,

Then you set H27 to a voltage between 0.00 and 5.00, default 1.60.

There is no note to explain this in the manual.

The two questions for Tec are:

1 The 'ptc' appears to get lower in resistance as it warms up?

2 How do I calculate the voltage as 1by my calculations 1.60 is far too high, but what is 'right'?

If Tec don't get back to me I'll be on to you as soon as your feet touch terra firma, John

Neil
2 June 2014 at 06:34 #154148
Les Jones 1
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@lesjones1
Hi Neil,

It looks like the motor has an NFC thermistor. I think you will have to transpose the position of the thermistor and resistor so that the voltage to the a2d increases as the temp rises. I am on holiday until June 10th so I do not have access to the manual. I suggest connecting a pot between 0 v and the reference voltage wit the slider to the a2d input. Use this to find out what voltage level trips the inverter off. When you know this you should be able to calculate the resistor value if you can find data on the thermistor that is actually .

Les.
2 June 2014 at 07:56 #154152
Les Jones 1
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@lesjones1
For NFC read NTC. (Noticed error too late to edit post.)

Les.
11 June 2014 at 10:11 #155008
Les Jones 1
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@lesjones1
Hi Neil,
Just got back from holiday. I've just been looking at your problem and I Googled PTC thermistor to confirm that my understanding is correct. (Resistance goes up with temperature.) It confirmed this but showed something I did not know. It showed that at lower temperatures the the resistance goes down as the temperature starts to rise before it starts to rapidly increase. I suspect this explains the effect you are seeing. I also looked at the data sheets on some PTC thermistors on Farnell's website and they confirmed this effect.

Les.
11 June 2014 at 11:13 #155012
Anonymous
A useful rule of thumb is that PTC thermistors are used for heating, in-rush current control and over-temperature protection. A sharp rise in resistance over a relatively narrow temperature is a useful attribute in these applications. For measurement of temperature NTC thermistors are more common, with a non-linear, but well defined curve based on the natural logarithm.

Regards,

Andrew
11 June 2014 at 12:23 #155014
Les Jones 1
Participant
@lesjones1
Hi Andrew,
One unusual use I have seen them used for is connected in series with the start winding of a motor. By doing this the start winding has a large current passing though it for a short time but as the PTC thermistor heats up the current drops to a low level. I saw this on the sealed compressor unit on a dehumidifier.

Les.
12 June 2014 at 07:13 #155084
frank brown
Participant
@frankbrown22225
Years ago I was involved with industrial PTC resistors fitted motors on pumps. The "standard" trip resistance then was 600 ohms. The were unreliable, as the PTC resistors were imbedded in the windings before baking and varnishing, they were subject to mechanical strain as the thermal stresses in the winding finally relaxed, so they went open circuit. I believe better motors had the resistors fitted into pockets so they could be replaced. Over the ten years I was involved with the 46 motors, we had about 6 thermistor failures and zero motor failures. If they fail, just fix a contact thermostat to the outside of the motor.

Frank
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