Large motor start up

[steviemuk]

I'm looking for the simplest way to start a large DC brushed motor on pressing a foot pedal.
The motor is rated at 72v, pulling 30 Amps under load but pulling over 60 Amps at start up.
I've fried several cheap relays and mosfets trying different soft start set ups and spark arrest component I've found on the net.

 

[duke37]

What is the power supply?

The motor field should be up to full whack before the rotor is energised. A resistor bank used to be used in trams to control torque.
If the supply is AC, then SCRs or triacs could give a gentle start and speed control.
The 250kW motor I once used had a solid state drive but this was supplied to match the motor.
Previously, the machine was driven with a Ward/Leonard set.

 

[Tha fios agaibh]

30 amps under load is a bit vague. Whats the load?
What does the motor name plate data say?
It depends on several things like the motor type and duty cycle but a 60a contactor usually is adequate. Sometimes you can reduce inrush current by adding a NTC thermistor.
But yes, a footswich and a contactor is probably the simplest.

 

[kellys_eye]

Why use a 'cheap' relay when you're expecting 60A+ to flow? You're talking 'car starter motor' type currents and the solenoid from one would handle that kind of current quite easily.

MOSFETs would equally work well but have to be 'driven' correctly. What process did you use to drive yours when you tried them?

 

[steviemuk]

The power supply is 6 leisure batteries, I did consider a resistor bank but I don't want to waist any energy if I can help it.
The load is the propeller of a 12 foot boat.
I can't find a solenoid or contractor that rated high enough "using RS Components".

 

[Bluejets]

One way would be to successively switch in one battery at a time using appropriate contactors if starting on no load.
For large current contactors I would expect it better to see an electrical wholesale supplier or just ask an electrician.

 

[kellys_eye]

A PWM motor drive using MOSFETs would work - if designed properly. It would make an ideal 'accelerator' for a boat motor.

Got any information on what you used previously?

You can get these on eBay: https://www.ebay.co.uk/itm/Relay-He...720121&hash=item3b03491b3e:g:3l0AAOSwgWBaltVf

 

[duke37]

A resistor could be used which is shorted after a few seconds. The loss of energy would be negligible.
If you use a starter solenoid, check that it can be used continuously without over heating.
I am trying to get a smaller waist, without much success.

 

[steviemuk]

A PWM motor drive using MOSFETs would work - if designed properly. It would make an ideal 'accelerator' for a boat motor.

Got any information on what you used previously?

You can get these on eBay: https://www.ebay.co.uk/itm/Relay-He...720121&hash=item3b03491b3e:g:3l0AAOSwgWBaltVf

I did make a PWM driver that I found instructions for, worked fine on 5 batteries- 60V but that was not quite fast enough and could not handle 72V

A PWM motor drive using MOSFETs would work - if designed properly. It would make an ideal 'accelerator' for a boat motor.

Got any information on what you used previously?

You can get these on eBay: https://www.ebay.co.uk/itm/Relay-He...720121&hash=item3b03491b3e:g:3l0AAOSwgWBaltVf

I've tried, https://www.ebay.co.uk/itm/HIGH-PER...e=STRK:MEBIDX:IT&_trksid=p2060353.m2749.l2649
60A one and 100A solid state relay from china (which I've seen aren't made very well) with a 47ohm resistor and a 100uf cap across.

I think when the batteries are fully charged and the sun is on the solar panels I just need a bit more suppression or contact protection before I spend £60 on a relay.

 

[Bluejets]

If you used one battery as supply for your control circuit , you could as I mentioned earlier, successively switch in one battery at a time, perhaps starting with 24v or 36v, using automotive type solenoids. They are usually rated at around 150A or more and usually fairly economical (maybe $20 a piece for reasonable quality).

You would naturally need a timer arrangement for the switching but this could be done fairly cheaply also with timer modules available on Ebay (again using 12v control supply voltage)

 

[kellys_eye]

If you are controlling a motor for a boat you will want a variable control. SImple on-off won't give you much in the way of control which is why the MOSFET PWM method would be the way to go.

It doesn't really matter what voltage/current you are faced with - properly designed it WILL work.

There are plenty of windlass controller systems out there - ok, not at 72 volts but it proves that the principle works.

 

[WHONOES]

PWM and some big fets.

 

[Mukund yadav]

I'm looking for the simplest way to start a large DC brushed motor on pressing a foot pedal.
The motor is rated at 72v, pulling 30 Amps under load but pulling over 60 Amps at start up.
I've fried several cheap relays and mosfets trying different soft start set ups and spark arrest component I've found on the net.

bud its your good operation for soft start
but need some construction for motor voltage variable control

 

[globecollector]

Go back to the PWM driver you had functioning on 60v and "work forward" and redesign the output stage for 72v. That way you start with 95% of the circuit and just modify what has already proven itself. Make sure the MOSFET(S) have sufficient Vds and have appropriate snubbers across them. Also check to make sure the mark of the PWM drive advances all the way to 100%.

Also, make sure the gate drive is of sufficient voltage to turn them fully on and get them past the linear region. Again, make sure the Vgs on their data sheet is not exceeded.

 

[KD6DYR]

For what you are wanting to do. I recommend that you go check out electromotorsports.com and get your self a controller. They have all kinds on their site along with motors and accessories. Go check them out. The problem you are experiencing with the relays that you are using they won't break the arc at 72 volts and let me just clear somthing up hear rule of thumb if that motor has a full load current of 30 amps then the lock rotor current can be as high as 300 amps. And when you close the contacts on that motor at a standstill for a few milliseconds is drawing locked rotor current. That's why you keep blowing stuff up.