DC Motor starting current

[SuryaGG]

Please let me know what is the starting current (in terms FLC) normally recommended for DC motors.

How to decide series resistance value in case of DC compound motors?

DC Motor vendor has offered Class H insulation with temp rise to class F in place of Class F/B. Is vendor recommendation (Class H/F) better than the specified values (class F/B)?

Thanks in advance.

 

[ScottyUK]

Your motor will run hotter as a class H machine with a class F rise. The surface temperature will be high enough to cause serious burns to personnel if they were to make contact with it. Whether this is a problem depends on the application. Other equipment or the product may also be affected by the higher temperature.

The maximum current depends on at least three factors: the ability of the source to deliver current, the maximum torque required by the load, and there will inevitably be some mechanical limit imposed by the motor manufacturer and possibly by the driven equipment too. I assume you are looking at a sequential resistance starter? I would normally keep the armature current below 4x FLC for awkward loads with high breakaway torque, lower for more forgiving loads such as a centrifugal pump.

Motors can of course be designed to deliver higher torque but that may result in the motor frame size increasing to keep within mechanical limits and to allow a larger commutator and brushgear to be fitted. TENV motors are often larger than their ventilated equivalents and are mechanically capable of handling higher peak torque without damage.



----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[Skogsgurra]

I would be concerned if starting current is above 2x rated current. There will be severe sparking at higher speeds if you run more than double rated armature current. Some motors don't even tolerate 1.6 times rated before arcing starts.

Of course, if this is a small motor built for direct start across line, you may have higher currents.

The series resistance (starting resistor) is calculated as (U - E)/I where U is line voltage, E is motor EMF and I is maximum allowed current. A starting resistor usually has three or more stages that are switched out (shorted) as speed increases.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[SuryaGG]

Thanks ScottyUK and Skogsgurra.

I forgot to mention the motor rating. It is 0.55 kW, 1400 rpm, 24 V DC rated, suitable for Zone 2 area.

Since the pump starting torque is high, the motor vendor has recommended the starting current as 5 times FLC. The required pump torque is 20Nm. Please refer the attached files for motor and pump speed torque curves. Can you please advise me whether the motor selection is correct?

How to find out the required operational torque (full load torque) of motor from the given pump curve?

 

[SuryaGG]

I think the the pump curve is not attached properly. I have attached it again.

 

[ScottyUK]

FL torque looks to be about 3.7Nm so the 5x FLC would put you in the right ballpark for starting the pump. Whether the commutator and brushgear are up to the job is another question, and one best answered by the motor manufacturer.



----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[Skogsgurra]

I think we are playing in different ball parks. I was thinking 400 - 600 V and 10+ kW. I think that your current source will limit current to safe values. 24 V sources usually have a rather large internal impedance (if it isn't a car battery). Just hook it up and run!

Hell is not as hot as the priests say...

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ScottyUK]

Skogs,

Agreed - this little pump will probably be ok.

Most of our big DC stuff is 110V DC off a battery, primarily providing lube oil to the turbines under emergency conditions: 15kW on 110V is a big machine, more so when it is a TENV design. Our starting currents are limited to 4x FLC and we don't see any abnormal commutator damage.

Have you been doing some consulting work for an unusual client?


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[Skogsgurra]

"Have you been doing some consulting work for an unusual client?"

Now - that's a somewhat enigmatic question. Yes, sometimes I think most of my clients are "unusual".

Any special "unusualness" you have in mind?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

E*R=I will give the starting current of a shunt DC motor.
E: Effective voltage. This is applied voltage minus back emf. The back EMF may be estimated by multiplying the motor speed as a percentage of no-load speed times the applied voltage.
R: The resistance of the armature circuit plus the supply resistance.
The sources of error are:
1> The brush resistance is non linear.
2> The field current.
3> The no-load losses, windage, winding losses and bearing losses. These have an effect on the estimate of the back EMF.
Given the magnitude of the starting current it is often acceptable to ignore these losses.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Based on "Hell is not as hot as the priests say..." I was envisaging a forked tongue, horns, cloven hooves and a trident. Either that or you have been to Teesside, England.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[Skogsgurra]

Haha! Got it.

Sorry to have misled you, Scotty.

It is a way to tell someone that he can break the rules (if he knows the rules, that is) and if that sin takes him to Hell, well, it might be comforting to know that it isn't quite as hot as the clerks say.

Closest to Hell I have been is probably an aluminium refinery. I think all ingredients were there - including the Master himself...

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[MotorExpert]

The final temperature and thus the insulation class that you need is determined by the time that you need the extra torque and the thermal resistance of the motor in question.

Different motor needs different insulation class.

If the motor vendor has recommended the starting current as 5 times FLC than I would not buy a motor from him that has any less.

What you may want to do is to get an offer from a second supplier and compare the products offered.

Edward Hage

http://www.specamotor.com

 

[waross]

This is a DC motor, MotorExpert.
The starting current is usually so high that it is limited with external resistors. If the vendor recomends a maximum of 5X starting current then external resistors are used to limit the starting current to not more than 5X.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[MotorExpert]

Not necessaraly so. You have to limit the starting-current with an amplifier. So you don't have to burn that energy with resistors.

The motor rating is 0.55 kW, 1400 rpm, 24 V DC. That is a fairly small motor which you can simply hook to an amplifier without external resistance (use motor with permanent magnets). Of course you have to look at the additional cost of the amplifier; but then you have to look at the cost of ownership.

Amplifier and motor with PM may have initial higher cost, but the savings in energy (oilprice!) may be larger in the long run.

Edward Hage

http://www.specamotor.com