Dyanamic Braking resistor

2rlp · Jul 3, 2014

Hi,

I am using a Delta C2000 Sensorless Vector VFD to control a 11KW motor on a Man Machine Hoist. As per the manual, and also as per the sales engineer, the value of Dyanamic Braking Resistor of 43 Ohms, 1500 Watts is sufficient.

However I feel, that the value should be around 43 Ohms, 8000 Watts for a 50% duty cycle.

Can anyone shed any light on this?

Thanks

Ron

danatomega · Jul 3, 2014

I wouldn't use a 50% duty cycle for the braking resistor. They are down to about 9.4% duty cycle which might be pretty close to what you need.

jraef · Jul 3, 2014

I would NEVER use SVC on a hoist, and ESPECIALLY a Man Hoist!! You could kill someone. When you have a hoist, you must have a MECHANICAL brake, because electronic braking has no effect when the power fails. So when you have a mechanical brake, you must have a sequence in which you set and release the brake. When you have the brake set and need to move, you have to release the brake. If it takes even a fraction of a second for the drive to create torque from the motor, the load will have run away and you may not be able to overcome that inertia any longer, so the load falls to the bottom. In a man lift, that can mean the death of the people in it. Seriously, don't do it.

Whenpeople use VFDs on hoists, they use drives that are capable of full Flux Vector Control, and a motor with an encoder feedback, so that the drive already knows exactly what position the rotor is in and can perform what is called a "torque proving" routine prior to allowing the mechanical brake to be released, the the full torque is ALREADY on the drive before that happens. You CANNOT accomplish that with Sensorless Vector Control, especially on a cheap little drive like a Delta.

So the details on how to do a brake resistor are moot as far as I'm concerned.

2rlp · Jul 4, 2014

danatomega said:
I wouldn't use a 50% duty cycle for the braking resistor. They are down to about 9.4% duty cycle which might be pretty close to what you need.

Thank you danatomega.
You suggest that the value of 43 Ohms, 1500 Watts is OK?

Ron

2rlp · Jul 4, 2014

Thanks jraef.

There is a mechanical brake on the system and in the past I have used Torque proving very succesfully on SVC, (on ABs 1336 Plus II). The brake will be governed by the torque in conjuction with the speed of the motor. However, my major query is for the wattage of the dynamic braking resistance to be connected to the drive chopper. I envisage that the hoist on its way down will take the DC bus voltage high and that the chopper circuit will be active through out its descent, I will definately need more than the prescribed value of 43 Ohms, 1500 Watts.

Thanks once again

Ron

rpoet · Jul 4, 2014

jraef said:
I would NEVER use SVC on a hoist, and ESPECIALLY a Man Hoist!! You could kill someone. When you have a hoist, you must have a MECHANICAL brake, because electronic braking has no effect when the power fails. So when you have a mechanical brake, you must have a sequence in which you set and release the brake. When you have the brake set and need to move, you have to release the brake. If it takes even a fraction of a second for the drive to create torque from the motor, the load will have run away and you may not be able to overcome that inertia any longer, so the load falls to the bottom. In a man lift, that can mean the death of the people in it. Seriously, don't do it.

Whenpeople use VFDs on hoists, they use drives that are capable of full Flux Vector Control, and a motor with an encoder feedback, so that the drive already knows exactly what position the rotor is in and can perform what is called a "torque proving" routine prior to allowing the mechanical brake to be released, the the full torque is ALREADY on the drive before that happens. You CANNOT accomplish that with Sensorless Vector Control, especially on a cheap little drive like a Delta.

So the details on how to do a brake resistor are moot as far as I'm concerned.

Interestingly enough, I have, for years, used simple v/Hz drives to build and operate performer flying winches for special effects; think Peter Pan and the like. I've tried several brands of drives; the (now obsolete) Mitsi E500 drives, and their replacement, the A700, worked flawlessly. The Automation Direct GS3 drives were terrible. My point is, v/Hz drives can work in hoisting applications; it just depends on the details. Nowadays, servos are cheap enough to justify their higher power density and positioning capabilities.

-rpoet

DickDV · Jul 5, 2014

1500 watts seems a bit thin in my view also. Brake resistors are often sized by the 10/1 rule but that assumes a cold resistor and a very short braking period. That is not what you have with a hoist.

The 8000watt size seems suitable if the braking period doesn't exceed 20 continuous seconds. If it does, I'd go up to 11-12000 watts as the long braking period is essentially continuous braking, as far as the resistor is concerned. (I'm assuming here that the 43 ohms is the full motor rated torque value for the drive and chopper)

2rlp · Jul 5, 2014

Thanks DickDV.
I received instructions from Delta to use a 43 Ohm 11000 W resistor.

Ron

Dyanamic Braking resistor

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