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Participant

Join Date: Mar 2017
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Start a PM 3KW DC Motor Without Controller

03/04/2017 10:26 AM

We're looking to start a machine without a controller- we don't need additional functions, just on/off. The working environment can be dusty, affecting sensitive controllers, and (of course), we'd like to eliminate the high cost of the controller for a simple starter. Possible?

Or, would all the "work arounds" (limited starts because of heat, etc) for a simple starter add up to the cost for a controller?

Incoming power: single phase 220V

Motor: Brushed 3KW DC permanent magnet

Inrush needs to be below 100amps

Thanks for expending brain cells on our quest!

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#1

Re: Start a PM 3KW DC motor without controller

03/04/2017 10:45 AM

You will need an armature series resistor for starting otherwise you will have lots of blown fuses.

A fairly simple two stage starter should do what you want.

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#8
In reply to #1

Re: Start a PM 3KW DC motor without controller

03/06/2017 12:19 PM

Thank you, sir, our team is looking at this.

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Guru

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#2

Re: Start a PM 3KW DC motor without controller

03/04/2017 11:38 AM

Doing DOL (Direct On Line) starting of single phase electric motors of ath KW rating and even far higher is normal and you can likely did the same with yto DC motor.

All you need to do is make sure your bridge rectifier is capable of handling at least 5 - 6 times the motor's maximum rated nameplate current which by rough estimate would probably be around 15 - 18 amps.

Given that most any honest rated bridge rectifier with a 600 volt 30 amp average (not peak) rating will work for you without issues.

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#3

Re: Start a PM 3KW DC Motor Without Controller

03/04/2017 3:12 PM
  1. What rated voltage and current and speed is motor?
  2. Is it a brushed commutator rotor with Permanent Magnet Field or what?
  3. Any series field windings or other variations?
  4. What winding resistances?
  5. Do you have voltage, current, speed characteristic curve?
  6. Is start "on-load" or "no-load"?
  7. What are "moments of inertia" of motor and load - for a stepped resistance start you will need to determine step times, if only to know "not critical"?

Rough guess is you need 1 ohm to keep to 100 amp peak. A length of 60 metres of 1 sq.mm insulated copper wire will give 1 0hm and stand 100 amp for 5 seconds with about 50 'C temperature rise.

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Guru

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#4
In reply to #3

Re: Start a PM 3KW DC Motor Without Controller

03/04/2017 3:22 PM

Apologies, brain misunderstood "100" on chart - that is 100 amp for 0.5 seconds.

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#5

Re: Start a PM 3KW DC Motor Without Controller

03/05/2017 1:34 AM

You will need a starter to limit firstly starting current - which can be upwards of 25 times full load current for wound field, and 12-15 times for PM motors, and secondly starting torque.

The armature current of a DC motor is limited by its Ohmic resistance and speed of rotation. As the resistance is extremely small (often less than 1 Ohm), then, lacking any external resistive control, speed of rotation becomes the main current limiter, and at start up, that speed is zero, so current is very high - it is not until the motor achieves RPM that sufficient back EMF is developed to limit armature current. Apart from blown fuses, armature winding failure is the most likely outcome.

With such high current, torque is also very high, the probability of damage to the motor (throwing the windings out of the slots and/or de-magnetisation of the field is not unheard of) and damage to driven equipment is also a risk due to the high torque.

Another consideration is that permanent magnet DC motors tend to be fully enclosed, mainly to prevent unwanted accumulation of ferrous detritus on the magnets, this feature tends to make them run a bit hot, and the problem could be exacerbated with higher than designed starting currents.

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#6
In reply to #5

Re: Start a PM 3KW DC Motor Without Controller

03/05/2017 10:50 AM

That would have to be one hell of a badly designed and built motor connected to one near impossibly oversized power source to experience such damage due to a common DOL startup.

In typical real life applications power source voltage droop due to multiple factors including and primarily related simple wiring series resistances and actual nearest transformer or other such local source voltage drop makes such things impossible.

In my shop I have a 15 HP air compressor with a rotating mass of over 100 pounds that does DOL starts all the time off of a 240 VAC single phase source without problems despite the fact that it has a ~ .3 -.4 ohm series resistance which going by theoretical maximum would imply it should pull at least 700 - 800 amps on start up.

Reality is it pulls ~ 130 - 140 amps and starts just fine off a common 60 amp circuit breaker due to the combination line voltage drop and its own inductive impedance effects limiting its start up current.

Which the OP's DC motor would still be very much affected and thusly current limited by as well being high ripple DC still makes its own counter EMF thus overruling what pure winding resistance based current consumption calculations would say it should draw.

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#7

Re: Start a PM 3KW DC Motor Without Controller

03/06/2017 11:57 AM

What did the motor manufacturer say during the recent telephone calls on this topic, please?

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#9
In reply to #7

Re: Start a PM 3KW DC Motor Without Controller

03/06/2017 12:32 PM

No phone calls to the manufacturer (China) were made. Our motor supplier designed a custom controller with many features (and high price tag) we just don't need- so, looking for a work around.

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