VFD stop mode options on a pallet hoist?

[504bloke]

I am currently trying to size/source a resistor.

Make sure the resistor is suitable for the drive ie dont go below the drives minimum brake resistor ohms. Also use a decent brake resistor not some of the rubbish thats around. Heres a good site with brake resistor info but its in the UK

http://www.reo.co.uk/braking_resistors

I normally have the resistor fitted with a thermal switch for overheat and feed this back to the drive or a plc.

 

[bikerdude]

Hi,

AB recommends either IPC or PowerOhm companies here in the states. I have a quote request in at PowerOhm now. Looks like 100 Ohm (62 Minimum) & somewhere around 300-400 watts. Does that seem like the right size for a 5 HP, 480vac motor @ 5% duty cycle? I'll get one with a thermo switch & run the enable input through it.
BD

 

[allscott]

What kind of drive? IIRC most of the new AB pflex drives have a provision for an internal braking resistor although you will obviously get a higher wattage resistor if mounted externally.

Secondly, to be clear, you can't just hook the resistor up to the DC bus because that would put DC voltage on the resistor all of the time. A braking kit includes a transistor to switch power from the bus to the resistor as the DC voltage rises.

 

[DickDV]

Bikerdude, if the drive already has the chopper installed, you will not have the DC bus rails available on the terminal strip. Instead, you will have two resistor terminals. All you need to do is attach the resistor to those terminals.

I think your resistor wattage is ok but the ohm value should go down closer to the drive minimum resistance of 62 ohms. It doesn't have to match exactly, just don't go under 62 ohms.

If your motor has an auxiliary blower, you have no reason to worry about motor temperature when braking. I would give some thought to connecting those P1/P2 thermostat leads to a drive external fault loop and turning off the overload protection in the drive. It's always better to measure rather than to estimate motor temperature and the happy extra result is that you will be able to get more out of the motor using the thermostats. That is because the estimate has to be overly conservative since it is only an estimate. The thermostats, if chosen correctly, will be right at the insulation temp limit giving more thermal overhead than otherwise.

Be sure to understand the difference between DC injection braking and snubber or dynamic braking. DC injection braking can only be used with the drive set to "coast to stop" so there is no direct motor speed control down the ramp to stop. The DC simply makes the motor resist rotation and stopping occurs faster. On the other hand, snubber braking works with the drive in "ramp to stop" mode. The drive commands the motor to decelerate at the preset rate regardless of changes in load or anything else. The ramp is enforced so stopping time is the same each time. The braking energy flows backward into the drive and the drive wastes it as heat in the resistor. With DC injection braking, the braking energy results in heat in the motor and the motor's fan is expected to carry it away the same as when motoring.

 

[bikerdude]

Hey Guys,

Here is the dirty right from the PowerFlex manual (pflex-rm001e-en-e.pdf).
Quote from page 221:
"
How Dynamic Braking Works
When an induction motor’s rotor is turning slower than the synchronous

speed set by the drive’s output power, the motor is transforming

electrical energy obtained from the drive into mechanical energy

available at the drive shaft of the motor. This process is referred to asmotoring. When the rotor is turning faster than the synchronous speed

set by the drive’s output power, the motor is transforming mechanical

energy available at the drive shaft of the motor into electrical energy that can be transferred back to the drive. This process is referred to as regeneration.

Most AC PWM drives convert AC power from the fixed frequency utility

grid into DC power by means of a diode rectifier bridge or controlled

SCR bridge before it is inverted into variable frequency AC power.

Diode and SCR bridges are cost effective, but can only handle power in

the motoring direction. Therefore, if the motor is regenerating, the

bridge cannot conduct the necessary negative DC current, the DC bus

voltage will increase and cause an overvoltage fault at the drive. More

complex bridge configurations use SCRs or transistors that can

transform DC regenerative electrical power into fixed frequency utility

electrical energy. This process is known as line regeneration.

A more cost effective solution can be provided by allowing the drive to

feed the regenerated electrical power to a resistor which transforms it

into thermal energy. This process is referred to as dynamic braking."

Here is a pic of the "Snubber" circuit.
A Dynamic Brake consists of a Chopper (the chopper transistor and

​

related control components are built into PowerFlex drives) and a
Dynamic Brake Resistor.

Chopper

The Chopper is the Dynamic Braking circuitry that senses rising DC bus

voltage and shunts the excess energy to the Dynamic Brake Resistor. A

Chopper contains three significant power components:

The Chopper Transistor is an Isolated Gate Bipolar Transistor (IGBT).

The Chopper Transistor is either ON or OFF, connecting the Dynamic

Brake Resistor to the DC bus and dissipating power, or isolating the

resistor from the DC bus. The most important rating is the collector

current rating of the Chopper Transistor that helps to determine the

minimum resistance value used for the Dynamic Brake Resistor.

