Motor Disconnect Switch on Motor With Brake

While not a PLC question, I figured there are people here who will have an answer for this question.

I have two situations:

• 460 VAC motor with a 460 VAC brake This motor is controlled with a VFD.
• 460 VAC motor with a 120 VAC brake. This motor is controlled with a reversing contactor

In either situation I am unable to power the motor brake directly from the electricity going to the motor windings.

What are the requirements/common practice for providing a motor disconnect? Do I provide:

• Single 3 pole disconnect for the motor 460 VAC and don't worry about the brake.
• Two 3 pole disconnects (one for motor, one for brake).
• Single 6 pole disconnect (one set for motor, other for brake).
• Other?

What do people here typically do?

In situations where I do have a disconnect at the motor with a brake not powered by motor voltage, it's a 6 pole disconnect so one disconnect isolates all power.

Most other situations involve exceptions (to my local code) where disconnects are not needed at the motor.

Tharon + 1 , however , I fit an aux. contact which is wired in series with the VFD enable circuit .

Paul

You need to risk assess all scenarios. Always assume the person working on the motor is stupid.

My preference would be one disconnect which covered motor and brake. Keep it simple, 2 disconnects for 1 motor, to a layman, just doesn't make sense.

keshik said:

While not a PLC question, I figured there are people here who will have an answer for this question.

I have two situations:

• 460 VAC motor with a 460 VAC brake This motor is controlled with a VFD.
• 460 VAC motor with a 120 VAC brake. This motor is controlled with a reversing contactor

In either situation I am unable to power the motor brake directly from the electricity going to the motor windings.

What are the requirements/common practice for providing a motor disconnect? Do I provide:

• Single 3 pole disconnect for the motor 460 VAC and don't worry about the brake.
• Two 3 pole disconnects (one for motor, one for brake).
• Single 6 pole disconnect (one set for motor, other for brake).
• Other?

What do people here typically do?

Click to expand...

You should not be powering the brake from the motor circuit when running from a VFD. You must power the brake coil separately. The VFD motor power is variable frequency and variable voltage and PWM, not something the coil will want to see. They burn out really fast.

When you power the brake coil from the motor power, the brake coil is on at the same time as the motor. The brake is SPRING LOADED to brake, then you RELEASE it and hold it open when the motor is running by applying power when you apply power to the motor with the starter. The brake circuit, usually a small rectifier and a DC coil, is embedded in the motor connection box, no separate wiring. So if powered by the motor starter, you are only sending 3 wires to the motor. No need for anything additional to the brake circuit. A disconnect at the motor disconnects all power going to the motor, as it should.

When you power the brake coil SEPARATELY, you must disconnect that power simultaneously when you disconnect power to the motor, regardless of the brake coil voltage. Brake coil currents on motors with integral brakes is in the area of a few amps single phase, so it's usually just done with an aux contact on the disconnect switch.

jraef said:

You should not be powering the brake from the motor circuit when running from a VFD. You must power the brake coil separately. The VFD motor power is variable frequency and variable voltage and PWM, not something the coil will want to see. They burn out really fast.

Click to expand...

Just so we are clear, the OP understood that, and said that he could not power the brake from the motor circuit because he was using a VFD (or in the case of the 120V brake circuit, the voltage was wrong from the start).

Brake coil currents on motors with integral brakes is in the area of a few amps single phase, so it's usually just done with an aux contact on the disconnect switch.

Click to expand...

Depending on the motor/brake manufacturer/model, the brake rectifier may be 3 phase as well. I've got a very large machine that has that, and has a bunch of 6 pole disconnects to switch motor and brake power off at the point of use.

jraef said:

You should not be powering the brake from the motor circuit when running from a VFD. You must power the brake coil separately. The VFD motor power is variable frequency and variable voltage and PWM, not something the coil will want to see. They burn out really fast.

Click to expand...

As already stated, I do understand the need to power the brake of a motor that is driven by a VFD separately.

jraef said:

When you power the brake coil SEPARATELY, you must disconnect that power simultaneously when you disconnect power to the motor, regardless of the brake coil voltage. Brake coil currents on motors with integral brakes is in the area of a few amps single phase, so it's usually just done with an aux contact on the disconnect switch.

Click to expand...

This is something that I haven't considered and the sort of response that I was hoping for. Thank you! For our company, it looks like the 6-pole disconnects are somewhere in the neighborhood of 6-7x more expensive than a 3-pole. However, an auxiliary on the disconnect switch is very reasonably priced. For us, I can't think of any brake rectifiers that we're using that are not single phase. Does anyone else use a motor disconnect aux for their brake circuit?

It looks like the ABB aux contacts (OA1G10) that we would use are rated for 10A at 600VAC so no problems there.

keshik said:

As already stated, I do understand the need to power the brake of a motor that is driven by a VFD separately.

Click to expand...

Sorry, I misread your original post as saying ABLE to power... not UNable to power. Brain ****...

This is something that I haven't considered and the sort of response that I was hoping for. Thank you! For our company, it looks like the 6-pole disconnects are somewhere in the neighborhood of 6-7x more expensive than a 3-pole. However, an auxiliary on the disconnect switch is very reasonably priced. For us, I can't think of any brake rectifiers that we're using that are not single phase. Does anyone else use a motor disconnect aux for their brake circuit?

It looks like the ABB aux contacts (OA1G10) that we would use are rated for 10A at 600VAC so no problems there.

Click to expand...

As pointed out by Tharon, there ARE much bigger brakes on larger motors, so make sure you investigate and confirm the brake coil current. But yes, I use "pilot duty" aux contacts rated NEMA A600 (make 60, break 6, 10A continuous) all the time for that purpose.