A small 1/2 hp induction motor is turned on and off with a motor-rated toggle
switch connected to a 208V 20A branch circuit. No other protection installed.


Obviously not enough protection so I'm planning to add a slow-blow 9A fuse
(rating of the motor) to keep it safe.

Any objections?

No internal overload protection?

As in a resettable overload button? Don't remember, I'll check. Can you give
me a couple of "what if's"? ("If it does, then..." and "If it doesn't,
then...")

It won't hurt anything to fuse it but you may be adjusting the fuse
size.

As in, the 9A may blow quickly and I'll have to adjust upward a little?

On Sun, 16 Feb 2014 22:23:31 -0800, Mike Cook
wrote:

A small 1/2 hp induction motor is turned on and off with a motor-rated toggle
switch connected to a 208V 20A branch circuit. No other protection installed.


Obviously not enough protection so I'm planning to add a slow-blow 9A fuse
(rating of the motor) to keep it safe.

Any objections?

Any reason that you didn't bother to disclose all the important motor
characteristics and details? Single phase or 3 phase? 1725 rpm or
something else? The nameplate of your 1/2 hp motor should specify the
operating current at 208v. What is the nameplate operating current?
Extra credit for the maker and model number.

The problem is that starting current can be 3 to 4 times the nameplate
oprating current. Your common 9A slow blow cartridge fuse isn't going
to work if your motor operates at about 4A.

You have some choices. Try plugging into this calculator and pick the
fuse type (non-time delay, dual element time delay, instant trip, or
inverse time trip) that you find appropriate.
http://www.electrician2.com/calculators/motor_ver_1.html

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On 17/02/2014 07:50, Jeff Liebermann wrote:
On Sun, 16 Feb 2014 22:23:31 -0800, Mike Cook
wrote:

A small 1/2 hp induction motor is turned on and off with a motor-rated toggle
switch connected to a 208V 20A branch circuit. No other protection installed.


Obviously not enough protection so I'm planning to add a slow-blow 9A fuse
(rating of the motor) to keep it safe.

Any objections?

Any reason that you didn't bother to disclose all the important motor
characteristics and details? Single phase or 3 phase? 1725 rpm or
something else? The nameplate of your 1/2 hp motor should specify the
operating current at 208v. What is the nameplate operating current?
Extra credit for the maker and model number.

The problem is that starting current can be 3 to 4 times the nameplate
oprating current. Your common 9A slow blow cartridge fuse isn't going
to work if your motor operates at about 4A.

You have some choices. Try plugging into this calculator and pick the
fuse type (non-time delay, dual element time delay, instant trip, or
inverse time trip) that you find appropriate.
http://www.electrician2.com/calculators/motor_ver_1.html


And does the motor have to start-up fully loaded

On 2/17/2014 1:11 AM, Mike Cook wrote:
No internal overload protection?

As in a resettable overload button? Don't remember, I'll check. Can you give
me a couple of "what if's"? ("If it does, then..." and "If it doesn't,
then...")


In the US the nameplate would typically say "thermally protected" and
there would not be a manual reset. If the motor overheats the internal
thermal protection will disconnect the motor and it will reconnect (and
restart) when the motor cools off.

Starting current is often 6 x running current.

On 17/02/14 14:23, Mike Cook wrote:
A small 1/2 hp induction motor is turned on and off with a motor-rated toggle
switch connected to a 208V 20A branch circuit. No other protection installed.


Obviously not enough protection so I'm planning to add a slow-blow 9A fuse
(rating of the motor) to keep it safe.

Any objections?

No go right ahead :-)
However I reckon you would be better off with thermal protection if the
motor does not have it.

However I reckon you would be better off with thermal protection if the
motor does not have it.

How does thermal protection work? I understand the safety mechanism, but if a
motor is heating up under load, isn't it drawing over-max current? Shouldn't
the (carefully-chosen value) fuse blow?

I need to learn about this, I know...

Mike Cook wrote:

However I reckon you would be better off with thermal protection if the
motor does not have it.

How does thermal protection work? I understand the safety mechanism, but
if a motor is heating up under load, isn't it drawing over-max current?
Shouldn't the (carefully-chosen value) fuse blow?

I need to learn about this, I know...
The phase angle of induction motors changes with varying load.
The current draw only changes a little with loads from zero
to the motor's rating. So, if the motor is rated
at 1/2 Hp, 208 V, the running current at rated load
will be something just over 2 A.
I = (745 W * 1/2 * 1.2 (efficiency))/208

At idle, the current may be 1.5 - 1.7 A with a very low power
factor (phase angle near 90 degrees lagging). At full load,
the current will be about 2.1 A with a phase angle of maybe
5 - 10 degrees. This, the motor draws more REAL POWER at
full load, but the current doesn't change much throughout the
range the motor is rated for. This is why external fusing
for motors is to protect the building wiring from fire, it
does not protect the motor from minor overload or overheating.
Internal thermal protectors actually measure the temperature
INSIDE the motor, and thus are a lot more effective.

Jon

"Mike Cook" wrote in message
...
However I reckon you would be better off with thermal protection if the
motor does not have it.

How does thermal protection work? I understand the safety mechanism, but
if a
motor is heating up under load, isn't it drawing over-max current?
Shouldn't
the (carefully-chosen value) fuse blow?

There's usually a fair bit of heaving and grunting as a motor starts
spinning - especially if it starts under load.

Even a slow-blow fuse that can handle the second or two of what is pretty
much stall-current, will carry enough steady running current to let the
magic smoke out if a fault/overload develops.

A fuse is not a replacement for a properly-sized motor OLP overload
protection device.

A fuse will protect wiring, as previously stated.. but you'll just keep
asking the question until you get the answer you want, right?

Overload protectors and TP thermal protection devices aren't the same, and
are intended for completely different situations.

The OLP will react rapidly to protect a motor in the event of a sudden
machine jam or other fault.

The application was previously stated to be a bandsaw which previously had a
smaller motor on it.

Generally, fuses aren't motor protection devices, and won't replace a proper
OLP.

--
Cheers,
WB
..............


"Mike Cook" wrote in message
...
A small 1/2 hp induction motor is turned on and off with a motor-rated
toggle
switch connected to a 208V 20A branch circuit. No other protection
installed.


Obviously not enough protection so I'm planning to add a slow-blow 9A fuse
(rating of the motor) to keep it safe.

Any objections?