I assume that the resistor will connect to the terminals marked BR1 & BR2, these terminals are connected to the chopper circuit shown above correct? Have I assumed incorrectly again? Sorry about the text formatting, it didn't copy/paste very well from the PDF.

BD

 

[kamenges]

Yes, you are correct. Connect to BR1 and BR2.

Keith

 

[allscott]

Just a couple more things on braking resistors.

- You will probably need to use high temperature wire.

- Put a label somewhere near it that says "Caution hot" or something to that effect. What you have amounts to a 400W electric heater and these things can get quite hot depending on the duty cycle.

- Try not to mount it to the control panel if you don't have to, no sense transfering heat into a panel unnecessarily. If you have no choice but to mount it to the panel I usually make some standoffs to lift them off the panel a little.

 

[Steve Bailey]

By all means, mount the brake resistor where it won't cook any devices in the neighborhood. I've seen many of them mounted on top of the cabinet housing the drive and with an expanded metal cage surrounding the resistor to keep inquisitive fingers away. Many braking resistors available from the drive manufacturers include a thermal switch that opens if the brake gets too hot. It's a good idea to monitor that switch and shut down the drive if it opens. An overheated brake is telling you that you didn't select the correct size for your application.

 

[allscott]

No doubt they are mounted on the top of cabinets all the time. The reason I use standoffs is because of one nasty incident that I had with a brake resistor mounted directly to the cabinet. I had a chopper on one drive fail on. The top of the cabinet got so hot it actually started to melt the plastic on the SLC cards that were mounted inside the cabinet. Not a good day....

I've never heard of this happening to anyone else so it's definately not a common occurence but for the little bit of effort it takes to mount them with some clearnce I always do.

 

[bikerdude]

Hi,

Thanks for the heads up on the mounting & heat issues. We have another machine with a chain driven hoist that has the expanded metal enclosure. I don't like the idea of exposing the circuit to the cleaning crew around here. Moisture seeping into the open enclosure would be bad! I'll probably have to install it externally with a drip shield & a warning sticker. Maybe the cabinet interior has room, maybe not. I'll have to wait & see how big the sucker is. If mounted internally I'll put a ventillation system in the cabinet to let the heat out.

I see that Automation Direct has a 80 ohm 500watt resistor assembly in stock for $90. Has anyone used the Automation Direct dynamic braking resistors before? Seems like a kind of cheap price compared to the other suppliers. Would I be getting what I pay for if I use the more expensive supplier?
BD

 

[alan_505]

If you use a PowerFlex700 it would have made the lifting application easier as this unit has torque proving built in, it does not allow the mechanical brake to release until the drive is supporting the load
, also the drive has full control of the brake and you can set the apply and release time in the drive to allow for mechanical timings.
I once replaced a 8kW servo motor controlled by an AB 1394 servo controller, with a 11kW induction motor and a PF700, using the built in torque proving it made setting up the system very easy.


Alan

 

[dirtyc]

Controlling motor brake via PLC....??

This is unrelated to the wiring/sizing of a brake, as this thread was started, but I think my tangent question is relevant to all controls guys:

Is it a good idea, safe (or even legal) to control a overhauling load's motor brake coil with a PLC output? Shouldn't it be a fail-safe system, like safety relays or a safety PLC?

If a PLC output point to your brake coil failed in the conducting position, then your brake would always be energized (brake off) and never apply even if your programmed logic was changing it to do so.

Anyways, this is the impression I'm under, as I was once taught.
However, I also see some frequent and long time posters commenting here and no one mentioning anything awry - so I must be mistaken. I'm wondering what I'm missing

 

[DSMR]

This is unrelated to the wiring/sizing of a brake, as this thread was started, but I think my tangent question is relevant to all controls guys:

Is it a good idea, safe (or even legal) to control a overhauling load's motor brake coil with a PLC output? Shouldn't it be a fail-safe system, like safety relays or a safety PLC?

If a PLC output point to your brake coil failed in the conducting position, then your brake would always be energized (brake off) and never apply even if your programmed logic was changing it to do so.

Anyways, this is the impression I'm under, as I was once taught.
However, I also see some frequent and long time posters commenting here and no one mentioning anything awry - so I must be mistaken. I'm wondering what I'm missing

What we usually do with Brakes is power the brake power through our safety system.

Now something that is very important to mention is that you may want to use a timed safety monitoring card or programmable safety controller so that you can issue a stop command programmatically before applying a safety stop. This allows your system to stop process using normal beefy methods rather than wear out your brake with a violent stop (Especially if you are using a holding brake).

Now I do not know all your specifics but the idea is E-stop hit -> Safety delay starts -> Try to stop the motion as fast as you can -> STO or Safety Rated Contacts drop power. This is a generalization.

Also consider if the pallet is at the top position and the motor was de-energized. If the brake was not there would there be any motion? If so it is a single point of failure and should be addressed.