DC motor question

newbie

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I am looking at an application that requires a dc motor to be physically rotated with the leads disconnected. Do I have to worry about induced EMF?

The simplest explanation is this: picture a cassette player that uses one motor for forward and one motor for reverse. The driven motor pulls the tape off the free spinning one. Both motors are controlled by the same drive and switched with a relay.
 
Leads would be connected to relay contacts that are in the open position while the motor is being turned - when the relay closes the contacts, the motor will get power and run.

So, as long as the leads are terminated I should be ok?
 
Last edited:
What is the DC Motor's field type?
If it is a wound shunt field, then if you can also shut down the field supply (common feature on many drives), then there will be minimal back EMF generated. If it is a permanent magnet field, or you can't shut down the field supply, then the back EMF at rated speed will be whatever the armature voltage is at rated speed.

A driven DC motor with a field is a generator if there is any field.

What do you mean 'Terminate the leads'?

Again, if you have any kind of field present, then if the armature leads are open, you will develop full rated armature voltage (in the opposite polarity of running voltage from your example), but as long as there is no load, that isn't a problem. Terminating the armature to a resistor will turn the motor into a brake. Terminating the armature by shorting it out is definitely not advisable.
 
rdrast said:
Again, if you have any kind of field present, then if the armature leads are open, you will develop full rated armature voltage (in the opposite polarity of running voltage from your example), but as long as there is no load, that isn't a problem. Terminating the armature to a resistor will turn the motor into a brake. Terminating the armature by shorting it out is definitely not advisable.
Click to expand...

A1 and A2 from the drive will connect to a 2 pole switch or relay which will direct the power to one motor or the other. The unpowered motor will have its' leads terminated to the switch, but the switch/relay will be open. I'm concerned that emf would build up and arc in the switch/relay or that there would be arcing when switching.

I guess I'm picturing a jacob's ladder being created between the switch/relay contacts.
 
Put A1 and A2 to a breaking resistor just before unloading the drive, one second delay. Placing the motor back online just remove the resistor at startup. Add more relays and a TOF timer to accomplish this.
 
newbie said:
A1 and A2 from the drive will connect to a 2 pole switch or relay which will direct the power to one motor or the other. The unpowered motor will have its' leads terminated to the switch, but the switch/relay will be open. I'm concerned that emf would build up and arc in the switch/relay or that there would be arcing when switching.

I guess I'm picturing a jacob's ladder being created between the switch/relay contacts.
Click to expand...

You are thinking too hard. As long as the armature leads are open and the switch/relay you are using is rated properly there will be no arcing, just make sure the motor is stopped before closing your switch.
 
Just for safety, when DC motor is not being used (other motor is working), disconnect field voltage ( use a contactor/relay) and also diconnect armature ( use contactor/realy), this way even if motor is rotating wil not generate any DC voltaje. Be careful with interlocks.
 
Guys, for a small cassete-player motor, the field is most likely a magnet, and the voltage is probably 9 or 12 volts. I doubt if it will ever build up enough to arc across the relay contacts. Besides, most manufacturers would think this is an advantage - it might burn out after a year or two, and have to be replaced sooner!
 
Lancie1 said:
Guys, for a small cassete-player motor, the field is most likely a magnet, and the voltage is probably 9 or 12 volts. I doubt if it will ever build up enough to arc across the relay contacts. Besides, most manufacturers would think this is an advantage - it might burn out after a year or two, and have to be replaced sooner!
Click to expand...

Actually, this is an industrial application with a fractional HP motor. The cassette player reference was to give you an idea what we are trying to do. We are modifying an existing piece of equipment, if designed from scratch it would have been a single motor with gearing.

Thanks again!
 
yes when motor turns it will have voltage on it, so when closing the relais before motor stopped, you could have a problem, however that is depending on the power source, normally there are diodes inside it to prevent this EMF, so no worries.
to prevent this use a change over contact this way you can short the motorleads, thus no Voltage, however some current will be flowing.
 
Actually, this is an industrial application with a fractional HP motor. The cassette player reference was to give you an idea what we are trying to do.
Click to expand...
Okay, but it was very misleading for me. I am not the brightest bulb on the tree.
 
One more question: the motor is a 90 volt permanent magnet - no field. What's the best way to determine that the motor has stopped? Do I even need to worry about it with dynamic braking? It's a KB drive.
 
newbie said:
One more question: the motor is a 90 volt permanent magnet - no field. What's the best way to determine that the motor has stopped? Do I even need to worry about it with dynamic braking? It's a KB drive.
Click to expand...

That would be a question to ask KB I would think.
 

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